2. Key Components of Air Pollution in the Black Sea Basin

2.1. What is Air Pollution

Air pollution is an environmental problem caused by an increase in the concentration of harmful substances in the atmosphere as a result of nature or human based activities. This phenomenon manifests itself as a decrease in air cleanliness and is particularly associated with industrial activities, transportation systems, and energy production in cities. Air pollution is not only a problem affecting specific regions but also a serious environmental degradation process threatening nature and life on a global scale.

Pollutants released into the atmosphere spread over time, some directly and others through chemical reactions, and settle in a way that harms the environment. This situation can affect not only local areas but also other ecosystems through spread of wind to distant regions. Air pollution, in this sense, causes both short-term and long-term environmental changes.

This phenomenon typically arises as a result of combustion of fossil fuels, agricultural activities, industrial production, and other human-induced actions. However, some natural events can also increase air pollution. For example, natural processes such as volcanic eruptions, forest fires, and desert dust can lead to the accumulation of toxic substances in the air.

Air pollution is a reality being felt in daily life, even though it is not visible to the naked eye. Traffic congestion, high numbers of buildings, and industrial facilities, especially in enclosed areas, cause a decline of air quality in the surrounding environment. While air pollution is more noticeable in densely populated residential areas, its effects could also be noticed in rural areas through agricultural activities and direct emissions.

Today, air pollution is not only an environmental issue but also an important subject in terms of sustainable development, health policies, and economic growth. It has indirect effects on sectors such as agriculture, tourism, industry, and trade, as well as on public health. Therefore, it is a global issue that must be addressed within the framework of both public policies and private sector decisions.

Air pollution has existed in various forms throughout history. With the rapid increase in factories and fossil fuel use following the Industrial Revolution, air pollution has reached serious levels. Today, this situation is becoming increasingly widespread with the rise in energy production, transportation, and consumption. With globalization, air pollution has become a cross-border problem.

In short, air pollution is a condition that arises as a result of the increase in the concentration of harmful substances in the atmosphere, threatening human health and nature. This phenomenon, which could pose serious consequences at both local and global level, is one of the most obvious indicators of the activities of human on nature. If it continues unnoticed for a long time, it may cause irreversible damages on environment, economy, and social welfare.

2.2. Air Pollutants Causing Air Pollution

The primary pollutants affecting air quality in the Black Sea Basin are particulate matters (PM10 and PM2.5), nitrogen oxides (NO and NO2), sulfur dioxide (SO2), ground-level ozone (O3), and carbon monoxide (CO). PM2.5 is directly linked to respiratory diseases and originates from both natural (e.g., sea salt, dust) and human-made (industry, transportation) emissions. NOx compounds play a role in both tropospheric ozone formation and acid rains. SO2 is primarily emitted from coal-fired power plants. Surface ozone is formed through photochemical reactions and can reach high levels during summer months.

Air pollution is known for the harm it causes to human health, ecosystems, and climate systems through the release of harmful chemical substances into the atmosphere. The most notable elements in this process are the primary pollutants that cause air pollution. Air pollutants are generally the result of human activities. These include the use of fossil fuels, industrial emissions, transportation systems, domestic heating, and open burning. These pollutants can remain in the atmosphere for long periods of time and could cause serious effects at both local and regional levels.

The main components of air pollution include particulate matters (PM_2.5 and PM₁₀), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ozone (O₃), carbon monoxide (CO), and volatile organic compounds (VOCs). These pollutants originate from different sources and have distinct health and environmental impacts. For example, particulate matter enters the body directly through inhalation, while ozone is formed in the troposphere through reactions between NO₂ and VOCs under sunlight. Therefore, each type of these pollutants should be analyzed separately.

2.2.1. Particulate Matter (PM10 and PM2.5)

Particulate matter (PM) consists of a mixture of solid and liquid particles suspended in the atmosphere. These particles vary in size, chemical composition, and source. These structures, commonly referred to as atmospheric aerosols, have significant environmental and health impacts. Particles defined as PM10 have a diameter of less than 10 micrometers, while PM2.5 particles have a diameter of less than 2.5 micrometers. PM10 can be trapped in the upper respiratory tract, while PM2.5 can reach the deeper regions of the lungs. Even smaller PM0.1 (ultrafine particles) can mix with the bloodstream and be transported to internal organs.

Particulate matter can originate as primary particles emitted directly from sources or as secondary particles formed through the chemical transformation of gaseous substances in the atmosphere. Natural sources include volcanic activity, forest fires, pollen, desert dust, and sea salt aerosols. Anthropogenic sources, or human-made sources, include fossil fuel use, diesel engines, thermal power plants, industrial processes, construction activities, and agricultural practices. Secondary particles are formed as a result of chemical reactions in the atmosphere involving compounds such as sulfates, nitrates, organic aerosols, and ammonium salts. These reactions typically occur through photochemical processes or cloud-based processes and are found in the PM2.5 fraction.

The shape of particulate matter can be spherical, fibrous, or irregular. Their density can range from 0.5 to 3 g/cm³, while nano-particles, in particular, have a very large surface area. Their water-holding properties vary depending on the hydrophilic or hydrophobic nature of the particle. In terms of chemical composition, inorganic ions (sulfate, nitrate, ammonium), carbon compounds (elemental and organic carbon), heavy metals (lead, arsenic, cadmium), and mineral structures such as silicates are prominent.

Their persistence in the atmosphere depends on their size. Coarse particles (PM10-2.5) generally remain in the atmosphere for a few hours to a few days and settle in areas close to their origin sources. Fine particles (PM2.5) can remain in the atmosphere for days and be transported over long distances by winds. Ultrafine particles, on the other hand, quickly aggregate into larger particles or settle onto surfaces. As a result, for example, Saharan dust can reach the Americas, while particles formed in China can travel as far as the North Pacific.

Particulate matter concentration is measured using various methods. In gravimetric methods, particles are filtered and weighed using high- or low-volume samplers. Automatic measurement devices use methods such as beta absorption, vibrating microbalances (TEOM), optical scattering, or electrostatic precipitation. Additionally, remote sensing is conducted using Lidar systems and satellite data such as MODIS and CALIPSO. PM concentration is typically expressed in micrograms per cubic meter (μg/m³) and reported as 24-hour or annual average values.

Particulate matter poses serious health risks to the respiratory system. The main effects include decreased lung function, increased asthma attacks, development of COPD, and increased risk of lung cancer. In the cardiovascular system, heart rhythm disorders, increased risk of heart attack, arterial stiffness, and high blood pressure may occur. Additionally, ultrafine particles can have adverse effects on the nerve system, increase the risk of diabetes, cause pregnancy complications, and lead to premature deaths, particularly in vulnerable groups. According to the World Health Organization, PM2.5 exposure is associated with approximately 4.2 million premature deaths worldwide each year.

Particulate matter also has significant environmental impacts. It has both direct and indirect effects on the climate system. Substances such as sulfates reflect sunlight, causing a cooling effect, while substances such as black carbon absorb light, causing warming. They contribute to indirect climate changes by affecting cloud formation. By accumulating on snow and ice, they reduce the reflection rate, which can lead to the melting of glaciers. On ecosystems, they can accumulate on leaf surfaces, inhibit photosynthesis, disrupt soil chemistry, and cause acidification or eutrophication in water bodies. They also have negative impacts on structures and materials; such as building facades may become dirty, historical artifacts may erode, and electronic devices may malfunction.

Some international standards for particulate matter are as follows: According to the World Health Organization, the annual limit value for PM2.5 is 5 μg/m³, and for PM10 it is 15 μg/m³. The European Union has set limits of 25 μg/m³ for PM2.5 and 40 μg/m³ for PM10. The United States Environmental Protection Agency (EPA) has established annual limit values of 12 μg/m³ for PM2.5 and 150 μg/m³ for PM10 (24-hour average). In China, these limits are 35 μg/m³ for PM2.5 and 70 μg/m³ for PM10.

Various strategies are being implemented to reduce PM pollution. Systems such as electrostatic precipitators and bag filters in industrial chimneys and particulate filters in diesel vehicles are being used. Dust suppression measures are being taken in construction areas, and clean combustion technologies are being promoted. At the city level, measures such as increasing green spaces, promoting public transportation, developing bicycle lanes, and limiting the use of coal for heating are being prioritized. At the individual level, it is recommended to monitor air quality indices, reduce time spent outdoors during high particulate levels, use indoor air purifiers, and wear N95 masks.

In recent years, technological developments have gained momentum in the fight against PM pollution. Low-cost sensor networks enable more widespread data collection, and artificial intelligence-based prediction models are being developed. New filter systems designed with nanomaterials and the integration of satellite data are also being utilized in this process. With climate change, changes in the nature of particulate pollution are also expected. In particular, increasing forest fires may increase particulate emissions from biomass sources. Therefore, PM pollution remains a priority issue for environmental and public health policies.

2.2.2. Nitrogen Dioxide (NO₂)

Nitrogen oxides (NOx) are gaseous compounds composed of various combinations of nitrogen (N) and oxygen (O) atoms, characterized by their reactive properties in the atmosphere. The term NOx typically refers to the compounds nitric oxide (NO) and nitrogen dioxide (NO₂). NO is a colorless and paramagnetic gas, while NO₂ is a reddish-brown, pungent-smelling, and toxic gas. Other nitrogen-containing gases include diazot monoxide (N₂O), diazot trioxide (N₂O₃), and diazot pentoxide (N₂O₅). NOx compounds are thermodynamically unstable in the atmosphere and can convert into one another. Nitric oxide, in particular, rapidly reacts with ozone or oxygen in the atmosphere to form nitrogen dioxide. This conversion rate varies depending on atmospheric conditions such as temperature, pressure, and the presence of other pollutants.

The main formation pathways of nitrogen oxides are grouped under three main mechanisms: thermal, fuel-related, and prompt NOx formation. Thermal NOx is formed primarily at high temperatures through the direct reaction of molecular nitrogen and oxygen. This process is known as the Zeldovich mechanism and plays a dominant role in high-temperature combustion systems such as gas turbines, coal-fired power plants, and industrial furnaces. Fuel NOx, on the other hand, is produced during the combustion of fuels with high nitrogen content, such as coal and fuel oil. In this process, nitrogen compounds in the fuel first convert into intermediate compounds (e.g., HCN and NH₃), which then oxidize to form NO or N₂. Prompt NOx formation occurs in low-temperature environments with abundant fuel, resulting from the reaction of hydrocarbon radicals with molecular nitrogen, and is also referred to as the Fenimore mechanism.

NOx emissions are divided into two categories: natural and anthropogenic. Natural sources include microbial activities in soil (nitrification and denitrification), lightning, stratospheric transport, forest fires, and gases emitted from the ocean surface. These sources account for approximately 50% of global NOx emissions. The transportation sector is the primary anthropogenic source. Road vehicles, particularly diesel-powered vehicles, are responsible for significant NOx emissions. Maritime transport and aviation are also notable emission sources. Energy production activities (thermal power plants, natural gas power plants) and industrial processes (nitric acid production, metallurgy, chemical industry) increase NOx production. Agricultural activities, fertilizer applications, and biomass burning are also added to this list.

Globally, the sectoral distribution of NOx emissions is approximately 44% from transportation, 28% from energy production, 18% from industry, and 10% from other sources.

NOx in the atmosphere undergoes various chemical reactions, creating significant effects on air quality and climate. The photochemical cycle between NO and NO₂ forms the basis for the formation of tropospheric ozone. In this process, NO₂ is photolyzed by sunlight (λ < 420 nm), and the resulting oxygen atoms combine with molecular oxygen to form ozone (O₃). At the same time, NO reacts with the formed ozone and is being converted back to NO₂. This equilibrium continues permanently in the presence of sunlight. Furthermore, NO₂ reacts with hydroxyl (OH) radicals in the atmosphere to form nitric acid (HNO₃). NO₃ radicals then react with VOCs to cause the formation of organic nitrates. HNO₃ reacts with ammonia to form ammonium nitrate (NH₄NO₃) in the particulate phase. Such transformations limit the atmospheric lifetime of NOx to 1–2 days, whose duration may vary depending on seasonal and geographical conditions.

NO₂ exposure has serious effects on human health. Respiratory disorders such as decreased lung function, bronchitis, and asthma attacks are among the main effects. Additionally, it can cause heart rhythm disorders and increase the risk of heart attacks. Children, the elderly, and individuals with chronic respiratory conditions are particularly vulnerable to this gas. Some compounds indirectly caused by NOx, such as nitrosamines, have potential carcinogenic effects.

Among the environmental effects, NOx can cause acid rain by converting into nitric acid, lead to eutrophication in freshwater ecosystems, create necrotic lesions on plant leaves, and inhibit photosynthesis. It can also disrupt soil chemistry, leading to acidification and disruption of nutrient cycles. From a climate perspective, NOx contributes to the greenhouse effect by supporting the formation of tropospheric ozone. Additionally, an indirect cooling effect can be observed through nitrate aerosols. Furthermore, the presence of NOx extends the lifespan of greenhouse gases such as methane in the atmosphere.

One of the primary methods used for NOx measurement is the chemical reaction technique. In this method, NO reacts with ozone to form excited NO₂, which then emits light and returns to its ground state. The light intensity is measured to determine the NO concentration. DOAS (Differential Optical Absorption Spectroscopy) calculates the average concentration over a long distance by measuring absorption at multiple wavelengths. Infrared spectroscopy techniques such as FTIR and NDIR are used, particularly for detecting compounds like N₂O. Continuous emission monitoring systems (CEMS), which are mandatory in industrial facilities, determine real-time NOx concentration and emission rates and transmit the data to environmental authorities. Satellite-based sensors and lidar systems also provide data for air quality modeling.

International standards for NO₂ are designed to protect public health. The World Health Organization (WHO) has set the annual average limit value for NO₂ at 10 μg/m³ and the 24-hour average limit at 25 μg/m³. The European Union has set an emission limit of 150–200 mg/Nm³ for large combustion plants, while the limit for diesel vehicles under the Euro 6 standard is 80 mg/km. The U.S. Environmental Protection Agency (EPA) has established limits of 53 ppb for the annual average and 100 ppb for the 1-hour average.

Policies to reduce NOx emissions have been developed on a sector-by-sector basis. In the transportation sector, measures such as improving fuel quality, catalytic converters (e.g., SCR, TWC), and exhaustgas recirculation (EGR) are widely used. In the energy sector, low NOx burners, post-combustion control systems, and switching from coal to natural gas are prominent strategies. In industrial processes, the focus is on process optimization, gas purification systems, and leak prevention.

Reduction technologies are divided into two groups as primary and secondary measures. Primary measures aim to reduce NOx formation during combustion and include low-NOx burners, staged combustion, air/fuel ratio control, and reducing the nitrogen content of the fuel. The most commonly used secondary measures are Selective Catalytic Reduction (SCR) systems. In this method, NO reacts with ammonia in the presence of a catalyst at 300–400°C to form nitrogen and water. It achieves efficiency of up to 90%. The Non-Selective Catalytic Reduction (SNCR) method, on the other hand, is applied at 900–1100°C with lower investment costs and offers 30–70% efficiency. Adsorption processes and oxidation methods are also additional solutions.

Current research focuses on the development of zeolite-based catalysts, new perovskite-structured materials, and single-atom catalysts. Approaches such as the direct removal of NOx using electrochemical methods and even its conversion into ammonia are also gaining popularity. Biological methods, particularly the use of denitrifying bacteria is promising ways of treating low-concentration emissions. Advances in sensor technology are enabling the development of low-cost, mobile, and artificial intelligence-supported NOx monitoring systems.

From a future perspective, significant trends are emerging in NOx control. With the widespread adoption of electric vehicles a decrease in transportation-related emissions is expected. The transition to a hydrogen economy may bring NOx formation in high-temperature combustion systems back into focus. Increasing forest fires due to climate change are increasing natural NOx emissions, while new policies for controlling agricultural emissions are also on the agenda. Industrialization and increasing energy demand, especially in developing countries, have made NOx emission control to be a global environmental policy priority.

2.2.3. Sulfur Dioxide (SO₂)

Sulfur dioxide (SO₂) is a sharp-smelling, colorless, and toxic gas that plays an important role in atmospheric chemistry. This compound, with the molecular formula SO₂, has a molecular weight of 64.066 g/mol. It melts at -72°C, boils at -10°C, and reaches a density of 2.6288 kg/m³ at 25°C. It is highly soluble in water (94 g/L at 20°C) and, due to this feature, it gets in to reaction rapidly in humid atmospheric conditions. Its molecular structure is polar, with two oxygen atoms bonded to a sulfur atom at the center, arranged at an angle of 119°. It reacts with water to form sulfurous acid (H₂SO₃), which is an important component of atmospheric acidification processes.

SO₂ is released into the atmosphere through both natural and human-induced processes. Among the natural resource volcanic activities are the most noticeable ones. Stratovolcanic eruptions, continuous volcanic degassing, and submarine volcanism cause large amounts of SO₂ emissions. Additionally, the decomposition of sulfur-containing organic matter, emissions from dimethyl sulfide (DMS) at the sea surface, and the metabolic activities of certain microorganisms are among the biogenic sources. Forest fires, geothermal activities, and dust storms are other natural factors that increase SO₂ emissions.

Anthropogenic SO₂ emissions primarily result from the combustion of fossil fuels. In particular, coal-fired power plants, industrial boilers, and residential heating using low-quality coal are the largest sources of this gas. Additionally, industrial processes such as the metallurgy sector (e.g., copper, lead, and zinc smelting), petroleum refining plants, sulfuric acid production, and paper pulp manufacturing produce significant amounts of SO₂. Agricultural applications such as diesel engines, waste incineration facilities, and fertilizer production also contribute to emissions. The sulfur content of fuels varies between 0.5–5% in bituminous coal, 0.5–10% in lignite, and 0.5–3.5% in fuel oil, while it is present in natural gas sparingly.

SO₂ in the atmosphere undergoes complex chemical reactions in both the gas and liquid phases, transforming in to be various secondary pollutants. SO₂ molecules excited by sunlight react with oxygen and water vapor to form sulfuric acid (H₂SO₄). Additionally, reactions initiated by OH radicals result in the formation of SO₃, which then combines with water vapor to produce H₂SO₄. In heterogeneous processes, SO₂ dissolves in cloud droplets or on particle surfaces and undergoes in to oxidation. Its reactions with sea salt aerosols may result in the formation of sulfate salts (e.g., Na₂SO₄) and hydrochloric acid (HCl).

The sulfate aerosols formed as a result of these reactions play an important role in the atmosphere. These particles reduce visibility by scattering light, influence cloud formation by acting as cloud condensation nuclei, and create a cooling effect in the climate system. The atmospheric lifetime of SO₂ is approximately 2 to 4 days, which may vary depending on meteorological conditions and the presence of other pollutants.

SO₂ also has numerous adverse effects on the environment. Its most well-known effect is causing acid rain. SO₂, which turns into sulfuric acid in the atmosphere, reaches the earth’s surface with precipitation, lowering soil pH, acidifying aquatic ecosystems, and damaging vegetation. In plants, it penetrates through stomata, causing cell death, inhibiting photosynthesis, and forming necrotic lesions on leaf surfaces. Additionally, inorganic carbonate-based construction materials (e.g., limestone, marble) react with SO₂ and undergo erosion. This can cause irreversible damage, particularly to historical artifacts. In metals, it accelerates corrosion.

The effect of SO₂ on the climate system is mostly occurs indirectly. Sulfate aerosols reflect sunlight, causing a cooling radiative forcing. They also, increase the number of cloud droplets which alter the reflectivity (albedo) of clouds. SO₂ reaching the stratosphere during large volcanic eruptions can cause global cooling effects lasting several years.

In terms of human health perspective, SO₂ has both acute and chronic effects. Short-term exposure at concentrations of 5–10 ppm can cause throat irritation, 10–50 ppm can cause eye irritation and coughing, and above 50 ppm can cause bronchospasm and pulmonary edema. Long-term exposure can lead to reduced lung function, emerge of chronic bronchitis, and an increase in asthma symptoms. An increase in the risk of cardiopulmonary mortality has also been observed. Children, the elderly, individuals with COPD and asthma, and smokers are among the sensitive groups. The World Health Organization reports that SO₂ exposure causes approximately 4 million premature deaths worldwide each year.

Various measurement techniques are used to determine SO₂ concentrations. The UV fluorescence method is based on the principle that SO₂ emits light in the 240–420 nm range when excited at 214 nm. In the chemical absorption method, SO₂ is dissolved in a hydrogen peroxide solution for analysis. Gas chromatography provides high-precision analysis using sulfur-selective detectors. In industrial facilities, continuous emission monitoring systems (CEMS) are mandatory and perform real-time concentration and emission rate calculations. Low-cost passive samplers are also widely used and analyzed in the laboratory using ion chromatography.

Various international air quality standards have been established for SO₂. The World Health Organization has set a limit value of 20 μg/m³ for the 24-hour average. The European Union has set a maximum value of 350 μg/m³ for 1 hour and a limit of 125 μg/m³ for 24 hours. The United States Environmental Protection Agency (EPA) has set a limit of 196 μg/m³ (75 ppb) for a 1-hour average.

Numerous technological solutions are applied to control SO₂ emissions. Methods such as coal washing, hydrodesulfurization of petroleum products, and microbial biosulfurization are used for fuel purification. Interventions during combustion include fluidized bed combustion, lime injection, and oxygen-enriched combustion technologies. One of the most common applications is flue gas treatment. In wet scrubbing systems, SO₂ reacts with limestone to form calcium sulfate (gypsum), achieving an efficiency of 90–98%. High removal rates can also be achieved in dry and semi-dry systems. Innovative methods such as membrane separation, electron beam oxidation, biological treatment systems, and oxygen-enriched combustion are among the new-generation technologies.

The economic dimension of SO₂ control should also be considered. The investment cost of wet FGD systems ranges from 150–300 $/kW, while dry systems cost 100–200 $/kW. The cost of desulfurization systems for maritime transport can vary between 2–5 million dollars per vessel. Reactive consumption, waste disposal, and energy usage constitute operational costs. However, the benefits gained in the health, agriculture, and construction sectors largely offset these costs.

Current research focuses on the use of metal-organic frameworks (MOFs), ionic liquids, and graphene-based materials for SO₂ capture. The evaluation of waste from flue gas treatment as construction materials, sulfuric acid recovery, and biological sulfur leaching (biorefining) techniques are also gaining popularity. High-resolution modeling based on satellite data is contributing to the improvement of emission inventories.

In conclusion, while SO₂ control is an area where significant progress has been made in air quality management, there are still significant implementation gaps, particularly in developing countries. Accelerating the transition for clean energy, promoting the widespread adoption of best available technologies, and implementation of sulfur recovery strategies in line with circular economy principles will be critical in reducing SO₂ emissions on a global scale.

2.2.4. Ozone (O₃)

Ozone (O₃) is an allotropic molecule composed of three oxygen atoms. Its molecular structure has a bent geometry with a bond angle of approximately 116.8° bond angle. Physically, it is a light blue, sharp-smelling gas with a molecular weight of 47.998 g/mol. It melts at -192.2°C and boils at -111.9°C. At 0°C, it has a density of 2.144 kg/m³ and plays a significant role in atmospheric chemistry due to its high reactivity.

Ozone can be found in three different layers of the atmosphere. Its highest concentration is found in the stratosphere, approximately 15–35 kilometers above the Earth’s surface. In this region, ozone forms a layer known as the ozone layer, which prevents harmful ultraviolet (UV) rays from reaching the Earth’s surface. Stratospheric ozone accounts for approximately 90% of total atmospheric ozone and reaches its maximum concentration at an altitude of approximately 25 kilometers.

In contrast, ozone found in the troposphere, the layer closest to the Earth’s surface, is referred to as “bad ozone” because it has adverse effects on human health and the environment. While the natural background concentration ranges from 20–45 ppbv, this value can rise to 100–200 ppbv. Additionally, the tropopause, the transition zone between the stratosphere and the troposphere, is important for the vertical transport of ozone and can influence atmospheric ozone balance.

Stratospheric ozone is primarily formed through photochemical processes known as the Chapman mechanism. In these processes, oxygen molecules (O₂) undergo photodissociation when exposed to short-wavelength UV radiation (λ < 242 nm), resulting in the formation of free oxygen atoms. These atoms then form ozone by merging with existing oxygen molecules. Ozone can also be broken down by UV radiation and decompose into oxygen molecules and free atoms. These processes occur in a balanced state.

Ozone in the troposphere is not a direct emitted pollutant but is emerge as a result of the reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) with sunlight. In this photochemical smog formation process, NO₂ photolyzes to form free oxygen atoms, which then combine with molecular oxygen to form ozone. This process is supported by radical chain reactions involving VOCs, leading to increased ground-level ozone concentrations, especially during sun shine rich sunny days in summer.

Ozone is influenced by both natural and anthropogenic sources. Naturally, it can be formed through processes such as lightning, biogenic VOC emissions (e.g., isoprene and terpenes), forest fires, and the photolysis of carbonyl compounds. Human-induced ozone production is associated with activities such as motor vehicle emissions, industrial processes, solvent use, and fossil fuel combustion.

Ozone contributes to the formation of secondary organic aerosols (SOA) through chemical reactions in the atmosphere. The reaction of ozone with alkenes results in the formation of low-volatility compounds, which eventually transition into the particulate phase and transform into SOA, negatively impacting air quality. Additionally, during winter months, especially under inversion conditions, the accumulation of NOx and the dissolution of temporary compounds like peroxyacetyl nitrate (PAN) can lead to the formation of “winter ozone.”

The effects of ozone on human health are particularly noticeable in the respiratory system. In cases of acute exposure, even at levels of 80–120 ppb, respiratory distress may be observed in sensitive individuals. At levels of 120–180 ppb, a decrease in lung function is observed, while levels above 180 ppb result in noticeable symptoms in the general population. Chronic exposure may contribute to the development of COPD, suppress lung development in children, and increase the risk of cardiovascular disease. Ozone can also affect the nervous system and increase the permeability of the blood-brain barrier. The most sensitive groups include asthma patients, the elderly, children, and occupational groups working outdoors.

Ozone also causes serious damage to plants. It penetrate plant leaves through stomata, damages cell membranes, inhibits photosynthesis, and reduces the activity of the Rubisco enzyme. These effects can result in yield losses of up to 10–30% in sensitive crops such as wheat, cotton, and soybeans. At the ecosystem level, ozone can lead to reduced biological productivity in forests, changes in species composition, chain reactions in aquatic ecosystems, and disruptions in soil microbial balance.

Ozone is also effective over building materials. It causes crackings in rubber and elastomers, color fading in paints, and loss of durability in textile fibers. Therefore, measuring and monitoring ozone is important.

Ozone concentrations can be monitored by use of various reference measurement methods. UV absorption spectrophotometry operates based on the Beer-Lambert law at a wavelength of 254 nm and is in compliance with international standards. The chemical method, on the other hand, is based on the principle of ozone reacting with ethylene to emit light and provides high sensitivity. DOAS (Differential Optical Absorption Spectroscopy) enables ozone measurement over long distances. Additionally, satellite-based measurements (such as OMI, TROPOMI), lidar systems, and passive samplers can also be used to monitor ozone.

Limit values set by organizations such as the World Health Organization (WHO), EPA, European Union (EU), and China vary depending on exposure durations of 8 hours and 1 hour. For example, the WHO recommends a limit of 60 μg/m³ for an 8-hour average. The EU uses the seasonal AOT40 index (ozone accumulation above 40 ppb) to measure the effects of ozone on agriculture.

In controlling ozone formation, reducing both NOx and VOC emissions is critical. Emission standards introduced for motor vehicles (such as Euro 6, Tier 3), selective catalytic reduction (SCR) systems used in industrial processes, promoting products with low VOC content, and vapor recovery units are among the effective measures in this area. Regional strategies include the trade of ozone precursors, VOC restrictions during summer months, and emergency response protocols.

Technological solutions to reduce ozone pollution include the use of clean fuels (e.g., LNG, hydrogen), electric vehicle infrastructure, bicycle lanes, smart traffic systems, and air quality early warning systems. Additionally, green infrastructure initiatives and industrial VOC control technologies (e.g., thermal oxidation, carbon adsorption, biofiltration) are among the strategies which aim at limiting ozone formation.

Ozone has complex interactions with climate change. Tropospheric ozone acts as a greenhouse gas with a radiative forcing effect of approximately 0.4 W/m². It also affects the carbon cycle by reducing plant carbon uptake. On the other hand, temperature increases can increase biogenic VOC emissions and alter stratosphere-troposphere dynamics, thereby affecting ozone balance.

Current research encompasses a wide range of technologies, from quantum dot-based sensors to artificial intelligence-supported modeling, high-resolution chemical transport models, and the use of local satellite data. Under international climate agreements such as the Paris Agreement, the control of short-lived climate pollutants (SLCPs) like ozone, sustainable urban planning, and circular economy practices have gained importance.

In conclusion, combating ozone pollution requires a multifaceted approach. Coordinated control of NOx and VOC emissions, integration with climate policies, and the development of new technologies will be among the cornerstones of sustainable air quality management.

2.2.5. Carbon Monoxide (CO)

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas composed of one carbon and one oxygen atom. This compound, with a molecular weight of 28.01 g/mol, melts at -205°C and boils at -191.5°C. At room temperature, CO exists in the gaseous phase with a density of 1.25 g/L, and its solubility in water is 27.6 mg/L at 25°C. Its molecular structure features a partial triple bond with a bond length of approximately 112.8 picometers. Despite these physical properties, carbon monoxide is an extremely hazardous air pollutant for human health and the environment.

The toxic effect of CO primarily stems from chemical reactions with hemoglobin (Hb). When inhaled, CO binds with hemoglobin in the blood to form carboxyhemoglobin (COHb), significantly reducing the blood’s ability to carry oxygen. The affinity of hemoglobin for carbon monoxide is 200 to 250 times greater than that for oxygen. Additionally, CO inhibits the mitochondrial cytochrome oxidase enzyme, which is responsible for cellular energy production, thereby preventing tissues from utilizing oxygen.

Carbon monoxide is released into the atmosphere through natural and human-caused (anthropogenic) processes. Natural sources include forest fires, activities of soil microorganisms, plant metabolism, emissions from the ocean surface, volcanic gases, and geothermal sources. However, anthropogenic sources play a more dominant role in the atmospheric load of CO. In particular, incomplete combustion of fossil fuels is the primary production pathway for this gas. Internal combustion engines, coal and wood stoves, industrial boilers, blast furnaces used in steel production, petrochemical plants, and certain chemical production processes (e.g., formaldehyde and methanol production) are the primary sources. Additionally, cigarette smoke and indoor heating devices can also contribute significantly to CO emissions. Globally, approximately 2,500 Tg of CO is emitted annually from natural sources, while 1,000 Tg comes from anthropogenic activities. The transportation sector accounts for 55% of these emissions, industrial processes contribute 20%, and residential heating makes up 15%.

CO is a reactive gas in the atmosphere and primarily reacts with hydroxyl (OH) radicals to form carbon dioxide (CO₂). This process also creates a significant impact on the atmospheric oxidant balance. Soil microorganisms can also oxidize CO and remove it from the atmosphere. However, the atmospheric lifetime of CO is generally between 1 and 3 months. Concentrations can reach up to 1-50 ppm in urban areas and 0.05-0.5 ppm in rural areas. It exhibits relatively homogeneous distribution in the troposphere; however, due to its long lifetime, it can be transported to the stratosphere and have indirect effects on ozone chemistry.

From a health perspective, CO is extremely dangerous. Effects ranging from mild headaches to respiratory failure can occur depending on the level of COHb caused by inhaled CO. At COHb levels below 10%, symptoms are generally not observed, while at levels of 20-30%, symptoms such as headaches and dizziness may appear. At levels of 40-50%, symptoms such as confusion, hallucinations, and syncope may occur. COHb levels above 60% can lead to respiratory arrest and may result in death. Long-term and low-level exposure may increase the risk of cardiovascular disease, impair neurocognitive functions, and cause fetal development disorders in pregnant women. Heart patients, pregnant women, newborns, children, chronic respiratory patients, and anemia patients are among the most sensitive groups.

The environmental effects of CO are also noteworthy. By reacting with the OH radical and consuming it, CO causes greenhouse gases such as methane to remain in the atmosphere for longer periods. This increases CO’s indirect greenhouse gas effect. Additionally, it triggers the formation of tropospheric ozone, affects the carbon cycle, and has a radiative forcing potential estimated at approximately 0.23 W/m². CO can influence microbial activity in soil, inhibit plant growth, and cause indirect harm by increasing oxygen consumption in aquatic ecosystems.

CO concentrations are being measured thorugh various methods. Infrared absorption spectroscopy provides accurate measurements, particularly at a wavelength of 4.6 μm. Gas chromatography offers high-resolution analysis using molecular sieve columns and FID detectors. Electrochemical sensors offer low-cost, portable solutions and are particularly suitable for personal exposure monitoring. In industrial facilities, continuous emission monitoring systems (CEMS) are used to collect and report real-time data.

There are established air quality limits for CO worldwide. The World Health Organization (WHO) recommends a limit of 6 mg/m³ for an 8-hour average, while the EPA has set a limit of 35 ppm for a 1-hour average and 9 ppm for an 8-hour average. Similar regulations are in place in different regions, such as the European Union and China. From an occupational health perspective, organizations such as OSHA, ACGIH, and NIOSH have established occupational exposure limits.

Various control technologies, such as combustion optimization, catalytic converters, thermal oxidation systems, oxygen enrichment, and alternative fuels, are used to reduce CO emissions. Catalytic converters used in vehicles, in particular, have the capacity to remove 90% of CO. Additionally, clean energy solutions such as electric vehicles, biofuels, and hydrogen-enhanced combustion systems are effective methods for reducing emissions.

Carbon monoxide buildup in enclosed spaces poses serious risks. Improperly used heaters, gas stoves, poorly ventilated garages, and tobacco smoke are significant indoor sources. Therefore, the use of CO detectors, regular chimney cleaning, and effective ventilation systems are of vital importance. In industrial facilities, fixed monitoring systems, personal protective equipment, emergency response plans, and employee training programs must be implemented to prevent CO-related workplace accidents.

In recent years, nanotube-based sensors, optical resonance systems, quantum dot detectors, and systems integrated with mobile devices have provided more sensitive and portable CO measurement capabilities. Additionally, the biological degradation of CO has become possible through the use of biological treatment systems and carboxylic acid-degrading microorganisms. New-generation catalysts (e.g., MOF and perovskite-based) are being developed to create more efficient and sustainable treatment systems.

Carbon monoxide is a priority air pollutant on a global scale due to its acute toxic properties that directly affect human health and its effects on atmospheric chemistry. Although significant technological and political progress has been made in combating CO, indoor air pollution and industrial emissions remain serious public health issues, particularly in developing countries. In future, policies for transition to clean energy and protection of vulnerable groups and integrated air quality management systems will be decisive in the fight against CO pollution.

2.2.6. Volatile Organic Compounds (VOCs)

Volatile Organic Compounds (VOCs) are carbon-based chemical compounds with high vapor pressure at room temperature and can exist in the atmosphere in a gaseous state. These compounds, which typically contain carbon between C₃ and C₂₀, have boiling points ranging from 50°C to 260°C and molecular weights between 30–300 g/mol. They have a high tendency to vaporize and can easily mix into the atmosphere under environmental conditions.

VOCs are classified into subgroups based on their chemical structure, such as aliphatic (e.g., hexane, butane), aromatic (benzene, toluene, xylene), oxygenated (acetone, ethanol, formaldehyde), halogenated (trichloroethylene, chloroform), and terpenes (α-pinene, limonene). Based on their sources, they are classified as biogenic (natural systems such as plants, soil, and ocean surfaces) and anthropogenic (industry, transportation, consumer products). Additionally, they are categorized into high, medium, and low reactive VOCs based on their atmospheric reactivity. Highly reactive VOCs (e.g., isoprene, terpenes) can cause significant environmental effects despite their short atmospheric lifetimes due to their rapid chemical reactions.

Natural VOC emissions, primarily consisting of isoprene, monoterpenes, and oxygen-containing compounds emitted by deciduous and coniferous trees, amount to approximately 1,150 Tg of carbon per year. Anthropogenic emissions, on the other hand, are approximately 150 Tg of carbon per year. Major human-generated VOCs include compounds such as toluene, benzene, and xylene. Activities such as motor vehicle exhaust, industrial facilities, paint and solvent use, cleaning products, and construction materials causes to VOC emissions.

VOCs undergo various photochemical reactions in the atmosphere, transforming them to be secondary pollutants. Reactions with hydroxyl (OH) and nitric (NO₃) radicals can result in the formation of ozone (O₃), formaldehyde, organic nitrates, and secondary organic aerosols (SOA). These processes, in the presence of sunlight and nitrogen oxides (NO_x), lead to the formation of photochemical smog. The atmospheric lifetimes of VOCs range from a few hours to several months. Highly reactive VOCs affect local air quality, while less reactive ones can travel long distances, causing regional and global impacts.

VOCs have both acute and chronic effects on human health. Acute effects include headaches, dizziness, coordination problems, eye and throat irritation, nausea, and heart rhythm disturbances. Chronic exposure may lead to carcinogenic effects such as benzene-related leukemia, nasopharyngeal cancer associated with formaldehyde, and an increased risk of lymphoma linked to 1,3-butadiene. Additionally, long-term VOC exposure can cause neurological damage, liver and kidney dysfunction, endocrine system disruption, and adverse effects on reproductive health. Children, the elderly, pregnant women, and individuals with chronic illnesses are more sensitive to these pollutants.

From an environmental perspective, VOCs trigger the formation of ground-level ozone, reducing plants’ photosynthesis capacity and causing the degradation of various surface materials. VOCs’ oxidative products are secondary organic aerosols, particulate matter increase their concentration, thereby reducing air quality and shortening visibility. Additionally, VOCs contribute to climate change directly or indirectly. For example, some VOCs like methane have a strong greenhouse gas effect, while others contribute to indirect radiative forcing through SOA and ozone formation. VOCs can also affect cloud microphysics by acting as cloud condensation nuclei.

Reference methods for measuring VOCs include gas chromatography with mass spectrometry (GC-MS/FID), proton transfer reaction mass spectrometry (PTR-MS), and Fourier transform infrared spectroscopy (FTIR). These techniques enable both laboratory and field measurements of complex and precise VOC concentrations. Additionally, online gas chromatography systems, PID detectors, and low-cost wireless sensors are used for continuous monitoring.

There are national and international regulations for the control of VOCs. The World Health Organization (WHO), EPA, and the European Union have established air quality limit values for specific VOC compounds. For example, the annual average limit value for benzene is 5 μg/m³, while the 30-minute limit value for formaldehyde is 0.1 mg/m³. To limit emissions, Euro 6 and Tier 3 standards are being applied to vehicles, process-based limits are being implemented for industrial facilities, and VOC content restrictions are being imposed on products.

Various technologies are used for VOC control. Water-based products, high-solid formulations, and UV curing systems are preferred for source control, while process modifications, closed systems, and automatic application technologies are becoming more widespread. In waste gas treatment, advanced methods such as regenerative thermal oxidizers (RTO), activated carbon adsorption, biological treatment systems, membrane separation, plasma treatment, and photocatalytic oxidation are employed.

Current research focuses on innovative technologies such as satellite-based measurement systems (e.g., TROPOMI) for remote sensing of VOCs, artificial intelligence-supported air quality prediction models, microchip-based portable GC systems, and MOF-based adsorbents. Additionally, solvents developed in line with biotechnological approaches and green chemistry principles contribute to sustainable production goals.

In conclusion, VOCs require integrated control strategies due to their multifaceted impacts on both human health and the environment. Personalized exposure monitoring systems, smart city solutions, sustainable production models, and climate-air quality integration will be key components of future VOC management.

2.2.7. Lead (Pb)

Lead (Pb) is a soft, malleable heavy metal with atomic number 82 and atomic weight 207.2. In the context of air pollution, lead is typically found in particulate form (PM) or as compounds in the gas phase. Elemental lead (Pb°), inorganic compounds (e.g., PbO, PbSO₄, PbCO₃), and organometallic compounds (such as tetraethyl lead) exist in various forms. Its melting point is 327.5°C, boiling point is 1749°C, and vapor pressure at 973°C is 1 mmHg.

Chemically, lead is classified into two groups: inorganic and organic. Inorganic lead primarily originates from industrial emissions, while organic lead has been used as a fuel additive in the past. Lead may be found for more than 90% in PM₁₀ and 50-70% in PM_2.5. Emissions can be direct (primary) from sources or secondary, resulting from chemical reactions in the atmosphere.

Natural sources of lead include geogenic processes such as volcanic activity, soil erosion, and sea salt aerosols, as well as biogenic processes such as forest fires and plant-based emissions. Annual natural emissions are around 25-50 Gg.

Anthropogenic sources are more concentrated. Historically, leaded gasoline has been one of the most significant sources, along with industrial processes such as coal combustion, metallurgical activities, battery and paint production, electronic waste recycling, and waste incineration facilities. These emissions are estimated to be between 350-400 Gg annually. The most common emission compounds are lead oxide (PbO), lead sulfate (PbSO₄), and tetraethyl lead ((CH₃CH₂)₄Pb).

Lead undergoes oxidation and acid-base reactions in the atmosphere, transforming into different compounds. Lead compounds in the gas phase can dissolve in cloud droplets and transition to the particulate phase. The atmospheric lifetime of lead in particulate form is 5–10 days, and its transport can range from local (up to 10 km) to regional (100–1,000 km) and global levels (stratospheric transport).

Exposure to lead typically occurs through inhalation (via PM), ingestion (contaminated food and water), and dermal exposure (especially organic lead compounds). Approximately 30–50% of inhaled lead is absorbed by the lungs and then binds to erythrocytes, entering the bloodstream. Lead undergoes various redox reactions in the body. Excretion primarily occurs through urine (75%), feces (15%), and hair/nails (10%).

Lead poses serious health risks to humans. It can negatively affect cognitive development in children, causing a 2-3 point decrease in IQ for every 10 μg/dL increase in lead levels. In adults, it can cause peripheral nervous system damage, anemia (inhibition of heme synthesis), kidney failure, cardiovascular disorders, and toxic effects on reproductive health.

Lead also has long-lasting effects on the environment. It does not break down easily in soil, so it is absorbed by plants and causes bioaccumulation. In water systems, it can accumulate in sediments and cause biomagnification throughout the food chain. Additionally, lead particles can act as cloud condensation nuclei, affecting climate processes by absorbing infrared radiation and disrupting radiative balance.

The primary analytical methods for lead concentrations are ICP-MS and XRF. The ICP-MS method can measure even very low concentrations (0.1 ng/m³) in all particle fractions, while XRF provides a faster and field-applicable analysis. Beta absorption monitors are used in conjunction with XRF in continuous monitoring systems. Laser ablation techniques also offer advanced analytical capabilities.

Air quality limits for lead are set by the World Health Organization (WHO) at an annual average of 0.5 μg/m³, the same level as EU standards. The US Environmental Protection Agency (EPA) has established a limit value of 0.15 μg/m³ for a three-month average.

Industrial filtration systems (electrostatic precipitators and bag filters) for emission control, fuel additive bans, and policies such as the use of unleaded gasoline play an important role. Promoting electric vehicles is another strategy. In waste gas treatment, dry (bag filters) and wet (lime-based scrubber systems) methods offer high efficiency.

Among new technologies, nanomaterial-based adsorption systems, biological treatment (bioremediation) techniques, and satellite-based global monitoring methods are gaining prominence. These developments offer promising solutions for reducing the health and environmental impacts of lead.

2.2.8. Other Pollutants

Heavy metals and aromatic hydrocarbons such as benzene, lead (Pb), arsenic, cadmium, and benzo(a)pyrene are also important components of air pollution. These pollutants are typically produced by industrial activities, waste incineration, and certain vehicle emissions. These substances particularly increase the risk of cancer and have adverse effects on the immune system. Polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene, are powerful carcinogenic substances released during combustion.

Among the pollutants posing the greatest threats to air quality in Türkiye are PM_2.5, PM₁₀, NO₂, SO₂, and O₃. According to reports in the Turkish Environment Press Bulletin, these pollutants occasionally exceed their limit values, especially in large cities. In metropolitan areas such as Ankara, Istanbul, and Izmir, traffic congestion and emissions from domestic heating are the main causes of air pollution. In Black Sea coastal cities, air pollution risks increase during winter months due to meteorological conditions and industrial activities.

The areas with the highest levels of air pollution in Romania are large cities, primarily Bucharest, Brașov, and Cluj-Napoca. The most common pollutants in Romania are PM₁₀ and PM_2.5. The main sources of particles are heating systems, construction activities, transportation, and industrial facilities. NO₂ and O₃ levels are also high. Ozone levels can reach dangerous levels, especially during the summer months.

The Russian Federation is affected by both local sources and regional transport in terms of air pollution in its southwestern regions, which are part of the Black Sea Basin. Industrial density, energy production methods, transportation infrastructure, and climatic effects are the main factors determining air quality in these regions. Industrial facilities in Krasnodar Krai, Rostov-on-Don, and surrounding areas are significant sources of air pollution.

Air pollution is also prevalent in countries such as Bulgaria and Georgia, particularly in large cities and industrial areas. The primary pollutants in these countries include PM_2.5, PM₁₀, NO₂, and SO₂. Outdated industrial facilities and inefficient heating systems are the main factors contributing to air pollution in these countries.

Air pollution knows no borders among the countries of the Black Sea Basin. Pollutants produced in one country can be carried by winds to other countries. For this reason, inter-country cooperation in the region is of vital importance. The Black Sea Economic Cooperation Organization (BSEC), which is active in the region, offers platforms for cooperation on environmental issues. However, such organizations need to be transformed into more concrete projects.

The World Health Organization (WHO) has recommended lower limit values for many pollutants in its new air quality guidelines published in 2021. For example, the annual average guideline value for PM_2.5 has been reduced from 10 μg/m³ to 5 μg/m³, with a 24-hour value set at 15 μg/m³. For PM₁₀, the WHO annual guideline is 15 μg/m³, with a 24-hour value of 45 μg/m³. The 2021 guidelines recommend an annual value of 10 μg/m³ and a 24-hour value of 25 μg/m³ for NO₂, while the 24-hour value for SO₂ is set at 40 μg/m³. Compliance with these values in Türkiye and other Black Sea countries is of critical importance for protecting public health.

According to data from the European Environment Agency (EEA), air pollution levels in urban areas in Türkiye are well above European standards. In particular, PM_2.5 and NO₂ levels exceed WHO limits. This situation highlights the need to revise Türkiye’s air quality regulations. Under the Air Quality Assessment Regulation (HKDY) in Türkiye, air quality limit values and target values have been established for 13 pollutants. These pollutants include SO₂, NO₂, NOx, PM₁₀, PM_2.5, O₃, CO, benzene, arcenic, cadmium, nickel, and benzo(a)pyrene.

For each pollutant, both short-term limit values (such as hourly or daily averages) and long-term limit values (annual averages) have been established. For example, the 24-hour limit value for PM₁₀ is 50 μg/m³, and it is permitted to exceed this value 35 times per year. However, considering the WHO’s 2021 guideline values, these values are still considered insufficient.

2.3. Relationship Between Pollutants and Meteorological Variables

The behavior, dispersion, transport, transformation, and accumulation of air pollutants in the atmosphere are to a large extent determined by meteorological conditions. Variables such as temperature, wind speed and direction, relative humidity, atmospheric pressure, precipitation, sunshine duration, and temperature inversions influence both the physical and chemical processes of pollutants, thereby playing a fundamental role in determining air quality.

Temperature is the primary variable affecting the rate of atmospheric reactions. Photochemical processes such as ozone and secondary particle formation increase at high temperatures. Tropospheric ozone is formed during hot summer days under sunlight through reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). As temperature rises, VOC emissions also increase, leading to significant increases in ozone levels. Additionally, the atmospheric lifetimes of pollutants such as carbon monoxide (CO) and nitrogen dioxide (NO₂) can vary depending on temperature. CO reacts with OH radicals and is broken down in the atmosphere; however, OH concentrations also depend on temperature. High temperatures also facilitate the vaporization of VOCs at ground level, which supports the formation of secondary organic aerosols (SOA).

Wind can prevent local accumulation by facilitating the horizontal transport of pollutants, but it may also, cause pollutants to be transported to other regions. In calm atmospheric conditions with low wind speeds, the air becomes stagnant, pollutants accumulate at ground level, and concentrations increase. This situation leads to high concentrations of NO₂, CO, and PM2.5, particularly in urban areas.

High wind speeds, on the other hand, can temporarily improve air quality by diluting particulate matter and gaseous pollutants. However, particles such as desert dust, sea salt, and industrial emissions can be transported over long distances, causing regional and even continental-scale pollution.

Humidity levels also play a decisive role in pollutant dynamics. Gases such as sulfur dioxide (SO₂) and nitrogen dioxide (NO₂) can react with water vapor under high humidity conditions to form sulfuric and nitric acids. These compounds combine with atmospheric particulate matter to form acidic aerosols. High humidity also supports the formation of secondary particles. Fine particles such as PM2.5 can grow larger in humid environments due to hygroscopic growth, becoming more dangerous to health. High humidity also promotes cloud formation, which initiates wet deposition processes that facilitate the deposition of pollutants.

Atmospheric pressure indirectly affects air quality by influencing the vertical movement of air masses. High-pressure systems are typically characterized by calm weather conditions and can cause inversion layers to form. In this case, cold air becomes trapped near the ground, while warm air remains in the upper layers, preventing the vertical mixing of pollutants. As a result, pollutants such as PM, CO, and NO₂ become concentrated near the surface. These inversion events, commonly observed during winter months, are one of the primary causes of air pollution in large cities. In low-pressure systems, vertical air movements increase, facilitating the dispersion of pollutants through convection.

Precipitation is one of the most effective natural processes in cleaning pollutants from the atmosphere. Particulate matter (PM10, PM2.5) and water-soluble gases (SO₂, NO₂, HNO₃) are washed away by rain droplets and transported to the ground. This process is called wet deposition and generally improves air quality temporarily. However, pollutants can also be transported to the ground surface through dry deposition; this process is slower. Snowfall also effectively removes particulate matter from the atmosphere. During long periods without precipitation, pollutant accumulation can accelerate.

Sunlight duration and radiation intensity are triggers of photochemical reactions. Especially, during the summer months, prolonged sunlight exposure increases the formation of ozone and secondary organic aerosols. The photolysis of NO₂, which is necessary for ozone formation, is directly dependent on sunlight. Additionally, high UV radiation accelerates the reactions of VOCs with radicals, thereby promoting the formation of secondary products (e.g., formaldehyde, glyoxal). These compounds contribute to both ozone formation and particulate matter formation.

Temperature inversions are among the most critical meteorological determinants of air pollution. Especially during nighttime hours, when the Earth’s surface cools rapidly while the upper atmosphere remains warmer, a temperature inversion occurs. This layering prevents pollutants from dispersing vertically, causing them to accumulate in areas close to the surface. This is one of the main reasons for high PM2.5 and NO₂ concentrations during winter months. The simultaneous occurrence of fuel use for heating, insufficient wind, and inversion significantly negatively impacts urban air quality.

In addition to the short-term effects of meteorological variables, seasonal and climatic scales are also known to play a decisive role in pollutant dynamics. Increased ozone and SOA levels in summer and rising PM and NO₂ concentrations in winter are examples of this seasonal pattern. Along with climate change, long-term increases in temperature, the frequency of extreme weather events, and the spread of forest fires are reshaping the behavior of air pollutants in the atmosphere. In particular, the increase in biogenic VOC emissions with temperature can trigger ozone and particulate pollution. Similarly, dust transport during drought periods can cause significant increases in PM10 levels.

In conclusion, the presence of air pollutants in the atmosphere depends not only on the amount of sources but also on the physical and chemical conditions of the atmosphere. Effective monitoring of meteorological variables and their use in integrated air quality models are critical for both short-term air quality forecasts and long-term sustainable environmental policies.

2.4. Air Quality Index (AQI) and Assessment Criteria

Air pollution is one of the most significant global environment issues of today and can cause serious harm to human health, ecosystems, and the climate system. Therefore, it is essential to continuously monitor air quality and communicate it to the public in an understandable manner. In this context, the Air Quality Index (AQI) plays a crucial role in informing the public by expressing pollutant levels as a single numerical value. The AQI is used as an indicator system that evaluates overall air quality by combining measurements of different pollutants.

AQI is usually calculated separately for each pollutant, and then the highest value is defined as the official AQI value for that day. For example, if the PM2.5 level is measured as 178, the O₃ level as 95, and the NO₂ level as 43 on a given day, the AQI value for that day is taken as 178. Thus, the most dangerous pollutant reflects the daily air quality status. This approach ensures that more realistic and meaningful information is provided to the public.

AQI values are explained using classifications established according to international standards. The generally used classification is as follows:

• 0–50: Good – No risk to the general public.
• 51–100: Moderate – Sensitive groups should take precautions.
• 101–150: Unhealthy for Sensitive Groups – Low risk for individuals other than those with respiratory conditions such as asthma or COPD.
• 151–200: Unhealthy – Adverse effects may be observed in the general population.
• 201–300: Very unhealthy – There is a health risk for the entire population.
• 301+: Hazardous – Serious health risks are present; it is recommended to stay indoors.

This classification is the basic structure adopted by the World Health Organization (WHO), the European Environment Agency (EEA), the US Environmental Protection Agency (EPA), and other international organizations. In Türkiye, the Air Quality Index (AQI) is used under the Air Quality Assessment and Management Regulation and is disclosed to the public by local authorities through publicly available data.

The primary pollutants considered in the calculation of the AQI include PM_2.5, PM₁₀, SO₂, NO₂, CO, and O₃. Threshold values are set for each pollutant index is created based on these values. For example, according to the WHO’s 2021 guideline values, the annual average for PM_2.5 is average of 5 μg/m³ and a maximum value of 15 μg/m³ for 24 hours. In Türkiye, according to the HKDYY regulation, the annual average for PM_2.5 is set at 15 μg/m³, which is three times the value recommended by the WHO.

The WHO 2021 guideline for PM₁₀ is an annual average of 15 μg/m³ and a 24-hour average of 45 μg/m³, while the limit applied in Türkiye is quite close to this value. However, this difference may be significant, particularly for individuals with chronic conditions such as asthma and COPD. For NO₂, the WHO recommends an annual average of 10 μg/m³ and a 24-hour limit of 25 μg/m³, while Türkiye’s annual limit is set at 20 μg/m³. This indicates that Türkiye’s air quality standards are more lenient compared to WHO guidelines.

For SO₂, the WHO recommends a 24-hour limit of 40 μg/m³, while Türkiye’s limit is 50 μg/m³. For CO, an 8-hour average limit of 10 mg/m³ and for O₃, an 8-hour limit of 60 μg/m³ are used. While these values are in line with EU directives, they are considered insufficient when compared to the WHO’s new guidelines.

In Türkiye, the AQI is calculated by the Air Quality Monitoring Network (HKİA) operated by the Ministry of Environment, Urbanization, and Climate Change. This network collects real-time data through fixed monitoring stations located in metropolitan municipalities and some industrial areas. These data are published in daily AQI reports at the provincial level. However, there are some shortcomings in terms of analyzing these data and making them available to the public. In particular, data gaps in small cities and rural areas make it difficult to make accurate decisions about the regional distribution of air pollution.

AQI data is an important tool not only for public information but also for decision-making processes by policymakers. Prolonged AQI levels at “unhealthy” or “very unhealthy” levels require emergency action plans. Such warnings may lead to measures such as reducing emissions from domestic heating sources, controlling traffic density, or implementing temporary production cuts at industrial facilities.

The purpose of the AQI is not only to produce data but also to provide timely warnings to protect the public. Therefore, the accurate interpretation of AQI values is of critical importance for public health. For example, when the AQI value exceeds 150, children, the elderly, and individuals with chronic illnesses are advised to avoid going outside. Additionally, steps such as canceling sports activities in schools and adjusting work hours for outdoor workers may be taken.

AQI values in the Black Sea Basin rise particularly during the winter months. The main reasons for this include the use of coal and wood for domestic heating, emissions from industrial facilities, and meteorological conditions. The humid climate of the Black Sea region causes particulate matter to remain in the atmosphere for longer periods, while temperature inversions create a layer where pollutants settle, further deteriorating air quality.

In cities such as Zonguldak, Samsun, and Trabzon in the region, AQI values frequently reach “unhealthy” levels during the winter months. This situation has negative effects, particularly on children and the elderly. Studies conducted in these cities have concluded that there is an increase in hospital visits on days when AQI levels are high. Increasing trends in respiratory infections, asthma attacks, and heart attacks clearly demonstrate the link between AQI and health.

AQI is used for both short-term and long-term assessments. Short-term assessments reflect daily air quality conditions, while long-term assessments are based on average AQI values over years. Long-term average AQI values are an important indicator for assessing the overall air quality status of a region and its impact on health. Average annual AQI values in Türkiye are well above WHO limits, especially in large cities. According to the data, more than 99% of the Turkish population breathes air that is considered polluted according to WHO standards.

Monitoring and reporting AQI values serve as a fundamental basis for policy development at both national and regional levels. AQI data is shared among the Black Sea Economic Cooperation Organization (BSEC) countries, and joint solutions are being developed within the framework of regional cooperation. However, the capacity to share and analyze this data varies among countries. Therefore, regional cooperation and technical support are of vital importance.

AQI is not merely a technical indicator but also a powerful tool for public communication. Therefore, the manner in which AQI values are communicated to the public is of great importance. Visual warning systems using color coding are highly effective in raising public awareness. For example, green means “good,” yellow means “moderate,” orange means “unhealthy,” red means “very unhealthy,” and purple means “hazardous.” Such visual systems can be more effective, especially among individuals with lower levels of education.

In Türkiye, AQI data is regularly published by the Ministry of Environment, Urbanization, and Climate Change. However, the way this data is communicated to the public is not always effective. In some cities, AQI data is only presented in technical reports, while in others, it is shared through social media channels using simple visuals. For effective public communication, AQI data must be provided in a simple, understandable, and visually supported format.

AQI data is also used to raise public awareness and develop policies. Organizations such as the Turkish Respiratory Research Association (TÜSAD), the Clean Air Rights Platform, and the Chamber of Environmental Engineers of the Turkish Chamber of Engineers and Architects (TMMOB) prepare reports based on AQI data and develop policy recommendations to combat air pollution. These organizations have emphasized that Türkiye’s 2029 targets are five times higher than the WHO guidelines. This highlights the need to reassess Türkiye’s air quality standards.

AQI data can also be used for educational purposes. In teaching programs recommended by UNESCO and the World Health Organization, students can learn about AQI data, conduct graphical analyses, and develop solution proposals in science, social studies, and citizenship courses. Such projects aim to increase environmental awareness among young generations while contributing to the creation of more informed societies in the future.

Activities can be organized in schools under the name of “clean air day.” For example, students can take samples with air quality measurement devices in the school garden and evaluate these data within the AQI classification framework. Such applications are beneficial both in terms of environmental education and the development of scientific thinking. In addition, awareness can be raised by organizing seminars and information brochures for parents to encourage family participation.

AQI data is also used in the evaluation of public policies. When evaluated based on AQI data, Türkiye’s air pollution control policies are found to be insufficient. According to IQAir data for 2022, Türkiye ranked 46th in the world, with Iğdır and Düzce listed among Europe’s most polluted cities. These data demonstrate how important AQI is as a tool for policymakers.

AQI data is also used in the strategic planning of local governments. Municipalities can take measures to control traffic based on AQI data, allocate budgets to increase green spaces, and launch campaigns to reduce fossil fuel use. However, access to data and analytical capacity are critical for local governments to make such decisions.

The Air Pollution Control Regulation (HKKY), Türkiye’s first comprehensive air quality legislation, was enacted in 1986. This regulation was developed in response to severe winter smog and coal-related pollution problems in major cities. The regulation established threshold values for key pollutants used as air quality indicators, with sulfur dioxide (SO₂) and particulate matter (PM) being the primary focus. The values set in 1986 were significantly more lenient compared to current standards.

Over the years, the legislation was updated, and the Air Quality Assessment and Management Regulation (AQMRA) came into effect in 2008. This new regulation classified air quality based on AQI values and updated the limit values, partially tightening them. However, these values are still considered insufficient according to the WHO 2021 guidelines. For example, while the WHO recommends an annual average of 5 μg/m³ for PM_2.5, this value is set at 15 μg/m³ in Türkiye.

AQI data is of great importance not only in terms of legislation but also in terms of health expenditures. According to World Bank data, air pollution can cause costs equivalent to 2–3% of Gross Domestic Product in some countries. Although these rates are low in Türkiye, they are increasing over time. According to data from the Ministry of Health, approximately 30,000 people die prematurely due to air pollution. This highlights how critical AQI data is for policymakers.

AQI data also has an impact on the agriculture, tourism, and energy sectors. High AQI values can reduce agricultural production, have negative effects on tourist numbers, and require new measures to be taken in energy production planning. For this reason, AQI data should be taken into account not only in the environment and health sectors but also in economic planning.

Monitoring AQI data should not only be supported by government agencies but also by civil society organizations and local communities. Communities living in the Black Sea Basin can monitor AQI data in collaboration with local authorities, develop local solutions, and make their voices heard by policymakers. Such community-focused initiatives are vital for a sustainable environmental future.

The number and quality of measurement stations are also important for the reliability of AQI data. There are approximately 150 fixed air quality measurement stations in Türkiye under the HKİA. However, this number is insufficient outside major cities. Strengthening the monitoring system, especially in the Black Sea Region, is important for more accurate and local analysis of AQI data. In this context, mobile measurement devices and community can help make AQI data accessible to a wider audience.

Public awareness is also crucial for the proper use of AQI data. Education programs, media campaigns, and awareness-raising activities in local communities can facilitate the understanding of AQI. As public awareness of AQI increases, individual behaviors may also change in a positive direction. For example, behaviors such as avoiding motor vehicles and reducing open-air burning may become more widespread on days with high AQI levels.

AQI plays a very important role in both scientific and political decision-making processes. By making complex chemical data accessible to the public, AQI is a powerful communication tool in the fight against air pollution. Therefore, it is of great importance that AQI data is monitored accurately and regularly, communicated to the public in a transparent manner, and used effectively in the policy-making process.

In the Black Sea Basin, AQI data can yield more effective results through an approach based on regional cooperation and community participation. In this context, local governments, civil society organizations, and university researchers can work together to play a role in both data collection and awareness-raising activities. Starting this fight today means leaving a more livable world for future generations.