Air Pollutants: Why Do They Settle In Low-Lying Areas?

Urban air pollution is caused by a variety of factors, including transportation, domestic fossil fuel use, industrialization, power generation, combustion, and agriculture. The use of private vehicles, particularly older diesel models, is the major source of urban air pollution. Industrialization also plays a significant role, as factories release numerous toxic gases and chemicals into the atmosphere. Urbanization exacerbates these issues by increasing population density and energy consumption, leading to poor air and water quality, insufficient water availability, and waste disposal problems. Additionally, urban runoff contributes to nonpoint source pollution, as rainwater and snowmelt carry pollutants from impervious surfaces into nearby water bodies. These pollutants can have detrimental effects on fish and wildlife populations, native vegetation, drinking water quality, and recreational areas.

Urban air pollution sources

Urban air pollution is a pressing issue that affects almost every city globally, with severe health and environmental consequences. While there are some natural sources of urban air pollution, such as volcanic eruptions, thunders, and dust, most sources are anthropogenic and closely tied to human activities. Here are the key sources of urban air pollution:

Transportation

The use of private vehicles, especially older diesel models, is a significant contributor to urban air pollution. In Europe, road traffic is estimated to generate almost 40% of nitrogen oxide emissions, and transportation accounts for about 75% of VOC emissions by weight. Vehicles also contribute to high local ambient concentrations of sulfur dioxide.

Industrialization

Industrial activities, particularly in areas with a high concentration of industries, significantly impact air quality. Factories release numerous toxic gases due to the burning of fossil fuels and the use of chemicals. Coal-powered power plants are a major source of pollution, releasing toxins like asbestos, dioxin, lead, and chromium.

Power Generation and Combustion

The combustion of fossil fuels releases harmful gases, including carbon dioxide (CO₂) and carbon monoxide (CO). Incomplete combustion processes further release carbon monoxide, a highly toxic, colorless, odorless, and tasteless gas. This combustion is common in residential, commercial, and institutional buildings, contributing to carbon monoxide and black carbon emissions.

Domestic Use of Fossil Fuels

The domestic burning of fossil fuels, such as coal, contributes to air pollution. Solid fuels like wood, charcoal, and coal are often used for cooking and heating, releasing large amounts of particulate matter and pollutants into the environment due to inefficient stoves.

Agriculture and Beauty Products

Agriculture releases gases like nitrogen dioxide (NO₂) and methane (CH₄) into the atmosphere. Additionally, the use of beauty products, such as cosmetics and perfumes containing volatile organic compounds (VOCs), contributes to air pollution.

Population Density and Urbanization

The increase in population and urbanization intensifies the demand for energy, leading to a greater reliance on fossil fuels for power generation. This, in turn, contributes to higher emissions and air pollution levels.

Urban landscape patterns

Air Quality

Ecosystem Health

The rapid expansion of artificial landscapes in urban areas poses a significant threat to ecosystem health. The conversion of pristine ecosystems, such as watersheds, wetlands, and farmland, for urbanization can lead to structural and integrity changes in the ecosystem. Additionally, changes in landscape patterns can impact material cycles and energy flows, affecting regional environmental factors like climate, soils, and hydrology. The size, density, and perimeter of urban patches, as well as their interspersion with other land covers, are crucial factors in maintaining ecosystem health. Adaptive landscape management is essential to ensure sustainable urbanization and preserve the foundation of healthy ecosystems.

Infectious Disease Risk

Urbanisation and river pollution

Urbanisation is an increasing source of multiple pollutants that contaminate rivers in the 21st century. The rise in urbanisation and industrialisation has led to a surge in toxic gases, such as nitrogen oxides, carbon monoxide, and sulphur oxides, being released into the atmosphere. These gases are emitted by power stations, vehicles, and industrial and domestic combustion processes, which then contribute to the formation of particulate matter (PM) and ground-level ozone.

The burning of fossil fuels, particularly in thermal power plants and vehicles, is a major source of anthropogenic sulphur dioxide emissions. In addition, the domestic combustion of coal and the use of beauty products also contribute to high local ambient concentrations of sulphur dioxide. Furthermore, industrial activities release various toxins, including asbestos, dioxin, lead, and chromium, into the atmosphere through the burning of fossil fuels and the use of chemicals.

The impact of urbanisation on river pollution is significant, with nutrients, microplastics, chemicals, and pathogens being released into water bodies. A study quantified the combined point-source inputs of these pollutants into 10,226 rivers in 2010, 2050, and 2100, finding that river pollution in Europe, Southeast Asia, and North America is already severe. The increase in population and industrial activities in urban areas has led to a higher demand for energy, resulting in the increased use of fossil fuels for energy generation. This, in turn, has contributed to the deterioration of water quality, as wastewater discharge from industrial, agricultural, and domestic sectors contains high levels of chemical oxygen demand (COD), a primary indicator of water pollution.

In the future, it is projected that around 80% of the global population will reside in sub-basins facing multi-pollutant issues, with river pollution in Africa expected to increase by 11–18 times compared to 2010 levels. This poses significant challenges in achieving Sustainable Development Goals. While advanced wastewater treatment technologies can help reduce pollution levels, it may not be sufficient to decrease pollution to pre-2010 levels in African rivers.

The relationship between urbanisation and water resources is complex and interdependent. Water resources are essential for the daily needs of city dwellers and the sustainable operation of industries and agriculture. However, rapid urbanisation can lead to an imbalance in water resource consumption, with urban stormwater and water pollution posing economic threats and hindering the sustainable development of urban areas.

Nonpoint source pollution

Nonpoint source (NPS) pollution is a significant issue that affects water quality. NPS pollution is caused by rainfall or snowmelt moving over and through the ground, picking up and carrying away natural and human-made pollutants, which are then deposited into lakes, rivers, wetlands, coastal waters, and groundwater. Unlike pollution from industrial and sewage treatment plants, NPS pollution arises from numerous diffuse sources, including runoff from farm fields, livestock facilities, construction sites, lawns, gardens, city streets, parking lots, surface coal mines, and forestry.

The major sources of NPS pollution in Illinois, for example, are agriculture, urban runoff, and habitat modification. Urban runoff, which flows directly into lakes and streams without undergoing treatment, can pick up various pollutants from city streets and parking lots. Habitat modification refers to the channelization of streams and the disturbance of riparian corridors, which are the areas immediately adjacent to stream banks.

Nutrients from fertilizers and animal waste are common NPS pollutants that can cause water discolouration and harm aquatic life. Sediment, which is soil eroded from farm fields, construction sites, and streambanks, can also carry other pollutants such as metals and toxic chemicals. Sedimentation reduces water clarity, impairs aquatic feeding and respiration, damages fish gills and insect breathing, reduces sunlight penetration, and affects plant growth.

The impact of NPS pollution on water quality is significant. States have reported that NPS pollution is the leading remaining cause of water quality issues, affecting drinking water supplies, recreation, fisheries, and wildlife. Federal programs and initiatives, such as the Nonpoint Source Management Program, aim to address and mitigate the effects of NPS pollution on aquatic ecosystems and human communities.

Air pollution and health

Air pollution is a major threat to global health, causing more than 4 million deaths annually, according to the World Health Organization (WHO). It is caused by various natural and anthropogenic sources, with the latter being the major contributor. Natural sources include volcanic eruptions, thunder, dust storms, and naturally occurring particulate matter. However, the dominant human-induced sources are transportation, domestic use of fossil fuels, industrialization, power generation, combustion, and agricultural and beauty products.

The health effects of air pollution are far-reaching and impact people of all ages, from children to adults, and even the unborn. Short-term exposure to high levels of particulate matter can lead to reduced lung function, respiratory infections, and aggravated asthma. In contrast, long-term exposure increases the risk of non-communicable diseases, including stroke, heart disease, chronic obstructive pulmonary disease, and cancer. Fine particulate matter, such as PM2.5, is of particular concern as these tiny particles can penetrate deep into the lungs, enter the bloodstream, and cause systemic damage to tissues and organs, including the heart, brain, and lungs.

Nitrogen oxides (NOx) are another significant pollutant, predominantly emitted by power stations, vehicles, and industrial and domestic combustion processes. These compounds contribute to the formation of ground-level ozone, which irritates the respiratory system and causes inflammation and other damage with both short-term and long-term exposure. Carbon monoxide (CO), a highly toxic and colorless gas produced by the incomplete combustion of fossil fuels, is another major air pollutant. It is odorless and tasteless, making it difficult to detect, and vehicle emissions are the largest anthropogenic source of this pollutant.

Additionally, air pollution has been linked to adverse pregnancy outcomes, with maternal exposure associated with low birth weight, pre-term birth, and small gestational age. There is also growing evidence of its impact on diabetes and neurological development in children. The health risks from air pollution exposure depend on the types and concentrations of pollutants, and vulnerable groups such as pregnant women, children, the elderly, and people with chronic conditions are at a higher risk of adverse health effects.

While air pollution affects everyone, it is important to recognize that the burden is not shared equally. An individual's level of risk is influenced by factors such as exposure and susceptibility. Those living, studying, or working in areas with high pollution levels are more susceptible to the detrimental health effects of air pollution.

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