8.3 Urban Air Pollution

Los Angeles sits in a low, bowl-shaped basin, open to the Pacific Ocean but walled in by mountains on its northern and eastern edges. On hot, sunny, still days, a distinctive brown haze can settle over the city and simply refuse to leave.

What is actually trapping that pollution in place over the city?

Urban air pollution is caused by inputs from human activities into atmospheric systems, including nitrogen oxides (NO_x), sulfur dioxide, carbon monoxide and particulate matter. Particulate matter is categorized by particle size: PM2.5 is fine particulate matter with a diameter of 2.5 micrometers or less, small enough to penetrate deep into the lungs and even enter the bloodstream, while PM10 is larger particulate matter with a diameter of up to 10 micrometers, still small enough to be inhaled but less able to penetrate as deeply.

Sources of primary pollutants, pollutants directly active at the point of emission rather than formed later through chemical reaction, are both natural and anthropogenic. Natural sources include forest fires, windblown dust and volcanic eruptions. Anthropogenic sources include burning for agricultural and forest clearance, burning of fossil fuels and biomass for energy production, and dust generated by construction and unpaved roads.

Sort each source below as natural or anthropogenic.

Most common air pollutants in the urban environment are either derived directly or indirectly from the combustion of fossil fuels. PM2.5, PM10, carbon monoxide and sulfur dioxide are all primary pollutants, released directly from combustion. Tropospheric ozone, by contrast, is a secondary pollutant: it does not come directly out of a tailpipe or chimney, but instead forms afterward, when primary pollutants already in the atmosphere undergo further chemical reaction. This primary-versus-secondary distinction matters for management: reducing a primary pollutant means intervening at the emission source itself, while reducing a secondary pollutant means intervening earlier in the chain, by cutting the primary pollutants that later react to form it. The full chemistry of how tropospheric ozone actually forms is covered in this subtopic's HL Extension tab.

A range of different management and intervention strategies can be used to reduce urban air pollution, including improved public transportation, cycling infrastructure, growing trees, natural screens and green walls, compulsory catalytic converters, limited car use, and pedestrianized town centers. No single strategy solves the problem alone: effective urban air quality management typically combines several of these at once, targeting different pollution sources simultaneously.

Beijing's case shows that measurable, large-scale air quality improvement is achievable, but it required sustained, multi-sector policy over more than a decade, not a single quick intervention.

NO_x and sulfur dioxide react with water and oxygen in the air to produce nitric acid and sulfuric acid, resulting in acid rain. In simplified terms, sulfur dioxide (SO₂) reacts with oxygen and water vapor in the atmosphere to form sulfuric acid (H₂SO₄), while nitrogen oxides react similarly with water and oxygen to form nitric acid (HNO₃). These acids dissolve into cloud droplets and precipitation, lowering the pH of rain, snow or fog well below the roughly neutral pH of unpolluted rainfall, and can also settle out of the atmosphere as dry deposition even without precipitation.

Acid rain has real, measurable impacts across ecology, human health and infrastructure. On terrestrial habitats, it leaches essential nutrients from soil and directly toxifies it, while also damaging plant foliage on contact. On freshwater habitats, lowered pH solubilizes aluminum naturally bound in surrounding soil and rock, releasing it into the water where it is directly toxic to fish, and it damages fish gills and invertebrate exoskeletons. It also causes real corrosion of marble, limestone, steel, paint and other construction materials, meaning acid rain carries direct economic costs through damage to buildings and monuments, not just ecological costs. On human breathing, the nitrate and sulfate particles involved cause tissue damage and lung inflammation, since they are components of PM2.5 and can also be inhaled directly as acid deposition.

Sort each described impact into the category it belongs to.

Management and intervention strategies to reduce the impact of sulfur dioxide and NO_x on ecosystems follow the same three-part pattern used for other forms of pollution across this course: altering human activity, such as switching to alternative energy sources that do not release these gases in the first place; controlling pollutants at the point of release, such as fitting scrubbers to industrial flues and catalytic converters to vehicle exhausts, both of which chemically convert pollutants into less harmful substances before they reach the atmosphere; or restoring already-damaged systems, such as providing healthcare for affected populations or adding limestone or fertilizer directly to acidified lakes to neutralize their pH and support recovery.