Photochemical Smog and Acid Deposition

Primary and secondary pollutants. A primary pollutant is emitted directly: oxides of nitrogen formed when combustion is hot enough to make atmospheric nitrogen and oxygen react, unburnt volatile organic compounds from fuel and solvents, carbon monoxide, sulfur dioxide and particulates. A secondary pollutant is not emitted at all but is formed in the atmosphere from primary pollutants. Tropospheric ozone is the standard example, and so are sulfuric and nitric acid. The distinction matters for policy: a secondary pollutant cannot be captured at the point of release, so control must target its precursors.

How photochemical smog forms. Nitrogen dioxide is split by sunlight into nitric oxide and an oxygen atom; that atom joins an oxygen molecule to make ozone. Volatile organic compounds keep the cycle running by converting nitric oxide back to nitrogen dioxide without consuming ozone, so ozone accumulates instead of being recycled. Because the key step requires sunlight, ozone concentrations peak in the early afternoon rather than during the morning traffic peak, and smog is worst in summer.

Why topography and weather decide severity. A thermal inversion places a layer of warm air above cooler surface air. Since the surface air is denser it cannot rise, convection stops, and pollutants accumulate under the warm lid instead of dispersing. Inversions form on calm clear nights when the ground radiates heat away, and under high-pressure systems where subsiding air warms. Cities in valleys or basins — Los Angeles, Mexico City, Santiago, Delhi in winter — suffer most, because the surrounding relief also prevents horizontal ventilation. The inversion does not create pollution; it concentrates it.

Good ozone and bad ozone. Stratospheric ozone and tropospheric ozone are the same molecule. In the stratosphere it shields the surface from ultraviolet radiation; in the air people breathe it inflames airways, reduces lung function, aggravates asthma and measurably reduces the yield of crops such as wheat and soya. Location, not chemistry, decides whether ozone is protective or harmful.

Acid deposition. Sulfur dioxide, mainly from burning coal containing sulfur impurities, and oxides of nitrogen from all high-temperature combustion, are oxidised in the atmosphere to sulfuric and nitric acid. These return to the surface as wet deposition in rain, snow, fog and mist, or as dry deposition of acidic gases and particles that acidify surfaces later when they are wetted. Unpolluted rain already has a pH of about 5.6 because dissolved carbon dioxide forms carbonic acid, so acid deposition is normally defined below about pH 5.0. Since pH is a logarithmic scale, pH 4 is ten times more acidic than pH 5 and one hundred times more acidic than pH 6.

Why the damage map does not match the deposition map. Acid deposition is a transboundary problem: tall chimneys, built to solve local air quality, put emissions high enough to be carried hundreds of kilometres by wind, so the country that burns the coal is often not the country whose lakes and forests are damaged. But deposition alone does not predict damage. A catchment on limestone contains carbonate that neutralises incoming acid, so its lakes may barely change pH; a catchment on granite has almost no buffering capacity, so pH falls quickly. The mechanism that kills fish is then usually not the hydrogen ion itself but aluminium, which acid mobilises from soil minerals into the water, where it damages gill tissue and causes suffocation. In forests, acid leaches calcium and magnesium from soils, so trees are weakened and become vulnerable to frost, drought and pests rather than being killed directly.