The Chemistry of Atmospheric Pollutants

Although some pollutants emitted into the atmosphere present a health risk in their own right, a major additional impact results from their oxidation in the atmosphere, which leads to the production of a range of secondary pollutants (eg. ozone), many of which are potentially more harmful than their precursors. An understanding of the chemical conversions of the various atmospheric pollutants, and the rates of these conversions, allows us to rationalise observational measurements made in the atmosphere. A brief description of the atmospheric formation and removal of the series of pollutants routinely measured is given below.

Nitrogen oxides (nitric oxide and nitrogen dioxide):

Nitrogen oxides are released into the atmosphere mainly in the form of nitric oxide, as a result of fossil fuel combustion. Nitric oxide is readily oxidised to nitrogen dioxide by reaction with ozone. Elevated levels of nitrogen oxides are generally observed in urban environments under stable meteorological conditions, when the airmass is unable to disperse. Together with hydrocarbons, they play an important role in the formation of ozone in the atmosphere, as described below. Nitrogen oxides have a lifetime of approximately 1 day with respect to conversion to nitric acid, which is removed from the atmosphere by direct deposition to the ground, or tranfer to aqueous droplets (eg. cloud or rainwater), thereby contributing to acid deposition.

Hydrocarbons:

Hydrocarbons (and other organic compounds) are released into the atmosphere from a variety of man-made sources (eg. fossil fuel combustion, solvent evaporation) and some natural sources (eg. vegetation). Some hydrocarbons present a direct health hazard, for example benzene and 1,3 butadiene are carcinogens. Although the levels of most hydrocarbons typically observed do not present a health risk, they are oxidised to form oxygenated organic products (some of which may be harmful) and carbon monoxide. Owing to the diversity of their structure and differences in their reactivity, the rates and products of hydrocarbon oxidation are highly varied. In the presence of nitrogen oxides, hydrocarbon oxidation also leads to ozone formation, as described below.

Carbon monoxide:

Carbon monoxide is both emitted into the atmosphere as a result of combustion processes, and is formed from the oxidation of hydrocarbons and other organic compounds. The highest concentrations are found close to combustion sources. It has an atmospheric lifetime of approximately one month with respect to further oxidation to form carbon dioxide.

Ozone:

The formation of ozone requires the presence of three ingredients: hydrocarbons, nitrogen oxides and sunlight. The sunlight provides the energy for the whole process to begin through near ultra-violet radiation which is able to dissociate certain stable molecules, leading to the formation of reactive species known as free radicals. In the presence of nitrogen oxides, these free radicals catalyse the oxidation of hydrocarbons to carbon dioxide and water vapour. Partially oxidised organic species such as aldehydes, ketones and carbon monoxide are intermediate oxidation products, and ozone is generated as a by-product. Since ozone itself is photodissociated to form free radicals, it promotes the oxidation chemistry, thereby catalysing its own formation (ie. it is an autocatalyst). Consequently, high levels of ozone are generally observed under sunny, summertime conditions in locations where the airmass has previously collected emissions of hydrocarbons and nitrogen oxides. Because of the time required for chemical processing, ozone formation tends to be downwind of pollution centres (ie. it is usual for the highest ozone levels to be in suburban or rural locations).

Sulphur dioxide:

In continental regions, atmospheric sulphur dioxide results mainly from fossil fuel combustion. The oxidation of sulphur dioxide leads to the production of sulphuric acid, which contributes to acid precipitation. Its atmospheric lifetime with respect to oxidation is typically a few days. However, the rate of oxidation is variable, since it may occur both in aqueous droplets (eg. clouds), and in the gaseous phase where the sulphuric acid itself may condense to form condensation nuclei. Since sulphuric acid is very hygroscopic, these nuclei can grow rapidly by adsorption of water, and are therefore classed as cloud condensation nuclei.

PM10:

PM10 is defined as the total mass (per unit volume of air) of particles of median aerodynamic diameter less than 10 micro metre, sometimes referred to as "fine" particulate matter. Larger particles are not readily inhaled, and are removed comparatively efficiently from the air by sedimentation. Sources of fine particulate matter are numerous, and their chemical compositions highly variable. The particles are often classed as either primary (those emitted directly into the atmosphere) or secondary (those formed or modified in the atmosphere from condensation and growth). A major source of fine primary particles results from combustion processes, in particular diesel combustion, where the transport of hot exhaust vapour into a cooler tailpipe or stack can lead to spontaneous nucleation of "carbon" particles prior to emission. Secondary particles are typically formed when low volatility products are generated in the atmosphere, for example the oxidation of sulphur dioxide to sulphuric acid referred to above. The atmospheric lifetime of particulate matter is a strong function of particle size, but may be as long as 10 days for particles of about 1micro metre in diameter.