Most gases, like greenhouse gases (CO2, CH4, etc.) or pollutants (NO2, CO, etc.) from natural processes and human activities are emitted into the troposphere. The main source of pollutants in the northern hemisphere is fossil fuel combustion (energy for traffic, industry and domestic heating) coupled with some biomass burning. In the southern hemisphere biomass burning is the dominating source of pollutants. Pollutants are emitted within urban and near-urban areas where they are dispersed over the surrounding countryside and, depending on the atmospheric lifetime of the pollutant or its secondary reaction product(s), are transported around the globe.
Tropospheric processes as sketched in figure 1-4 are well known to exhibit strong variability influenced by meteorology, diurnal variations in the sources of the emissions and solar illumination. Photolysis of O3 initiates the production of OH that determines, to a large extent, the oxidative (or cleansing) capacity of the troposphere. The role of the halogen oxides in the boundary layer as oxidants is currently a research matter. Many of the tropospheric trace gases are transformed into acids and other soluble products which are removed from the atmosphere by precipitation or by uptake on aerosols and subsequent dry deposition on surfaces. The atmospheric oxidation efficiency is vital in the control of radiatively and chemically active pollutants. Therefore, any change in the atmospheric oxidation efficiency directly affects the air quality, atmospheric chemical and radiative budgets and global biogeochemical cycles.
The lack of information on the temporal and spatial distributions of the relevant species, as well as the source strengths of CO, CH4 and NOx (NO and NO2), severely limits the quantitative understanding of the processes involved in tropospheric ozone production and destruction. This is also a prerequisite for quantitative estimates of the hydroxyl radical distribution and thus of the cleansing power of the atmosphere which is expected to be changing as a result of increasing emissions and resulting concentrations of O3, CH4, NOx and CO. One of the major challenges facing atmospheric science is to assess, understand and quantify the impact on air quality of a changing climate and atmospheric composition. (see fig. 1-4)