scope:

Scope and objective

This evaluation guideline describes the criteria to be satisfied by reaction mechanisms for modeling the chemical reaction dynamics of trace gases in the troposphere. For the greater part of the applications, the chemical reaction mechanism will have to be combined with a meteorological dispersion model. The combination of the chemical and meteorological schemes is exemplified by three model types pursuing different objectives and differing in the magnitude of the computational task. Furthermore, examples of the major applications of combined chemical and transport models are given.

• Typical applications of chemical reaction schemes in combination with dispersion models as presented in this guideline are:
• Sensitivity studies to estimate the effect of emission control measures on the formation of photooxidants and radicals in the troposphere
• Calculation of ozone and other photo-oxidants during summer smog and other meteorological episodes without meso-scale formation of precipitation
• Determination of the lifetime of primary and secondary air pollutants
• Estimating the effect of technological and urban planning measures for emission control on precursors of secondary air pollutants within the scope of air pollution control programs

Gaseous compounds may be directly released into the atmosphere by natural or anthropogenic sources. They are referred to as primary trace gases or gaseous pollutants. In the course of their chemical transformation, which is dominated by gas-phase reactions, secondary trace gases and gaseous pollutants are formed, among them photo-oxidants like ozone. Based on current knowledge, gas-phase reactions are the key chemical processes determining the atmospheric concentrations of primary and secondary trace gases during summer smog episodes. For this reason, heterogenous processes or aqueous-phase reactions are not considered in this guideline.

Prediction of secondary and primary trace gas production and degradation requires a chemical reaction mechanism which reflects the relevant chemical processes in the atmosphere. Balance equations for the species encountered are then derived from the chemical reaction mechanism. These balance equations have to be coupled with appropriate meteorological models and solved by numerical techniques.

The selection of the reactions to be considered in a chemical mechanism is already based on assumptions about usual trace gas concentrations and the resulting relative importance of chemical transformation processes. This evaluation guideline relates exclusively to chemical reaction mechanisms for the conditions encountered in the continental, non-polluted or polluted troposphere. It is not applicable to processes in the immediate vicinity of pollution sources, in a smog chamber or a reaction vessel. Furthermore, it is assumed that the air is not so heavily polluted that nitrogen oxides or longer-chain hydrocarbons are present at mixing ratios above 1 ppm.

Accordingly, the reaction mechanisms here presented relate to the mean composition of the slightly to moderately polluted continental troposphere and the associated typical trace gas composition without major concentrations of aerosols. Thus, this evaluation guideline is not applicable to

• fires, explosions and accidental industrial releases
• chemical-physical transformation due to heterogenous
• processes processes in the immediate vicinity of the emission
• source smog caused by SO₂ and particles

This evaluation guideline does not apply for defining or setting new emission limits. It is destined for engineering offices, regulatory authorities and scientific institutes to the extent they are active in the above areas of work.