A 2-DIMENSIONAL STUDY OF ETHANE AND PROPANE OXIDATION IN THE TROPOSPHERE

The oxidation chemistry of ethane (C2H6) and propane (C3H8) in the troposphere was studied using a global two-dimensional model, adopting observed surface volume mixing ratios of C2H6 and C3H8 as a function of latitude and season. From the calculated distribution of OH, C2H6, and C3H8 the source strengths, which compensate the chemical loss of these hydrocarbons in the atmosphere, are estimated at 16 Tg C2H6/a and 23 Tg C3H8/a. Uncertainties involved in the calculations are discussed. The resulting seasonal and latitudinal distribution of various organic compounds, such as acetaldehyde, acetone, peroxyacetyl nitrate (PAN), peroxypropyl nitrate (PPN), and alkyl nitrates were derived. The contribution of various nitrogen species to the unidentified NO(y) observed during measurement campaigns was examined. C2 - C3 alkyl nitrates and HNO4 formed at mixing ratios of a few tens of pptv could account only for some of the unidentified NO(y). PAN is calculated to be the most abundant organic nitrate, with mixing ratios exceeding 100 pptv at mid-latitudes to high latitudes in spring in the northern hemisphere. These values are low compared to observations, however. Regionally, up to 10 times more odd nitrogen may be transported in the form of PAN than NO(x). The influence of C2H6 and C3H8 chemistry on calculated mean tropospheric NO(x) mixing ratios and, subsequently, on O3 and OH concentrations is limited. Therefore major effects on global O3 and OH concentrations must be due to PAN formation in the low troposphere from NO(x) and reactive hydrocarbons other than C2H6 and C3H8. Such hydrocarbons are required to explain the observed high PAN mixing ratios.