A two‐dimensional study of ethane and propane oxidation in the troposphere

Abstract

The oxidation chemistry of ethane (C 2 H 6 ) and propane (C 3 H 8 ) in the troposphere was studied using a global two‐dimensional model, adopting observed surface volume mixing ratios of C 2 H 6 and C 3 H 8 as a function of latitude and season. From the calculated distribution of OH, C 2 H 6 , and C 3 H 8 the source strengths, which compensate the chemical loss of these hydrocarbons in the atmosphere, are estimated at 16 Tg C 2 H 6 /a and 23 Tg C 3 H 8 /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. C 2 −C 3 alkyl nitrates and HNO 4 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 C 2 H 6 and C 3 H 8 chemistry on calculated mean tropospheric NO x mixing ratios and, subsequently, on O 3 and OH concentrations is limited. Therefore major effects on global O 3 and OH concentrations must be due to PAN formation in the low troposphere from NO x and reactive hydrocarbons other than C 2 H 6 and C 3 H 8 . Such hydrocarbons are required to explain the observed high PAN mixing ratios.