Transport and distribution of primary and secondary nonmethane volatile organic compounds in east Asia under continental outflow conditions

Abstract

A regional scale three‐dimensional atmospheric chemistry model is used to study the transport and chemistry of nonmethane volatile organic compounds (NMVOCs) in east Asia. A springtime episode during the month of May 1987 is investigated. The emission estimates of ethane, propane, and ethene from our calculations in east Asia are ∼1.6, ∼1.3, and ∼5.2 Tg C/yr, respectively. The model calculated hydrocarbon mixing ratios are compared with observations from a network of stations in China and Japan and also with the Pacific Exploratory Mission ‐ West B data. The model predictions generally agree favorably with observations over China and Japan, where hydrocarbon mixing ratios are between 1 and 5 ppb. The long‐lived ≥C4 alkanes are better modeled than the highly reactive short‐lived ≥C3 alkenes having mixing ratios <0.5 ppb, which are generally underpredicted by the model. The differences between these model predicted and observed values are attributed to incomplete information about the intensity of the NMVOC emissions as well as their distribution. Positive correlations are obtained for the predicted hydrocarbons against CO and SO2, an indication of similar anthropogenic emission sources. Secondary organic compounds are also analyzed to assess the extent of the oxidation of primary hydrocarbons in the troposphere. It is observed that over continental China the contributions of the oxidation products of isoprene are >80% of the pool of oxygenates. The reverse is true at marine locations, where oxidation products of long‐lived alkanes dominate. Sensitivity analysis to increase in the NOx and NMVOC emissions suggest that for the simulation conditions of the model, ozone production over most of east Asia is NOx sensitive. A 20–30% increase in ozone is predicted over China for doubling of NOx emissions.