Sensitivities of gas-phase dimethylsulfide oxidation products to the assumed mechanisms in a chemical transport model

Abstract

The gas-phase products of dimethylsulfide (DMS) oxidation are simulated in the global 3D Model of Atmospheric Transport and Chemistry (MATCH). The focus is on the sensitivities to the assumed mechanisms of DMS oxidation chemistry in large-scale models, and hence volcanic and anthropogenic sources Of SO 2 are ignored. Four representations of DMS chemistry are considered, including two comprehensive mechanisms (about 50 reactions) and two parameterized schemes (four and five reactions). The gas-phase yields of DMS, SO 2, methanesulfonic acid (MSA) and H 2 SO 4 are compared between these four cases as a measure of the sensitivity to uncertain DMS chemistry. Among the four cases, DMS is largely invariant, while S0 2 using parameterized chemistry is three times higher than its levels using comprehensive chemistry in the tropical upper troposphere. For MSA and H 2 SO 4, there are order-of-magnitude inter-mechanistic differences at high and low altitudes in the extratropics, respectively. The differences are attributed to fixed branching yields and the absence of important intermediate species and pathways in the parameterized mechanisms. Regional budgets are also analyzed within the remote Southern Ocean and central tropical Pacific. While the DMS budget varies primarily between the regions, the SO 2, MSA and H 2 SO 4 budgets vary strongly between the regions and mechanisms. The DMS oxidation products, therefore, are as sensitive to the assumed mechanisms as to the external conditions between the tropics and extratropics. To distinguish between the mechanism cases, the MATCH simulations are also compared to campaign measurements. The simulated values of DMS and SO 2 are found to approximate the observations, but do not provide mechanism differentiation. The gas-phase H 2 SO 4 and MSA simulations differ more extensively from the observations, yet the comprehensive chemistry cases give a better fit to the MSA measurements. Better fits to the MSA observations are also achieved for the two mechanisms that consider dimethylsulfoxide (DMSO) as an MSA precursor. Copyright 2005 by the American Geophysical Union.