Dimethylsulfide Chemistry: Annual, Seasonal, and Spatial Impacts on Sulfate

Abstract

We incorporated oceanic emissions and atmospheric chemistry of dimethylsulfide (DMS) into the hemispheric Community Multiscale Air Quality model and performed annual model simulations without and with DMS chemistry. The model without DMS chemistry predicts higher concentrations of sulfate $\left( {{\text{SO}}_{4}^{2 - } } \right)$ over land compared to the low concentrations over seawater. Including DMS chemistry substantially increases $\left( {{\text{SO}}_{4}^{2 - } } \right)$ concentrations over seawater and many coastal areas. It enhances the annual mean surface $\left( {{\text{SO}}_{4}^{2 - } } \right)$ by >0.8 μg/m³ in some areas of the Pacific Ocean, Atlantic Ocean, Arabian Sea, and Caribbean Sea. The largest enhancement occurs in summer in which DMS chemistry increases surface ${\text{SO}}_{4}^{2 - }$ by 70% over seawater. The model without DMS chemistry underestimates the summer-time observed ${\text{SO}}_{4}^{2 - }$ in the U.S. while the model with DMS chemistry improves model performance in the U.S. coastal areas.

Questions and Answers on ITM Paper No. 6.40

1. Questioner name: Dick Derwent

Question: There is an oxidation product from the OH + DMS reaction which has a competitive fate between NO₂ and O₂ reactions. The NO₂ route produces NO which acts as an ozone sink in high NO_x environments. It would be interesting to see if a small increase in detail in the OH + DMS chemistry could produce this ozone sink along the coast of North America.

Answer: This reaction is not included in our current DMS chemistry. However, we can explore its impact in a future study.

2. Questioner name: Pavel Kishcha

Question: Could we expect any significant increase in sulfate aerosols in the future? Water temperature increases due to global warming which is followed by an increase in DMS emissions in one hand. On the other hand, anthropogenic SO₂ emissions could decrease. What factor is more effective?

Answer: Anthropogenic SO₂ emissions have decreased during the last 20 years and are likely to further decrease in the future. DMS emissions may increase in the future due to warmer water temperature. Thus, the relative importance of sulfate aerosols from DMS is likely to increase further in the future.

3. Questioner name: Pius Lee

Question: The enhancement plots you showed for sulfate and SO₂ production from considering DMS is much stronger in the Gulf Coast than in the Pacific Coast. It looks to me that there is a strong temperature dependency. Is it as great as halogen that is roughly dependent on water temperature (exponentially)?

Answer: Inorganic halogen (iodine) emissions depend on seawater iodide concentration, atmospheric O₃, and wind speed. Seawater iodide concentration has a strong dependency on seawater temperature. DMS emissions depends on seawater DMS concentration and transfer velocity. Seawater DMS concentrations are obtained from a global database and the Nightingale et al. (2000) parameterization is used to calculate transfer velocity. While both the wind speed and temperature affect transfer velocity, wind speed has a stronger influence on transfer velocity and DMS emissions.