Chemistry-climate interactions: Boundary layer ozone in the United States and free tropospheric methanesulfonic acid over the tropics

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Zhang, Yuzhong
Wang, Yuhang
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This work presents a multi-scale modeling analysis approach to explore two subtopics pertaining to the chemistry-climate interaction: (1) ground-level ozone pollution in the United States and (2) the marine sulfur chemistry over the tropical oceans. To understand the drivers for high ozone episodes during the fall over the Southeast US, we use a 3-D chemical transport model to analyze the month of October 2010, the most recent ozone extreme over the Southeast in October. In addition to enhanced photochemical production and suppressed pollution ventilation, modeling analysis shows that the high ozone concentrations are also due to enhanced emission of biogenic isoprene under a dry and warm weather condition from water stressed plants, which is corroborated by recent field and laboratory studies. This finding implicates that a drying and warming future, projected by climate models, will likely lead to an extended ozone season from summer to fall and an increase of secondary organic aerosols in the Southeast US. We also use vertical profiles observed in the DISCOVER-AQ aircraft campaign in July 2011 to examine how NOx, an important ozone precursor, distributes vertically in the boundary layer. The observed average vertical profile of NOx shows a large negative gradient with increasing altitude in the boundary layer. Our analysis suggests that the magnitude of the NOx gradient is highly sensitive to meteorological factors such as atmospheric stability. Using a 1-D chemical transport model, we reasonably reproduce the vertical profiles of NOx under different boundary layer stability conditions. We then use the model to assess the impact of parameterizations of the boundary layer and land-surface processes on vertical profiles. Using model simulations, we evaluate the impact of boundary layer NOx gradient on the calculation of the ozone production rate and satellite NO2 retrieval. Using the 1-D model, we also examine the sulfur chemistry during the PASE mission in the tropical Pacific, with a focus on methanesulfonic acid (MSA). The observed sharp decrease in MSA from the surface to 600m implies a surface source of 4.0×10^7 molecules/cm2/s. We also find that the observed large increase of MSA from the boundary layer into the lower free troposphere (1000-2000 m) results mainly from the degassing of MSA from dehydrated aerosols. We estimate a source of 1.2×10^7 molecules/cm2/s to the free troposphere through this pathway. This source of soluble MSA could potentially provide an important precursor for new particle formation in the free troposphere over the tropics, affecting the climate system through aerosol-cloud interactions.
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