5.2      Prediction of acidity in WRF-Chem.

Barth, Mary C., National Center for Atmospheric Research, and Rahul Zaveri, Pacific Northwest National Laboratory

The acidity of cloud water and aerosols moderates the chemistry that occurs in clouds and aerosols. WRF-Chem determines the pH of cloud water in its cloud chemistry module and the pH of aerosols in the aerosol module, yet the WRF-Chem pH values have not been evaluated with observations or other models. As part of a contribution to a review on the acidity of clouds and aerosols, pH of cloud water and aerosols have been included in the WRF-Chem output files. The model configuration used is the MOZART gas-phase chemistry with 4-bin aerosol and cloud chemistry as depicted in the MOSAIC aerosol scheme. This configuration was applied to the conterminous U.S. (CONUS) with 12-km grid spacing for 1-14 June 2013. With MOSAIC able to dynamically partition trace gases to size-resolved aerosols, we find that aerosol pH increases as the size of the bin increases, and coarse-mode aerosols are near neutral over the oceans where sea salt dominates. However, since the gas chemistry does not include HCl, the MOZART-MOSAIC scheme does not fully include the displacement of chlorine from sea salt by sulfate and nitrate. Hence, aerosol acidity over the oceans should be more acidic than the present MOZART-MOSAIC prediction. Temporally-averaged, model-predicted fine mode aerosol pH in Pasadena, CA and Centreville, AL show good agreement with observations obtained during the CalNex (May-June 2010) and SOAS (June 2013) field experiments. WRF-Chem predicted cloud water pH (that is a column average weighted by liquid water content) over CONUS shows pH ~4 over the Ohio River Valley, northern Mexico, and New Mexico-Colorado. In the western U.S., Great Plains, and North Carolina, higher cloud water pH (pH ~6) is found. Over the East Coast of the U.S. a pH of 4.5-5.0 is predicted by WRF-Chem. These cloud water pH values are roughly similar to measurements, which show a gradient of high pH along the West Coast with values of 6 to 7 to lower values (pH of 4-5) near Houston and on the East Coast.