Sensitivity assessment of PM2.5 simulation to the below-cloud washout schemes in an atmospheric chemical transport model

This study analyses the sensitivity of PM2.5 simulation and source apportionment results by integrating different below-cloud washout (BCW) schemes from various models into the CAMx model during the rainy days (3-13 September 2010). Furthermore, this study has also considered the influence of different raindrop size distribution parameterizations on the simulation. PM2.5 time series, spatial maps and the average concentration of the study region using different BCW schemes are presented. Our results show that different BCW schemes can cause over 50gm(-3) discrepancies in a PM2.5 simulation during the heavy rain periods. The source apportionment results for some cities (e.g. Hong Kong) are also sensitive to the choice of the BCW scheme. After implementing the composition dependent BCW coefficients calculated by using the field observation data, the PM2.5 simulation performance was improved and mean bias was reduced to 0.5gm(-3) during the study period. Future BCW studies should focus on the effects caused by aerosol compositions and raindrop size distributions in order to produce reliable simulation results for the rainy season.