A model validation study of the washout/rainout contribution of sulfate and nitrate in wet deposition compared with precipitation chemistry data in Japan

We simulated washout/rainout ratios of non-sea-salt (nss)-SO42− and NO3− using a chemical transport model and compared the estimates with precipitation measurements sampled at 0.5/1 mm intervals at sites located in Kobe (urban), Toyo-oka (suburban), and Tamba (rural) cities, Japan. The 25th and 75th percentile range of the simulated washout contributions was 30-70%. The simulated washout contribution range of NO3− (40-70%) was greater than that of nss-SO42− (30-60%). There was good agreement between the simulated and observed values, and the observed washout contribution of NO3− was also greater than that of SO42−. The simulated washout contribution range was higher (60-75%) in emission source regions and lower (40-55%) in downwind areas. The wet deposition process is one of the key causes of uncertainty in chemical transport modeling. Comparing model results with such high-frequency precipitation chemistry data has been extremely rare. Thus the current study is providing useful information for evaluating and improving wet deposition modeling and for the better understanding of the wet deposition mechanism.