Long-range transport of acidifying substances in East Asia—Part I: Model evaluation and sensitivity studies

This study has conducted a comprehensive model evaluation to help identify major uncertainties of regional air quality model in predicting long-range transport and deposition of acidifying substances in East Asia. Annual predictions of the Community Multiscale Air Quality (CMAQ) model are carried out at two horizontal scales: an 81 km domain over East Asia and a 27 km domain over Northeast Asia. The model successfully reproduces the magnitudes and diurnal variations of SO2 mixing ratios at most sites of the Acid Deposition Monitoring Network in East Asia (EANET). Through the comparison with tropospheric NO2 columns from the Global Ozone Monitoring Experiment (GOME), the model is shown to be able to capture major spatial and seasonal variations of NO2 observed from space over East Asia. Regarding the magnitudes, however, CMAQ underpredicts the GOME retrieval over industrial area of eastern China in March and December, and over the remote western China in July. Primary reasons for the discrepancy over eastern China are the uncertainties both in emission inventory and in the GOME retrieval in wintertime. For the wet season the soil-biogenic NO emission estimates need to be reviewed regarding the intensity and timing of fertilizer applications, and the magnitude of rain-induced pulsing. The sensitivities of predicted NO2 columns, NOx mixing ratios, and wet nitrate deposition to 50% increase of NOx emissions are studied. Due to the underpredictions of NOx and also to the uncertainty in modeled precipitation and nitrate formation, CMAQ has a tendency to underpredict annual wet deposition loads of nitrate observed by the EANET network.