CORRECTING RADM SULFATE UNDERPREDICTION - DISCOVERY AND CORRECTION OFMODEL ERRORS AND TESTING THE CORRECTIONS THROUGH COMPARISONS AGAINST FIELD DATA

A serious underprediction of ambient sulfate (SO42-) by two comprehens ive, Eulerian models of acid deposition, the Regional Acid Deposition Model (RADM) and the Acid Deposition and Oxidant Model(ADOM), was foun d in the National Acid Precipitation Assessment Program phase of the E ulerian Model Evaluation Field Study (EMEFS) model evaluation. Two hyp otheses were proposed to explain the cause of the underprediction in R ADM: insufficient SO42- production by nonprecipitating convective clou ds and insufficient primary SO42- emissions. Modifications of the RADM cloud and scavenging module to simulate nonprecipitating cumulus clou ds better are described in detail, Three contrasting pairs of tests us ing data from the EMEFS were applied to these hypotheses: source vs do wnwind regions, mid-summer vs late summer seasons and sunny-dry vs clo udy-wet synoptic types. The SO42- emissions hypothesis, tested by arti ficially boosting SO42- emissions, fared better than expected but was rejected because of its poor performance on the regional and seasonal contrast tests. The RADM nonprecipitating cumulus modification success fully captured the seasonal and the late summer synoptic contrasts but improvement is still needed for the regional and mid-summer synoptic contrasts.