Correcting RADM's sulfate underprediction: Discovery and correction of model errors and testing the corrections through comparisons against field data

A serious underprediction of ambient sulfate (SO 4 2−) by two comprehensive, Eulerian models of acid deposition, the Regional Acid Deposition Model (RADM) and the Acid Deposition and Oxidant Model (ADOM), was found in the National Acid Precipitation Assessment Program phase of the Eulerian Model Evaluation Field Study (EMEFS) model evaluation. Two hypotheses were proposed to explain the cause of the underprediction in RADM: insufficient SO 4 2− production by nonprecipitating convective clouds and insufficient primary SO 4 2− emissions. Modifications of the RADM cloud and scavenging module to simulate nonprecipitating cumulus clouds better are described in detail. Three contrasting pairs of tests using data from the EMEFS were applied to these hypotheses: source vs downwind regions, mid-summer vs late summer seasons and sunny-dry vs cloudy-wet synoptic types. The SO 4 2− emissions hypothesis, tested by artificially boosting SO 4 2− 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 successfully captured the seasonal and the late summer synoptic contrasts but improvement is still needed for the regional and mid-summer synoptic contrasts.