The Relative Importance of Oxidation Pathways and Clouds to Atmospheric Ambient Sulfate Production as Predicted by the Regional Acid Deposition Model

Abstract

The development and use of a version of the Regional Acid Deposition Model/Engineering Model (RADM/FM) called the Comprehensive Sulfate Tracking Model (COMSTM) is reported. The COMSTM is used to diagnose the relative contributions of each sulfate production pathway to the total atmospheric ambient sulfate predicted by RADM. Thirty meteorological cases are used to aggregate the results into annual estimates. For the operational RADM (denoted RADM 2.6), nonprecipitating cloud production of ambient sulfate dominates over precipitating cloud production, and the hydrogen peroxide pathway dominates over four other aqueous formation routes. Gas-phase production of sulfate contributes ten than 40% of the ambient budget. Further, the COMSTM is used to explore the sensitivity of the RADM's cloud water and rainwater pH's and ambient sulfate predictions to uncertainties in the ammonia emissions inventory. By developing correction floors to improve in-cloud and deposited ammonia, and utilizing them in the COMSTM, it is shown that the uncertainties should have a minimal effect on predicted cloud water and rainwater pH's and on the overall ambient sulfate budget in the operational PADM 2.6.