The Chemistry of Acid Rain

Much progress has been made in our understanding of the sources, formation and deposition of acid and sulfate. Large field studies can be conducted with good quality control of analyses and data. In the gas phase, ·OH radicals are known to be capable of converting SO2 to sulfate fast enough to be important. Rates for H2O2, O3 and O2 in cloud droplets are fast under certain conditions. However, serious gaps in our knowledge still exist, especially methods for measuring and predicting dry deposition and estimates ofthe supply of reactants to active cloud systems. A focal point for development of the U. S. control strategy is the Regional Acid Deposition Model (RADM). Uncertainties in some features of the model are likely to be so large that it may not provide credible predictions in a time soon enough to be useful to legislators or regulators. Hybrid receptor models may be able to provide some answers for sulfur species more quickly, although RADM should ultimately yield more detailed predictions for more species. Many problems attributed to acid rain, especially damage to trees at high altitudes, may be largely due to some other species, e.g., H2O2 or O3.