Influence of Gaseous Nitric Acid on Sulfate Production and Acidity in Rain

A physlco-chenmal subcloud rain model IS used to simulate the effect of gaseous HNO, and NO, on pH and SOiproduction in a IOmmh-‘, 1000-m fall distance rain event. The ambient gases constdered m the chemistry were SO,, NO, NO,, HNO,. 0,. and CO,. Ramdrops mtttally at a pH of 5.5 absorbed these gases, and as they fell through a polluted zone, produced SOiand NO; by the oxidatron of dissolved SO, by O3 and the dissociation of HN03, which reduced the pH. For the chemical mechanism and the below-cloud washout rain events considered, it was observed that: absorption of gaseous HN03 controlled the actdification in the initial stages of a ram event, and inhibited the productton of SO: _ ; NO and NO, played no dtrect role m theacidification or formation of NO; or SO:-; pre-acidified raindrops (pH of 4) were further acidified only by absorbing HNOJ.