On the equilibrium partial pressures of nitric acid and ammonia in the atmosphere

Abstract Water-soluble salts of sulfate and nitrate have been shown to constitute a substantial portion of the ambient aerosol. Unlike sulfuric acid which has a high affinity for water, NH 3 and HNO 3 are relatively volatile and may transfer between the gaseous phase and the suspended solution droplets. The chemical equilibrium for the NH 3 -HNO 3 -H 2 SO 4 -H 2 O system at 25°C is considered. Equilibrium partial pressures of NH 3 and HNO 3 ranging from a few hundredths to a few hundred ppbv are calculated for droplets of various sulfate and nitrate compositions. The effects of relative humidity and solution pH value on the atmospheric NH 3 and HNO 3 concentrations are studied analytically. The results indicate that, while the HNO 3 partial pressure depends strongly on relative humidity and the nitrate-to-sulfate ratio in solution, the NH 3 partial pressure varies only slightly with humidity but inversely with the hydrogen-ion concentration.