Kinetic and thermodynamic behaviour of volatile ammonium compounds in industrial and marine atmospheres

Abstract

Measurements of volatile ammonium aerosol compounds and respective acidic and basic gaseous precursors were taken at an industrial site emitting HCl, HNO 3 and NH 3 and at a marine suburban location. Because of fresh industrial emissions the gaseous acid and base concentration products show values orders of magnitude higher than thermodynamic equilibrium previsions, at the industrial site. To our knowledge these are the first published field measurements that strongly confirm that gas-particle equilibrium conditions are not attained instantaneously in the atmosphere and that kinetic constraints play an important role on gas-particle conversion for ammonium chloride and ammonium nitrate compounds. The results permit to conclude that, for temperature conditions lower than 15°C, transformation rates are not high, several minutes being necessary, at least, for the equilibrium between gaseous precursors and ammonium particulate products to be reached. At the marine suburban location a diurnal variation was observed for gaseous and particulate compounds in agreement with known emission and transformation mechanisms. The behaviour of nitric and hydrochloric acids in the marine atmosphere can be better explained if kinetic limitations are considered for the reaction of HNO 3 with NaCl aerosol particles and for the transference of acids and ammonia to the particulate phase.