KINETICS OF EVAPORATION OF AMMONIUM-CHLORIDE AND AMMONIUM-NITRATE AEROSOLS

The evaporation rates of ammonium chloride and ammonium nitrate were measured by continuously and rapidly removing gaseous NH3 and HNO3 or HCl from aerosols in an annular denuder. The experiments gave the evaporation rates in terms of mass loss of chloride or nitrate which can be expressed conveniently as the rates of reduction of aerosol radius with time. Both dry aerosols (humidity 30-60% r.h.) and aqueous aerosols (humidity ca 97% r.h.) were studied. Dry aerosols evaporate at rates of -1.05 Å s-1 for NH4Cl and -0.45Å s-1 for NH4NO3, while the evaporation rates of aqueous aerosols expressed as for equivalent dry particles are -4.52 Å s-1 for NH4Cl and -0.49 Å s-1 for NH4NO3. The experimentally measured rates are independent of particle radius and remarkably low compared with those predicted from existing theories of aerosol evaporation, thus implying that there is an unknown kinetic constraint to the achievement of equilibrium at atmospheric temperature and pressures. © 1990.