Abstract: Ammonium nitrate (AN) is a semivolatile inorganic component of atmospheric aerosol particles typically accounting for 10-30% of the fine aerosol mass. It can evaporate from the particle phase as gas phase ammonia and nitric acid and vice versa, gas phase ammonia and nitric acid can condense into aerosol particles and dissociate to ammonium and nitrate ions in the aqueous phase.

Gas–particle partitioning of ammonium nitrate depends strongly on temperature with significantly larger mass being present in the particle phase in winter. Due to successful reduction of atmospheric sulfate concentrations during recent years, more gas phase ammonia has become available for reaction with nitric acid to form condensed phase ammonium nitrate and as a result the particle sulfate to nitrate ratio has increased globally within the last decade.

To calculate equilibrium partitioning of ammonium nitrate its vapor pressure needs to be known at the relevant atmospheric temperatures. We will present laboratory based experiments deducing vapor pressures from evaporation rates of single, levitated AN particles in its solid and aqueous, liquid state and compare these with literature data.

We will also show that evaporation rates of AN in viscous organic aerosol particles are significantly reduced, indicative of mass transfer limitations within the condensed phase. The impacts for atmospheric gas–particle partitioning will be discussed.

Dr. Ulrich Krieger is from the Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland.