A novel model to predict the physical state of atmospheric H2SO4/NH3/H2O aerosol particles
The physical state of the tropospheric aerosol is largely unknown despite its importance for cloud formation and for the aerosol’s radiative properties. Here we use de- tailed microphysical laboratory measurements to perform a systematic global modelling study of the physical state of the H2SO4/NH3/H2O aerosol, which constitutes an impor- tant class of aerosols in the free troposphere. The Aerosol Physical State Model (APSM) developed here is based on Lagrangian trajectories computed from ECMWF (European Centre for Medium RangeWeather Forecasts) analyses, tak- ing full account of the deliquescence/efflorescence hystere- sis. As input APSM requires three data sets: (i) deliques- cence and efflorescence relative humidities from laboratory measurements, (ii) ammonia-to-sulfate ratios (ASR) calcu- lated by a global circulation model, and (iii) relative hu- midities determined from the ECMWF analyses. APSM re- sults indicate that globally averaged a significant fraction (17–57%) of the ammoniated sulfate aerosol particles con- tain solids with the ratio of solid-containing to purely liq- uid particles increasing with altitude (between 2 and 10 km). In our calculations the most abundant solid is letovicite, (NH4)3H(SO4)2, while there is only little ammonium sul- fate, (NH4)2SO4. Since ammonium bisulfate, NH4HSO4, does not nucleate homogeneously, it can only form via het- erogeneous crystallization. As the ammonia-to-sulfate ra- tios of the atmospheric H2SO4/NH3/H2O aerosol usually do not correspond to the stoichiometries of known crystalline substances, all solids are expected to occur in mixed-phase aerosol particles. This work highlights the potential impor- tance of letovicite, whose role as cloud condensation nucleus (CCN) and as scatterer of solar radiation remains to be scru- tinized.