Global distribution of solid and aqueous sulfate aerosols: effect of the hysteresis of particle phase transitions

The partitioning between solid and aqueous phases of tropospheric sulfate-ammonium particles is simulated with a global 3D chemical transport model (CTM). The simulation explicitly accounts for the hysteresis of particle phase transitions by transporting aqueous sulfate and three solid sulfate forms (ammonium sulfate, letovicite, and ammonium bisulfate). Composition-dependent deliquescence relative humidities (DRH) and crystallization relative humidities (CRH) are based on recent laboratory data. We find that the solids mass fraction on a sulfate basis is 0.34, partitioned as 93% ammonium sulfate, 6% letovicite, and 1% ammonium bisulfate. The solids mass fraction increases with altitude from 0.10-0.20 in the boundary layer to 0.60-0.80 in the upper troposphere. The dominance of solids in the upper troposphere arises in part from high sulfate neutralization, reflecting in our simulation a low retention efficiency of NH3 upon cloud freezing. High sulfate neutralization is consistent with the few available observations. High acidity with a dominant aqueous phase, however, can occur in the upper troposphere during volcanic eruptions. Seasonal variation of the solids mass fraction in global average is modulated by emissions of NH3 from the terrestrial biosphere and biomass burning and of dimethylsulfide from the ocean biosphere. The timescale of phase transitions as driven by changes in relative humidity varies from 10- 50 hr in the boundary layer to 150-400 hr in the upper troposphere. Omission of the hysteresis effect in global models by assuming that particle phase follows the lower side of the hysteresis loop increases the solids mass fraction from 0.34 to 0.56. An upper-side assumption decreases the fraction to 0.17. Lower- and upper-side assumptions better approximate particle phase for high and low altitudes, respectively. Fluctuations in the CRH, which can be induced by other constituents in sulfate particles such as minerals or organic molecules, strongly affect the solids mass fraction in the boundary layer but not at higher altitudes.