Abstract
The growth of monodisperse particles (0.07 to 0.5 µm) exposed to SO2 (0–860 ppb), H2O2 (0–150 ppb) and sometimes NH3 (0–550 ppb) in purified air at 22 °C at relative humidities ranging from 25 to 75% were measured using the Tandem Differential Mobility Analyzer technique. The experiments were performed in a flow reactor with aqueous (NH4)2SO4 and Na2SO4 droplets. For (NH4)2SO4 droplets the fractional diameter growth was independent of size above 0.3 µm but decreased with decreasing size below that. When NH3 was added the fractional growth increased with decreasing size. Measurements were compared with predictions of a model that accounts for solubility of the reactive gases, the liquid phase oxidation of SO2 by H2O2, and ionic equilibria. Agreement between measured and predicted droplet growth is reasonable when the ionic strength effects are included. Theory and experiments suggest that NH3 evaporation is responsible for the decrease in relative growth rates for small aqueous ammonium sulfate particles. The observed droplet growth rates are too slow to explain observed growth rates of secondary atmospheric sulfate particles.
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