Modeling studies of the sulfur cycle in low-level, warm stratiform clouds

A one-dimensional cloud model with size-resolved microphysics has been fully coupled with size-resolved aqueous-phase chemistry. The model, driven by prescribed dynamics, has been used to study the sulfur cycle in low-level, warm stratiform clouds. Model evaluation and sensitivity tests show that the model can reasonably simulate the time-evolution of cloud properties, interstitial aerosols, gas to aqueous conversions and size-resolved cloud water acidity.Model results show that gaseous NH3 and H2O2 can be removed quickly by the aqueous-phase process within the cloud layer due to the very small size of cloud droplets. 10% to 50% of SO2 within the cloud layer can be scavenged in a 1-h period depending on the concentrations of other gaseous species, especially those of NH3 and H2O2. Aqueous-phase chemistry can effectively transfer mass from gas to aqueous particles in low-level, warm stratiform clouds. The rate of transfer from gaseous SO2 to aqueous sulfate depends on the cloud properties, which are controlled by the initial cloud condensation nuclei (CCN) from which the cloud forms, and on gaseous concentrations of other species such as NH3 and H2O2. Aqueous-phase chemistry might broaden the cloud droplet spectra by increasing the number concentrations of both smallest and largest droplets, and thus enhance precipitation formation.