Observations and modelling of aerosol growth in marine stratocumulus - Case study

Airborne measurements of the growth of the marine accumulation mode after multiple cycles through stratocumulus cloud are presented. The nss- sulphate cloud residual mode was log-normal in spectral shape and it's mode radius was observed to progressively increase in size from 0.78 to 0.94 μm over 155 min of air parcel evolution through the cloudy marine boundary layer. The primary reason for this observed growth was thought to result from aqueous phase oxidation of SO₂ to aerosol sulphate in activated cloud drops. An aqueous phase aerosol-cloud-chemistry model was used to simulate this case study of aerosol growth and was able to closely reproduce the observed growth. The model simulations illustrate that aqueous phase oxidation of SO₂ in cloud droplets was able to provide enough additional sulphate mass to increase the size of activated aerosol. During a typical cloud cycle simulation, ≃4.6 nmoles kg(air)^-1 (0.44 μg m^-3) of sulphate mass was produced with ≃ 70% of sulphate production occurring in cloud droplets activated upon sea-salt nuclei and ≃ 30% occurring upon nss-sulphate nuclei, even though sea-salt nuclei contributed less than 15% to the activated droplet population. The high fraction of nss-sulphate mass internally mixed with sea-salt aerosol suggests that aqueous phase oxidation of SO₂ in cloud droplets activated upon sea-salt nuclei is the dominant nss-sulphate formation mechanism and that sea-salt aerosol provides the primary chemical Sink for SO₂ in the cloudy marine boundary layer.