The impact of cloud processing by trade-wind cumulus on the light scattering efficiency of aerosol particles

Conserved thermodynamic tracers, wet equivalent potential temperature and total water mixing ratio (θ_q and Q respectively) are used to characterize sub-saturated air parcels in a cloudy trade-wind cumulus layer. A mixing diagram of {θ_q,Q} samples close to a cloud shows that sub-saturated air, sampled at a constant altitude in the cloud layer upwind and downwind of a cloud band, is composed of air parcels that have originated from two levels; near cloud base and a level in the free troposphere. The only manner in which air from the sub-cloud layer can make its way into the free troposphere is via convection and, therefore, contains cloud-processed air. The largest values of θ_q identify air that has most recently been detrained from cloud and experienced less mixing with environmental air than air parcels with lower θ_q. As such, parcels with larger θ_q are more indicative of the effect of cloud processing on the aerosol particle and SO₂ concentration than parcels with a smaller θ_q. We stratify the aerosol size distribution, condensation nuclei concentrations, sulfur dioxide concentration and the aerosol extinction coefficient and mass extinction efficiency with θ_q. This enables physical interpretations about cloud processing of aerosol particles and SO₂ to be inferred. Although there is evidence of mass addition of sulfate to aerosol particles and an observable modification of the aerosol size distribution, we find no modification of the aerosol extinction coefficient nor the aerosol mass extinction efficiency.