Expected impact of an aged biomass burning aerosol on cloud condensation nuclei and cloud droplet concentrations

[ 1] During May of 2003, smoke from fires on the Yucatan Peninsula was transported across the Gulf of Mexico and into Texas where it caused a significant enhancement in measured aerosol concentrations. The 24-hour average PM2.5 concentration measured in Austin on 10 May was 50.1 mu g/m(3), which was more than twice that of the highest daily average concentration measured during any other month in 2003. During this event, a differential mobility analyzer/tandem differential mobility analyzer (DMA/TDMA) system was used to characterize the size distribution and size-resolved hygroscopicity and volatility of the aerosol. The hygroscopicity data were used to isolate the less hygroscopic biomass burning particles from other aerosol types. Biomass burning aerosol-only size distributions were then constructed by coupling the size-resolved fraction of particles attributed to the fires with the overall size distribution. These distributions, and the aerosol properties derived from the TDMA data, were used to examine the impact of the smoke on predicted cloud condensation nuclei (CCN) spectra. The influence of the smoke on cloud droplet concentrations and the influence of other aerosol types present on the activation efficiency of the smoke were evaluated using a cloud parcel model. For a subset of the updraft speeds considered, the model predicted that the cloud droplet concentration would sometimes be lower when both smoke and pollution aerosols entered cloud relative to that when only smoke was present. Whereas these cases in which an increased aerosol concentration resulted in a decreased cloud droplet concentration were rare, the inclusion of the pollution aerosol in the model always reduced the activation efficiency of the smoke aerosol, which would influence both its evolution during transport and its atmospheric removal rate.