Aerosol characteristics observed in southeast Queensland and implications for cloud microphysics

Regular cloud base aerosol measurements were collected by a research aircraft over a 4 month period during the wet season in southeast Queensland, near Brisbane, Australia. In situ cloud microphysical measurements were also collected in many of these clouds. Brisbane is situated on the east coast of Australia and, depending on the synoptic weather conditions, can experience influences from inland or from the adjacent ocean. In situ aerosol measurements were used to compile a climatology of the aerosol conditions in the region and a back trajectory model was run for 120 h from each measurement to determine the possible air mass influences on each measurement. The most influential factor on the measured aerosol concentrations was the time the trajectory spent over land beneath 2 km (a proxy for the boundary layer). Using this criterion, the measurements were divided into two regimes: maritime for those with minimal time histories over land and continental for the remainder. Other influential factors on the aerosol concentration were also diagnosed and quantified with a regression model, including the proximity of the trajectory to the city of Brisbane and the number of fires along the trajectory. Thermodynamic, aerosol, and cloud microphysical characteristics are presented for each regime. The maritime regime tended to have more coarse mode (larger) aerosol particles and a tail of larger drops in the cloud base droplet spectra, which could be due to nucleation on the larger aerosol particles. The city influence on maritime regime trajectories yielded enhanced nucleation (fine) particle concentrations.