Abstract

Aerosols play a major role in the climate system, acting both directly to perturb radiative transfer and atmospheric heating by scattering and absorbing solar radiation, and indirectly to modify cloud albedo and lifetimes. This talk will present recent work relevant to aerosol direct effects, motivated by the need to improve our fundamental understanding of the optical behaviour of different aerosol types as a function of wavelength, atmospheric age and environmental condition.

New instrumentation developed for the accurate and sensitive airborne measurement of aerosol absorption (using photoacoustic spectroscopy) and extinction (using cavity ringdown spectroscopy) will be introduced and applications in three areas presented. First, airborne measurements from the California Basin will be used to examine the impact of ageing on aerosol properties affecting climate and visibility in the Los Angeles plume. This analysis will highlight the key role of semi-volatile component partitioning in this region. Second, ground-based observations from Boulder, Colorado will be used to provide the first direct attribution of absorption to black carbon, brown carbon and absorption enhancement sources in a biomass burning plume. This study will highlight the importance of several absorption sources currently missing from global climate model simulations. Third, laboratory measurements will be used to demonstrate the utility of photoacoustic spectroscopy for aerosol absorption measurement at high relative humidity, for which no other direct methods currently exist. In addition to technical evaluation, these measurements also provide new constraint of the water mass accommodation coefficient, a key parameter controlling the kinetics of water condensation. Implications for potential kinetic limitations to cloud droplet formation will be discussed.