Direct measurement of aerosol absorption at elevated relative humidity using photoacoustic spectroscopy

The impacts of atmospheric aerosol on radiative forcing depend strongly on relative humidity (RH). As particles take up water at elevated humidity, both scattering and absorption properties can be modified. Experimental measurements to date have focused heavily on characterising the RH dependence of scattering. These efforts have yielded a large body of data, which has guided the construction of parameterisations needed for large-scale climate modelling. Similar measurements of the RH dependence of aerosol absorption are however limited, largely due to difficulties associated with making measurements using traditional methods. New approaches are needed. One technique that has shown promise is photoacoustic spectroscopy. In this presentation we will provide a comprehensive laboratory validation of the performance of the NOAA aerosol photoacoustic instrument at elevated RH. This section will highlight key factors that must be considered when operating photoacoustic instruments at elevated RH, such as variable microphone sensitivities, background corrections and evaporative heat transfer effects. We will also present measurements of absorption at elevated RH made in a dense forest fire smoke plume sampled aboard the NASA DC-8 aircraft during the recent Deep Convective Clouds and Chemistry (DC3) project. The significance of these measurements for understanding the impacts of this aerosol class on radiative forcing will be discussed.