On the evaporation of semi-volatile compounds during aerosol drying and its effect on CCN activity

The water uptake and cloud droplet nucleating potential of ambient aerosol are an important factor in determining cloud properties such as albedo and thus an important factor and uncertainty in climate prediction. Organic compounds contribute a large fraction to ambient aerosol and can include semi-volatile materials in the gas phase as well as the condensed phase. These semi-volatiles may evaporate during particle drying, which would make them invisible to measurement methods that dry the aerosol during sampling. In this study we test a new approach to quantify the amount of evaporating material that is irreversibly lost in the aerosol drying process. Size distributions are measured by a scanning mobility particle sizer (SMPS) that characterizes the aerosol corresponding to three thermodynamic states: unperturbed, dried, and dried then re-humidified. The unperturbed state measures the size distribution at ambient relative humidity with minimal perturbation of the gas-phase surrounding the particles. Drying is achieved by a cold-trap (dT = -30 K) that removes water and semi-volatile compounds from the gas phase, followed by reheating the sample to ambient temperature. In the third configuration the dried sample is brought back to ambient relative humidity via a Nafion membrane humidifier. A Cloud Condensation Nuclei (CCN) counter downstream of the SMPS determines the diameter for aerosol activation as CCN at a supersaturation of ~0.3%. Sampling during the dried state corresponds to typical size-resolved CCN measurements. Sampling during the other states reveals the following new information: first, the difference in activation diameter between the dried state and the unperturbed state is a measure of the hygroscopic growth factor at ambient relative humidity. Second, because the only difference between the unperturbed and re-humidified states is the drying process, the difference in activation diameter between these states quantifies the irreversible loss of semi-volatile material during drying. Ambient data using this method were collected over a two week period in July 2013 as part of the Southern Oxidant and Aerosol Study (SOAS) at the main research site near Brent, Alabama, USA. The observed CCN-derived hygroscopicity parameter ranged from 0.1 to 0.3, and the data comparing the three states suggest that semi-volatile material was not irreversibly lost upon drying the aerosol.