AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
An Online Method to Characterize the Reversibility of Secondary Organic Aerosol Formed in Aerosol Liquid Water
MARWA EL-SAYED, Christopher Hennigan, University of Maryland, Baltimore County
Abstract Number: 13 Working Group: Instrumentation and Methods
Abstract A new method is presented for the characterization of the reversible/irreversible nature of secondary organic aerosol formed in aerosol water (aqSOA). The relative contribution of reversible and irreversible uptake processes is a major unknown in our understanding of atmospheric aqSOA formation. The method utilizes simultaneous measurements of water soluble organic carbon in the particle (WSOC$_p) and gaseous (WSOC$_g) phases. These bulk measurements are surrogates for SOA and for secondary oxygenated organic gases, respectively. The central feature of this measurement approach is the behavior of WSOC$_p under conditions of drying. To characterize the effect of aerosol water evaporation on aqSOA, the WSOC$_p sample is alternated between an unperturbed ambient channel (WSOC$_p) and a 'dried' channel in which the air sample passes through a silica gel diffusion dryer (WSOC$_(p,dry)). The dried channel does not remove all particle-bound water, but rather approximates the lowest relative humidity (RH) that particles are exposed to in the lower troposphere to simulate natural drying processes. The enhancement in SOA formation due to aqSOA is inferred based on the enhancement in the fraction of the total WSOC in the particle phase, F$_p, as a function of RH. A decrease in the WSOC$_p concentration through the dried channel indicates the evaporation of SOA due to water evaporation – and hence, reversible aqSOA. On the other hand, irreversible aqSOA is inferred if no statistically significant difference is observed in the WSOC$_p concentrations through the two channels. The completely automated system is able to run for weeks with minimal intervention. A single WSOC$_p-WSOC$_(p,dry)- WSOC$_g measurement cycle is completed in 14 min, allowing for the characterization of dynamic changes in the factors influencing reversible/irreversible uptake processes. Several measures were undertaken to validate the method, minimize particle losses within the system and ensure fully automated and reliable measurements across diverse ambient conditions.