Particle-Phase Diffusion Limitations in Fresh Isoprene Secondary Organic Aerosol

Description

Environmental chambers have been used to investigate secondary organic aerosol (SOA) formation and generate model parameterizations based on semi-volatile equilibrium partitioning theory, which have successfully improved the prediction of SOA formation in atmospheric models. However, recent studies have shown that traditional equilibrium partitioning theory sometimes fails to predict SOA mass generated from oxidation of binary or more complex precursor VOC mixtures. Our previous experimental work has shown that fresh isoprene SOA did not rapidly partition into a-pinene SOA and isoprene SOA that are aged for 15−18 hours (at OH concentrations of ~ 2 × 10⁶ molecules/cm³) as equilibrium partitioning theory would predict, even under humid condition (RH ~ 70%). In a new study, we probed the aging time scale required to trigger these mass transfer limitations. Isoprene SOA seed was prepared by irradiating gas-phase isoprene and H₂O₂ in the presence of dry ammonium sulfate seed in the PNNL environmental chamber at ~ 23 °C and RH ~50%. Once the initial isoprene SOA growth had stopped, the resultant isoprene SOA was aged under either light or dark for 0 – 6 hours. This was followed by a second photochemical growth period initiated by the injection of additional isoprene vapor. Surprisingly, equilibrium partitioning overpredicted the observed SOA yield when we assume that the fresh SOA is soluble in the aged SOA seed for the second photochemical growth period regardless of the aging time and irradiation condition. A box model that includes mass transfer limitations was used to simulate the observed particle size distribution evolution during the second growth phase. Reproducing the experimental observations required assuming mass transfer limitations in the model. These results suggest diffusion limitations may exist even in freshly-formed SOA particles.

This research is supported by the ICLASS SFA, Jerome Fast, PI