PNNL

Abstract

Organic aerosol (OA) is a complex matrix of various constituents – fresh (primary organic aerosols – POA) and aged via oxidation (secondary organic aerosols – SOA), generated from both biogenic and anthropogenic sources. The viscosity of OA can be critical in influencing new particle formation, the processing of reactive uptake that impact evaporation-growth kinetics, and the lifetime of particles in the atmosphere. This work utilizes a well-defined relationship between volatility and viscosity for pure compounds to determine the phase state via viscosity of bulk OA during the dry-to-wet transition season (September-October) in the Amazon rainforest during 2014. Our simulations indicate spatial and temporal heterogeneity in aerosol phase state often not captured by global-scale models. We show the strong role of water associated with organic aerosol (w_s) as the predominant factor that can be used to quantitatively estimate OA viscosity. We find a strong inverse log-linear relationship between w_s and OA viscosity with a correlation coefficient approaching 1, both in the background and biomass burning influenced conditions. At high altitudes where RH and temperatures are low, our simulations indicate that OA exists in a semi-solid/solid like phase-state. OA hygroscopicity is also strongly correlated (~ -0.8) to OA viscosity at RH ~ 30-50% under background conditions, but OA hygroscopicity is uncorrelated with viscosity at lower and higher RH regimes. Supported by DOE Early Career Award, Manish Shrivastava PI