Modeling Volatility-Based Aerosol Phase State Predictions in the Amazon Rainforest

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 biogenic, anthropogenic, and biomass burning sources. The viscosity of OA can be critical in influencing new particle formation, reactive uptake processes 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, which we incorporated within the Weather Research and Forecasting Model coupled to chemistry (WRF-Chem) to simulate the phase state and 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 dominant factor that can be used to quantitatively estimate OA viscosity. Analysis of WRF-Chem simulations across the entire atmospheric column indicates a strong inverse log-linear relationship between w s and OA viscosity with a correlation coefficient approaching 1, in the background and biomass burning-influenced conditions. At high altitudes where relative humidity (RH) and temperatures are low, our simulations indicate that OA exists in a semisolid-/solid-like phase state, consistent with previous studies. OA hygroscopicity is strongly correlated (ca. −0.8) with OA viscosity at RH ca. 30–50%, but this RH range is found mostly at low OA concentrations and the middle troposphere (ca. 6–10 km altitudes) in our simulated domain. OA hygroscopicity is uncorrelated with viscosity at higher-RH (near surface) and lower-RH (upper troposphere) regimes. At the urban site near surface, where day–night differences in RH are significant, RH is found to drive the phase state. At the background forested site near surface, where day–night RH differences are small, biomass burning-influenced OA is semisolid and a significant OA associated with background conditions is liquid-like. Simulations indicate a long tail of OA viscosity frequency distributions extending in the semisolid/solid regimes over background biogenic-influenced conditions due to the role of low-volatility OA components such as monoterpene oxidation products.

Tight Coupling of Surface and In-Plant Biochemistry and Convection Governs Key Fine Particulate Components over the Amazon Rainforest

“…We calculated the viscosity of OA (η org ) based on the glass transition temperature of OA (T g,org ), which is essentially the temperature at which the transition to the amorphous solid-phase state occurs. 23 Viscosity was calculated online at each WRF-Chem model time step and grid cell as a function of temperature, RH, and OA composition (volatility and O/C ratio), as described in Rasool et al, 24 and further details of these calculations are provided in the Supporting Information. Calculations of OA viscosity and particle-phase diffusivity are performed.…”

Glyoxal as a Potential Source of Highly Viscous Aerosol Particles

“…The T g values measured here refer to dried aerosol samples with the main component being glyoxal, i.e., without the presence of water. Nevertheless, such experimentally determined values are crucial for the parametrization development and subsequent modeling of the phase state or viscosity of atmospherically relevant particles. ,,− Our results also indicate the importance for implementing more complex aqueous phase chemistry in such models, rather than applying simplified mixing rules. At colder temperatures, the equilibration time of aqueous glyoxal aerosol droplets after water uptake or loss is longer than at room temperature.…”

Particles' phase state variability in the North Atlantic free troposphere during summertime determined by different atmospheric transport patterns and sources

“…These effects have been previously shown in laboratory studies by measuring the viscosity of organic and inorganic mixtures (Dette and Koop, 2015;Power et al, 2013;Rovelli et al, 2019;Saukko et al, 2012;Schill and Tolbert, 2014;Song et al, 2021;Wang et al, 2015) and in ambient samples (Liu et al, 2019;Slade et al, 2019). Moreover, the effects of internally mixed inorganics on the particle's viscosity are not considered in current climate models (Rasool et al, 2021;Schmedding et al, 2020;Shiraiwa et al, 2017). These liquid particles are expected to be less stable and have faster 490 heterogeneous reaction rates, be more reactive, and be quickly photodegraded in the FT (Berkemeier et al, 2016;Kuwata and Martin, 2012;Lienhard et al, 2015;Liu et al, 2018a;Marshall et al, 2016;Pöschl and Shiraiwa, 2015b;Renbaum-Wolff et al, 2013).…”

“…DeRieux et al, 2018 predicted the viscosity of 400 biomass burning particles using their chemical composition, and they found that the viscosity of biomass burning particles varied between 10 -2 Pa S and 10 12 Pa S depending on the RH. Liu et al, 2021 found that ambient and lab-generated biomass burning particles are in a non-solid state at RH between 20 % and 50 %. Therefore, these studies support our finding that biomass burning can emit particles with a broad viscosity distribution.…”