Modeling the Impact of the Organic Aerosol Phase State on Multiphase OH Reactive Uptake Kinetics and the Resultant Heterogeneous Oxidation Timescale of Organic Aerosol in the Amazon Rainforest

Journal Article · Mon Apr 24 00:00:00 EDT 2023 · ACS Earth and Space Chemistry

DOI:https://doi.org/10.1021/acsearthspacechem.2c00366· OSTI ID:1974533

Rasool, Quazi Z. ^[1]; Shrivastava, ManishKumar B. ^[1]; Liu, Ying ^[1]; Gaudet, Brian J. ^[1]; Zhao, Bin ^[2]

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1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Tsinghua Univ., Beijing (China)

Accurate predictions of chemical lifetime, i.e., oxidation timescale and change in the mass of organic aerosols (OAs) in atmospheric models, are critical to quantify the impacts of OA on aerosol–cloud interactions, radiative forcing, and air quality. The heterogeneous oxidation of OA by hydroxyl (OH) radicals is a key process governing OA mass changes during their oxidation. Recently, laboratory observations of the OH uptake coefficient (γ_OH) and heterogeneous reaction rate (k_het,OH) for different OA systems with varying phase states have been combined to develop a new parameterization of γ_OH and k_het,OH as a function of the OA phase state, i.e., viscosity (η_OA). Here, in this work, we use a recently published viscosity prediction framework to analyze the new γ_OH and k_het,OH parameterization with a box model. Subsequently, we implement this box model to predict γ_OH and k_het,OH over the Amazon rainforest in the dry-to-wet transition season with both significant biomass burning and biogenic influences. Relevant parameters within our box model are specified based on detailed regional model simulations over the Amazon using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). η_OA is predicted as a function of species volatility, OA composition (including water uptake) along with ambient relative humidity (RH), and temperature. Based on ambient conditions simulated by WRF-Chem over the entire atmospheric column, we use the box model to predict the upper bounds of the heterogeneous oxidation timescale of OA. Based on previous laboratory measurements, we assume that this heterogeneous OH oxidation causes fragmentation (carbon loss) of OA, followed by the evaporation of the fragmented molecules that results in the exponential decay of OA. We predict that the oxidation timescale of OA is ~1 month near the Earth’s surface in the pristine Amazonian background because of low OH concentrations. But OA is oxidized more rapidly within urban and wildfire plumes near the Earth’s surface with an order of magnitude higher OH concentrations compared to the pristine background, causing the simulated oxidation timescale of OA to be much shorter, ~3–4 days. At 3–5 km altitudes where biomass burning OA is predicted to be semisolid, the heterogeneous oxidation timescale is estimated as ~6 days to 3 weeks and decreases with increasing OH concentrations within plumes. We show that the simulated mass loss of OA is strongly size-dependent, where smaller particles oxidize more rapidly compared to larger particles due to their greater per-particle surface area-to-volume ratio. Increasing urbanization and deforestation in the Amazon in the future might increase OH concentrations in the background Amazon, causing faster oxidation of OA. At higher altitudes above liquid clouds, especially in the upper troposphere where temperatures approach ~250 K, future measurements are needed to reduce the uncertainties related to the mass loss of OA at colder temperatures and low relative humidity conditions due to its heterogeneous OH oxidation.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1974533
• Report Number(s): PNNL-SA-178543
• Journal Information: ACS Earth and Space Chemistry, Journal Name: ACS Earth and Space Chemistry Journal Issue: 5 Vol. 7; ISSN 2472-3452
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Amazon
OH heterogeneous uptake
aerosols
biomass burning plume
liquid OA
liquids
organic aerosol (OA) viscosity
oxidation
oxidation timescale
phases of matter
reaction kinetics
semisolid OA
viscosity