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Oxidation mechanism and organic aerosol formation from the ?�-pinene and pinonaldehyde reaction with NO3 radicals under near-ambient conditions

Introduction of the problem: The reaction of ?�-pinene with the nitrate (NO3) radical exerts significant control over the formation of secondary organic aerosol (SOA) and organic nitrates in the Southeast US, and other regions with joint biogenic and anthropogenic influence. Yet, there are large uncertainties in both the organic nitrate yield (10-30%) and the SOA yield (0-30%) for this reaction in the literature, and an incomplete understanding of the role of the organic nitrates as NOx reservoirs in ambient aerosols. These large uncertainties undermine accuracy in modeling SOA and reactive nitrogen from BVOC+NO3 sources in nature.

Rationale for proposed work: Discrepancies in yields and knowledge gaps in the reaction mechanism of ?�-pinene + NO3 may be due to the large range of chemical and physical conditions under which this reaction has been studied in past chamber studies, including those where the reaction is shifted toward higher volatility products instead of SOA. This work will thoroughly characterize the reaction of ?�-pinene+NO3 under near-ambient conditions by finely tuning reaction parameters, and provide a systematic understanding of (1) how surface area and liquid water content of inorganic seeds affect SOA yields in the chamber; (2) how the reaction responds to changing chemistry regimes, specifically to NO3 and HO2 levels that approach those representative of the atmosphere; and (3) how the second-generation chemistry of pinonaldehyde+NO3 contribute to ?�-pinene�??s ability to produce SOA and organic nitrates.

Brief summary of work: A series of atmospheric chamber experiments are planned to measure the yield and molecular composition of SOA and organic nitrates in the ?�-pinene+NO3 and pinonaldehyde+NO3 reactions, under a variety of conditions that mimic the reaction branching observed in the Southeast US nighttime (e.g., mainly RO2+HO2 and RO2+NO). A range of NO3 production rates (PNO3), relative humidity, and particle surface area will be investigated for each reaction subset. In addition to SOA yields, the oxidized VOC products (including gaseous organic nitrates) will be measured in-situ with a CF3O- CIMS, the SOA molecular composition (including aerosol organic nitrates) will be characterized with high-resolution nanospray mass spectrometry, along with other planned measurements.

Relevance to the AC4 Program: This work directly addresses two out of three priorities in the AC4 Program, Priority 1 and Priority 2, i.e., (a) promote a better understanding of BVOC+NO3 oxidation mechanisms; (b) provide better constraints on the yields of SOA and gaseous products from BVOC+NO3; and (c) provide accurate measurements of aerosol organic nitrate production and loss to understand their role in atmospheric NOx removal. Relevance to NOAA�??s climate goals This research is expected to help reduce uncertainties in atmospheric model simulations of SOA and reactive nitrogen in certain regional environments. Our focus on the dependence of the reactions on PNO3 rates has relevance to understanding a changing future atmosphere, which is a benefit to society in general. As emission controls further reduce NOx in the United States, we will see lower NO2 and O3 concentrations (and thus, lower PNO3).

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