Author: Carley FredricksonPublisher:
ISBN:
Category :
Languages : en
Pages : 72
Book Description
Phenolic compounds emitted from wildfires and biomass burning (BB) are highly reactive and yield secondary organic aerosol (SOA) and brown carbon (BrC) upon oxidation initiated by the hydroxyl radical (OH) and nitrate radical (NO3). In high nitrogen dioxide (NO2) environments, such as BB plumes, phenolic oxidation is expected to form nitroaromatics in high yield which can explain in part the BrC content of associated SOA. We conducted a set of experiments as part of the Monoterpene and Oxygenated aromatics Oxidation at Night and under LIGHTs (MOONLIGHT) campaign to evaluate the chemical and physical drivers of phenolic compound evolution in high nitrogen oxide (NOx = NO + NO2) wildfire plumes, specifically investigating the composition, volatility, and absorption of the SOA components formed under OH and NO3 oxidation, with catechol as the focus of this thesis. Oxidation products in both the gas and particle phases were measured using an I- adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF I- CIMS) coupled with the Filter Inlet for Gases and Aerosols (FIGAERO). Oxidation of catechol produced BrC, defined by light absorption at 405 nm, at the highest yields out of all the phenolics studied. Particle-phase nitrocatechol (C6H5NO4) was found to account for 28% and 79% of organic aerosol (OA) mass formed from OH-initiated or NO3-initiated oxidation, respectively, and was strongly associated with BrC. Effective molar yields, i.e., including chemical and physical losses, of nitrocatechol were measured to range from 0.65 to 1 for NO3-initiated oxidation, and 0.03 for OH oxidation conditions. Maximum SOA mass yields from catechol oxidation were strongly tied to formation of nitrocatechol, ranging from 0.38 to 1.63 for the different experiments, lower than previously reported values. Higher SOA mass yields from catechol oxidation were found for NO3 rather than OH oxidation. The effective volatility of the SOA measured with the FIGAERO thermograms decreased significantly with subsequent aging after formation. Gas-particle partitioning measurements imply the saturation vapor concentration of nitrocatechol to be roughly 5 [micrograms] m-3, while the FIGAERO thermogram model estimate is lower but in the same order of magnitude, implying that wildfire gas-particle partitioning of nitroaromatics is likely dynamic. Group contribution method estimates of nitrocatechol saturation concentration range across 8 orders of magnitude with 3 [micrograms] m-3 from the Nannoolal method paired with the Joback and Reid boiling point method being closest to our observational estimates. In extended photochemical aging experiments, BrC formed from catechol oxidation had a photochemical lifetime of ~12 hours, while that of particulate nitrocatechol ranged from 7 hours if formed by NO3 oxidation to 18 hours if formed by OH oxidation. Implications for atmospheric evolution of BrC in wildfire and mechanisms of particulate nitroaromatic losses are discussed.
