Identifying Isoprene and Toluene Gas-Phase Oxidation Products to Better Constrain Ozone and Secondary Organic Aerosol Formation in the Atmosphere

Abstract

Anthropogenic pollutants such as NOx interact with volatile organic compounds (VOCs) such as isoprene and toluene to produce ozone (O3) and oxidized low volatility compounds that are responsible for forming secondary organic aerosol (SOA). Understanding the processes that form O3 and SOA from VOCs is important for understanding climate interactions and air quality. Both O3 and SOA are harmful air pollutants. O3 directly contributes to warming while the influence of aerosols is far more complicated, but ultimately leads to regional cooling. Understanding the chemistry that produces O3 and SOA will help better predict how future regulations will influence climate and air quality. A series of experiments using the Caltech chamber facility were designed and performed to better understand the influence of isoprene and toluene gas-phase oxidation products on O3 and SOA formation. First, in order to conduct experiments, the new Caltech chamber facility was characterized. Second, to understand the oxidation products from isoprene in the presence of anthropogenic pollutants such as NOx, a chemical ionization mass spectrometer (CIMS) was used to identify the gas-phase products from the oxidation of isoprene by the nitrate radical (NO3). First-generation nitrates were identified to be predominantly δ-nitrates while first-generation nitrates formed from OH oxidation of isoprene in the presence of NO are predominantly β-nitrates. This has important consequences for NOx recycling and O3 generation because these β- and δ-nitrates react with O3 and OH at different rates and form different products. Photooxidation products from nitrooxy hydroperoxide, a product from isoprene + NO3 oxidation (in the presence of hydroperoxy radical-HO2), were identified to be predominantly propanone nitrate and nitrooxy hydroxy epoxide. Nitrooxy hydroxy epoxide undergoes reactive uptake to seed aerosol similar to isoprene dihydroxy epoxide, suggesting it may be important for SOA formation. Lastly, first- and later-generation photooxoidation products from cresol and benzaldehyde oxidation were identified. Cresol and benzaldehyde are products from toluene OH oxidation. Low volatility ring-retaining products produced from cresol oxidation were detected in the gas phase by the CIMS and in the particle phase using offline direct analysis in real time mass spectrometry (DART-MS). Products detected included polyols such as dihydroxy, trihydroxy, tetrahydroxy, and pentahydroxy toluenes and benzoquinones such as hydroxy, dihydroxy, and trihydroxy methyl benzoquinones. These results suggest that even though the cresol pathway only contributes ∼20% to gas-phase toluene oxidation, products from the cresol channel potentially generate a significant fraction ( ∼20-40%) of toluene SOA.