Organic nitrates from nitrate radical oxidation of monoterpenes: Effects of dilution, temperature, humidity, and photochemical aging

Secondary organic aerosols (SOA) contribute a substantial fraction of ambient fine particulate matter (PM) and have important implications on climate and health. Results from field measurements have shown that particulate organic nitrates are ubiquitous in ambient aerosols. Organic nitrates play an important role in the cycling of NOx, SOA, and ozone formation, yet their formation mechanisms and fates remain highly uncertain. The reactions of nitrate radicals with monoterpenes at night is an important source of organic nitrates. In this work, we investigate the formation and fates of organic nitrates formed from the oxidation of a-pinene, b-pinene, limonene, which are the most abundant monoterpenes. Experiments are conducted in the Georgia Environmental Chamber facility (GTEC) under dry and humid conditions. Speciated gas-phase and particle-phase organic nitrates are continuously monitored by a Filter Inlet for Gases and AEROsols High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (FIGAERO-HR-ToF-CIMS). Bulk aerosol composition is measured by a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). A large suite of highly oxygenated gas- and particle-phase organic nitrates are formed rapidly. We investigate the fates of organic nitrates via dilution, temperature change, hydrolysis, and further photochemical oxidations. These experiments are used to simulate the properties and fates of nighttime organic nitrates during the morning period as night transitions into day. Results from this study provide new fundamental insights into the chemical life cycles of organic nitrates, which will aid interpretation of ambient data and provide valuable inputs for atmospheric model developments to better constrain organic nitrate chemistry.