AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Photochemical Aging of a-Pinene and b-Pinene Secondary Organic Aerosol Formed from Nitrate Radical Oxidation
THEODORA NAH, Javier Sanchez, Christopher Boyd, Nga Lee Ng, Georgia Institute of Technology
Abstract Number: 41 Working Group: Aerosol Chemistry
Abstract The nitrate radical (NO$_3), one of the most important oxidants in the nocturnal atmosphere, can react rapidly with a variety of BVOCs to form high mass concentrations of SOA and organic nitrates. Since NO$_3 is derived almost exclusively from the reaction of NO$_2 with O$_3, SOA production from NO$_3-BVOC reactions represents a mechanism for positively correlating anthropogenic pollutants with biogenic emissions. Organic nitrates, which account for a large fraction of ambient nighttime SOA, play important roles in the atmosphere as their fates may influence the global nitrogen budget and ozone production. Despite the importance of NO$_3-BVOC reactions, their reaction mechanisms and products remain poorly characterized. There is also a dearth in knowledge on how SOA and organic nitrates formed from NO$_3-BVOC reactions evolve as they age in the atmosphere. Aerosol lifetimes in the atmosphere typically range between 1 and 2 weeks; half of it is in the day where OH photochemical oxidation is likely dominant.
We investigated how the composition and mass loading of a-pinene and b-pinene SOA formed from NO$_3 oxidation under dark humid conditions changes when they are photochemically aged with OH through a series of chamber experiments performed at the Georgia Tech Environmental Chamber (GTEC). The a-pinene and b-pinene SOA are characterized using real-time gas- and particle-phase measurements, which are used to propose mechanisms for SOA and organic nitrate formation and aging. Using a Filter Inlet for Gases and AEROsols (FIGAERO) coupled to a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS), we observed the formation of highly oxygenated particle-phase organic nitrates (possessing 6 to 8 oxygen atoms) during the NO$_3-b-pinene reaction, but not during the NO$_3-a-pinene reaction. Results from this study aims to explain ambient observations from the recent Southern Oxidant and Aerosol Study (SOAS), which showed that the NO$_3-monoterpenes reactions account for a substantial fraction of nocturnal SOA and organic nitrates in the southeastern US, as well as provide new insights into how the transition from night to day oxidation environments affects the fates of nocturnal monoterpene SOA and organic nitrates.