Synergisms Between Ozone and Nitrate Radical Chemistry in the Formation and Composition of Secondary Organic Aerosol

Veronique Perraud University of California
Emily Bruns University of California
Michael Ezell University of California
Stanley Johnson University of California
Yong Yu California Air Resources Board
Lizabeth Alexander Pacific Noerthwest National Laboratory
Alla Zelenyuk-Imre Pacific Northwest National Laboratory
Dan Imre Imre Consulting
Barbara Finlayson-Pitts University of California

Category: Aerosol Properties

Working Group: Aerosol Life Cycle

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Schematic representation of the α-pinene oxidation reactions and resulting particle composition observed in this study using NO2+O3 as the source of NO3 radicals.

The three major atmospheric oxidants involved in the formation of SOA from biogenic volatile organic compounds are O3, OH, and NO3. While O3 and OH-initiated oxidation are most important during the day, NO3 radical-initiated oxidation is a major contributor to the nighttime chemistry of volatile organic compounds in the troposphere. The reaction of biogenic hydrocarbons with NO3 is relatively fast (for example, the lifetime of α-pinene is only ~11 min at 2.5 x 10^8 NO3 molecules cm-3), so this reaction is expected to be a major sink for organics at night, and also to contribute to the removal of NOx from the atmosphere. Furthermore, it has the potential to form SOA of different composition (e.g., containing organic nitrates) than the ozone and OH oxidations. Previous studies by other groups have shown that the SOA yield from the NO3 reaction is relatively small due to the high volatility of the reaction products. However, studies to date have been in relatively simple systems where NO3 was the sole oxidant, which is not necessarily representative of atmospheric conditions. We report the results of experiments designed to probe the influence of O3 on particle formation and composition in the NO3 oxidation of α-pinene. The experiments were performed in a unique large diameter, high volume, slow-flow tube using the reaction of NO2 with O3 as the source of NO3 radicals. Particle size distributions were measured using a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS). Real-time aerosol mass spectrometers (SPLAT-II and AMS) as well as integrated collection of particles on ZnSe impaction disks and quartz fiber filters were used to characterize the chemical composition of the particles. Organic nitrates were major components of the particles at high ratios of NO2/O3 but decreased as this ratio decreased. The particle size distribution also shifted from a very few-but-large particle distribution to a numerous-much-smaller particle distribution. In addition, the formation of low vapor pressure products such as carboxylic acids was observed due to the increasing contribution from the ozonolysis of α-pinene. The results of these experiments suggest that organic nitrates may contribute more to SOA than expected, based on oxidations carried out using NO3 in the absence of other SOA forming reactions such as ozonolysis reactions, and may potentially modify particle properties such as water uptake and their ability to act as CCN.

This poster will be displayed at ASR Science Team Meeting.