Probing the Model-Measurement Discrepancy in SOA Formation: A Case Study of Alpha-pinene Oxidation by NO3 Radicals

VERONIQUE PERRAUD, Emily A Bruns, Michael J Ezell, Stanley N Johnson, Yong Yu, M Lizabeth Alexander, et al.

University of California Irvine

     Abstract Number: 513
     Last modified: April 4, 2011

Abstract
Measurements of secondary organic aerosol (SOA) formed from the NO$_3 radical initiated oxidation of alpha-pinene were conducted in a large volume, slow-flow aerosol flow tube at 1 atm in air and 295 K. The source of NO$_3 radicals was the reaction of a constant concentration of O$_3 with varying concentration of NO$_2. Particle concentrations and chemical composition were measured simultaneously using scanning mobility and aerodynamic particle sizers, two aerosol mass spectrometers (HR-ToF-AMS and SPLAT-II MS), and two integrative sampling techniques (impaction on ZnSe discs with subsequent analysis by FTIR and collection on quartz-fiber filters followed by LC-UV analysis). Ozone chemistry was shown to play the dominant role in determining the particle formation, growth, and composition, even at relatively small O$_3/NO$_2 ratios. Surprisingly, partitioning of the semi-volatile organic nitrate products into the particles is lower than expected for equilibrium gas/liquid partitioning. For comparison, experiments were carried out in presence of liquid polyethylene (PEG) seed-aerosols with N$_2O$_5 as the source of NO$_3 radicals. Under these conditions, the organic nitrate products reached equilibrium between the gas phase and liquid particles, as expected. These results suggest that the SOA generated from the NO$_2 + O$_3 + alpha-pinene system might not be liquid, consistent with recent published work from other laboratories. Model studies were also carried out using the UCI-CIT airshed model for the South Coast Air Basin of California to explore the impact of semi-volatile organics not attaining equilibrium between the gas phase and a liquid particle phase. The results of the experimental and modeling studies will be presented, along with the implications for the current discrepancies between model predictions and field measurements of SOA. In addition, the impacts on other important pollutants for air quality and climate change will be discussed.