Atmos. Chem. Phys. Discuss., 11, 9217-9248, 2011
www.atmos-chem-phys-discuss.net/11/9217/2011/
doi:10.5194/acpd-11-9217-2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Effect of humidity on the composition and yield of isoprene photooxidation secondary organic aerosol
T. B. Nguyen1, P. J. Roach2,*, J. Laskin2, A. Laskin3, and S. A. Nizkorodov1
1Department of Chemistry, University of California, Irvine, California 92697, USA
2Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 9935, USA
3Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA
*now at: Roach & Associates LLC Seymour, WI, USA

Abstract. The effect of relative humidity (RH) on the composition and concentrations of gas-phase products and secondary organic aerosol (SOA) generated from the photooxidation of isoprene under high-NOx conditions was investigated. The yields of most gas-phase products were the same regardless of initial water vapor concentration with exception of hydroxyacetone and glycolaldehyde, which were considerably affected by RH. A significant change was observed in the SOA composition, with many unique condensed-phase products formed under humid (90% RH) vs. dry (<2% RH) conditions, without any observable effect on the rate and extent of the SOA mass growth. There is a 40% reduction in the number and relative abundance of distinct particle-phase organic nitrogen (ON) compounds detected by high resolution mass spectrometry. The suppression of condensation reactions, which produce water as a product, is the most important chemical effect of the increased RH. For example, the total signal from oligomeric esters of 2-methylglyceric acid was reduced by about 60% under humid conditions and the maximum oligomer chain lengths were reduced by 7–11 carbons. Oligomers formed by addition mechanisms, without direct involvement of water, also decreased at elevated RH but to a much smaller extent. The observed substantial reduction in the extent of condensation-type oligomerization at high RH may have substantial impact on the phase characteristics and hygroscopicity of the isoprene aerosol. The reduction in the amount of organic nitrates in the particle phase has implications for understanding the budget of ON compounds.

Citation: Nguyen, T. B., Roach, P. J., Laskin, J., Laskin, A., and Nizkorodov, S. A.: Effect of humidity on the composition and yield of isoprene photooxidation secondary organic aerosol, Atmos. Chem. Phys. Discuss., 11, 9217-9248, doi:10.5194/acpd-11-9217-2011, 2011.
 
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