Study of SOA growth mechanism: kinetic uptake of organic nitrates from the oxidation of monoterpenes
Authors
Veronique Michele Lucie Perraud — University of California
Michael Ezell — University of California
Lisa M Wingen — University of California, Irvine
Matthew L Dawson — University of California, Irvine
Barbara J Finlayson-Pitts — University of California
Category
Aerosol Properties
Description
A typical LC-UV chromatogram of particles collected on a quartz-fiber filter from the simultaneous oxidation of alpha-pinene by ozone and nitrate radicals.
Secondary organic aerosol (SOA), generated from the oxidation of volatile organic compounds (VOCs), contributes to a large proportion of the total organic aerosol mass encountered in the atmosphere. Understanding and quantifying the processes that lead to SOA formation and growth are critical, as SOA has adverse impacts on visibility, human health, and climate. The mechanism for SOA growth is currently described in most atmospheric models by an instantaneous thermodynamic equilibrium partitioning of semi-volatile organic compounds (SVOCs) into liquid pre-existing organic particles. However, there have been substantial discrepancies between model-predicted and field-measured SOA concentrations. There is increasing evidence that a significant part of the discrepancy may lie in model assumptions, especially that equilibrium between the gas phase and particles is achieved on very short time scales. There are a number of recent observations in both laboratory systems and field campaigns that indicate that some SOA may be much more viscous and less “liquid-like” than initially assumed. Thus, the diffusion coefficients would be smaller than assumed, resulting in longer times to reach equilibrium. A recent study from our group showed that in the case of simultaneous oxidation of α-pinene by ozone and nitrate radicals, the uptake of semi-volatile organic nitrate products into the SOA that is generated does not follow thermodynamic equilibrium partitioning. However, the data are consistent with a kinetically limited/condensation growth mechanism. We report here a series of new experiments performed using a large-volume slow flow aerosol flow tube system to measure the partitioning of organic nitrates into SOA formed under the same experimental conditions from a series of atmospherically relevant monoterpene precursors. Implications for model treatment of SOA growth will be discussed.
