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Project Status: complete

Title: Effect of Vapor Wall Loss in Laboratory Chambers on Yields of Secondary Organic Aerosols

Principal Investigator / Author(s): Seinfeld, John

Contractor: California Institute of Technology

Contract Number: 13-321


Research Program Area: Atmospheric Processes

Topic Areas: Modeling, Toxic Air Contaminants


Abstract:

Secondary organic aerosol (SOA), formed from the atmospheric oxidation of gas-phase hydrocarbons, comprises a large fraction of ambient particulate matter, and has important impacts on climate and air quality. However, air-quality and climate models have not been able to accurately simulate ambient SOA concentrations. Quantitative simulation of SOA depends on the translation of SOA formation observed in laboratory chambers into robust parameterization. Comprehensive comparisons of predicted versus observed organic aerosol concentrations found that predicted concentrations were considerably below the observed concentrations; sometimes referred to as "missing carbon". Losses of particles to the walls of chambers are routinely accounted for, but there has been little evaluation of the effects on SOA formation of losses of semi-volatile vapors to chamber walls. Teflon-lined chambers are used to study atmospheric chemistry; however, SOA formation is typically underestimated, due to deposition of SOA-forming vapors on chamber walls. This study provided an experimental protocol and a model framework to constrain these vapor-wall interactions. Wall deposition rates were measured for 25 compounds generated from the photo-oxidation of isoprene, toluene, α-pinene, and dodecane in three chambers; one that had been extensively used, and in two new chambers. Among the 25 compounds studied, the maximum wall deposition rate is exhibited by the most highly oxygenated and least volatile compounds. By optimizing the model output to the observed vapor decay profiles, the result indicates that the dominant parameter governing the extent of wall deposition of a compound is its wall accommodation coefficient, which can be correlated through its volatility with the number of carbons and oxygen in the molecule. By doing so, the wall-induced deposition rate of intermediate/semi-volatile organic vapors can be reasonably predicted based on their molecular constituency.


 

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