Chamber Studies and Modeling of Secondary Organic Aerosol Formation

Citation

Chan, Arthur Wing Hong (2010) Chamber Studies and Modeling of Secondary Organic Aerosol Formation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/61XH-D105. https://resolver.caltech.edu/CaltechTHESIS:05182010-161102525

Abstract

Secondary organic aerosol (SOA), formed from atmospheric oxidation of gas-phase hydrocarbons, comprise a large fraction of ambient particulate matter. Significant uncertainties exist in identifying the sources and mechanisms responsible for SOA formation, making it difficult to understand its impact on global climate and local air quality. Laboratory chambers have been a valuable tool to study underlying chemical mechanisms of SOA formation and to quantify SOA formation from select hydrocarbons in a controlled environment. However, a good understanding of the chemical processes involved is required to be able to extrapolate data acquired from smog chamber studies. This thesis presents results from experimental investigation of SOA formation from atmospherically important compounds, and model simulations of kinetic mechanisms involved in SOA formation.

The distinguishing mechanism of SOA formation is the partitioning of semivolatile hydrocarbon oxidation products between the gas and aerosol phases. While SOA formation is typically described in terms of partitioning only, the rate of formation and ultimate yield of SOA can also depend on the kinetics of both gas- and aerosol-phase processes. Here a general equilibrium/kinetic model of SOA formation is presented to provide a framework for evaluating the extent to which the controlling mechanisms of SOA formation can be inferred from laboratory chamber data. Current atmospheric models systematically underpredict SOA formation, suggesting that in current models, 1) signicant SOA precursors could be missing and 2) SOA forming processes could be misrepresented. Aerosol formation from oxidation of 2-methyl-3-buten-2-ol (MBO) and polycyclic aromatic hydrocarbons (PAHs), two important classes of compounds previously assumed to be an insignicant SOA source, is studied. Upon photooxidation, MBO produces glyoxal (an important SOA intermediate), but the yields are too low to be atmospherically important. Photooxidation of napthalene and other 2-ring PAHs leads to substantial amounts of aerosol, and can account for a large fraction of SOA formed from oxidation of diesel exhaust and other primary emissions.

Isoprene is a signicant source of atmospheric organic aerosol; however, the oxidation pathways that lead to SOA have remained elusive. Under remote low-NO_x conditions, epoxydiols are formed from gas-phase photooxidation of isoprene, and are found to undergo reactive uptake to lead to low-volatility compounds, such as C₅-methyltetrols and organosulfates observed in ambient particulate matter. Under urban high-NO_x conditions, methacrolein, an important C₄ aldehyde formed from isoprene oxidation, is found to form SOA via reaction with NO₂ to form peroxy methacryloyl nitrate, which subsequently forms low-volatility oligoester products. As a result of radical chemistry of aldehydes, SOA formation from isoprene depends critically on the NO₂/NO ratio, and the implications on ambient aerosol formation are discussed.

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