ACS PRF | ACS
All e-Annual Reports
43935-AC6
Secondary Organic Aerosol Formation from Polycyclic Aromatic Hydrocarbons
Particulate matter (PM) emitted to and formed in the troposphere is important regionally and globally because of its effects on visibility, climate, and human health. Species contributing to this aerosol mass include organics, ammonium, sulfate, nitrate, sodium, chloride, crustal materials, elemental carbon, and others, with organic material being ubiquitous in atmospheric PM. Primary organic aerosol is emitted directly to the atmosphere as condensed-phase material from processes such as combustion. Gas-phase oxidation of volatile organic compounds (VOCs) yields products that often contain hydroxyl, carboxyl, carbonyl, nitro, and nitrate functional moieties and that are typically more soluble and less volatile than the parent VOC. If large enough, this increase in solubility and/or decrease in volatility results in an affinity for the condensed phase, forming secondary organic aerosol (SOA). An important class of organic compounds that may lead to SOA formation is the polycyclic aromatic hydrocarbons (PAHs) that consist of two or more fused benzene rings and that are formed from the incomplete combustion of organic matter such as coal, oil, wood, and gasoline fuel. However, experimental work has not characterized the SOA yields from oxidation of gas-phase PAH compounds. Such a characterization of SOA yields from 2- and 3-ring PAH compounds (defined by the number of constituent benzene rings) is the focal point of this project. The specific research objectives for this project are to: a.) Perform chamber experiments to characterize the SOA yields from hydroxyl radical (OH)-initiated oxidation of the 2-ring PAHs naphthalene, 1- and 2-methyl-naphthalene, fluorene, acenaphthylene, and acenaphthalene; b.) Perform chemical characterization of the 2-ring PAH systems using several mass spectrometric techniques; and c.) Repeat parts a.) and b.) for the 3-ring PAHs anthracene and phenanthrene.
During the first year of this project, an experimental system has been constructed/updated at the University of New Hampshire (UNH) for these experiments. A new 9-cubic-meter chamber made of 50-micron thick Teflon film has been purchased for this project and is mounted on a metal framework about 0.30m above the ground. The metal framework is surrounded by a black cloth curtain for the purpose of light screening and temperature control. A gas chromatograph (GC) with a flame ionization detector (FID) and a DB-5 column is used to measure gas-phase PAH mixing ratios in the chamber during experiments. A Scanning Mobility Particle Sizer (SMPS) is likewise used to measure aerosol particle number size distributions. The SMPS was calibrated prior to the set of experiments using atomized solutions of National Institute of Standards-traceable polystyrene latex spheres of known diameter. A zero air generator is used for filling the chamber, and the air from this generator is further stripped to remove hydrocarbons, oxides of nitrogen, and water. An illumination system used to initiate photochemistry. This illumination system emits light in the germicidal range (265 nm), which is appropriate to photolyze hydrogen peroxide, which is used as the source of OH in this system. The capability to measure the mixing ratio of both oxides of nitrogen and ozone in the chamber also exists.
The protocol for these experiments includes the following steps: a.) Inject the PAH, oxides of nitrogen, hydrogen peroxide, and a standard to the dark chamber; b.) Measure initial conditions using the SMPS, GC/FID, and nitrogen oxide monitor; c.) Initiate the reaction through lighting the illumination system; d.) Monitor the PAH, oxides of nitrogen, and ozone with the appropriate technique until reactions are complete; and e.) Monitor the aerosol particle size distribution and number concentration using the SMPS until observed SOA formation reaches a plateau.
To date, experimental results are available for naphthalene, 1-methyl-naphthalene, and 2-methyl naphthalene. For these compounds, the SOA yields (defined as the ratio of the mass concentration of SOA formed to the mass concentration of parent PAH reacted) at the end of experiments range from 2 to 17%, with apparently little influence of the presence or location of the substituent methyl group in the case of the methyl-naphthalene isomers. In summary, this work has begun to elucidate the atmospheric chemical degradation pathways of PAH compounds with a specific focus on 2- and 3-ring compounds being oxidized by OH in the presence of oxides of nitrogen. Particular attention has been paid so far to calculating SOA yields under a range of atmospheric conditions for 2-ring PAH compounds.
This project has had a significant impact on the involved scientists, particularly the primary investigator and the graduate student performing the experiments. The doctoral student performing the experiments is Nepalese. His presence at UNH brings additional diversity and points of view to the research group of the primary investigator. The training that the student receives while studying at UNH while supported by this project will allow him to work to improve air quality in his native land when he returns at the end of his degree program.
