43935-AC6
Secondary Organic Aerosol Formation from Polycyclic Aromatic Hydrocarbons
Particulate matter emitted to and formed in the troposphere is important because of its numerous consequences. Species contributing to aerosol mass include organics, ammonium, sulfate, nitrate, sodium, chloride, crustal materials, and elemental carbon, with organic material being ubiquitous. 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 functional moieties that cause an increase in solubility and a decrease in volatility. These property changes can result in affinity for the condensed phase, forming secondary organic aerosol (SOA). An important class of organic compounds that leads to SOA is the polycyclic aromatic hydrocarbons (PAHs) that are formed from the incomplete combustion of organic matter. Experimental work to characterize the SOA yields from oxidation of gas-phase PAHs is lacking. Such a characterization of SOA yields from 2- and 3-ring PAH compounds (defined by the number of constituent benzene rings) is the focus of this project. The specific 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 an aerosol mass spectrometer; and c.) Repeat a.) and b.) for the 3-ring PAHs anthracene and phenanthrene.
An experimental system was constructed for these experiments. A 9-cubic-meter chamber made of 50-micron thick Teflon film was purchased 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 has been used to measure gas-phase PAH mixing ratios in the chamber during experiments with all compounds except fluorene, anthracene, and phenanthrene. Method development is underway for these compounds. 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 (254 nm), which is appropriate to photolyze hydrogen peroxide, which is used as the source of OH in this system for some experiments. Additional experiments have been performed with 365-nm wavelength lamps with nitrous acid (HONO) as a source of OH and nitric oxide (NO) to investigate the effects of lamp wavelength on aerosol formation. The capability to measure the mixing ratio of both NO and ozone in the chamber also exists.
The protocol for these experiments is to: a.) Inject the PAH, NO and hydrogen peroxide (or HONO), and a standard to the dark chamber; b.) Measure initial conditions using the SMPS, GC/FID, and NO monitor; c.) Initiate the reaction with light; d.) Monitor the PAH, NO, 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, experiments are complete for naphthalene, 1-methyl-naphthalene, 2-methyl naphthalene, acenaphthene, and acenaphthylene. For these compounds, the SOA yields (defined as the ratio of the mass concentration of SOA formed to the mass concentration of PAH reacted) at the end of experiments range from 2 to 37%, with apparently little influence of the presence or location of the substituent methyl group in the case of the methyl-naphthalene isomers (for the same wavelength). Highest yields are observed for acenaphthene and acenaphthylene, likely because of increased OH-oxidation associated with the third (non-benzene) ring in the parent structure. For 2-methyl-naphthalene, shorter wavelength light leads to greater SOA formation, but this could also from different nitrogen oxide levels. This work has begun to elucidate the atmospheric chemical degradation pathways of PAH compounds. Particular attention has been paid to estimating SOA yields under a range of atmospheric conditions for 2-ring PAH compounds. In the upcoming months, remaining yield and additional aerosol characterization experiments will be performed. The characterization experiments will involve two additional students.
This project has significantly impacted the involved scientists, particularly 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.
