Environ. Sci. Technol. 2008, 42, 5461–5466
Light Intensity and Light Source Influence on Secondary Organic Aerosol Formation for the m-Xylene/NOx Photooxidation System BETHANY. WARREN, CHEN. SONG,† AND DAVID R. COCKER III* Department of Chemical and Environmental Engineering, Bourns College of Engineering, Center for Environmental Research and Technology (CE-CERT), University of California, Riverside, California 92521
Received November 29, 2007. Revised manuscript received April 23, 2008. Accepted May 7, 2008.
A series of m-xylene/NOx photooxidation experiments were conducted to determine the influence of light intensity and radiation spectrum on secondary organic aerosol (SOA) formation within the UC Riverside/CE-CERT environmental chamber. The environmental chamber is equipped with 80 115-W black lights and a variable voltage 200 kW argon arc lamp that emits a wavelength spectrum more similar to natural light. SOA formation increased significantly with light intensity, measured as the photolysis rate of NO2 to NO (k1), increased from 0.09 to 0.26 min-1. The argon arc lamp produced ∼20% more SOA than black lights at a k1 of 0.09 min-1 for similar amounts of m-xylene consumed. These results may help explain the variation of SOA formation between environmental chambers and the differences between measured SOA in the ambient atmosphere versus environmental chamber predictions.
Introduction Secondary organic aerosol (SOA) is formed from the oxidation of a parent hydrocarbon into products with lower vapor pressures which partition into the particle phase. A significant portion of the organic fraction of fine particulate matter, PM2.5, in the urban atmosphere is attributed to SOA (1). PM2.5 has been linked to impaired human health, reduction in visibility, and can impact the global radiation budget (2–4). Environmental chambers are often used to simulate atmospheric processes leading to SOA formation. Daytime simulations require a light source, which is achieved by placing the reactor outdoors or by an artificial light source, commonly black lights. Currently there is a large disconnect between the SOA measured in the atmosphere versus the amount estimated on the basis of environmental chamber experiments (5–7). Aromatic hydrocarbons have been extensively investigated as precursors for SOA formation. The photochemical production of SOA from m-xylene has been studied under a variety of environmental conditions including differing HC/ NOx ratios, no NOx conditions (H2O2), and the addition of various light hydrocarbons to increase peroxy (HO2) and * Corresponding author phone: +1-951-781-5695; fax: +1-951781-5790; e-mail:
[email protected]. † Currently at Pacific Northwest National Laboratories. 10.1021/es702985n CCC: $40.75
Published on Web 07/03/2008
2008 American Chemical Society
hydroxyl (OH) concentrations (8–12). While it is accepted that the main oxidant to react with m-xylene is the OH radical, there exist several hypotheses for mechanisms leading to SOA formation within this system. One theory is the reaction pathway through the alkylperoxy radical (RO2) reacting with an HO2 radical to form a less volatile hydroperoxy (ROOH) which can partition into the aerosol phase (9, 13). RO2 plus RO2 reactions can also occur to form large peroxides that can partition into the aerosol phase (14). Accretion reactions have also been proposed between aldehyde and alcohols in an acidic particle that may result in larger SOA mass produced (15). Natural light intensity, measured as the photolysis rate of NO2 to NO (k1), depends on geographic location, time of year, and time of day. On a cloudless, sunny, August day in Riverside, CA (34°00′00N, 117°20′08”W), the k1 is approximately 0.45 min-1 at 1:00 p.m. PST (16). Light intensity has been shown to affect gas phase chemistry by altering the relative concentration of products, such as ozone and PAN, and directly impacts the amount of oxidants present within a photooxidation system (17). These results indicate that light intensity may play an important role in photooxidation chemistry resulting in SOA formation; however, there is no data currently available to evaluate the potential effect light intensity may have on SOA formation. Furthermore, many environmental chambers use black lights as an irradiation source to provide the necessary wavelengths for NO2 photolysis (
