LaPorte Ground Site Measurements
Statement of Work
Environmental Chamber Studies of Chlorine-Promoted Ozone Formation and Secondary Organic Aerosol Formation in Houston, Texas
David Allen, CEER and the Dept. of Chemical Engineering, The University of Texas at Austin, Austin, TX 78758
Phone (512) 475-7842, FAX (512) 471-1720, Email: allen@che.utexas.edu
Charles Mullins, Dept. of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712
Phone: (512) 471-5817, FAX (512) 471-7060, Email: mullins@che.utexas.edu
Paul Tanaka, Dept. of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712
Phone: (512) 471-7988, FAX (512) 475-7824, Email: tanaka@che.utexas.edu
Understanding the chemical and physical processes that control air quality in Houston poses substantial challenges. The region contains a high density of stationary emission sources for volatile organic compounds (VOCs), oxides of nitrogen (NOx), and hazardous air pollutants (HAPs). In addition, the region is home to a large number of mobile sources of air pollution. Although the details of how some of these pollutants combine to form ozone in the urban atmosphere are fairly well understood, some episodes of extreme ozone concentrations that occur in Houston are not consistent with known mechanisms. In addition, the details of how the pollutants emitted in Houston react to form organic aerosol are not known. This study will provide data to help understand several critical issues related to atmospheric chemistry in Houston.
The objectives of this study are to: 1) investigate the ozone-forming potential of molecular chlorine when chlorine is emitted into air masses typical of those found in Houston and 2) investigate how chlorine affects the formation of secondary organic aerosols from organics commonly emitted in Houston.
Background
Chlorine Promoted Ozone Formation
Preliminary experiments performed in environmental chambers located at the University of Texas at Austin have indicated that molecular chlorine (Cl2), a photolytic source of chlorine atoms, can promote the formation of ozone in a model atmosphere containing volatile organic compounds (VOCs) and nitrogen oxides (NOx) typical of those found above Houston. The results of these studies indicated that 5 to 10 additional moles of ozone could be produced per mole of Cl2 injected, compared to control experiments where no chlorine is injected. We hypothesize that molecular chlorine can be a significant contributor to ozone formation in the Houston area given that approximately 200,000 pounds of molecular chlorine are emitted per year by anthropogenic sources in Harris County (1993-1996 TRI reported air releases).
Secondary Organic Aerosol Formation
Exposure to fine particulate matter (dp<2.5 mm) reduces lung function and increases the incidence of respiratory ailments and mortality. While it is not clear whether the particles are responsible or act as a surrogate for responsible agents, mortality rates have been shown to increase by up to 26% between cities with high and low levels of fine particulate exposure. In the Houston area, approximately two-thirds of aerosol mass is composed of inorganic aerosols, for which sources and compositions are relatively well known. The other third, however, consists of both primary and secondary organic aerosols. Primary organic aerosols are organic particles emitted directly into the atmosphere, whereas by definition, secondary organic aerosols (SOA) are formed in the gas-phase photooxidation of hydrocarbons that condense into the aerosol phase.
Although SOA represent less than a third of total aerosol mass, they tend to be concentrated in smaller sized particles (less than 2 mm in diameter). Particles in this size range are capable of reaching the deep lung, where removal by the body is difficult. Because of the potential hazards to health that SOA pose, it would be beneficial to understand the formation mechanisms of these SOA to help develop strategies to reduce their formation. Unfortunately, the nature and formation mechanism of these SOA are not well understood. By adequately characterizing the organic content of the air and monitoring particle formation in a mobile environmental chamber, we will gain a better understanding of the physical and chemical processes important to the formation of SOA in the Houston area.
Approach
Experiments will be performed in three mobile, teflon environmental chambers. Each chamber is an identical, 4-foot cube mounted on casters. The chambers will be run simultaneously to minimize chamber-to-chamber variations in temperature and lighting. Each chamber will be conditioned prior to use by rinsing with distilled water and treatment with ozone. After conditioning, the chambers will be opened to allow ambient air inside. The chambers will then be closed, covered with an opaque tarp, and connected to several monitoring instruments. A surrogate hydrocarbon (propane, propene, or methane) will then be injected into the two non-control chambers. Of these two chambers, one chamber will receive an injection of molecular chlorine just prior to uncovering the chambers. For each run, the chambers will be dedicated as follows: Chamber 1 – Control Chamber (Ambient air only), Chamber 2 – ambient + surrogate hydrocarbon, and Chamber 3 – ambient + surrogate hydrocarbon + injected chlorine.
Ozone concentration, nitrogen oxides (NOx) concentration, temperature, and dew point temperature will be monitored during each experimental run. Ozone concentrations will be measured in each chamber with an ultraviolet photometric ozone analyzer, and nitrogen oxide concentrations will be measured with a nitrogen oxides analyzer operating on the principle of chemiluminescence. Temperature and dew point measurements will be made using a digital hygrometer in Chambers 1 and 3. Primary actinic flux data will be provided by the TexAQS 2000 study.
In addition to the field instrumentation, samples will be collected for subsequent analysis in a remote laboratory. Size-segregated aerosol samples will be collected using an eight stage low-pressure impactor. These samples will be collected on zinc selenide discs for later analysis by Fourier Transform Infrared Spectroscopy. The composition and size distribution of aerosols collected before each run will be compared to that of the aerosols collected after each run. Gas-phase grab samples will be collected in stainless steel gas sampling canisters and analyzed by gas chromatograph to identify the hydrocarbons present in each chamber and to track hydrocarbon concentrations during each experimental run.
The data collected from this study will help in understanding the effectiveness of chlorine in promoting ozone formation and how chlorine affects secondary organic aerosol formation in captive ambient Houston air.