IMPACT OF REACTIVE HALOGEN GASES ON CA AIR QUALITY

Project No. A-62

Leaders:    J. C. Ball

                  E. McCauley

Scope and Objective

The objective of the proposed work is to measure reactive halogen gases and associated pollutants at a Southern California coastal site and to assess the potential significance of reactive halogen chemistry on air quality.

Current Status and Future Program

Hydroxyl radical (OH) is widely considered the dominant daytime oxidizing species, initiating ozone chemical formation in the presence of volatile organic carbons (VOC) and NO_x.  However, there is increasing evidence that halogen atoms, specifically chlorine and bromine, are also significant oxidants in coastal areas.  Reactive halogen gases (Cl₂, BrCl, Br₂) are produced from chemical reactions on sea salt particles and readily photolyze in the early morning to produce halogen atoms.  Cl-atoms oxidize hydrocarbons 100 times faster than OH, thus initiating ozone production and aerosol formation earlier than possible from OH chemistry alone.  For these reasons, it is suspected that this accelerated chemistry could lead to higher daytime ozone concentrations, affect aerosol particle formation and composition, and potentially increase human exposure in highly populated coastal cities of California, including Los Angeles, San Diego, and San Francisco. Conversely, Br-atoms do not react rapidly with most organics, but do react with ozone and lead to its destruction. Regional photochemical models used to evaluate the effectiveness of emission control strategies in ozone non-attainment areas do not account for halogen reactions and therefore may be flawed when applied to coastal regions.  Measurements of reactive halogen gases in coastal areas are needed to assess their potential importance and to determine if halogen chemistry should be included in air quality models.

Direct measurements of up to 150 pptv Cl₂ on the Long Island, NY, coast and indirect measurements of up to 127 pptv Cl₂ on the Florida coast have been made.  Direct measurements of up to 27 pptv Br₂ and 35 pptv BrCl have been made in the Arctic prior to polar sunrise. Until Project A-62 was started, no halogen measurements had been made on the U.S. West Coast.  Laboratory studies have established that Cl₂ is likely present in on-shore marine air flow due to reactions involving sea-salt particles, which are ubiquitous in California coastal regions.  Modeling studies of Southern California have shown that including chlorine chemistry increases ozone levels by as much as 12 ppb over a base case at Long Beach.  However, chlorine and bromine effects on California photochemical air pollution cannot be truly assessed until measurements of these gases are made and analyzed.

A two-week early fall field measurement campaign occurred during Year 1 (2006) in the Los Angeles area.  The gases Cl₂, BrCl, and Br₂ were measured on-line by atmospheric pressure chemical ionization mass spectrometry to quantify their diurnal variation.  Differential optical absorption spectroscopy was used to measure halogen oxide concentrations, as well as O₃, NO₂, and HCHO.  On-line size resolved aerosol composition measurements were made to characterize the urban/marine aerosol and the evolution of its composition (SO₄, NH₄, NO₃, Cl, Br, Na, K, organics).  In Year 2, these data will be used in an appropriate gas-aerosol model to analyze the impacts of measured halogen gases on urban air ozone and aldehyde concentrations, and on aerosol composition and size distributions. This collaborative effort involves scientists from Battelle Memorial Institute (who performed the only existing direct measurements of Cl₂, BrCl and Br₂ and also have extensive expertise with on-line, size-resolved aerosol composition measurements), UCLA (who first identified ClO in the troposphere near Salt Lake City), and researchers at Washington State University (WSU).

The knowledge to be gained from this project is needed to assess emission controls for reducing ozone concentrations.  The committee approved proposals received from the research team.  CARB is supporting work at UCLA and one-half the work at WSU.  CRC is supporting the research at Battelle and one-half the WSU research in separate agreements, respectively.  Researchers from the University of New Hampshire and the University of California at Irvine are also contributing to the study supported by the National Science Foundation. 

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