Halogen atoms have the potential to produce and destroy ozone in polluted air, but there are few field studies of reactive halogens and their precursors in coastal urban environments. Here we present API/MS/MS measurements of dihalogen mixing ratios in Malibu, CA during October, 2006, as part of a field campaign to determine the impact of halogens on urban ozone photochemistry and air quality. During this study, Cl2 mixing ratios ranged from <1.2-11 ppt with a geometric mean of 1.1±1.5. Cl2 was observed both day and night with no dependence on wind direction. Br2 levels ranged from <0.3-11 ppt with a geometric mean of 0.3±0.6. Br2 was frequently observed at night and the highest levels were detected in off-shore winds, suggesting a possible continental or anthropogenic source. BrCl was observed above detection limit for the first time in marine air, at levels ranging from 0.5-2.2 ppt, with a geometric mean of 0.2±0.8. I2 ranged from <0.3-1.4 ppt with a mean of 0.2±0.4. The highest I2 mixing ratios were observed in westerly winds that recently passed over local kelp beds. I2 exhibited a consistent diurnal cycle with typical daytime levels 0.2-0.4 ppt higher than nighttime levels. A photochemical box model was used to infer the impact of dihalogen chemistry on ozone under the moderately polluted conditions encountered during this study. The results suggest that chlorine oxidation of hydrocarbons may be responsible for up to 5% of the observed O3 levels. At the observed Br2 levels, destruction of ozone by bromine radicals is minor, lowering O3 levels by only 1-2%. Iodine radicals generated from photolysis of I2 contribute to O3 destruction, partially off-setting the O3 produced from chlorine radicals. The efficiency of ozone destruction by Br and I atoms in polluted air is reduced in polluted air compared to clean air, due to halogen-NOx interactions that short-circuit catalytic O3 destruction cycles.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0345 Pollution: urban and regional (0305;
- 0365 Troposphere: composition and chemistry;
- 0394 Instruments and techniques