Observations of Cl2, Br2, and I2 in coastal marine air

Cl-2, Br-2, and I-2 were measured in coastal Pacific air from 2 to 29 January 2006. Air was sampled at 10 m over the sea surface near the end of Scripps Pier (La Jolla, California). The measurements were made using atmospheric pressure chemical ionization with tandem mass spectrometry (APCI/MS/MS). Over the course of this study, Cl-2, Br-2, and I-2 levels ranged from below detection limits of 1.0, 0.5, and 0.2 ppt, respectively, to maxima of 26, 19, and 8 ppt, respectively. Mean dihalogen levels for the study period were 2.3 +/- 1 ppt for Cl-2, 2.3 +/- 0.4 ppt for Br-2, and 0.7 +/- 0.1 ppt for I-2 (expressed as geometric mean +/- 1 geometric standard error). The mixed dihalogens BrCl, ICl, and IBr had geometric mean levels below 0.3 +/- 1 ppt and never exceeded their detection limits of 0.5 ppt. Consistent patterns of diurnal variability were observed for Cl-2 and I-2, with Cl-2 maxima during daytime and I-2 appearing almost exclusively at night. The detection of I-2 appeared to be related to the passage of air over nearby kelp beds. The observed dihalogen levels suggest that (1) chlorine atom oxidation of hydrocarbons makes a significant contribution to the formation of ozone and (2) halogen atom oxidation of airborne mercury contributes to mercury deposition in polluted coastal air.