Cl₂, Br₂, and I₂ 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, Cl₂, Br₂, and I₂ 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 Cl₂, 2.3 ± 0.4 ppt for Br₂, and 0.7 ± 0.1 ppt for I₂ (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 Cl₂ and I₂, with Cl₂ maxima during daytime and I₂ appearing almost exclusively at night. The detection of I₂ 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.

Measurements of Cl₂ in coastal urban air were made in Irvine, California from Aug.-Nov., 2005, using negative ion atmospheric pressure chemical ionization with tandem mass spectrometry. Cl₂ ranged from <2.5 to 20 ppt with a two month mean of 3.5 ± 3.5 ppt (1σ) and an average noon photolysis rate of 44 ppt hr⁻¹ Cl₂. Model calculations suggest that Cl₂ photochemistry results in an increase of 5–8 ppb in daily maximum O₃ levels, relative to a base case scenario without Cl₂.

This study examines atmospheric reactive chlorine chemistry at the Cape Verde Atmospheric Observatory in the eastern tropical Atlantic. During May–June, 2007, Cl₂ levels ranged from below detection (∼2 ppt) to 30 ppt. Elevated Cl₂ was associated with high HNO₃ (40 to 120 ppt) in polluted continental outflow transported in the marine boundary layer (MBL) to the site. Lower Cl₂ was observed in recently subsided air masses with multiday free tropospheric oceanic trajectories and in air containing Saharan dust. Model simulations show that the observations of elevated Cl₂ in polluted marine air are consistent with initiation of Cl chemistry by OH + HCl and subsequent heterogeneous, autocatalytic Cl cycling involving marine aerosols. Model estimates suggest that Cl atom reactions significantly impact the fates of methane and dimethylsulfide at Cape Verde and are moderately important for ozone cycling.