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The magnitude and sources of chlorine atoms in marine air remain highly uncertain but have potentially important consequences for air quality in polluted coastal regions. We made continuous measurements of ambient ClNO₂ and Cl₂ concentrations from May 15 to June 8 aboard the Research Vessel Atlantis during the CalNex 2010 field study. In the Los Angeles region, ClNO₂ was more ubiquitous than Cl₂ during most nights of the study period. ClNO₂ and Cl₂ ranged from detection limits at midday to campaign maximum values at night reaching 2100 and 200 pptv, respectively. The maxima were observed in Santa Monica Bay when sampling the Los Angeles urban plume. Cl₂ at times appeared well correlated with ClNO₂, but at other times, there was little to no correlation implying distinct and varying sources. Well-confined Cl₂ plumes were observed, largely independent of ClNO₂, providing support for …