Chlorine activation within urban or power plant plumes: Vertically resolved ClNO₂ and Cl₂ measurements from a tall tower in a polluted continental setting

Abstract

Abstract Nitryl chloride (ClNO 2 ) is a chlorine atom source and reactive nitrogen reservoir formed during the night by heterogeneous reactions of dinitrogen pentoxide on chloride‐containing aerosol particles. The main factors that influence ClNO 2 production include nitrogen oxides, ozone, aerosol surface area, soluble chloride, and ambient relative humidity. Regions with strong anthropogenic activity therefore have large ClNO 2 formation potential even inland of coastal regions due to transport or local emissions of soluble chloride. As part of the Nitrogen, Aerosol Composition, and Halogens on a Tall Tower field study, we report wintertime vertically resolved ClNO 2 and molecular chlorine (Cl 2 ) measurements taken on a 300 m tall tower located at NOAA's Boulder Atmospheric Observatory in Weld County, CO, during February and March of 2011. Gas and particle phase measurements aboard the tower carriage allowed for a detailed description of the chemical state of the nocturnal atmosphere as a function of height. These observations show significant vertical structure in ClNO 2 and Cl 2 mixing ratios that undergo dynamic changes over the course of a night. Using these measurements, we focus on two distinct combustion plume events where ClNO 2 mixing ratios reached 600 and 1300 parts per trillion by volume, respectively, aloft of the nocturnal surface layer. We infer ClNO 2 yields from N 2 O 5 ‐aerosol reactions using both observational constraints and box modeling. The derived yields in these plumes suggest efficient ClNO 2 production compared to the campaign average, where in‐plume yields range from 0.3 to 1; the campaign average yield in the boundary layer is 0.05 ± 0.15, with substantial night‐to‐night and within night variability similar to previous measurements in this region.