Vertically Resolved Nitryl Chloride Observations and Yield Estimates

Nitryl chloride (ClNO2) is a Cl atom source produced during the night by heterogeneous reactions of dinitrogen pentoxide (N2O5) on chloride-containing aerosol particles. The main drivers of ClNO2 production include ambient relative humidity, aerosol surface area, aerosol and gas phase chloride, NOx (NO2 + NO) and O3 levels. ClNO2 formation has implications on tropospheric halogen budgets as a potential anthropogenic source of reactive halogen atoms. As part of the Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT 2011) field study, we report wintertime vertically resolved ClNO2 measurements taken on a 300 meter tall tower located at NOAA's Boulder Atmospheric Observatory (BAO) in Erie, CO, during February and March of 2011. Gas phase and particle phase measurements aboard the tower carriage allow for a detailed description of the chemical state of the nocturnal atmosphere at the various heights. These observations show the formation of distinct chemical layers that undergo dynamic changes over the course of a night. Using these measurements we are able to infer ClNO2 yields from N2O5 aerosol surface reactions that range from less than 5% to 75% with a campaign mean of 0.15% +/- 0.1. We compare these with yields predicted by an observationally constrained chemical box model as well as with previous estimates of ClNO2 yields in the Boulder, CO, area.