The effect of aerosol thermodynamics on ClNO2 production: vertically resolved measurements of ClNO2 and its precursors

ClNO₂ is species whose importance in the nocturnal troposphere has recently been recognized. It is produced during the night by hydrolysis of N₂O₅ on chloride containing aerosol. The thermodynamics of the aerosol change the partitioning of chloride between the gas and aerosol phase and allow transfer of chloride from supermicron aerosol to submicron aerosol through gas phase HCl. The night time atmosphere is stable and layered, and the differences in temperatures, relative humidity, and aerosol composition in these layers lead to variability in the production of ClNO₂. In February of 2011 ClNO₂ and its precursors were measured from a movable instrument carriage on a 300 m tower north of Denver, CO (Erie, CO) as part of the NACHTT field study. The work presents an analysis of the layers of the nocturnal atmosphere and variability of ClNO₂ production in these layers. An aerosol thermodynamic model is used to investigate the effect of relative humidity and temperature on aerosol composition and ClNO₂ production. A vertically resolved budget of chloride is also presented.