Modeled Effects of Observed Nitryl Chloride Concentrations in the Houston Area

The recent TexAQS II field study involved intensive air pollutant measurements over Texas during the summer and fall of 2006. During TexAQS II, NOAA researchers measured concentrations of nitryl chloride of up to 1 ppb in the Houston urban area. Nitryl chloride is potentially important to atmospheric chemistry in urban environments because its photolysis products include both NO2 and chlorine radicals. Chlorine radicals have previously been shown to significantly increase ozone formation in urban Houston. If the values of nitryl chloride measured in Galveston Bay are widespread, this compound has the potential to significantly affect the local reactive chlorine budget and ultimately ozone mixing ratios. Photochemical modeling was performed using the comprehensive air quality model with extensions (CAMx) to investigate the possible effects of measured nitryl chloride concentrations on local chemistry. CAMx was modified to include nitryl chloride and its photolysis reaction (the dominant loss mechanism of this compound). The rate constant of nitryl chloride photolysis was calculated within the model using a scaling factor to the photolysis rate constant of formaldehyde. This scaling was done to simplify the execution of the model. It was found that under standard conditions (ozone column of 300 Dobson units, albedo of 0.06, and a measurement height of 640 m) this scaling factor varied very little with shifting zenith angles. Calculations at zenith angles of 0, 10, 20, 30, 40, 50, 60, 70, 78, and 86 degrees, showed that scaling factor values had a standard deviation of less than five percent of the mean value. The photochemical modeling runs were performed for a series of days during the TexAQS II study. After an initial 3 day ramp-up period, model runs were stopped at 7am every morning (the approximate time of sunrise). At that time, measured concentrations of nitryl chloride were inserted into the modeling domain in the three by three grid cell region around the measurement point (a 12 square km region). The parcel of air which contained the initial nitryl chloride concentrations was followed throughout the day. Chlorine radical and ozone concentrations in this model run were compared to those predicted in a basecase model run which did not include any nitryl chloride. The authors would like to acknowledge the NOAA research team led by James Roberts and thank them for allowing access to their nitryl chloride measurement data.