Modeling the impact of ClNO2 on ozone formation in the Houston area

During the summer of 2006, nitryl chloride (ClNO2) mixing ratios of over 1 ppb were measured 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 atoms. Chlorine atoms have previously been shown to significantly increase ozone formation in urban Houston. Photochemical modeling was performed using the Comprehensive Air quality Model with extensions (CAMx) to estimate the effects of observed nitryl chloride concentrations on local chemistry in southeast Texas. CAMx was modified to include a formation mechanism for nitryl chloride as well as its photolysis reaction. Comparisons between model predictions and ambient measurements showed that the model-predicted ClNO2 concentrations were within the range of observed data. Model simulations predict that ClNO2 increases total reactive chlorine mass by 20-40% in the atmosphere of southeast Texas. Despite the high reactivity of chlorine, nitryl chloride caused only modest increases in ozone concentrations ( up to 1.0-1.5 ppb when baseline 1-h average ozone concentrations were between 60 and 85 ppb). The chemistry and physical processes which affect ozone formation were further investigated using box model simulations and a Lagrangian process analysis tool (LPA) within the gridded photochemical modeling simulations. These analyses showed that vertical dispersion and local atmospheric composition moderated the effect of nitryl chloride on ozone mixing ratios.