Observationally Constrained Estimates of Nitryl Chloride Production on Regional and Global Scales

Nitryl chloride (ClNO2) is a Cl atom source produced during the night by heterogeneous reactions of dinitrogen pentoxide (N2O5) on chloride-containing aerosol particles. We observed ClNO2 in Boulder, CO, about 1400 km from any source of sea salt on nearly every night when we intercepted the urban NOx-rich plume. We use these observations as motivation to investigate the potential importance of ClNO2 formation over the contiguous United States and extend the relationships to the global scale. For this purpose, we use available ambient aerosol composition data from the IMPROVE network and from large-scale field intensives, precipitation composition from the NADP, nitrogen oxide emission databases, and global model outputs of the fraction of nitrogen oxide radicals that react on aerosol particles via N2O5. Uncertainty concerning the reactivity of N2O5 and the distribution of chloride mass across the particle size distribution ultimately limits the accuracy of these extrapolations, further supporting the need for better constraints on N2O5 reactivity and for single particle composition measurements. Nonetheless, a number of different approaches suggest that between 1 - 5 Tg Cl of ClNO2 are produced each year across the U.S. alone. This source is similar to that estimated for global coastal and marine boundary layer regions. Based on the results for the United States, we extrapolate to the global scale under the assumption that NOx and particulate chloride sources are distributed similarly to that in the U.S. We predict that continental scale ClNO2 production leads to a global Cl atom source of 8 - 22 Tg Cl per year, which is of the same order as that inferred from methane isotopes over the remote MBL. These estimates indicate that ClNO2 plays a substantial role in the global chlorine atom budget, much larger than was previously thought, with implications for the role of human activities on the tropospheric halogen budget.