A Study of Mid-Continent Halogen Activation: Wintertime Measurements of N2O5 and ClNO2

Nitryl Chloride (ClNO2) has recently been shown to be a important source of atomic chlorine and possibly responsible for a large faction of the global atomic Cl budget. Additionally, it short circuits the nocturnal conversion of NOx into nitric acid providing a photolytic NOx reservoir available for morning ozone production. N2O5, a ClNO2 precursor, exists in chemical equilibrium with NO3, which is the product oxidation of NO2 by O3. N2O5 is taken up by aerosol, where it reacts with water to form nitric acid. However if the aerosol contains chloride, production of ClNO2 is favored. After ClNO2 is produced in the aerosol it readily partitions into the gas phase and builds up over the course of the night. It is only lost in the morning when it is photolyzed by uv/vis light into atomic Cl and NOx. Measurements of ClNO2 concentration have become available only recently, and have been focused mainly on coastal areas where chloride-containing aerosol is expected to be found; no measurements have been made at continental sites. In February 2009, we undertook field measurements to study the production of ClNO2, N2O5 and related species in Boulder, CO, ~1400 km from any coastal sources of sea salt chloride. ClNO2 was observed every night during the two week measurement period usually in the range of 50 - 200 pptv, but with a maximum concentration of 600 pptv. In most, though not all, cases the ClNO2 concentration was less than and broadly correlated with the N2O5 concentration. This work presents several detailed analyses of these measurements, in support of recent estimates of regional and continental-scale estimates of ClNO2 production. First, we determine N2O5 uptake coefficients and ClNO2 yields. The yields allow us to identify nights when the production is chloride-limited or limited by uptake of N2O5. Second, because the concentration of ClNO2 regularly exceeds the chloride available in the aerosol, we infer that there is a significant gas phase HCl reservoir which partitions chloride into the aerosol, indicating the need for future measurements of gas phase HCl. Third, we use an aerosol thermodynamic model to calculate the aerosol water content and size. During most of the measurement period, the aerosol was dry and during the few nights with significant aerosol water content the ClNO2 concentration exceeded the N2O5 concentration. Fourth, based on back-trajectory analysis from FLEXPART and the local meteorology we speculate on possible sources of aerosol chloride and gas-phase HCl.