During the nighttime, NO_x is converted into the nitrate radical, NO₃, and dinitrogen pentoxide, N₂O₅. The heterogeneous reaction of N2O5 with chloride containing mixtures produces nitryl chloride, ClNO₂.  The nitryl chloride production channel is of interest since during the night ClNO2 remains largely unreactive but is photolyzed during the daytime, liberating NO₂ and atomic Chlorine.  Thus ClNO₂ is a nighttime NO_x reservoir, the formation of which reduces the efficiency of NOx removal by nighttime reactions of NO₃ and N₂O₅.  Additionally, atomic Cl is an extremely strong oxidant, 10-100 times stronger than OH, the major tropospheric oxidant.  These reactions are largely thought of as marine phenomena, but through the use of long term chemical measurement databases and aerosol thermodynamic models it is shown that chemistry is likely widespread as is suggested by recent field measurements of ClNO₂ during the ACCRONIM field campaign in Boulder, CO.

During February 2009 in Boulder, Colorado, ClNO₂ was routinely measured at concentrations higher  than 100pptv whenever the urban plume was sampled. The measurement site was located about 150 m above Boulder proper thus isolating it from direct influences of local emission.  These observations are consistent with predictions showing significant ClNO₂ production potential in continental regions. 

Using a variety of long term databases (NO_x emissions, aerosol and precipitation composition) along with an aerosol thermodynamic equilibrium model and global chemical transport model, the nitryl chloride production over the United States can be predicted.    

Full time series of the ACCRONIM measurements of ClNO2 (blue) and N2O5 (red).

Annual fraction of NOx that reacts as N2O5.

Annual NOx emissions over the U.S. in kg/year (logarithmic scale).

Annual yield of ClNO2 from heterogeneous N2O5 reactions.

Predicted annual ClNO2 production in Tg Cl/year (logarithmic scale).

According to these estimates the largest production takes place in wintertime in high NO_x environments near coastal areas.  However, significant production is also expected well inland, in areas far removed from direct marine influences.  The ACCRONIM measurements support these estimates of significant continental ClNO₂ production.

View of Boulder, CO, from ACCRONIM measurement site.