Quantification of Nitryl Chloride at Part Per Trillion Mixing Ratios by Thermal Dissociation Cavity Ring-Down Spectroscopy

Nitryl chloride (ClNO2) is an important nocturnal nitrogen oxide reservoir species in the troposphere. Here, we report a novel method, thermal dissociation cavity ring-down spectroscopy (TD-CRDS), to quantify ClNO2 mixing ratios with tens of parts-per-trillion by volume (pptv) sensitivity. The mixing ratios of ClNO2 are determined by blue diode laser CRDS of NO2, produced from quantitative thermal dissociation of ClNO2 in an inlet heated to 450 degrees C, relative to NO2 observed in an unheated reference channel. ClNO2 was generated by passing Cl-2 gas over a slurry containing a 1:10 mixture of NaNO2 and NaCl. The TD-CRDS response was evaluated using parallel measurements of ClNO2 by chemical ionization mass spectrometry (CIMS) using I as the reagent ion and NOy (= NO + NO2 + HNO3 + Sigma RO2NO2 + Sigma RONO2 + HONO + 2N(2)O(5) + ClNO2 + ...) chemiluminescence (CL). The linear dynamic range extends from the detection limit of 20 pptv (1 sigma, 1 min) to 30 parts-per-billion by volume (ppbv), the highest mixing ratio tested. The ClNO2 TD profile overlaps with those of alkyl nitrates, which has implications for nocturnal measurements of total alkyl nitrate (Sigma AN = Sigma RONO2) abundances by thermal dissociation (with detection as NO2) in ambient air.