Heterogeneous formation of nitryl chloride and its role as a nocturnal NOx reservoir species during CalNex‐LA 2010

The nocturnal conversion of dinitrogen pentoxide (N 2 O 5 ) to nitryl chloride (ClNO 2 ) on chloride‐containing aerosol can be a regionally important NO x (= NO + NO 2 ) recycling and halogen activation pathway that affects oxidant photochemistry the following day. Here we present a comprehensive measurement data set acquired at Pasadena, California, during the CalNex‐LA campaign 2010 that included measurements of odd nitrogen and its major components (NO y = NO x + NO 3 + 2N 2 O 5 + ClNO 2 + HNO 3 + HONO + peroxyacyl, alkyl, and aerosol nitrates) and aerosol size distribution and composition. Nitryl chloride was present during every night of the study (median mixing ratio at sunrise 800 pptv) and was usually a more significant nocturnal NO x and odd oxygen (O x = O 3 + NO 2 + 3N 2 O 5 + ClNO 2 ) reservoir species than N 2 O 5 (whose concentrations were calculated from its equilibrium with NO 2 and NO 3 ). At sunrise, ClNO 2 accounted for 21% of NO z (=NO y − NO x ), 4% of NO y , and 2.5% of O x , respectively (median values). Kinetic parameters for the N 2 O 5 to ClNO 2 conversion were estimated by relating ClNO 2 concentrations to their time‐integrated heterogeneous production from N 2 O 5 and were highly variable between nights. Production of ClNO 2 required conversion of N 2 O 5 on submicron aerosol with average yield (φ) and N 2 O 5 reactive uptake probability (γ) of γφ = 0.008 (maximum 0.04), scaled with submicron aerosol chloride content, and was suppressed by aerosol organic matter and liquid water content. Not all of the observed variability of ClNO 2 production efficiency could be rationalized using current literature parameterizations.