In the troposphere, nitryl chloride (ClNO₂), produced from uptake of dinitrogen pentoxide (N₂O₅) on chloride containing aerosol, can be an important nocturnal reservoir of NO(x) (= NO + NO₂) and a source of atomic Cl, particularly in polluted coastal environments. Here, we present measurements of ClNO₂ mixing ratios by chemical ionization mass spectrometry (CIMS) in Calgary, Alberta, Canada over a 3-day period. The observed ClNO₂ mixing ratios exhibited a strong diurnal profile, with nocturnal maxima in the range of 80 to 250 parts-per-trillion by volume (pptv) and minima below the detection limit of 5 pptv in the early afternoon. At night, ClNO₂ constituted up to 2% of odd nitrogen, or NO(y). The peak mixing ratios of ClNO₂ observed were considerably below the modeled integrated heterogeneous losses of N₂O₅, indicating that ClNO₂ production was a minor pathway compared to heterogeneous hydrolysis of N₂O₅. Back trajectory analysis using HYSPLIT showed that the study region was not influenced by fresh injections of marine aerosol, implying that aerosol chloride was derived from anthropogenic sources. Molecular chlorine (Cl₂), derived from local anthropogenic sources, was observed at mixing ratios of up to 65 pptv and possibly contributed to formation of aerosol chloride and hence the formation of ClNO₂.

Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO(2)), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO(2), together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClNO(2) exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClNO(2) source from aerosol uptake of N(2)O(5) throughout the boundary layer and a HONO source from dry deposition of NO(2) to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO(2) was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO(2) produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.

Measurements of hydroxyl (OH) and hydroperoxy (HO2*) radical concentrations were made at the Pasadena ground site during the CalNex‐LA 2010 campaign using the laser‐induced fluorescence‐fluorescence assay by gas expansion technique. The measured concentrations of OH and HO2* exhibited a distinct weekend effect, with higher radical concentrations observed on the weekends corresponding to lower levels of nitrogen oxides (NOx). The radical measurements were compared to results from a zero‐dimensional model using the Regional Atmospheric Chemical Mechanism‐2 constrained by NOx and other measured trace gases. The chemical model overpredicted measured OH concentrations during the weekends by a factor of approximately 1.4 ± 0.3 (1σ), but the agreement was better during the weekdays (ratio of 1.0 ± 0.2). Model predicted HO2* concentrations underpredicted by a factor of 1.3 ± 0.2 on the weekends, while measured weekday concentrations were underpredicted by a factor of 3.0 ± 0.5. However, increasing the modeled OH reactivity to match the measured total OH reactivity improved the overall agreement for both OH and HO2* on all days. A radical budget analysis suggests that photolysis of carbonyls and formaldehyde together accounted for approximately 40% of radical initiation with photolysis of nitrous acid accounting for 30% at the measurement height and ozone photolysis contributing less than 20%. An analysis of the ozone production sensitivity reveals that during the week, ozone production was limited by volatile organic compounds throughout the day during the campaign but NOx limited during the afternoon on the weekends.

[1] 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 À 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.