Seasonal variation in nitryl chloride and its relation to gas-phase precursors during the JULIAC campaign in Germany

Ambient measurements of nitryl chloride (ClNO₂) were performed at a rural site in Germany, covering three periods in winter, summer, and autumn 2019, as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed to understand the photochemical processes in air masses typical of midwestern Europe. Measurements were conducted at 50 m aboveground, which was mainly located in the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO₂ is produced at night by the heterogeneous reaction of dinitrogen pentoxide (N₂O₅) on chloride (Cl^-) that contains aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO₂ mixing ratio was 1.6 ppbv (parts per billion by volume; 1min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO₂ mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N₂O₅ necessary for ClNO₂ formation comes from the reaction of nitrate radicals (NO₃) with nitrogen dioxide (NO₂), where NO₃ itself is formed by a reaction of NO₂ with ozone (O₃). Measured concentrations of ClNO₂, NO₂, and O₃ were used to quantify ClNO₂ production efficiencies, i.e., the yield of ClNO₂ formation per NO₃ radical formed, and a box model was used to examine the idealized dependence of ClNO₂ on the observed nocturnal O₃ and NO₂ concentrations. Results indicate that ClNO₂ production efficiency was most sensitive to the availability of NO₂ rather than that of O₃ and increased with decreasing temperature. The average ClNO₂ production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO₂ production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO₂ chemistry in rural environments in midwestern Europe.