Seasonal variation of 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 3 periods in winter, summer, and autumn 2019 as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed for understanding the photochemical processes in air masses typical for mid-west Europe. Measurements were conducted at 50 m above ground, which was most located mainly at the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO₂ is produced at nighttime by heterogeneous reaction of dinitrogen pentoxide (N₂O₅) on chloride ion (Cl-) containing aerosol. Its photolysis at day is of general interest as it produces chlorine (Cl) atoms that react with different atmospheric trace gases forming radicals. The highest observed ClNO₂ mixing ratio was 1.6 ppbv (15-min average) in the middle of one night in 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 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 29 (NO₂), where NO₃ itself is formed by 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 increase 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 literature indicating the importance of ClNO₂ chemistry in rural environments in mid39 west Europe. [ABSTRACT FROM AUTHOR]