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<p>Ambient measurements of nitryl chloride (ClNO<span class="inline-formula">₂</span>) 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. ClNO<span class="inline-formula">₂</span> is produced at night by the heterogeneous reaction of dinitrogen pentoxide (N<span class="inline-formula">₂</span>O<span class="inline-formula">₅</span>) on chloride (Cl<span class="inline-formula">⁻</span>) 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 ClNO<span class="inline-formula">₂</span> mixing ratio was 1.6 ppbv (parts per billion by volume; 15 min 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 ClNO<span class="inline-formula">₂</span> 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 N<span class="inline-formula">₂</span>O<span class="inline-formula">₅</span> necessary for ClNO<span class="inline-formula">₂</span> formation comes from the reaction of nitrate radicals (NO<span class="inline-formula">₃</span>) with nitrogen dioxide (NO<span class="inline-formula">₂</span>), where NO<span class="inline-formula">₃</span> itself is formed by a reaction of NO<span class="inline-formula">₂</span> with ozone (O<span class="inline-formula">₃</span>). Measured concentrations of ClNO<span class="inline-formula">₂</span>, NO<span class="inline-formula">₂</span>, and O<span class="inline-formula">₃</span> were used to quantify ClNO<span class="inline-formula">₂</span> production efficiencies, i.e., the yield of ClNO<span class="inline-formula">₂</span> formation per NO<span class="inline-formula">₃</span> radical formed, and a box model was used to examine the idealized dependence of ClNO<span class="inline-formula">₂</span> on the observed nocturnal O<span class="inline-formula">₃</span> and NO<span class="inline-formula">₂</span> concentrations. Results indicate that ClNO<span class="inline-formula">₂</span> production efficiency was most sensitive to the availability of NO<span class="inline-formula">₂</span> rather than that of O<span class="inline-formula">₃</span> and increased with decreasing temperature. The average ClNO<span class="inline-formula">₂</span> production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO<span class="inline-formula">₂</span> 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 ClNO<span class="inline-formula">₂</span> chemistry in rural environments in midwestern Europe.</p>