Reply on RC1

Abstract

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 (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 mid-west Europe.