Nocturnal production of N₂O₅ and ClNO₂ in Delhi: driving factors and impacts

Abstract. Nitryl chloride (ClNO₂) is an important Cl· precursor, originating from the heterogeneous reactions of dinitrogen pentoxide (N₂O₅) on chloride-containing particles. This N₂O₅-ClNO₂ chemical process plays critical roles in chloride activation, nitrate formation, and thus air pollution. Here we present field measurements made in the early springtime of 2023 in Delhi and compare with a previous study conducted during the winter of 2019. We found elevated nocturnal levels of N₂O₅ and ClNO₂, averaging 13 and 80 ppt, respectively, which are approximately doubled compared to observations in 2019. This change is primarily driven by the reduced nighttime NO levels, from 124±25 ppb in 2019 to 44±9 ppb in 2023. In addition, the chloride concentration (nighttime average 4.7 μg/m³) in Delhi is among the highest reported globally, driving efficient conversion of N₂O₅ to ClNO₂. Decreased NO and elevated ClNO₂ levels lead to higher NO₃· and Cl· production that promote the oxidation of organics. Consistently, we observed increased fractions of gaseous nitrogen- and chlorine-containing organic products and a higher oxidation state of the organic aerosols. Our findings highlight the need for increased attention to atmospheric secondary pollution and stringent chlorine emissions control with the reduction of NO_x in Delhi.