Enhanced nocturnal N 2 O 5 -ClNO 2 chemistry in Delhi under reduced NO conditions

Nitryl chloride (ClNO 2 ) is an important Cl⚫ precursor, originating from the heterogeneous reactions of dinitrogen pentoxide (N 2 O 5 ) on chloride-containing particles. This N 2 O 5 -ClNO 2 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 N 2 O 5 and ClNO 2 , 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 m −3 ) in Delhi is among the highest reported globally, driving efficient conversion of N 2 O 5 to ClNO 2 . Decreased NO and elevated ClNO 2 levels lead to higher NO 3 ⚫ 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 anthropogenic chlorine emissions control under reduced NO x conditions in Delhi.