Dominant Contribution of NO 3 Radical to NO 3 - Formation during Heavy Haze Episodes: Insights from High-Time Resolution of Dual Isotopes Δ 17 O and δ 18 O.

δ 18 O is widely used to track nitrate (NO 3 - ) formation but overlooks NO 3 radical reactions with hydrocarbons (HCs), particularly in heavily emitting hazes. This study introduces high-time resolution Δ 17 O-NO 3 - as a powerful tool to quantify NO 3 - formation during five hazes in three cities. Results show significant differences between Δ 17 O-NO 3 - and δ 18 O-NO 3 - in identifying NO 3 - formation. δ 18 O-NO 3 - results suggested N 2 O 5 hydrolysis (62.0-88.4%) as the major pathway of NO 3 - formation, while Δ 17 O-NO 3 - shows the NO 3 - formation contributions of NO 2 + OH (17.7-66.3%), NO 3 + HC (10.8-49.6%), and N 2 O 5 hydrolysis (22.9-33.3%), revealing significant NO 3 + HC contribution (41.7-56%) under severe pollution. Furthermore, NO 3 - formation varies with temperatures, NO x oxidation rate (NOR), and pollution levels. Higher NO 2 + OH contribution and lower NO 3 + HC contribution were observed at higher temperatures, except for low NOR haze where higher NO 2 + OH contributions were observed at low temperatures ( T ← 10 °C). This emphasizes the significance of NO 2 + OH in emission-dominated haze. Contributions of NO 2 + OH and NO 3 + HC relate to NOR as positive ( f P1 = 3.0*NOR 2 - 2.4*NOR + 0.8) and negative ( f P2 = -2.3*NOR 2 + 1.8*NOR) quadratic functions, respectively, with min/max values at NOR = 0.4. At mild pollution, NO 2 + OH (58.1 ± 22.2%) dominated NO 3 - formation, shifting to NO 3 + HC (35.5 ± 16.3%) during severe pollution. Additionally, high-time resolution Δ 17 O-NO 3 - reveals that morning-evening rush hours and high temperatures at noon promote the contributions of NO 3 + HC and NO 2 + OH, respectively. Our results suggested that the differences in the NO 3 - pathway are attributed to temperatures, NOR, and pollution levels. Furthermore, high-time resolution Δ 17 O-NO 3 - is vital for quantifying NO 3 + HC contribution during severe hazes.