Dominant Contribution of NO<sub>3</sub> Radical to NO<sub>3</sub><sup>–</sup> Formation during Heavy Haze Episodes: Insights from High-Time Resolution of Dual Isotopes Δ<sup>17</sup>O and δ<sup>18</sup>O
Xinxin Feng, Yingjun Chen, Shaofeng Chen, Peng Yu, Zeyu Liu, Minjun Jiang, Yanli Feng, Lina Wang, Li Li, Jianmin Chen
Abstract
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.