Quantification of Hydroperoxyl Radical Based on Peroxynitric Acid Measurement

Hydroperoxyl radicals (HO2) play a central role in atmospheric oxidation and secondary pollutant formation, yet their direct detection remains limited. Peroxynitric acid (HO2NO2), a reservoir species in the HO2–NO2–HO2NO2 equilibrium system, offers an alternative pathway for HO2 quantification under equilibrium assumptions. In this study, we deployed nitrate-CIMS, I-CIMS, and LIF instruments during field campaigns in summer Nanjing (2023) and springtime Lulang (2021), spanning a wide temperature range (265 to 308 K). We assessed the feasibility of using HO2NO2 observations as a proxy for HO2 and examined the impact of ambient temperature and reagent-ion chemistry on proxy performance. Our results show that at warmer temperatures (>295 K), the fast thermal decomposition of HO2NO2 maintains the equilibrium, enabling accurate inference of HO2 from nitrate-CIMS observations with strong agreement to LIF data (R2 = 0.70). However, at colder temperatures (e.g., Lulang), the prolonged HO2NO2 lifetime (>60 s) leads to significant deviations from equilibrium, resulting in systematic overestimation of HO2. Furthermore, intercomparison between nitrate-CIMS and I-CIMS highlights that nitrate-based detection is more robust under high-humidity conditions, where I-CIMS tends to underestimate HO2NO2 due to reagent-ion clustering. These findings establish both the temperature applicability limit (∼295 K) for equilibrium-based HO2 inference and the superior humidity resilience of Nitrate-CIMS, providing critical guidance for future field deployments of HO2 proxies in diverse atmospheric environments.