PM1.0-Nitrite Heterogeneous Formation Demonstrated via a Modified Versatile Aerosol Concentration Enrichment System Coupled with Ion Chromatography

Particulate nitrite is a critical source of hydroxyl radicals; however, it lacks high-resolution methods due to its low abundance and stability to explore its formation mechanism. In this study, a modified versatile aerosol concentration enrichment system (VACES) coupled with ion chromatography (IC) was used to measure particulate NO2- hourly online and achieve a lowered detection limit of 10-3 μg m-3. VACES-IC was used to observe a high- and low-concentration events of PM1.0-NO2- in Shanghai, corresponding to the ambient-level concentrations of 0.34 and 0.05 μg m-3, respectively. The morning peak concentrations of NO2- even exceeded 3σ (standard deviation) in the high-concentration event due to the reduction of NO2 by aerosol SO32- based on kinetics and regression analysis. This implies that controlling SO2 emissions would be an effective strategy to decrease morning NO2- concentrations, correspondingly reducing the kinetic formation of SO42- by 20.8-34.8%. However, after sunrise, NO2- formation was primarily attributed to NO2 hydrolysis at pH 4.97-6.14. In the low-concentration event, NO2 hydrolysis also accounted for an overwhelming proportion (∼90%) of NO2- formation. This work estimates the contribution of different paths to particulate NO2- formation based on newly established high-resolution measurements.