PM_1.0-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 PM_1.0-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 SO₃^2- 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 SO₄^2- 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.