[Observations of Reactive Nitrogen and Sulfur Compounds During Haze Episodes Using a Denuder-based System]

Reactive nitrogen and sulfur compounds are chemically active in the atmosphere and play an important role in secondary particle formation. Among them, sulfate, nitrate, and ammonium (SNA) are important components of particulate matter (PM) that account for approximately one-third of fine particles. The precursors of SNA including HNO₃, SO₂, and NH₃ are all involved in haze formation in China. To date, the concurrent measurements of SNA and their precursors have been limited to single sites and short terms because of the high cost of the instruments. This study aimed to use DELTA (Denuder for Long-Term Atmospheric sampling) to characterize the daily concentrations of reactive nitrogen and sulfur species during haze episodes. The results showed that the background interface of NH₄⁺, NO₃^-, and SO₄^2- in the denuder was minor and could be used to determine 24-48 h concentrations of NH₃, HNO₃, SO₂, NH₄⁺, and NO₃^-. However, the SO₄^2- concentrations in the blank filter was so high that they could only be used for sampling weekly or for longer periods of time. During the campaign between May 9 and June 7, 2016 in urban Beijing, the concentrations of NH₃, HNO₃, NH₄⁺, and NO₃^- showed distinct daily variations at different wind directions, i.e., higher values were observed during southerly winds and lower values during northerly winds. The time series of these reactive nitrogen compounds coincided with that of PM_2.5, CO, SO₂, and NO₂, indicating the combustion of fossil fuels. The mean concentrations of NH₃, HNO₃, NH₄⁺, and NO₃^- were twice the concentrations during clean days, further highlighting the effect of local emissions on the urban environment. The ratios of HNO₃/NO₃^- and NH₃/NH₄⁺ were 1.2 and 4.5, respectively, without significant differences between hazy and clean days. The findings demonstrated that both the reduced and oxidized nitrogen were preferred in gaseous phase rather than particulate phase due to relatively high temperatures during the transition season of spring/summer.