Chemical characteristics of inorganic ammonium salts in PM_2.5 in the atmosphere of Beijing (China)

Abstract. The atmospheric concentrations of gaseous HNO₃, HCl and NH₃ and their relative salts have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China), as part of CAREBEIJING (Campaigns of Air Quality Research in Beijing and Surrounding Region). In this study, annular denuder technique used with integration times of 2 and 24h to collect inorganic and soluble PM_2.5 without interferences from gas–particle and particle–particle interactions. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. Fine particulate Cl⁻, NH₄⁺ and SO₄²⁻ exhibited distinct temporal variations, while fine particulate NO₃⁻ did not show much variation with respect to season. Daily mean concentrations of fine particulate NH₄⁺ and SO₄²⁻ were higher during summer (12.30 μg m⁻³ and 18.24 μg m⁻³, respectively) than during winter (6.51 μg m⁻³ and 7.50 μg m⁻³, respectively). Daily mean concentrations of fine particulate Cl⁻ were higher during winter (2.94 μg m⁻³) than during summer (0.79 μg m⁻³), while fine particulate NO₃⁻ showed similar both in winter (8.38 μg m⁻³) and in summer (9.62 μg m⁻³) periods. The presence of large amounts of fine particulate NO₃⁻ even in summer are due to higher local and regional concentrations of NH₃ in the atmosphere available to neutralize H₂SO₄ and HNO₃, which is consistent with the observation that the measured particulate species were neutralized. The composition of fine particulate matter indicated the domination of (NH₄)₂SO₄ during winter and summer periods. In addition, the high relative humidity conditions in summer period seemed to dissolve a significant fraction of HNO₃ and NH₃ enhancing fine particulate NO₃⁻ and NH₄⁺ in the atmosphere. All measured particulate species showed diurnal similar patterns during the winter and summer periods with higher peaks in the early morning, especially in summer, when humid and stable atmospheric conditions occurred. These diurnal variations were affected by wind direction suggesting regional and local source influences. The fine particulate species were correlated with NO_x and PM_2.5, supporting the hypothesis that traffic may be also an important source of secondary particles.