Figure 1.

(A) Time series of SO₃ during the winter (January 20 to March 31, 2019) and summer (June 1 to July 10, 2019), (B) median diurnal patterns of SO₃, UVB, and atmospheric water dimer concentrations during the winter and summer, and (C) median normalized intensities of the atmospheric ion SO₃·NO₃^–. Rainy and snowy days were excluded. The shadows show the values from the 25th to 75th percentile. In panel B, the dashed lines show diurnal variations of SO₃ during haze and nonhaze days. The water dimer concentration was calculated on the basis of temperature and relative humidity.^26,39 In panel C, the signals of atmospheric ion SO₃·NO₃^– were normalized by the sum of NO₃^– and HNO₃·NO₃^– that are dominant natural charged ions in urban Beijing (Figure S7).