[Characteristics and Sources of PM_2.5 and Its Light-scattering Properties with Different Chemical Compositions in...

In recent years, the concentration of PM_2.5 in the Beijing urban area has decreased with the increase in the proportion of secondary inorganic ions. In order to explore the characteristics and sources of the light scattering of PM_2.5 with different chemical compositions, PM_2.5 with its chemical components and scattering coefficient were continuously measured at hourly resolution in the Beijing urban area from December 2020 to November 2021. The components, scattering characteristics, and sources of PM_2.5 were analyzed. The results showed that NO₃^- was the major component of PM_2.5 in the Beijing urban area, and the ω(NO₃^-) and ω(SNA) were 24% and 46% in PM_2.5, respectively. PM_2.5 could be divided into six types according to mass concentration and component proportion. The occurrence frequency of the good-type was the highest during the study with a similar duration in the four seasons, and the ω(SNA), ω(OM), and ω(FS) were 32%, 32%, and 28% in PM_2.5, respectively. The dust(D)-type and the OM(O)-type appeared mainly in spring and summer with the lowest frequency during the study. FS and OM were their major components, and the ω(FS) and ω(OM) were 66% and 46% in PM_2.5, respectively. The OM+SO₄^2-(OS)-type, OM+NO₃^-(ON)-type, and NO₃^-(N)-type appeared mainly in the afternoon in summer, in the early morning and morning in winter, and at approximately 07:00 every day in spring. Under the condition of low humidity[relative humidity (RH)<40%], the MSE of N-type PM_2.5 was the highest (4.3 m²·g^-1), and that of D-type PM_2.5 was the lowest (2.1 m²·g^-1), reflecting the high scattering ability of SNA. The MSE increased with relative humidity. Under the condition of high humidity (RH>80%), the MSE of all types of PM_2.5 rose to 1.5 to 1.8 times the values under low humidity. The variation trends of SAE showed that particle size increased with the rising of RH level. Under non-high humidity conditions, the scattering coefficients reconstructed by the revised IMPROVE formula fitted well with the measured values at hourly resolution, the correlation coefficients were between 0.81 and 0.97, and the slopes were between 1.00 and 1.21 except for that of D-type. The N-type fitting result was the best. Under high-humidity conditions, the R and the slopes were from 0.82 to 0.84 and from 0.48 to 0.53, respectively. The annual B_sca was 203.8 Mm^-1, and N-type PM_2.5 contributed the most, accounting for 53%, in which the large particles of NH₄NO₃ were the major contributor. B_sca of good-type PM_2.5 was 67.2 Mm^-1, in which small particles of OM were the major contributor. B_sca was 1.5 times the annual B_sca(dry), whereas the B_sca values of SNA were 1.8 to 2.1 times the B_sca(dry). The peak value of NO₃^- and RH simultaneously appeared around 07:00, resulting in the maximum B_sca of NH₄NO₃ at this time. The peak value of SO₄^2- and the B_sca of (NH₄)₂SO₄ mainly appeared at 16:00 and at 04:00, respectively. The diurnal variation curves of OM concentration and B_sca were consistent, and the bimodal peaks appeared at 13:00 and 20:00, respectively. In spring and winter, NO₃^-, SO₄^2- and OM mainly came from the plains east of the Taihang Mountains, and their potential source regions were not in any particular place in summer and autumn; the main potential source regions of FS were the northwest areas of Beijing in spring and autumn. The flow with high RH across the south and southeast of the north China plain and the eastern rim of Bohai Sea was likely to increase the weighted potential source contribution factor values of B_sca of SNA in this region.