Size-resolved Refractive Index of Scattering Aerosols in Urban Beijing: A Seasonal Comparison

The refractive index (RI) of atmospheric aerosols is a critical parameter in aerosol direct radiative forcing (DRF) calculation that exhibits considerable uncertainties owing partly to substantial spatiotemporal changes in the RI. A tandem system comprising a single-particle soot photometer connected to a differential mobility analyzer is developed. Two field campaigns are conducted to derive the size-resolved real part of the RI of scattering (i.e., BC-free) particles at 1064 nm (RRIsp1064nm) in urban Beijing. Measurements of the RRIsp1064nm within a mobility diameter (D-mob) range of approximately 200 nm to 400 nm in summer and winter clearly show seasonal variations in the RRIsp1064nm, including magnitude and size dependence. A decreasing RRIsp1064nm trend is observed in summer as the D-mob enlarges from 237 nm to 422 nm, whereas the RRIsp1064nm increases in winter as the D-mob grows. This finding likely indicates varying aerosol physicochemical properties with particle size in summer and winter. Size-resolved RRIsp1064nm values within the D-mob range are generally larger in summer than in winter, thereby resulting in a larger bulk RRIsp1064nm of 1.50 +/- 0.01 in summer compared with winter (1.47 +/- 0.02). The low RRIsp1064nm in winter is likely attributable to the high contribution of organic aerosols (OA), particularly less oxidized OA, which generally have lower RRI values than secondary inorganic aerosols. Pronounced low RRIsp1064nm values are observed during polluted episodes in winter, implying significant OA contribution. Photooxidation is likely an important regulator of the RRIsp1064nm in summer. This inference is supported by the fact that RRIsp1064nm values in the afternoon are larger than those at night, which corresponds well to diurnal variations in O-x (O-x=O-3+NO2) concentrations. Results indicate that seasonal variations in the RRIsp1064nm should be considered carefully in research, such as in DRF assessments, aerosol property parameterization in chemical models, and aerosol optical property and composition remote sensing. Copyright (c) 2021 American Association for Aerosol Research