Variations of particle size distribution, black carbon, and brown carbon during a severe winter pollution event over Xi’an, China

Real-time particulate matter (PM) size distributions, 4-hour time resolution, PM_2.5, carbonaceous materials, and their optical properties were measured during a severe pollution event in Xi’an, China. High PM_2.5/PM₁₀ ratios were observed on both pollution (0.83) and non-pollution (0.73) days, emphasizing the abundance of fine particles during sampling days. The particle number (PN) first peaked with a wide size range (30–100 nm) before morning rush hours (approximately 01:00–05:00) on pollution and non-pollution days, demonstrating that PN was governed by the accumulation of freshly emitted diesel particles and characterized by distinct aerosol condensation growth. By contrast, the second peak time and size range differed between pollution and non-pollution days because of different formation mechanisms. The light-absorbing coefficients of both black carbon (BC, b_{abs-880nm, BC}) and brown carbon (BrC, b_{abs-370nm, BrC}) were high on pollution days and decreased to approximately half of those values on non-pollution days, indicating that the degree of light absorption is reduced by rain. The diurnal variation in b_{abs-880nm, BC} pollution peaked with traffic on January 1 and 2. By contrast, it remained in relatively stable and high ranges (120–160 Mm^–1) in the second period (January 3–5) without traffic peaks, illustrating that the dominant sources changed even during the same pollution period. High values of both b_{abs-370nm, BrC} and b_{abs-880nm, BC} coincided in the afternoon and evening due to emissions from primary sources, and abundant aqueous secondary organic carbon, respectively. A highly variable mass absorption coefficient of BrC also indicated the variety of fuel combustion sources of primary BrC in Xi’an.