Real-time particulate matter (PM) size distribution, 4-hour time resolution, PM2.5, carbonaceous materials, and their optical properties were measured during a severe pollution event in Xi’an, China. High PM2.5/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 (approximately 01:00–05:00) morning rush hours 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 times and size ranges differed between pollution and non-pollution days because of different formation mechanisms. The light-absorbing coefficients of both black carbon (BC, babs-880nm, BC) and brown carbon (BrC, babs-370nm, BrC) were high on pollution days and decreased by approximately half of that on non-pollution days, indicating that the degree of light absorption is reduced by rain. The diurnal variation of pollution babs-880nm, BC 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 babs-370nm, BrC and babs-880nm, BC coincided in the afternoon and evening because of 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.