Source Apportionment of Fine Particulate Matter during the Day and Night in Lanzhou, NW China

Source apportionment of PM_2.5 in Lanzhou, China, was carried out using positive matrix factorization (PMF). Seventeen elements (Ca, Fe, K, Ti, Ba, Mn, Sr, Cd, Se, Pb, Cu, Zn, As, Ni, Co, Cr, V), water-soluble ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca², Cl^-, NO₃^-, SO₄^2-), and organic carbon (OC) and elemental carbon (EC) were analyzed. The results indicated that the mean concentration of PM_2.5 was 178.63 ± 96.99 μg/m³. In winter, the PM_2.5 concentration was higher during the day than at night, and the opposite was the case in summer, and the nighttime PM_2.5 concentration was 1.3 times higher than during the day. Water-soluble ions were the dominant component of PM_2.5 during the study. PMF source analysis revealed six sources in winter, during the day and night: salt lakes, coal combustion, vehicle emissions, secondary aerosols, soil dust, and industrial emissions. In summer, eight sources during the day and night were identified: soil dust, coal combustion, industrial emissions, vehicle emissions, secondary sulfate, salt lakes, secondary aerosols, and biomass burning. Secondary aerosols, coal combustion, and vehicle emissions were the dominant sources of PM_2.5. In winter, the proportions of secondary aerosols and soil dust sources were greater during the day than at night, and the opposite was the case in summer. The coal source, industrial emissions source, and motor vehicle emissions source were greater at night than during the day in winter. This work can serve as a case study for further in-depth research on PM_2.5 pollution and source apportionment in Lanzhou, China.