Measurement Report: Size distributions of inorganic and organic components in particulate matter from a megacity in northern China: dependence upon seasons and pollution levels

Abstract. Size distributions of inorganic and organic components in particulate matter (PM) provide critical information on its sources, fate and pollution processes. Here, ions, elements, carbon fractions, n-alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes and steranes in size-resolved (9 stages) PM were analyzed during one year in a typical northern Chinese industrial megacity (Tianjin). We found that the concentrations of organic carbon fraction OC3, NO₃⁻ (or SO₄²⁻) and the sum of crustal elements were the highest in the pseudo-ultrafine (< 0.43 μm), fine (0.41–2.1 μm) and coarse (> 2.1 μm) modes, respectively. The diagnostic ratios of organic components consistently suggest that the traffic influence was stronger during summer and coal combustion during winter. Nitrate and high molecular weight PAHs were concentrated in the fine mode during winter, while nitrate and low molecular weight PAHs showed bimodal distributions especially during summer due to repartitioning. Long-chain n-alkanes showed a peak in the coarse mode during spring and summer, indicating a relatively stronger vegetation source and resuspended dust. Furthermore, we found a major difference in the size distribution of aerosol components during heavy pollution episodes (PM₁₀ > 233 μg m⁻³) in different seasons: in spring, OC fractions, 4- and 5-ring PAHs, hopanes and C18–C33 n-alkanes were enhanced at 1.1–3.3 μm, implying that they may arise from local combustion sources which emit relatively large particles; in summer PM mass, SO₄²⁻, NH₄⁺, Al, and C26–C33 n-alkanes were enhanced mainly in the coarse mode, peaking at 5.8–9.0 μm, indicating a large contribution from resuspended dust or heterogeneous reactions on dusts; in the winter and autumn, NO₃⁻ was significantly enhanced followed by SO₄²⁻, NH₄⁺, OC and EC with their peaks shifting from 0.43–0.65 μm to 0.65–2.1 μm, indicating strong atmospheric processing. These results reveal that the size distributions of inorganic and organic aerosol components are dependent on the seasons and pollution levels as a result of the differing sources and physicochemical processes.