Industrial Condensable Particulate Matter: A Missing Source in PM 2.5 Source Apportionment Driving Winter Pollution in China

Fine particulate matter (PM 2.5 ) from industrial sources remains a major health concern. However, current emission controls and source apportionment models ignore condensable particulate matter (CPM). Here, we provide the first nationwide quantification of industrial CPM contributions to atmospheric PM 2.5 in China through an integrated framework of field measurements, receptor modeling, and chemical transport simulations. Industrial CPM exhibits a chemical profile dominated by sulfate and ammonium, distinct from filterable PM, which creates a risk that its contribution may be misattributed to secondary aerosols in traditional source apportionment. Across industrial sources, CPM mass proportions vary significantly ( p < 0.01) for NH 4 +, NO 3 –, and Cl – . Based on receptor modeling, CPM contributes a median of 3.6% (95% CI: 0.9%–14.5%) to urban PM 2.5 across all four seasons in 2023, in the industrialized cities examined (Shanghai, Xi’an, Hefei, Shijiazhuang). The contribution peaks in winter (9.5%, 95% Monte Carlo-derived intervals: 2.4%–23.1%) due to enhanced gas-to-particle condensation at low temperature in these cities. From 2014 to 2023, the iron and steel sector emerged as the dominant CPM source, with its contribution increasing by up to 31.0% in key regions during winter. This overlooked source appears to substantially compress the apportioned contribution of secondary inorganics (by an estimated 5.6%–16.6%), suggesting that omission of CPM may lead to overestimation of secondary inorganic aerosol in traditional source apportionment. The results underscore the priority control of CPM emissions from industrial sources.