Fine particulate matter pollution in selected cities of eastern and southern China : compositions, sources, and impacts

Abstract

Fine particulate matter (PM₂.₅) pollution has become a perennial environmental issue and triggered health problems in China, especially in densely populated regions such as the Yangtze River Delta (YRD) in the east and the Pearl River Delta (PRD) in the south. For health-oriented pollution control, it is necessary to dissect PM₂.₅ from both the chemical and the biological perspective and further identify the key toxic components and source contributors of adverse health effects. However, integrated investigation of PM₂.₅ composition and closed-loop analysis from sources to impacts have not yet been completed. Towards this end, PM₂.₅ was collected simultaneously for one year across geographical locations under different land-use impacts in selected typical cities of the YRD and PRD regions, to characterise the inter/intra-regional differentiation of chemical and biological features of PM₂.₅ with linkage to emission sources and the resultant health impacts. First, the chemical composition profile (e.g., organic carbon and elemental carbon, water-soluble ions, and metals) of PM₂.₅ showed notable regional differences, while the intra­regional heterogeneity was more exhibited by trace metal profiles, with the highest concentrations of anthropogenically enriched metals at industrial sites. Incorporating the speciation and bioaccessibility information, a population-based risk assessment of chronic exposure to metal was conducted. Chromium and arsenic were major contributors of the estimated metal-induced carcinogenic risk (CR) over the safety level in both regions, whereas the non-carcinogenic risk (NCR) within the threshold was largely attributed to manganese. Traffic emission and combustion (coal/waste/biomass burning) were identified as two common dominant sources of metal-induced CR to be controlled in both regions, besides an elevated contribution from industrial emissions in the YRD sites. The differentiation of source profiles between environmental risk and the total mass of metals further implied the necessity of a regulatory control shift from a mass concentration-based to a risk-based framework for public health benefit. Second, investigations on the biological components of PM₂.₅ focused on bacteria and embedded antibiotic resistance genes (ARGs) for their potential as microbial hazards to humans and provided the following findings: (1) more pronounced seasonal variations in bacterial and ARG abundance at the (semi)rural site compared with other land-use types in both regions; (2) inter-regional differentiation in ARG profiles in contrast to similar intra-regional antibiotic resistance patterns independent of land-use types and seasonal cycles; (3) the targeted ARGs associated with limited bacterial genera and their relative abundance commonly correlated to mobile genetic elements (MGEs) across regions; (4) ARG and MGE enrichment in PM₂.₅ with high variability covering the span of terrestrial/aquatic samples from natural to engineered environments and human/animal excreta; and (5) PM₂.₅ as a unique exposure pathway for bacteria and ARGs via inhalation with region-specific importance, compared with ingestion of drinking water and food. To further quantify the contributions of anthropogenic sources to PM₂.₅-associated bacteria and ARGs, a metagenomics-based study was conducted in a wastewater treatment plant (WWTP) in Hong Kong, supplemented with analysis of literature data on other potential sources. As evidenced by the more distinct disparities in bacterial and ARG profiles across environmental matrices (PM₂.₅ and sewage/sludge/effluent) compared with sampling locations (PM₂.₅ in WWTP and urban/coastal sites), PM₂.₅ was reaffirmed as a unique bacterial and ARG reservoir. Dominated by multidrug resistance, the increasing ARG enrichment in PM₂.₅ along the coastal-urban-WWTP gradient illustrated the impacts of human activities on the atmospheric accumulation of ARGs. However, a co-occurrence analysis suggested that the horizontal transfer of ARGs, especially to pathogens, could hardly occur in the atmosphere. Finally, ~18% of the bacteria and >36% of the relevant ARGs in urban and WWTP PM₂.₅ were attributable to sewage/sludge/effluent in WWTP. The results showed the differences in source profiles between chemical and microbial hazards in PM₂.₅ and indicated WWTPs as an important source of airborne bacteria and ARGs to be controlled. Collectively, the current integrated study on PM₂.₅ from both chemical and (micro)biological dimensions quantitatively links geographical disparities in chemical-induced health risk (e.g., metal) and microbial components (e.g., bacteria and associated ARGs) to their sources. The results contribute to a better understanding of PM₂.₅ pollution and provide valuable information for health-oriented pollution control that can be adapted for regional-specific circumstances, thus helping to improve regional air quality and protect public health.