Urban versus rural air pollution in northern Vietnam

Air pollution affects not only the cities but also many rural areas of Vietnam, which has been experiencing a high rate of industrialization over the last years. The comparison of air pollution in urban and rural areas, therefore, will provide insights into the sources and properties of air pollutants, especially those affecting large areas in the region. For this purpose, air samples were collected concurrently at Hanoi (urban) and Lucnam (rural), about 65 km to the northeast. Samples were analysed for black carbon, water-soluble ions and chemical elements to provide inputs for receptor mofeling. The mean coarse PM10 mass concentration (CM) at the urban site is 37.8 mg m-3, about two times higher than that at the rural site (19.1 mg m-3). The mean fine PM10 mass (FM) concentrations, however, are almost similar at the two sites (31 mg m-3). Positive Matrix Factorization (PMF) was applied to aerosol chemical compositions to derive sources. Six similar sources were found at both sites for the coarse fraction, i.e. soil dust incorporating metal sulphate and nitrate, soil dust containing soil organic matter and ammonium sulphate, coal fly ash from distant and local sources and sea salt. Traffic (road) dust was found only at the Hanoi site contributing 8% of the coarse mass. But vehicles are the main contributor to coarse soil dust, which is about three times more abundant in Hanoi than in Lucnam. Coal-derived particles are slightly more abundant in Hanoi (16% of CM) than in Lucnam (36% of CM). Six siimilar sources were found for fine particulates at the two sites, namely long-range transport (LRT), Cl-depleted marine aerosol, distant and local fly ash, soil dust and biomass burning. Again, vehicle emission was found only in Hanoi contributing 8% of FM. LRT is the largest source, accounting for 36% of FM at the two sites. Coal fly ash accounts for 30 and 37% of FM at the urban and rural site, respectively. Biomass burning is much stronger at the rural site (11% vs 6%). Thus, although having similar sources, urban and rural settings considerably differ by the abundance of the coarse fraction, reflecting the contrast in traffic and construction activities. The difference, however, is insignificant for the fine fraction as LRT aerosols and coal fly ash are similarly affecting the two sites. Furthermore, the contribution from vehicle emissions at the urban site is balanced by the stronger biomass burning emission at the rural site. Keeping the streets clean would reduce significantly soil dust in urban areas. High priority should also be given to reducing the burning of low quality coal and biomass, which is the source of abundant fly ash and sulphate, which affect both urban and rural areas. To address LRT aerosol that makes up more than one third of FM, regional cooperation measures are required.

Keywords: coarse and fine PM10, chemical compositions, PMF, long-range transport, traffic, coal, biomass