The contribution of residential coal combustion to atmospheric PM 2. 5 in northern China during winter
A vast area in northern China, especially during wintertime, is currently suffering from severe haze events due to the high levels of atmospheric PM 2. 5. To recognize the reasons for the high levels of PM 2. 5, daily samples of PM 2. 5 were simultaneously collected at the four sampling sites of Beijing city (BJ), Baoding city (BD), Wangdu county (WD) and Dongbaituo (DBT) during the winter and spring of 2014–2015. The concentrations of the typical water-soluble ions (WSIs, such as Cl −, NO 3−, SO 42− and NH 4+) at DBT were found to be remarkably higher than those at BJ in the two winters, but almost the same as those at BJ in the two springs. The evidently greater concentrations of OC, EC and secondary inorganic ions (NO 3−, SO 42−, NH 4+ and Cl −) at DBT than at WD, BD and BJ during the winter of 2015 indicated that the pollutants in the rural area were not due to transportation from neighbouring cities but dominated by local emissions. As the distinct source of atmospheric OC and EC in the rural area, the residential coal combustion also made a contribution to secondary inorganic ions through the emissions of their precursors (NO x, SO 2, NH 3 and HCl) as well as heterogeneous or multiphase reactions on the surface of OC and EC. The average mass proportions of OC, EC, NO 3− and SO 42− at BD and WD were found to be very close to those at DBT, but were evidently different from those at BJ, implying that the pollutants in the cities of WD and BD, which are fully surrounded by the countryside, were strongly affected by the residential coal combustion. The OC ∕ EC ratios at the four sampling sites were almost the same value (4.8) when the concentrations of PM 2. 5 were greater than 150 µg m −3, suggesting that the residential coal combustion could also make a dominant contribution to atmospheric PM 2. 5 at BJ during the severe pollution period when the air parcels were usually from southwest–south regions, where a high density of farmers reside. The evident increase in the number of the species involved in significant correlations ( p < 0. 05) from the countryside to the cities further confirmed that residential coal combustion was the dominant source of key species in the rural area. However, the complex sources including local emissions and regional transportation were responsible for the atmospheric species in the cities. Strong correlations among OC, EC, Cl −, NO 3− and NH 4+ were found at the four sampling sites but only a strong correlation was found between OC (or EC) and SO 42− at BJ, implying that the formation rate of SO 42− via heterogeneous or multiphase reactions might be relatively slower than those of NO 3−, NH 4+ and Cl −. Based on the chemical mass closure (CMC) method, the contributions of the primary particle emission from residential coal combustion to atmospheric PM 2. 5 at BJ, BD, WD and DBT were estimated to be 32, 49, 43 and 58 %, respectively.