Levels, Chemical Compositions, and Sources of PM2.5 of Rural and Urban Area under the Impact of Wheat Harvest

Wheat harvesting significantly alters the characteristics of PM2.5 in both rural regions and the adjacent urban areas. We conducted a systematic PM2.5 sampling campaign at two sites, one rural (ARS) and the other urban (UA), in the center of the Beijing-Tianjin-Hebei megalopolis during and after the wheat harvest (WH). The PM2.5 concentrations at ARS and UA decreased from 156 to 75.5 mu g m(-3) and from 137 to 53.1 mu g m(-3), respectively, between the periods during (DWH) and after (AWH) the wheat harvest. The hysteresis of the PM2.5 peaks at UA confirmed the rural-to-urban migration of pollution. Additionally, we found high geo-accumulation index (I-geo) values for crustal elements at both sampling sites, indicating that the dust emissions originated from the WH. Between DWH and AWH, the share of soil dust in the PM2.5 decreased from 21.3% to 7.98% and from 9.40% to 6.75% at ARS and UA, respectively. Furthermore, the concentrations of the biomass burning markers Cl- and K+ increased from 5050 to 9370 mu g g(-1) and from 5480 to 8090 mu g g(-1) at ARS, respectively, and from 3360 to 6650 mu g g(-1) and from 3630 to 7500 mu g g(-1) at UA. Positive Matrix Factorization (PMF) identified six PM2.5 sources, viz., coal combustion (CC), vehicle exhaust (VE), industrial sources (ISs), biomass burning (BB), secondary inorganic aerosol (SIA), and fugitive dust (FD). FD dominated ARS during DWH and exhibited an increase at UA during AWH, and the contribution of BB at both sites rose between DWH and AWH, which can be ascribed to the burning of biomass for maize planting. Surprisingly, owing to the operation of harvesters and cultivators, the proportion of VE emissions was larger at ARS than UA. However, UA displayed a far greater percentage of industry-derived PM2.5, suggesting that local ISs should be controlled more strictly. Finally, the sizable share attributed to CC at both ARS and UA demonstrates the continued use of this fuel source, despite the governmental decree limiting it.