Abstract
ABSTRACT 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 µg m–3 and from 137 to 53.1 µg 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 (Igeo) 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 µg g–1 and from 5480 to 8090 µg g–1 at ARS, respectively, and from 3360 to 6650 µg g–1 and from 3630 to 7500 µg 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.
Highlights
North China Plain (NCP), one of two largest agricultural bases in eastern China, where the behaviors include winter wheat harvest in June and subsequent open burning of agricultural residues in field for maize planting are common
The PM2.5 peaks appeared later at UA than adjacent rural site (ARS), demonstrating the rural-to-urban migration of pollution. Both locations displayed higher Igeo values for the crustal elements than the other elements, confirming that the dust emissions originated from the wheat harvest
The mass ratios of the Trace element oxides (TEOs) to the PM2.5 increased from 0.254% to 0.280% and from 0.079% to 0.240% at ARS and UA, respectively, between DWH and AWH, because of straw burning emissions following the harvest
Summary
North China Plain (NCP), one of two largest agricultural bases in eastern China, where the behaviors include winter wheat harvest in June and subsequent open burning of agricultural residues in field for maize planting are common. The subsequent straw burning for maize planting can result in the releasing of drastic amounts of pollutants such as CO2, CO, NH3, non-methane hydrocarbon, elemental carbon, organic carbon, and particulate matter, which influence air quality, air visibility, human health, and the climate (Chen et al, 2018; Zhang et al, 2020). The particulate matter (PM) emissions related to agricultural operations has constituted an emerging issue about air quality, especially in the areas between rural and urban environments (Qiu and Pattey, 2008; Wang et al, 2015). The current studies mainly focused on the emission factors of PM during wheat harvest or the impact of subsequent biomass burning on local and urban areas (Qiu and Pattey, 2008; Yao et al, 2017; Chen et al, 2018)
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