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 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-tourban 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.

that the emission rate and factor for PM10 during WH period were up to 0.58 ± 0.12 g m -2 and 9 0.74 ± 0.12 kg ha -1 , respectively. Exposure to high level of agricultural dust can result in adverse 10 health effects including acute and chronic bronchitis, mucous membrane irritation, allergic 11 asthma and hypersensitivity, etc (Ou et al., 2020;Rao et al., 2020). Also the subsequent straw 12 burning for maize planting can result in the releasing of drastic amounts of pollutants such as 13 CO 2 , CO, NH 3 , non-methane hydrocarbon, elemental carbon, organic carbon, and particulate 14 matter, which influence air quality, air visibility, human health, and the climate (Chen et al., 15 2018; Zhang et al., 2020). The carbonaceous aerosols related to biomass burning contribute 16 approximately 42% and 74% of black carbon (BC) and organic carbon (OC) globally, in which 17 open burning of biomass accounts for 95% and 88%, respectively (Yao et al., 2017). More than 18 50% emissions of PM2.5, OC, EC, K, K + , and Clin eastern China was associated with wheat 19 straw burning (Li et al., 2014). 20 As a large agricultural country, China contributed more than 24% of global carbonaceous 21 aerosols, especially from field burning of biomass. The particulate matter (PM) emissions 22 related to agricultural operations has constituted an emerging issue about air quality, especially 23 in the areas between rural and urban environments (Qiu and Pattey, 2008; Wang et al., 2015). 24 Biomass burning has a significantly adverse effect on regional air quality, global climate change 25 through complex feedbacks with radiation and clouds, and public health ascribed to exposure    deviation and ensure its accuracy. Both QFs and TEFs were purified prior to PM2.5 sampling.

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The heating temperature was set as 450 ℃ and 60 ℃ for QFs and TEFs before PM2.5collections.

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A room with constant temperature of 20 ℃ and relative humidity of 50% was used to store 27 filters before and after sampling aiming to the obtainment of PM2.5 mass. 28

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The detailed detection procedure documented in Li  OES system (Agilent 5100). For ICP-MS system, the half TEF filter was heated using the both 3 aqua regia and HF acid at 120 ℃ and lasted for 2 hours, and then ramped to 130 ℃ for dryness. 4 Then it was heated again with HCl acid and stored in a plastic comparison tube for analysis.

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For ICP-OES, the other half TEF filter was heated at 300 ℃ and then raised to 550 ℃ for 6 completely ashing. Then it was heated together with added absolute ethanol and NaOH, and 7 boiled with water later. Finally, 2 mL HCl was added and diluted to 10 mL for analysis. This (RSD) values were in the range of 2.12% to 6.15%. 13 In regard to WSIs, half QF filter was firstly put into the centrifuge tube and underwent 20 14 mins ultrasonic extraction with 10 ml ultra-pure de-ionized water for three times, and then 15 filtered by a PTFE membrane with pore size as 0.22 µm (Whatman, Middlesex, UK). Finally, 16 the extract was transferred into a plastic bottle and stored at 4 ℃ before analysis.

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Igeo=Log 2 (C n /B n ) (4) 14 The pollution levels of individual elements including uncontaminated, uncontaminated to 15 moderately contaminated, moderately contaminated, moderately to heavily contaminated, 16 heavily contaminated, heavily to extremely contaminated, and extremely contaminated were 17 identified according to the Igeo values of ≤ 0, 0-1, 1-2, 2-3, 3-4, 4-5, and > 5, respectively.   16 Concerning the mass contents, the sum of 39 elements and 9 ions ranged from 134 mg g -1 of 17 June 30 to 963 mg g -1 of July 3 and averaged at 444 mg g -1 for ARS. They varied from 224 mg 18 g -1 of July 5 to 951 mg g -1 of June 23 with a mean value of 450 mg g -1 for UA. Higher dust mass shares attributing to ARS reflected the emissions of large amounts of dust 3 associating with wheat harvest. UA possessed much higher dust fractions of 9.40% than the However, TEO contents exhibited the reverse variation trends, higher values occurred during 16 AWH instead of DWH. The TEO contents increased from 0.079% to 0.240% for UA, and from  Sn, Mo, and Pb possessed high Igeo values reflecting the impact of industrial production 12 processes and coal combustion. 14 The sum of mass contents of 9 water soluble ions (WSIs) were in the range of 0.051 g g -1 of 15 June 30 to g g -1 of June 21 averaging at 0.321 g g -1 for UA. The corresponding values for ARS 16 varied from 0.095 g g -1 of June 30 to 0.871 g g -1 of July 3 with a mean value as 0.394 g g -1 . Acting as the markers of biomass burning, Cland K + increased along with the transition 31 from DWH to AWH for both UA and ARS (Fig. 4). ARS possessed higher contents of these two 32

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ions than UA indicating that the strong influence of local biomass burning during AWH in rural 1 area. Concerning UA, Clcontents fluctuated from 3080 to 3510 µg g -1 during DWH, and from 2 1390 to 29100 µg g -1 during AWH. For ARS, they ranged from 3870 to 6300 µg g -1 for ARS 3 during DWH, and from 1710 to 18000 µg g -1 during AWH. K + contents varied from 2960 to 4 4210 µg g -1 and 2040 to 35900 µg g -1 during DWH and AWH for UA, and from 3930 to 8050 5 µg g -1 and 2310 to 21600 µg g -1 for ARS. However, the mass concentrations (reported in µg

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between UA and ARS, and higher BB shares in ARS proved that the impact of open biomass 1 burning for maize planting after wheat harvest in rural areas (Li et al., 2020a(Li et al., , 2020b(Li et al., , and 2021   2) The calculated dust mass ratios based on IEs increased from 6.75% to 9.40%, and 7.98% 11 to 21.3% for UA and ARS with the transition from AWH to DWH, which further proved the 12 abovementioned migration effect. Unlike dust, mass ratios of trace element oxides (TEO) 13 decreased from 0.240% to 0.079% for UA, and from 0.280% to 0.254% along with the 14 transition from AWH to DWH owning to their emissions from straw burning during AWH.

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3) Biomass markers Cland K + elevated significantly from DWH to AWH for both UA and  to the operation of agricultural machineries. CC occupied 12.0% and 9.66% of total PM2.5 mass 26 during whole sampling period for UA and ARS indicating that coal was still an important fuel 27 for cooking or industrial production.

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for the Central Universities (2020MS125).

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Mooibroek, D.    Source apportionment of PM2.5 at a regional background site in North China using PMF 10 linked with radiocarbon analysis: insight into the contribution of biomass burning. Atmos. 11 Chem. Phys. 16:11249-11265. https://doi.org/10.5194/acp-16-11249-2016     Mass contents of trace element oxides during every sampling day and periods for UA and ARS.

Fig. 4.
Contents of Cland K + during each sampling day and period for UA and ARS.