关中典型城市及农村夏季PM_2.5的化学组成对比

PM_2.5 samples were collected on a day/night basis at an urban site in Xi'an in July 2016 and at Lin Village, a rural site 80km northeast to Xi'an, in August 2016. The samples were determined for organic carbon (OC), elemental carbon (EC), and inorganic ions to investigate the differences in chemical compositions and sources of PM_2.5 between the urban and rural areas of Guanzhong Plain. The results showed that PM_2.5 in Xi'an and Lin Village during the sampling period were (49.7±22.8)μg/m³ and (62.6±14.2)μg/m³, respectively. Concentrations of OC and EC in PM_2.5 were similar between Xi'an [(6.5±2.5)μg/m³ and (3.2±1.8)μg/m³] and Lin Village [(6.8±1.8)μg/m³ and (3.8±2.3)μg/m³]. The OC/EC mass ratios in Xi'an were higher in the daytime (2.6) than in the nighttime (1.9), in contrast to those in Lin Village, which were lower in the daytime (1.6) and higher in the nighttime (2.7). Such diurnal differences can be explained by an enhanced emission of EC due to the nighttime occurrence of heavy-duty trucks in the urban area and a decreased emission of EC due to the nighttime absence of anthropogenic activities (e.g., biomass burning and coal combustion) in the rural area. Inorganic ions of PM_2.5 were (20.2±14.6)μg/m³ and (30.1±10.5)μg/m³ in Xi'an and Lin Village and accounted for 40.6% and 47.6% of PM_2.5 in the two regions, respectively. SO₄^2- of PM_2.5 in the rural area was 19.0μg/m³ and accounted for one-third of the fine particle mass, which was much higher than that in Xi'an (9.4μg/m³ and 18.9%). Concentrations of NO₃^- and Ca²⁺ in Xi'an, their masses relative to PM_2.5 and NO₃^-/SO₄^2- mass ratio were significantly higher than those in Lin Village, suggesting that the urban atmosphere was more affected by motor vehicle exhausts and emissions from road-dust and construction activities. K⁺ in the samples showed stronger correlations with Ca²⁺ and Mg²⁺ in Xi'an due to the dominant source of dust. In contrast, K⁺ was correlated with EC in the rural area, mainly due to the dominance of biomass burning emissions.