【摘要】 为探讨汉中市秋季PM_2.5昼夜变化特征。于2015年9月7日至9月17日利用中流量大气颗粒物采样仪在汉中市三个不同站点分昼夜采集PM_2.5滤膜样品,并分别利用热光碳分析仪（DRI—2011）和离子色谱（Dionex—600）分析PM_2.5中碳组分和水溶性离子组分,主要探讨PM_2.5及其碳组分和水溶性离子昼夜变化特征。结果显示:汉中秋季PM_2.5浓度低于国家空气质量一级标准;PM_2.5中主要化学组分包括SNA （硫酸盐、硝酸盐、铵盐）和有机类物质,白天和夜间占比分别达到32.3%、39.6%和28.9%、39.6%; PM_2.5颗粒物呈酸性。除SO₄^2-、Mg²⁺和Ca²⁺之外,PM_2.5及其化学组分均呈现夜间浓度高于白天的特征。离子的赋存形态分析表明:SO₄^2-更多以（NH₄⁺）₂SO₄^2-的形式存在于PM_2.5中。本文相关结果可为地方环保政策的制定提供参考和基础数据。更多还原

【Abstract】 Background, aim, and scope Fine particulate matter(PM_2.5) has been widely concerned by people and scientists due to its negative impacts on climate, human health and ecological environment. To better understand air pollution in China, more characteristics of PM_2.5 chemical components should be considered in urban areas.Hanzhong（105°30.5’-108°24.6’E, 32°15.3’-33°56.6’N） is located in southwest of Shaanxi Province and has a special basin topography. The air pollution in Hanzhong has gradually become severe with economic development, but research on PM_2.5 in Hanzhong was limited. In the present study, PM_2.5 samples were collected at three urban sites in Hanzhong to investigate the characterizations of chemical species in PM_2.5 during autumn.This study could provide valuable information and dataset for air pollution prevention in Hanzhong. Materials and methods PM_2.5 samples including daytime（08:00-20:00） and night（20:00-08:00） were collected at three observation sites in Hanzhong from 7 to 17 September 2015. The samples were collected on pre-baked quartz fiber filters（QM/A, Whatman Inc., U.K.） by mid-volume samplers（TE-5030, TISCH Inc., USA）.The PM2 5 mass concentrations of each sample was weighted by a microbalance（± 1 μg sensitivity,Mettle M3,Switzerland）. The organic carbon（OC） and element carbon（EC） were analyzed by a Thermal/Optical Carbon Analyzer（DRI-2001） with IMPROVE_A protocol. Water-soluble inorganic ions(C1^-,NO₃^-,SO₄^2-,Na⁺,NH₄⁺,K⁺,Mg²⁺,Ca²⁺)were analyzed by an Ion Chromatograph（Dionex-600） using the aqueous extracts of the aerosol filters. Results The average mass concentrations of PM_2.5 was 27.04 μg·m^-3 in autumn, the main components in PM_2.5 were SNA(sulfates, nitrates and ammonium, calculated by SO₄^2-,NO₃^-and NH₄⁺) and organic matter with both contribution of～30% to PM_2.5 mass. The average concentrations of OC, EC, SO₄^2-,NO₃^-and NH₄⁺ were3.85 μg·m^-3, 2.93 μg·m^-3, 1.62 μg·m^-3, 5.43μg·m^-3, 1.67 μg·m^-3 during daytime and 5.76 μg·m^-3, 3.29μg·m^-3,1.96 μg·m^-3, 4.95μg·m^-3, 1.75 μg·m^-3 at night, respectively. Furthermore, the concentrations of anions followed the order of SO₄^2->NO₃^->C1^-,while in cations followed the order of NH₄⁺ > K⁺>Ca²⁺>Na⁺>Mg²⁺. A strong correlation（R²>0.93） was found between anions and cations for all samples, indicating that the five cations and three anions were the major ions extracted from filters. The slope（cation/anion） of linear regression during daytime and at night were 0.86 and 0.81, respectively, which indicates the aerosol particles were acidic in Hanzhong. Furthermore, OC correlated strongly with EC（R²>0.82）, which indicated similar sources for carbonaceous species. Discussion The diffusion of pollutants was difficult at night due to low atmospheric boundary layer, which led to higher PM_2.5 loadings at night compared with the daytime. The strong correlation among SO₄^2-, NO₃^-and NH₄⁺ and the results of regression analysis indicated that main existing forms of the three ions were（NH₄）₂ SO₄, NH₄ HSO₄, NH₄ NO₃ in PM_2.5.Further, the estimated NH₄⁺ concentrations correlated well with the measured values（R²>0.93） with a slope of 1.2 indicating that SO₄^2-was maily in the form of（NH₄）₂ SO₄.The well correlation between K⁺ and C1^-at night（R²>0.73） revelated that they may come from biomass burning.Conclusions The average concentration of PM_2.5 was lower than the first level of national ambient air quality standards. The concentrations of SNA and organic matter accounted for 32.3% and 39.6% of PM_2.5 mass during daytime, 28.9% and 39.6% abundance at night, respectively. PM_2.5 was generally acidic. The PM_2.5 concentration at night was higher than daytime in autumn. Recommendations and perspectives This research provides a significant scientific basis for understanding the characteristics of PM_2.5 and its chemical components. However,for the future pollution mitigation, more researches on aerosol in Hanzhong should be conducted.更多还原