Abstract: Background, aim, and scope Fine particulate matter(PM2.5) 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 PM2.5 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 Shannxi province and has a special basin topography. The air pollution in Hanzhong has gradually become severe deterioration with economic development limited research about PM2.5. In the present study, PM2.5 samples were collected at three urban sites in Hanzhong to investigate the characterizations of chemical species in PM2.5 during autumn. This study could provide valuable information and dataset for air pollution prevention in Hanzhong. Materials and methods PM2.5 samples including daytime (8:00-20:00) and night (20:00-8:00) measurements were collected at three observation sites in Hanzhong from 7 to 17 September 2015. The samples were collected on pre-backed quartz fiber filters (QM/A, Whatman Inc., U.K.) by mid-volume samplers (TE-5030, TISCH Inc., USA). The PM2.5 mass concentrations on each sample was weighted by a microbalance (±1μg sensitivity Mettle M3，Switzerland). The organic carbon(OC) and element carbon(EC) were analyzed by a Thermal/Optical Carbon Analyzer(DRI-2001) under IMPROVE_A protocol. Water-soluble inorganic ions (Cl-, NO3- , SO42- , Na+, NH4+ , K+, Mg2+, Ca2+) were analyzed by an Ion Chromatograph (Dionex-600) using the aqueous extracts of aerosol filters. Results The average mass concentration of PM2.5 was 27.04μg·m-3 in autumn, and the main components in PM2.5 were SNA (SO42- , NO3- and NH4+ ) and organic matter with both contribution of ~30% to PM2.5 mass. The average concentrations of OC, EC, SO42- , NO3- and NH4+ were 3.85, 2.93, 1.62, 5.43, and 1.67μg·m-3 during daytime and 5.76, 3.29, 1.96, 4.95, and 1.75μg·m-3 during nighttime, respectively. Furthermore, the concentrations of anions followed the order of SO42- > NO3- > Cl-, while the 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 slopes (cation/anion) of linear regression during daytime and nighttime were 0.86 and 0.81, respectively, which indicates the aerosol particles were acidic in Hanzhong. Further, 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, and this led to higher PM2.5 loadings at night compared with the daytime. The strong correlation among SO42- , NO3- and NH4+ and the results of regression analysis indicated that main existing forms of the three ions were (NH4)2SO4, NH4HSO4, NH4NO3 in PM2.5. Further, the estimated NH4+ concentrations correlated well with the measured values(R2 > 0.93) with a slope of 1.2 indicating that SO42- was maily in the form of (NH4)2SO4. The well correlation between K+ and Cl- at night (R2 > 0.73) revealed that they may come from biomass burning. Conclusions The average concentration of PM2.5 was lower than the first level of national ambient air quality standards. The concentrations of SNA and organic matter accounted for 32.3 and 26.9% of PM2.5 mass at daytime and 28.9 and 27.8% abundance at night, respectively. PM2.5 was generally acidic. The PM2.5 concentration at night was higher than that in daytime. Recommendations and perspectives This research provides a significant scientific basic for understanding the characteristics of PM2.5 and its chemical components. However, for the future pollution mitigation, more researches on aerosol in Hanzhong should be conducted.
Keywords: Hanzhong; PM2.5; carbonaceous components; water-soluble ions