Chemical Characterization And Source Apportionment Of PM_2.5 In Lanzhou,China

Fine particular matter(PM_2.5)not only plays a key role in environment and human health,but also affects the global climate change.With rapid development in economy and enhancement of human energy consumption,PM_2.5 contamination in urban areas has becoming increasingly serious.Frequent occurrences of haze events in many cities across China have aroused great concern in governments,public,and scientific community.China has introduced PM_2.5 into its daily air quality monitoring index in the revised ―Ambient Air Quality Standards‖（GB3095-2012）that came into force in 2012.Lanzhou,the capital city of Gansu province,is one of the most polluted cities in China.Understanding of major characteristics,spatial-temporal variation,and emission sources is essential for implementing emission reduction and control measures in the course of remediation of local air quality.The present study made use of mid-volume proactive samplers to collect PM_2.5 samples in spring,summer,autumn,and winter of 2014 at Xigu（a suburb of Lanzhou）and Chengguan（downtown of Lanzhou）in Lanzhou.The level of PM_2.5,water-soluble inorganic ions（Cl-、SO42-、NO3-、Na+、NH4+、K+、Mg2+、Ca2+）,organic carbon,elemental carbon,and elements（Ca、Fe、Al、Mg、Ti、Mn、Zn、Pb、Cu、As、Ni）in four seasons were determined at two sites.Based on these observed data,we assessed the seasonal variability,difference in concentrations between Xigu and Chengguan,factors influencing the formation and chemical components levels of fine particles.Chemical mass closure（CMC）and potential source contribution function（PSCF）were used to study the contribution of chemical substances to PM_2.5 mass and their potential area of sources,respectively.Finally,the positive matrix factorization（PMF）model was employed to apportion the sources of PM_2.5.The conclusions of this study are summarized below:（1）The levels and spatiotemporal characteristics of PM_2.5 in Lanzhou show that the annual averages of PM_2.5 were 93.7 μg m-3 in Xigu and 88.9 μg m-3 in Chengguan,respectively.Both concentrations of PM_2.5 at the two sites were higher than annual mean PM_2.5（35 μg m-3）specified in the national standard for ambient air quality.The levels of PM_2.5 in spring,summer,autumn,winter were 100.2,46.1,91.4,and 137.2 μg m-3,respectively,which are factors of 2.9,1.3,2.6,and 3.9 higher than the standard,at the Xigu industrial site.At the Chengguan downtown site,measured seasonal levels of PM_2.5 were 83.2 in spring,37.9 in summer,93.3 in autumn,and 141.1 μg m-3 in winter,respectively,which were factors of 2.4,1.1,2.7,and 4.0 higher than the standard.As shown,PM_2.5 levels in winter were much higher than that in summer at the two sites.（2）The levels and spatiotemporal characteristics of water-soluble inorganic ions in PM_2.5 in Lanzhou are summarized below.The annual averages of Cl-、SO42-、NO3-、Na+、NH4+、K+、Mg2+,and Ca2+ were 3.1,8.2,7.9,0.9,5.3,0.5,0.2,and 1.4 μg m-3,at the Xigu industrial site,and 2.3,5.6,5.2,0.5,2.9,0.9,0.2,and 1.3 μg m-3,at the Chengguan downtown site.Secondary inorganic ions SO42-,NO3-,NH4+ accounted for 78.1% and 72.8% of the total ions at the Xigu industrial site and Chengguan downtown site,respectively.The levels of SO42-,NO3-,NH4+ and Cl-at Xigu were higher than that at Chengguan in all seasons.Conversely,the levels of K+ were higher in Chengguan than Xigu.Concentrations of Cl-、K+、SO42-、NO3-and NH4+ were highest in winter,the highest concentrations of Ca2+ were found in spring at the both sites.The ISORROPIA II thermodynamic equilibrium model was used to quantify the combinations of inorganic ions.Result indicated that Cl-and NO3-were combined with NH4+ in the form of NH4 Cl and NH4NO3,respectively.However,SO42-was found to be also associated with K+、Ca2+、Na+、Mg2+,besides NH4+.（3）Spatiotemporal characteristics and the sources of OC and EC in Lanzhou are summarized below: The annually averaged OC and EC were 15.3 and 6.7 μg m-3 at the Xigu industrial site,and 17.7 and 9.0 μg m-3 at the Chengguan downtown site.The levels of OC and EC were higher at the Chengguan site than that at Xigu site in all seasons.The levels of OC,EC were highest in winter and lowest in summer at both Xigu industrial site and Chengguan downtown site.Correlations and slopes between OC and EC,K+ and EC,the levels of OC1,OC2,OC3,OC4,EC1,EC2,EC3 as well as their contributions to total carbon mass suggested that the dominant sources of carbonaceous aerosols at the both sites were coal combustion and vehicle exhaust,rather than biomass burning.（4）The levels and spatiotemporal characteristics of elements in Lanzhou: The annual average of Ca,Fe,Al,Mg,Ti,Mn,Zn,Pb,Cu,As,Ni were 2874,1710,1554,746,39.2,54.4,112,108,73.7,11.4,and 31.0 ng m-3,respectively,at the Xigu industrial site.At the Chengguan downtown site,these elements were 3618,2066,1271,753,36.0,100,167,339,106,8.6,and 39.2 ng m-3,respectively.Ca,Fe,Al,Mg,and Ti were originated from soil and Mn was influenced by both natural and anthropogenic sources.Zn,Pb,Cu,As,Ni were mainly derived from significant anthropogenic sources.（5）The results of PM_2.5 source apportionment in Lanzhou: The results of chemical mass closure（CMC）showed that the main components of PM_2.5 were mineral dust,water-soluble inorganic ions,organic matter,and elemental carbon.Potential source contribution function（PSCF）was used to analyze the potential source area of these main chemical substances.Results revealed that the main source areas of these components were located in and proximate to Lanzhou.Source apportionment by PMF model manifested that coal combustion was ranked as the largest contributor to PM_2.5 mass,followed by soil dust,traffic emissions,secondary inorganic aerosols,refined copper industry,and biomass burning whereas waste incineration was the smallest contributor to PM_2.5 mass.The contributions of these sources to PM_2.5 mass were 22.3%,21.8%,21.7%,16.6%,9.6%,and 8.0%,respectively.Results from the present study can provide useful information and theoretical basis for PM_2.5 pollution control in Lanzhou.Based on the PM_2.5 source apportionment results,to improve the air quality in Lanzhou,actions should be taken in changing energy structure,restraining the number of vehicles,and slowing down industrial activities,etc.