| Atmospheric aerosols can change the earth radiation balance by scattering solarradiation or increasing cloud condensation nuclei; they also provide surface areas foroccurring heterogeneous reactions in the atmosphere. Acid deposition or nitrogendeposition associated with these aerosols has been found to exert a significant influence onvenerable ecosystems. PM2.5had important environmental impacts because of the natureof their small size, large surface areas and long residence time in the ambient air. NO3-,NO2-, Cl-, SO42-and NH4+are important chemical components in PM2.5and they aremainly produced through secondary transformation of their precursors in the gas phasesuch as HNO3, HNO2, HCl, SO2, and NH3. The equilibriums between these particulatespecies and their partners in the gas phase influence their concentrations as well aslong-distance transport of these species. In order to improve understanding of the sourcesof PM2.5and formation mechanisms, it is important to study not only concentrations ofwater-soluble inorganic ions in PM2.5, but also their precursor gases and relatedgas-particle partitioning.In this paper, Qingdao, a coastal city in China, is selected as the study area.224setsof PM2.5samples and gases were collected from June in2012to April in2013. Theconcentrations of water-soluble inorganic ions and acid and alkaline gases weredetermined for studying their temporal variations in concentrations and analyzingequilibriums between them. ISORROPIA II model is used to simulate the partitioning ofthese species between in the gas and particle phases. A few sensitivity tests wereconducted to study the concentrations of these species in PM2.5responding to the changesof their precursors.During this observation, the average concentrations of HNO3, HNO2, HCl and SO2inQingdao were1.84μg·m-3,1.50μg·m-3,1.75μg·m-3and23.23μg·m-3while the averageconcentration of NH3was3.18μg·m-3. The average concentrations of NO3-, NO2-, Cl-andSO42-were9.96μg·m-3,0.16μg·m-3,2.16μg·m-3and12.46μg·m-3, while the averageconcentration of NH4+was7.79μg·m-3. Compared with other regions in the world, theconcentration of SO2ranked at a high level, the concentrations of HNO3and HCl ranked at the middle level, and the concentration of HNO2and NH3ranked at a low level. All ions inPM2.5ranked at the middle level except NO2-ranking at a low level.A distinctive seasonal variation was observed, e.g., the decreasing orders for theseasonal average concentrations of HNO3, HCl and NH3were spring> autumn> winter>summer, summe> spring> winter> autumn and summer> autumn> spring> winter,respectively. The decreasing orders for the seasonal average concentrations of HNO2andSO2were the same, i.e., winter> autumn> spring> summer. The concentrations of NO3-,NO2-in PM2.5were higher in winter and spring, lower in summer and autumn. Thedecreasing orders for the seasonal average concentrations of SO42-and NH4+were winter>summer> spring> autumn and winter> summer> autumn> spring, respectively.When diurnal variations were examined, the average concentrations of HNO3, HCl,SO2and NH3were higher during daytime, and the ratios of day to night were1.2,1.6,1.3and1.0, respectively. The average concentration of HNO2was lower during daytime, andthe ratio of day to night was0.5. The average concentrations of NO3-, NO2-, Cl-, SO42-andNH4+were higher during daytime, and the ratios of day to night were1.59,1.28,2.45,1.59and1.31, respectively. Independent sample test results showed that only HNO2, HCl andNO2-showed statistically significant differences between daytime and nighttimeconcentrations (p<0.05), and other components did not show significant differences.Concentrations of chemical components were found to be related to specific weatherconditions. The highest concentrations of HNO3, HNO2and SO2occurred on haze daysand the lowest concentrations occurred on rainy days. The concentrations of HNO3, HNO2and SO2on haze days increased by about170%-210%relative to those on rainy days. Theconcentrations of HCl and NH3on haze days were also the highest but the lowestconcentrations occurred on fog days. The concentrations of HCl and NH3on haze daysincreased by about50%-120%relative to those observed on fog days. The higherconcentrations of NO3-, SO42-, NH4+and Cl-occurred on haze days and fog days and lowerconcentrations occurred on sunny days and rainy days. Their concentration on fog or hazedays increased by approximately40%-260%relative to those observed on sunny days.The concentrations of NO2-in PM2.5showed little difference in different weatherconditions.Significant correlations existed between SO2andd SO42-and between NH3and NH4+,but there were no correlation between HNO3and NO3-, between HNO2and NO2-, betweenHCl and Cl-. During the observation, the atmosphere appeared to be NH3-richenvironment. Sulfate and nitrate in PM2.5were found to be completely neutralized. NO3-, SO42-and NH4+were the most important aerosol acidic and alkaline ions, and NH4+existedmainly in the form of (NH4)2SO4. Formation of NO3-was mainly limited by concentrationsof HNO3. Formation of SO42-in PM2.5was limited by the oxidation of SO2with OH inspring, summer and winter, but the oxidation reaction of SO2with O3might play animportant role on the generation of SO42-in the autumn in Qingdao.ISORROPIA II thermodynamic equilibrium model is used to investigate the controlfactor of NO3-, SO42-and NH4+in Qingdao. Sensitivity analysis showed that1) formationof NO3-was sensitive to the changes of total HNO3(TNO3);2) formation of SO42-wassensitive to the changes in total H2SO4(TSO4),3) formation of NH4+was sensitive to thechanges of TNO3and total NH3(TNH4) respectively. These results implies that1) NO3-reduce in PM2.5requires to control emissions of TNO3, and NH4+reduce needs to controlemissions of TNO3and TNH4, and TSO4reduce appeared to be more effective in loweringthe concentrations of SO42-in PM2.5. |
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