| During the past several decades, rapid economic development and urbanization process have led to deterioration of air quality in central eastern China. The fine particle (PM2.5) pollution leads to frequent occurrence of regional haze, which has a negative impact on human health and public safety. Thus it is urgent and important to investigate the characteristics and formation mechanism of atmospheric aerosols in this heavily polluted area to supply basis for the air quality control measurements.In the present study, several intensive or long-term field campaigns were carried out at five representative ground sites and one mountain site (Mt.Tai,1534m a.s.l), which is the highest peak of central eastern China. Multiple pollutants were measured by off-line and online techniques. A large suite of air pollutants data were obtained and analyzed with the aid of many statistical tools and modeling techniques including a backward trajectory model and cluster analysis. Several characteristics of water soluble ions and acidity in PM2.5were investigated, including temporal and spatial variations, source analysis, chemical transformation and transport processes. Another major objective of this study is to investigate the formation of secondary organic aerosol (SOA) under near real ambient atmospheric conditions. Simulation experiments were designed and performed in the University of North Carolina (UNC)270m3dual outdoor smog chamber in order to determine the effects of relative humidity (RH) and seed aerosol on the SOA formation from tolunene/xylens in the presence of an eleven component mixture of non-SOA-forming dilute urban hydrocarbon (HCmix). Condensed SOA formation mechanisms was built and evaluated based on these experiments.The observation of PM2.5was carried out in three continental urban sites (SD, ZB and ZZ, short for Jinan Shanda, Zibo and Zaozhuang), one coastal site (QD, short for Qingdao) and one rural site (MP, short for Jinan Miaopu) during Jan.2006-Feb.2007. More serious aerosol pollution was observed at these sites comparing to other sites over the world. The average concentration of total water soluble ions (TWSI) was 60.74±40.12μg/m3, accounting for about half of the PM2.5mass concentration. SO42-, NO3-and NH4+, as major secondary ions with an average concentration of47.50μg/m3, accounted for38.7%of PM2.5mass concentration. The average concentration of total organic ions was0.42μg/m3, accounting for0.3%of the PM2.5mass concentration. The TWSI concentration at ZZ was the highest among these sites. ZB and SD sites showed lower TWSI concentrations than that at ZZ. The coastal site of QD and the rural site of MP presented a low level of TWSI. The acidity of PM2.5, calculated from the pH of water solution of aerosol samples, showed an average concentration of6.40±7.00nmol/m3, with higher values at the rural site (MP), but relatively lower values at the urban sites, especially in ZB and ZZ. The seasonal variation showed the lowest acidity was in spring at most of the sites except Qingdao, where the acidity was the lowest in summer. Back trajectories analysis demonstrates high level of ions and aerosol acidity was observed when the south or local air masses prevailed and high oxalic acid concentration was obtained in PM2.5when the air masses from south direction dominated.Near real-time measurements of PM2.5ionic compositions were performed at the summit of the highest mountain in the North China Plain in the spring and summer of2007. Concurrently, gas phase species such as O3, SO2and NOy were also measured. The average concentrations of TWSI were27.52and36.65μg/m3in the spring and summer, respectively, indicating serious aerosol pollution in the high altitude over central eastern China. Diurnal patterns of SO42-, NH4+and NO3-showed a broad peak in the afternoon in both campaigns, which is attributed to the upslope/downslope transport of air masses and the development of the planetary boundary layer (PBL). The average SO2oxidation ratio (SOR) in summer was57%, more than twice that in spring (24%). This result indicates strong summertime production of sulfate aerosols. Overall,57.2%and81.3%of the aerosol samples were acidic in spring and summer, respectively. Strong (H+strong) and aerosol acidities (H+air) in summer were higher than those in spring. The diurnal variation of H+strong was coincide with that for SO42-, NO3- and NH4+, while the variation of H+air was consistent with RH and H2Op. Principal component analysis (PCA) showed that transport of pollutants, cloud processing, and crustal source were the main factors affecting the variability of the measured ions (and other trace gases and aerosols) at Mt. Tai. Back trajectory analysis indicated the medium/long-range transport of air mass coming from north/northwest direction, regional transport of air masses from Shanxi and Henan Provinces dominated at Mt. Tai in spring. Serious fine particle pollution was observed when the air masses from Henan and Shanxi or from northwest with high humidity. In summer, Mt. Tai was influenced by the air masses from east/northeast direction which passed through the ocean, from west direction with low altitude and from northwest direction. High level of ions concentration was observed when the air masses were primarily from east/northeast or from south direction.The detailed process of specific events including dust storm, biomass burning and cloud scavenging events were analyzed based on continuous measurements of ions along with other pollutants concentrations. The study on dust event showed a complex mixed pollution of dust and urban air mass with high RH. In this situation, elevated secondary ions concentrations were observed, suggesting the large surface area supplied by dust particles and the particle water (H2OP) enhanced the heterogeneous reactions to produce secondary ions. The ionic composition of aerosol at Mt. Tai was influenced by serious biomass burning in June with high contributions of K+and oxalate. The study of fog and clouds scavenging cases demonstrates the scavenging rate was positively correlated with cloud water content but negatively correlated with PM2.5mass concentrations. The relationship between acidity and secondary organic aerosol formation during daytime in summer was also investigated in this study. Over half of the investigating days, the secondary organic carbon (SOC) increased with high acidity and high H2OP levels, but low photochemical activity, suggesting acidity might enhance the heterogeneous reactions or liquid phase reactions involved in the SOC formation. The cases of rising SOC with low acidity often accompanied with low H2Op but high photochemical activity, suggesting gas phase oxidation dominated the SOC formation.In order to investigate the effects of RH and seed aerosol on secondary organic aerosol (SOA) formation, four aromatic systems were simulated by using UNC outdoor smog chamber, including toluene/o-xylene/p-xylene-NOx-HCmix and mixture of toluene with o-xylene/p-xylene-NOx-HCmix system. Ammonium sulfate particles in the presence of an atmospheric hydrocarbon mixture and NOX in sunlight under a dry atmosphere (RH range:6to10%) showed reduced SO A formation when compared to similar gas phase conditions with lower ammonium sulfate and higher relative humidities (RH range:40to90%). This suggests high ammonium sulfate together with high RH was responsible for the enhanced SOA formation, indicating H2OP maybe the key factor influencing SOA formation. The experiments for the other systems also demonstrate that a particle water phase was highly related to SOA formation. The yield curves generated from a one-product Odum model suggested high SOA formation under wet condition. P-xylene had a lower SOA yield compared with o-xylene and the mixtures of toluene and xylenes. A new condensed aromatic kinetic chemical mechanism employing uptake of organics in H2OP as a key parameter was developed based on the experimental data. The model showed reasonable fits to the observed data. Such information would provide useful information for developing local and regional air quality model.
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