Study On Identification Techniques Of Organic Compounds Of PM_2.5 Based On Solid Phase Extraction/Microextraction

Organic aerosol is one of the critical pollutants to involve regional and complex air pollution in Beijing.Daily PM_2.5 samples were collected in Tsinghua University for the measurements of compositions and concentrations,Beijing.This study characterized the temporal variation of aerosol,based on these data and hence various approaches including correlation and cluster analysis,density functional theory calculation,and diagnostic ratio.Further,chemical mechanism of regional aerosol pollution was interpreted.An off-line analysis by solid phase mocroextraction（SPME）coupled with gas chromatography mass spectrometry（GC-MS）was used to determine the particle-phase volatile and semi-volatile aroma compounds in fine aerosol(PM_2.5).Identification of compounds in this study has revealed the remarkable diversity,including oxygenated and nitrated aromatic compounds,some of which have not been previously determinated in airborne analyses.An enhanced analytical procedure to measure identify and quantify 16 EPA priority polycyclic aromatic hydrocarbons（PAHs）in fine aerosol by SPME coupled to gas chromatography tandem mass spectrometry（GC-MS/MS）.A solid-phase extraction（SPE）pretreatment procedure allowing organic acids separated from methyl esters in fine aerosol has been developed.The procedure first separates the organic acids from fatty acid methyl esters（FAMEs）and other nonacid organic compounds by aminopropyl-based SPE catridge,then,quantifies them by GC-MS.The procedure prevents the fatty acids and dimethyl phthalate from being overestimated,and so allows us to accurately quantify the C₄-C₁₁ dicarboxylic acids（DCAs）and the C₈-C₃₀ monocarboxylic acids（MCAs）.Results for the extraction of DCAs,MCAs and AMAs in eluate,and FAMEs in effluate by SAX and NH₂ SPE cartridges exhibited that the NH₂ SPE cartridge gave higher extraction efficiency than the SAX cartridge.Through extraction and cleanup using this procedure,17 aromatic acids in eluate were identified and quantified by gas chromatography-tandem mass spectrometry,including five polycyclic aromatic hydrocarbon（PAH）acids 2-naphthoic,biphenyl-4-carboxylic,9-oxo-9H-fluorene-1-carboxylic,biphenyl-4,4’-dicarboxylic,and phenanthrene-1-carboxylic acid.The resulting correlations between the orgnic acids and FAMEs suggest that the FAMEs had sources similar to those of the carboxylic acids,or were formed by esterifying carboxylic acids,or that aliphatic acids were formed by hydrolyzing FAMEs,and the first three PAH-acids were secondary atmospheric photochemistry products,the last two mainly primary.It was noteworthy that nitro-phenols were close related with the concentration of NO₂ and the RH of the atmosphere,which we suggested heterogeneous electrophilic mononitration occur at ortho-or para-positions of phenols at aqueous phase instead of hydroxylation of nitro-phenols based on the hydrogen chemical shift of nuclear magnetic resonance.The density functional theory calculation was performed to determine part of the photochemical oxidation and hydroxylation products of PAHs.A mechanism of votile and semivoltile organic compounds oxidization and hydroxylation was proposed in this paper,which we suggested that the oxygenated PAHs were more favourable for OH radical attachment,while the naphthol homologues could be formed from the hydroxylation of naphthalene with OH radicals,and several hydroxylated PAHs were isomerization products of oxygenated PAHs via hydrogen/charge transfer.The unsaturated n-alkenoic acids are unstable and susceptible to be oxidized and degraded to DCAs and aldehyde by free radicals,ozone and other oxidants in the atmosphere with the degree of aerosols aging.The PAHs were more aboudant at tetra-and penta-ring,the oxygenated PAHs were aboudant at tri-and tetra-ring,which discharged from fossil fuel burning and motor vehicle.The distributions of n-alkanes have the conventional higher plant pattern of high carbon preference index（CPI）and odd-numbered carbon dominance at C₂₃-C₃₄,rather than the even-numbered predominance at C₁₃-C₂₂.The fatty alcohols,which emitted from higher plants,have even-numbered predominan.The emissions of chemical additives from anthropogenic sources were serious in this period,including phthalic acid esters and chemical antioxidants.