| Biomass burning, which includes forest fires, savanna fires, agricultural wasteburnings, and domestic biofuel combustions, is one of the major sources of airborneparticulate matter (PM) and gaseous pollutants world-wide. Biomass burning hasgreat potential impact on regional air quality, the earth’s radiative budget, ecosystemand human health. In addition to local and regional scale effects, biomass burningplumes can be transported over long distances, exerting an influence on the globalenvironment.Studies on the influences of biomass burning in china,are still quite limited. Thepublished results about the contribution of biomass burning are mainly for Beijing andGuangzhou, and the data on the influence of biomass burning is few in Shanghai.Besides, the organic matters in the atmosphere are easily degraded in summer due tothe high temperature and strong solar radiation. Large uncertainties may exist in theestimated contribution of biomass burning obtained by conventional methods, andevaluation on the reliability of the results is needed.To better understand the influence of biomass burning on the air quality in thesummer of Shanghai,50PM2.5samples were collected with high-volume samplers atfive sites, Shanghai Academy of Environmental Sciences (SAES), Songjiang (SJ),Fengxian (FX), Chongming (CM) and Huinan of Pudong (HN), in this study. Thesampling period was from July11to20in2012at SAES, SJ, FX and CM, but July22to31in2012at HN. The chemical compositions of water-soluble ions andorganic matters in the samples were analyzed with ion chromatography and gaschromatography-mass spectrometry. The contribution of biomass burning toatmospheric fine particles in the summer of Shanghai were estimated with tracerbased emission factors. The results from the two conventional methods werecompared and evaluated based on the analysis of the advantages and disadvantagesof the methods. The influence on the estimated contribution of biomass burningbrought by the photo-chemical degradation of the organic tracer was evaluated.The main conclusions of this study are listed as below: 1) The analysis results of levoglucosan (LG) using the high-performanceanion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD)showed good agreement with that using gas chromatography-mass spectrometry(GC-MS), but the HPAEC-PAD method suffered from the co-elution of differenthydrocarbons and was incompetence for the summer airborne particulates samplesthat were low in concentration and very complicated.2) The highest average concentration of levoglucosan in summer was found atSAES, and the lowest at HN, but the differences in concentration was small amongSAES, SJ, FX, and CM, indicating that biomass burning had the regional features inShanghai. The averaged concentration of levoglucosan at the sites excluding HN was44.2ng/m3. Comparing with other regions in China, the concentration oflevoglucosan in summer airborne particulates in Shanghai was low.3) The average concentrations of water-soluble potassium was the highest at SJ,and the lowest at HN, FX, CM and SAES had concentrations in between in sequence.The spatial distribution of K+was different from that of levoglucosan, which partlyshowed the disparities of sources. The averaged concentration of non-sea-salt K+atsites except HN was0.45μg/m3, lower than other regions in China.4) The LG/nss-K+ratio had significant negative correlation with ozone,showing that levoglucosan endured obvious photochemical degradation underincreased atmospheric oxidative capacity.5) The spatial distribution of n-alkanes, hopanes, polycyclic aromatichydrocarbons (PAHs) and water-soluble secondary ions (SO42-, NO3-and NH4+, SNA)in the atmospheric fine particles in summer sampling period were in line with that oflevoglucosan and nss-K+, showing good spatial uniformity or regional features. Theaveraged concentration of n-alkanes, hopanes, PAHs and SNA at the other four sitesexcluding HN was23.4ng/m3,0.4ng/m3,7.1ng/m3and21.7μg/m3respectively. Thecompositional analysis indicated that the n-alkanes, were from mixed contributionsof plant wax and fossil fuel combustion, and the ubiquitous existence of hopanesshowed the universality of vehicle exhaust pollution.6) The tracers of biomass burning (levoglucosan and nss-K+) had good correlation with alkanes (n-alkanes and hopanes) and PAHs which indicated that theemission structure of pollutants was stable during the sampling period andthe meteorological conditions was the major factor controlling the pollution level.Biomass burning might be an important source of the n-alkanes and PAHs in summer.The correlations between biomass burning tracers and secondary ionswere significantly weaker than that of primary pollutants. Meanwhile, the biomassburning tracers had stronger correlations with NO3-than that with SO42-, and thedifferences could be attributed to the different chemical properties and formationpathways of SO42-and NO3-. Biomass burning may be an important source of NO3-.7) The estimated average contribution of biomass burning to organic carbon(OC) in PM2.5in summer at SAES, SJ, FX and CM were5.7%,5.5%,5.5%and5.2%respectively, and that to PM2.5mass were2.0%,2.6%,1.5%and1.6%respectively when using measured levoglucosan as the typical tracer. Two cases thatthe mineral dust could be from construction dust and soil dust were considered whenK+was used as a tracer to estimate the contribution of biomass burning, and no bigdifferences were found. The average contributions of biomass burning to OC atSAES, SJ, FX and CM were13.7%,21.3%,16.3%and20.7%, and that were4.4%,11.6%,4.4%and5.8%to PM2.5mass, respectively. The estimated contribution usingpotassium as the tracer was significantly higher than that using levoglucosan.8) After the concentrations of levoglucosan were corrected for photochemicaldegradation based on the correlation between LG/nss-K+and O3and the measuredO3concentrations, the contributions of biomass burning estimated by levoglucosandoubled approximately at all sites, and the estimated contributions at SAES and FXwere very close with that obtained using K+as the tracer, but the contributions aftercorrection at SJ and CM still had vast disparity with that obtained by using K+as thetracer, indicating the existence of significant uncertainties in the current methods. |
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