| Biomass combustion has become an important source of PM2.5.5 pollution in atmosphere.Levoglucan is widely used as a biomass combustion marker for the analysis of atmospheric particle sources.Levoglucan and other carbohydrates are water-soluble.They are obviously different from these semi-volatile POPs such as PCBs and PAHs,which are hydrophobic and lipophilic.So levoglucan and other carbohydrates are specifical in sampling and the environmental behaviors.At present,researches of levoglucan and other carbohydrates mainly focus on the particle phase.Few studies on the gas phase have been conducted relatively.And the analytical methods need organic solvent extraction and derivatization,making the analysis process tedious and complex.In addition,the domestic researches on the sources of carbohydrates in PM2.5.5 are mainly concentrated in the key prevention areas,such as Pearl River Delta and Yangtze River Delta.In this paper,taking the problems above into consideration,a rapid analysis method of levoglucan was established through the comparison of sampling and extracting methods.Then the field samples were analyzed by this method.As a result,we have a preliminary understanding of levoglucan levels and its gas/particle partitioning in the atmosphere.In the same time,based on the ratios of characteristic molecular markers and combined with actual conditions,the possible sources of carbohydrates in PM2.5.5 of the typical city in South China were identified.The main research contents and results of this study are as follows:1.The establishment of sampling and analysis methods of levoglucan in the gas and particle phases?1?Comparison of different sampling methods.Three different gas-phase sampling methods were compared:PUF alone?Method 1?,the PXP sandwich?Method 2?and sampling by ultra-pure water?Method 3?.Medium flow samplers?300 L/min?were used in Method 1 and Method 2,and a small flow sampler?01 L/min?was used in Method 3.Limited by the small sampling flow rate or the large mass transfer resistance,the concentrations of gaseous and particle phase levoglucosan in Method 3 were lower than the method detection limit?MDL?.When the concentrations of levoglucosan and other carbohydrates in the particle phase collected by Method 1 and Method 2 were very close?RSD<15%?,their concentrations in the gas phase collected by Method 2 were far higher than that by Method 1,and the gas-phase concentration ratios of Method 2 to Method 1ranged from 4 to 6.Therefore,a medium volume active sampler?300 L/min?with quartz filters?8×10 IN?and PXP sandwiches was used to collect levoglucosan and other carbohydrates in the gas and particle phases.?2?Comparison of different extracting methods.Taking the water solubility of levoglucosan into consideration,three different gas-phase extracting methods were compared:aqueous ultrasonic extraction?Method 1?,aqueous Soxhlet-extraction?Method 2?and back extraction?Method 3?.The chromatogram of Method 2 showed an extremely high baseline,which caused a great interference in the analysis.As for Method 3,there was a high interference peak near the peak of levoglucosan.Method 1 showed good levoglucosan peak effect and the extracting operation is simple.Therefore,the PXP sandwiches spiked at four concentrations?0.01,0.02,0.1 and 1.0 ppm?were used to evaluate the recovery effect of the ultrasonic extraction.The results showed that the recovery rates ranged from 145%to 154%,119%to 123%,103%to 111%and 95.8%to 98.9%,respectively.The RSDs of the parallel samples were less than 5.0%.Therefore,the ultrasonic extraction was used to extract levoglucosan and other carbohydrates in the atmospheric samples.?3?Instrument analysis and detection limit.Carbohydrates are analyzed by HPAEC-PAD with a Dionex ICS-3000 ion chromatograph using CarboPac MA1 analytical and protection columns.The mobile phase is consisted of NaOH solution and deionized water.The instrument detection limit ranges from 1 to 5 ng/mL.The MDL of levoglucosan for gaseous and particle phases was 0.46 ng/m3 and 5.11 ng/m3 with 432 m3 of sampling volume,respectively.The detection limit of this method can quantify the concentration of levoglucosan and other carbohydrates in the atmosphere.2.Primary researches on gas/particle partitioning of levoglucosan in the atmosphereOn the roof of a building in Guangzhou,actual samples of gaseous and particle phases were collected in spring,summer and autumn,and the carbohydrates were analyzed by the established rapid analysis method.The results showed that the concentration of levoglucosan in spring,summer,and autumn was 157 ng/m3,35.0 ng/m3 and 121 ng/m3,respectively.The reasons of the seasonal changes may be that there are some activities such as spring sowing,autumn harvest and straw burning near the sampling site,leading to higher contents of levoglucosan in spring and autumn.And lower wind speed in spring was not conducive to the spread of pollutants,while the high temperature and rainy climate in summer was conducive to the spread and settlement clearing of levoglucosan.Affected by the gas/particle partitioning,it also showed that the content of levoglucosan in the gas phase was higher in summer than that in spring,while levoglucosan was below the detectable limit in autumn.According to Yamasaki's formula,lgKp and 1/T showed good linear correlation in spring and summer.The values of R2 were 0.80 and 0.83?P<0.05?.It indicates that the ambient temperature is one of the main factors affecting gas/particle partitioning of levoglucosan.3.Identify the main sources of carbohydrates in PM2.5.5 of urban citiesDuring the sampling period,the content of total carbohydrates in Qingyuan showed that the highest in winter?629.2 ng/m3?,followed by autumn?528.8 ng/m3?,and the lowest in spring?303.0 ng/m3?.The reasons may be that biomass burning in the rural areas near the sampling site in autumn and winter results in serious pollution.In winter,biomass was used as a fuel for heating and combustion activities were more intense.In addition,we used the valuesoflevoglucosan/mannosan?L/M?andlevoglucosan/?mannosan+galactosan?[L/?M+G?]to analyze the biomass burning sources of the city.It is inferred that the mixture of cork and crop residues are the main biomass combustion sources in PM2.5.5 of Qingyuan. |
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