Chemical Composition of PM10 and PM2.5 Collected at Ground Level and 100 Meters during a Strong Winter-Time Pollution Episode in Xi'an, China

Abstract

ABSTRACT An intensive sampling of aerosol particles from ground level and 100 m was conducted during a strong pollution episode during the winter in Xi'an, China. Concentrations of water-soluble inorganic ions, carbonaceous compounds, and trace elements were determined to compare the composition of particulate matter (PM) at the two heights. PM mass concentrations were high at both stations: PM10 (PM with aerodynamic diameter ≤10 μm) exceeded the China National Air Quality Standard Class II value on three occasions, and PM2.5 (PM with aerodynamic diameter ≤2.5 μm) exceeded the daily U.S. National Ambient Air Quality Standard more than 10 times. The PM10 organic carbon (OC) and elemental carbon (EC) were slightly lower at the ground than at 100 m, both in terms of concentration and percentage of total mass, but OC and EC in PM2.5 exhibited the opposite pattern. Major ionic species, such as sulfate and nitrate, showed vertical variations similar to the carbonaceous aerosols. High sulfate concentrations indicated that coal combustion dominated the PM mass both at the ground and 100 m. Correlations between K+ and OC and EC at 100 m imply a strong influence from suburban biomass burning, whereas coal combustion and motor vehicle exhaust had a greater influence on the ground PM. Stable atmospheric conditions apparently led to the accumulation of PM, especially at 100 m, and these conditions contributed to the similarities in PM at the two elevations. Low coefficient of divergence (CD) values reflect the similarities in the composition of the aerosol between sites, but higher CDs for fine particles compared with coarse ones were consistent with the differences in emission sources between the ground and 100 m. IMPLICATIONS An intensive observation of aerosol particles from ground level and 100 m during a winter-time pollution episode in Xi'an, China, showed that PM levels often greatly exceeded national standards. High concentrations of sulfate indicated that emissions from coal combustion dominated the PM mass both at the ground and at 100 m. No dramatic differences in PM or chemical species loadings were found between the different elevations, and this was most likely due to relatively stable meteorological conditions. Nevertheless, the data suggested greater impacts from fresh emissions at the ground site and more particle aggregates and a stronger influence from biomass burning at the higher site. The results imply that monitoring programs need to take this variability into account. The air quality in Xi'an would be improved by reducing the emissions from coal and biomass burning.