Chemical composition and source characterization of spring aerosol over Horqin sand land in northeastern China

Abstract

In spring 2005, daily particulate matter (PM 2.5 ) aerosol samples were collected at Tongliao, a site in the Horqin sand land of northeastern China. The concentrations of 20 elements, 9 water‐soluble ions, and elemental and organic carbon (EC and OC, respectively) were determined in the filter samples. Crustal material was the major contributor to the PM 2.5 mass, but rural biomass burning and local urban pollution also influenced the composition of the aerosol. The mean PM 2.5 mass concentration was 126 ± 71 μ g m −3 (arithmetic mean ± standard deviation), with higher loadings during five dust storms (DS, 255 ± 80 μ g m −3 ) than for normal days (ND, 104 ± 43 μ g m −3 ) or pollution episodes (PE, 118 ± 52 μ g m −3 ). During the DS, crustal material accounted for 69% of the PM 2.5 mass, followed by carbonaceous matter (14%), sulfate (4%), nitrate (2%), ammonium (1%), and chloride (1%). The observed Si/Al, Ca/Al, and Fe/Al ratios during the DS events were different from those in dust from western or central/northern Asia. On normal days the percentage of crustal material decreased to 43%, and the mass of carbonaceous matter increased 2 times over that during DS. During the pollution episodes the contributions of sulfate and nitrate were 3 times those on DS while ammonium increased four‐fold. Secondary aerosols (NH 4 + , SO 4 2− , and NO 3 − ) were the dominant species during the pollution episodes, but SO 4 2− and NO 3 − also were important components of the aerosol during DS events, suggesting that mineral dust was mixed with other materials. Ion balance calculations indicate that the DS samples were alkaline, the ND samples were weakly alkaline, and the PE samples were slightly acidic. A deficit of measured anions during DS implied the presence of carbonate; this evidently accounts for ∼5.5% of the PM 2.5 mass. The average OC and EC concentrations were 16.3 ± 7.3 μ g m −3 and 3.4 ± 1.7 μ g m −3 , respectively. Noncrustal K was correlated with OC and EC, indicating that biomass burning was a major contributor to the regional carbonaceous aerosol.