Day–night differences and seasonal variations of chemical species in PM₁₀ over Xi’an, northwest China

Abstract

To investigate day–night differences and seasonal variations of PM₁₀ and its chemical composition in an urban environment in Xi’an, northwest China, day- and nighttime PM₁₀ mass and its chemical components including water-soluble ions (Na⁺, NH₄ ⁺, K⁺, Mg²⁺, Ca²⁺, F⁻, Cl⁻, NO₃ ⁻, and SO₄ ²⁻), organic carbon (OC), elemental carbon, and water-soluble organic carbon (WSOC) were measured on selected representative days from 20 December 2006 to 12 November 2007. Annual mean PM₁₀ concentration in this city was five times of the China Ambient Air Quality Standard for annual average (70 μg m⁻³). Carbonaceous fractions and water-soluble ions accounted for nearly one third and 12.4 %, respectively, of the annual mean PM₁₀ mass. No dramatic day–night differences were found in the loadings of PM₁₀ or its chemical components. Spring samples were highlighted by abundance of Ca²⁺, while the secondary aerosol species (SO₄ ²⁻, NO₃ ⁻, and NH₄ ⁺) and OC dominated in summer, autumn, and winter samples. Relatively low NO₃ ⁻/SO₄ ²⁻ ratio suggested that stationary source emissions were more important than vehicle emissions in the source areas in this city. Strong relationships between WSOC and biomass markers (water-soluble K⁺, OC1, and OP) were observed in winter and autumn, indicating that WSOC was derived mainly from biomass burning in these seasons. This was also supported by analysis results on the biomass burning events. In contrast, poor correlations between WSOC and biomass markers were demonstrated in summer and spring, implying that WSOC was mainly formed as secondary organic carbon through photochemical activities.