Day-night differences, seasonal variations and source apportionment of PM₁₀-bound PAHs over Xi'an, Northwest China

Day-night PM₁₀-bound PAHs were studied at an urban site of Xi'an from 20 December 2006 to 28 October 2007. The annual mean concentration of nighttime PAHs (285.0 ng m^-3) was higher than that in daytime (239.4 ng m^-3). A significant difference of PAH concentrations between daytime and nighttime was found in autumn with a coefficient of divergence (CD) of 0.23 (significant level 0.2). However, no distinct difference was observed in other seasons (with CD values < 0.2), although the difference of PAHs partition capacity in PM₁₀ between daytime and nighttime was significant in the four seasons. Remarkable seasonal variations were observed in the total PAH levels, with a highest mean concentration of 344.6 ng m^-3 in winter and a lowest mean concentration of 177 ng m^-3 in summer. Positive matrix factorization results revealed that residential emission for heating is the major contributor of the elevated PAH levels in winter, accounting for 49% of the total PAH levels. The coal combustion including industrial and residential usage, contributed over 40% of the PAH emissions in PM₁₀ of Xi'an during the one-year sampling period. These results can provide guidance for taking measures in reducing PAHs levels in the air.