Characterization of biomass burning tracers in particulate matter at 12 sites in China: Significant increase of coal combustion emitted levoglucosan in northern China during winter

Biomass burning (BB) is the largest contributor to carbonaceous aerosols globally. Specific organic tracers can track BB particles and identify BB types. At present, there is limited information on the composition of BB tracers on a continental scale. In this study, we conducted year-round sampling of particulate matter (PM) at 12 sites in China. Nine BB tracers were measured in PM with aerodynamic diameters <1.1 μm (PM_<1.1), in the range of 1.1-3.3 μm (PM_1.1-3.3), and > 3.3 μm (PM_>3.3). The annual average concentration of these nine BB tracers (∑₉ BB tracers) in the total PM was 366 ng m^-3 with the majority of levoglucosan (66 %). The concentration of ∑₉ BB tracers was higher in northern China than in southern China, especially in winter. ∑₉ BB tracers were most enriched in PM_<1.1 (50-61 % in mass), followed by PM_1.1-3.3 and PM_>3.3. The highest concentrations of ∑₉ BB tracers were observed in winter, while satellite-recorded fire spots were intensive in autumn and spring. The mismatch of seasonal trends between them indicated that the high levels of BB tracers in winter was not due to open BB. The composition of 4-hydroxybenzoic acid, syringic acid and vanillic acid suggested that the burning of crop residues and softwoods were the major BB types in China. The ratio of levoglucosan to mannosan could neither identify the major BB types in China nor distinguish between BB and coal combustion. Correlation analysis and the PMF model demonstrated that non-BB sources contributed 7 %-58 % to levoglucosan at the 12 sites, with coal combustion being the predominant non-BB source in China, especially in northern urban sites during winter. Our findings suggest that caution should be taken in application of these organic tracers to identify BB types and estimate BB aerosols.