Characteristics and sources of formic, acetic and oxalic acids in PM_2.5 and PM₁₀ aerosols in Beijing, China

Chemistry of formic, acetic and oxalic acids was studied at four sites representing the urban and rural conditions in Beijing from March 2002 to October 2003. The investigation was based on the PM_2.5 and PM₁₀ aerosols collected with virtual samplers. The total concentrations of these carboxylic acids averaged at 541 ng m^{- 3} in PM_2.5 and 615 ng m^{- 3} in PM₁₀, contributing 0.4% and 0.3% to the total mass of the aerosol, respectively. Oxalic acid was the most abundant carboxylic acids in aerosols. Formic and acetic acids displayed different seasonal variations (formic: spring < summer < autumn < winter; acetic: spring > summer > autumn > winter), and the variations of these acids were consistent among different sites in urban area. Formic and oxalic acids had a diurnal variation of nighttime < daytime. Formic and acetic acids had mass both in the fine and in the coarse modes, while oxalic acid predominated in the fine mode. The coarse mode fraction of these acids was elevated in summer. The traffic/dust/soil/vegetation emissions, coal/waste/biomass burnings, cooking and secondary formation from anthropogenic or natural gas-phase precursors could be the major sources of these acids. Acetic-to-formic acid ratio (A/F) was used to distinguish the primary sources and the secondary sources, and it indicated that the contribution of the primary sources was higher at rural site than at urban sites. A new method was developed to study the contribution of the biomass burning to these acids, which was estimated to be 30-60% for formic and oxalic acids in aerosols.