High abundances of water-soluble dicarboxylic acids, ketocarboxylic acids and α-dicarbonyls in the mountaintop aerosols over the North China Plain during wheat burning season

Aerosol (TSP) samples were collected at the summit of Mount Tai (elevation: 1534 m a.s.l., 36.25 N, 117.10 E) located in the North China Plain using a high- volume air sampler and pre-combusted quartz filters. Sam- pling was conducted on day/night or 3 h basis in the pe- riod from 29 May to 28 June 2006 during the field burn- ing of wheat straw residue and the post-burning season. The filter samples were analyzed for low-molecular-weight di- carboxylic acids, ketoacids and -dicarbonyls using capil- lary gas chromatography (GC) and GC-MS employing wa- ter extraction and butyl ester derivatization. Molecular dis- tributions of dicarboxylic acids (C 2-C11, 220-6070 ng m 3 ) were characterized by a predominance of oxalic (C2) acid (105-3920 ng m 3 ) followed by succinic (C4) or malonic (C3) acid. Unsaturated aliphatic diacids, including maleic (M), isomaleic (iM) and fumaric (F) acids, were also de- tected together with aromatic diacids (phthalic, isophthalic and terephthalic acids). !-oxocarboxylic acids (C2-C9, 24- 610 ng m 3 ) were detected as the second most abundant compound class with the predominance of glyoxylic acid (11-360 ng m 3 ), followed by -ketoacid (pyruvic acid, 3- 140 ng m 3 ) and -dicarbonyls (glyoxal, 1-230 ng m 3 and methylglyoxal, 2-120 ng m 3 ). We found that these lev- els (>6000 ng m 3 for diacids) are several times higher than those reported in Chinese megacities at ground levels. The concentrations of diacids increased from late May to early June, showing a maximum on 7 June, and then sig- nificantly decreased during the period 8-11 June, when the wind direction shifted from southerly to northerly. Similar temporal trends were found for ketocarboxylic acids and - dicarbonyls as well as total carbon (TC) and water-soluble organic carbon (WSOC). The temporal variations of water- soluble organics were interpreted by the direct emission from the field burning of agricultural wastes (wheat straw) in the North China Plain and the subsequent photochemical oxida- tion of volatile and semi-volatile organic precursors emitted from field burning as well as dark ozonolysis of volatile or- ganic compounds and other organics, accretion reactions and oxidation of nonvolatile organics such as unsaturated fatty acids. This study demonstrates that the field burning of agri- cultural wastes in early summer strongly influenced the air quality of the free troposphere over the North China Plain.