Atmospheric oxalic acid and related secondary organic aerosols inQinghai Lake, a continental background site in Tibet Plateau

Summertime PM2.5 aerosols collected from Qinghai Lake (3200ma.s.l.), a remote continental site in the northeastern part of Tibetan Plateau, were analyzed for dicarboxylic acids (C2-C11), ketocarboxylic acids and &alpha;-dicarbonyals. Oxalic acid (C2) is the dominant dicarboxylic acid in the samples, followed by malonic, succinic and azelaic acids. Total dicarboxylic acids (231&plusmn;119ngm-3), ketocarboxylic acids (8.4&plusmn;4.3ngm-3), and &alpha;-dicarbonyls (2.7&plusmn;2.1ngm-3) at the Tibetan background site are 2-5 times less than those detected in lowland areas such as 14 Chinese megacities. Compared to those in other urban and marine areas enhancements in relative abundances of C2/total diacids and diacids-C/WSOC of the PM2.5 samples suggest that organic aerosols in the region are more oxidized due to strong solar radiation. Molecular compositions and air mass trajectories demonstrate that the above secondary organic aerosols in the Qinghai Lake atmosphere are largely derived from long-range transport. Ratios of oxalic acid, glyoxal and methylglyoxal to levoglucosan in PM2.5 aerosols emitted from household burning of yak dung, a major energy source for Tibetan in the region, are 30-400 times lower than those in the ambient air, which further indicates that primary emission from biomass burning is a negligible source of atmospheric oxalic acid and &alpha;-dicarbonyls at this background site.</p

Comparison of dicarboxylic acids and related compounds in aerosol samples collected in Xi'an, China during haze and clean periods

PM10 aerosols from Xi&#39;an, a mega city of China in winter and summer, 2009 were measured for secondary organic aerosols (SOA) (i.e., dicarboxylic acids (DCA), keto-carboxylic acids, and &alpha;-dicarbonyls), water-soluble organic (WSOC) and inorganic carbon (WSIC), elemental carbon (EC) and organic carbon (OC). Molecular compositions of SOA on haze and clean days in both seasons were compared to investigate their sources and formation mechanisms. DCA in the samples were 1843&plusmn;810ngm-3 in winter and 1259&plusmn;781ngm-3 in summer, respectively, which is similar and even higher than those measured in 2003. Oxalic acid (C2, 1162&plusmn;570ngm-3 in winter and 1907&plusmn;707ngm-3 in summer) is the predominant species of DCA, followed by t-phthalic (tPh) in winter and phthalic (Ph) in summer. Such a molecular composition is different from those in other Asian cities where succinic acid (C4) or malonic acid (C3) is the second highest species, which is mostly due to significant emissions from household combustion of coal and open burning of waste material in Xi&#39;an. Mass ratios of C2/diacids, diacids/WSOC, WSOC/OC and individual diacid-C/WSOC are higher on the haze days than on the clean days in both seasons, suggesting an enhanced SOA production under the haze condition. We also found that the haze samples are acidic while the clean samples are almost neutral. Such a difference in particle acidity is consistent with the enhanced SOA production, because acid-catalysis is an important aqueous-phase formation pathway of SOA. Gly/mGly mass ratio showed higher values on haze days than on clean day in both seasons. We comprehensively investigated the ratio in literature and found a consistent pattern. Based on our observation results and those documented data we proposed for the first time that concentration ratio of Gly/mGly can be taken as an indicator of aerosol ageing.</p

Seasonal characteristics of oxalic acid and related SOA in the free troposphere of Mt. Hua, central China: Implications for sources and formation mechanisms

PM10 aerosols from the summit of Mt. Hua (2060 m a.s.l) in central China during the winter and summer of 2009 were analyzed for dicarboxylic acids, ketocarboxylic acids and alpha-dicarbonyls. Molecular composition of dicarboxylic acids (C-2-C-11) in the free tropospheric aerosols reveals that oxalic acid (C-2, 399 +/- 261 ng m(-3) in winter and 522 +/- 261 ng m(-3) in summer) is the most abundant species in both seasons, followed by malonic (C-3) and succinic (C-4) acids, being consistent with that on ground levels. Most of the diacids are more abundant in summer than in winter, but adipic (C-6) and phthalic (Ph) acids are twice lower in summer, suggesting more significant impact of anthropogenic pollution on the wintertime alpine atmosphere. Moreover, glyoxal (Gly) and methylglyoxal (mGly) are also lower in summer (12 +/- 6.1 ng m(-3)) than in winter (22 +/- 13 ng m(-3)). As both dicarbonyls are a major precursor of C-2, their seasonal variation patterns, which are opposite to those of the diacids, indicate that the mountain troposphere is more oxidative in summer. C-2 showed strong linear correlations with levoglucosan in winter and oxidation products of isoprene and monoterpane in summer. PCA analysis further suggested that the wintertime C-2 and related SOA in the Mt Hua troposphere mostly originate from photochemical oxidations of anthropogenic pollutants emitted from biofuel and coal combustion in lowland regions. On contrast, the summertime C-2 and related SOA mostly originate from further oxidation of the mountainous isoprene and monoterpene oxidation products. The AIM model calculation results showed that oxalic acid concentration well correlated with particle acidity (R-2 = 0.60) but not correlated with particle liquid water content, indicating that particle acidity favors the organic acid formation because aqueous-phase C-2 production is the primary mechanism of C-2 formation in ambient aerosols and is driven by acid-catalyzed oxidation.</p

Airborne particulate organic markers at the summit (2060 m,a.s.l.) of Mt. Hua in central China during winter: Implications for biofuel and coal combustion

Sugars, n-alkanes and PAHs in PM10 and size-segregated samples collected from the summit (2060 m, altitude) of Mt. Hua in Guanzhong Plain, central China during the winter of 2009 were characterized using a GC/MS technique. Concentrations of sugars, n-alkanes and PAHs in PM10 are 107&plusmn;52, 121&plusmn;63, 7.3&plusmn;3.4 ng m&minus;3, respectively. Levoglucosan and fossil fuel derived n-alkanes are more abundant in the air masses transported from southern China than in those from northern China with no spatial difference found for PAHs, suggesting that emissions from biomass burning and vehicle exhausts are more significant in southern part of the country. Dehydrated sugars, fossil fuel derived n-alkanes and PAHs presented a unimode size distribution, peaking at the size of 0.7&ndash;1.1 &mu;m, whereas non-dehydrated sugars and plant wax derived n-alkanes showed a bimodal pattern, peaking at 0.7&ndash;2.1 and 5.8&ndash;9.0 &mu;m, respectively. Principal component analysis showed that biofuel combustion plus plant emission is the most important source in Mt. Hua, being different from the cases in Chinese urban areas where fossil fuel combustion is the major source. By comparison with previous mountain and lowland observations and aircraft measurements we found that wintertime PAHs in China are still characterized by coal burning emissions especially in the inland regions, although in the country increasing rate of SO2 emission from coal combustion has decreased and emissions of vehicle exhaust has sharply increased.</p

Chemical composition and size distribution of wintertime aerosols in the atmosphere of Mt. Hua in central China

TSP, PM(10) and size-segregated aerosols were collected at the summit (2060 m, a.s.l.) of Mt. Hua in central China during the winter of 2009, and determined for organic (OC) and elemental carbon (EC), pH of water-extracts and inorganic ions. OC in TSP and PM(10) are 6.9 +/- 2.9 and 5.9 +/- 2.5 mu g m(-3), while EC in TSP and PM(10) are 0.9 +/- 0.6 and 0.9 +/- 0.5 mu g m(-3), respectively. SO(4)(2-), NO(3)(-), NH(4)(+) and Ca(2)(+) are major ions in PM(10) with concentrations of 5.8 +/- 3.7, 2.7 +/- 1.6, 1.6 +/- 0.9 and 1.5 +/- 0.7 mu g m(-3), respectively. OC/EC ratios (8.2 +/- 3.1 in TSP and 6.6 +/- 1.8 in PM(10)) at the mountaintop are 2-4 times higher than those in lowland surface, suggesting an enhanced transformation of organics from gas- to solid- phase because of an increased photochemical oxidation and/or an increased condensation due to lower temperature, as well as an increased organic input from mountain plant emission. Air mass backward trajectories showed that compared with those derived from north/northwest China aerosols transported from the south contained higher concentrations of SO(4)(2-) and NH(4)(+) and lower concentrations of Ca(2+). Size distributions of NH(4)(+) and K(+) presented as an accumulation mode with a peak at 0.7-1.1 mu m, in contrast to Ca(2+) and Mg(2+), which maximized at the size 4.7-5.8 mu m as a coarse mode. SO(4)(2-) and NO(3)(-) showed a bimodal pattern with a large peak at the range 0.7-1.1 mu m and a small peak at the size of 4.7-5.8 mu m, whereas Na(+) and Cl(-) displayed a bimodal pattern with two equivalent peaks in the fine (&lt; 2.1 mu m) and coarse (&gt;= 2.1 mu m) ranges. pH values of the water-extracts demonstrate that aerosols originate from southern China are more acidic than those from the north/northwest, and the particles with a diameter of 0.7-11 mu m are most acidic.</p

Molecular Distribution and Stable Carbon Isotopic Composition of Dicarboxylic Acids, Ketocarboxylic Acids, and alpha-Dicarbonyls in Size-Resolved Atmospheric Particles From Xi'an City, China

Size-resolved airborne particles (9-stages) in urban Xi&#39;an, China, during summer and winter were measured for molecular distributions and stable carbon isotopic compositions of dicarboxylic acids, ketocarboxylic acids, and alpha-dicarbonyls. To our best knowledge, we report for the first time the size-resolved differences in stable carbon isotopic compositions of diacids and related compounds in continental organic aerosols. High ambient concentrations of terephthalic (tPh, 379 +/- 200 ng m(-3)) and glyoxylic acids (omega C-2, 235 +/- 134 ng m(-3)) in Xi&#39;an aerosols during winter compared to those in other Chinese cities suggest significant emissions from plastic waste burning and coal combustions. Most of the target compounds are enriched in the fine mode (&lt;2.1 mu m) in both seasons peaking at 0.7-2.1 mu m. However, summertime concentrations of malonic (C-3), succinic (C-4), azelaic (C-9), phthalic (Ph), pyruvic (Pyr), 4-oxobutanoic (omega C-4), and 9-oxononanoic (omega C-9) acids, and glyoxal (Gly) in the coarse mode (&gt;2.1 mu m) are comparable to and even higher than those in the fine mode (&lt;2.1 mu m). Stable carbon isotopic compositions of the major organics are higher in winter than in summer, except oxalic acid (C-2), omega C-4, and Ph. delta C-13 of C-2 showed a clear difference in sizes during summer, with higher values in fine mode (ranging from -22.8 parts per thousand to -21.9 parts per thousand) and lower values in coarse mode (-27.1 parts per thousand to -23.6 parts per thousand). The lower delta C-13 of C-2 in coarse particles indicate that coarse mode of the compound originates from evaporation from fine mode and subsequent condensation/adsorption onto pre-existing coarse particles. Positive linear correlations of C-2, sulfate and omega C-2 and their delta C-13 values suggest that omega C-2 is a key intermediate, which is formed in aqueous-phase via photooxidation of precursors (e.g., Gly and Pyr), followed by a further oxidation to produce C-2.</p