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

Day-night differences and seasonal variations of chemical species in PM10 over Xi'an, northwest China

To investigate day-night differences and seasonal variations of PM10 and its chemical composition in an urban environment in Xi&#39;an, northwest China, day- and nighttime PM10 mass and its chemical components including water-soluble ions (Na+, NH4 (+), K+, Mg2+, Ca2+, F-, Cl-, NO3 (-), and SO4 (2-)), organic carbon (OC), elemental carbon, and water-soluble organic carbon (WSOC) were measured on selected representative days from 20 December 2006 to 12 November 2007. Annual mean PM10 concentration in this city was five times of the China Ambient Air Quality Standard for annual average (70 mu g m(-3)). Carbonaceous fractions and water-soluble ions accounted for nearly one third and 12.4 %, respectively, of the annual mean PM10 mass. No dramatic day-night differences were found in the loadings of PM10 or its chemical components. Spring samples were highlighted by abundance of Ca2+, while the secondary aerosol species (SO4 (2-), NO3 (-), and NH4 (+)) and OC dominated in summer, autumn, and winter samples. Relatively low NO3 (-)/SO4 (2-) ratio suggested that stationary source emissions were more important than vehicle emissions in the source areas in this city. Strong relationships between WSOC and biomass markers (water-soluble K+, OC1, and OP) were observed in winter and autumn, indicating that WSOC was derived mainly from biomass burning in these seasons. This was also supported by analysis results on the biomass burning events. In contrast, poor correlations between WSOC and biomass markers were demonstrated in summer and spring, implying that WSOC was mainly formed as secondary organic carbon through photochemical activities.</p

Characteristics of surface O-3 over Qinghai Lake area in Northeast Tibetan Plateau, China

Surface 03 was monitored continuously during Aug. 12, 2010 to Jul. 21, 2011 at a high elevation site (3200 m above sea level) in Qinghai Lake area (36 58&#39;37 &#39;&#39; N, 99 degrees 53&#39;56 &#39;&#39; E) in Northeast Tibetan Plateau, China. Daily average O-3 ranged from 21.8 ppbv to 653 ppbv with an annual average of 41.0 ppbv. Seasonal average of O-3 followed a decreasing order of summer &gt; autumn &gt; spring &gt; winter. Diurnal variations of O-3 showed low concentrations during daytime and high concentrations during late night and early morning. An intensive campaign was also conducted during Aug. 13-31, 2010 to investigate correlations between meteorological or chemical conditions and O-3. It was found that O-3 was poorly correlated with solar radiation due to the insufficient NOx in the ambient air, thus limiting O-3 formation under strong solar radiation. In contrast, high O-3 levels always coincided with strong winds, suggesting that stratospheric O-3 and long range transport might be the main sources of O-3 in this rural area. Back-trajectory analysis supported this hypothesis and further indicated the transport of air masses from northwest, northeast and southeast directions.</p

Chemical composition, sources, and deposition fluxes of water-soluble inorganic ions obtained from precipitation chemistry measurements collected at an urban site in northwest China

Precipitation samples were collected at an urban site in Xi&#39;an, northwest China during March to November in 2009 and were then analyzed to determine the pH and concentrations of water-soluble inorganic ions (Na(+), NH(4)(+), K(+), Mg(2+), Ca(2+), SO(4)(2-), NO(3)(-), Cl(-), and F(-)) in precipitation. The pH of precipitation ranged from 4.1 to 7.6 for all of the samples with an annual volume-weighted mean of 6.4. While a large portion of the precipitation events were weakly acidic or alkaline, around 30% of the precipitation events in the autumn were strongly acidic. Precipitation events with air masses from the northeast and the southeast were weakly acidic while those with air masses from the northwest and the southwest were alkaline. SO(4)(2-), Ca(2+), NH(4)(+), and NO(3)(-) were dominant ions in the precipitation, accounting for 37%, 25%, 18%, and 9%, respectively, of the total analyzed ions. Ca(2+) and NH(4)(+) were found to be the major neutralizers of precipitation acidity; however, the contribution of Mg(2+), although much lower than those of Ca(2+) and NH(4)(+), was important, in many cases, in changing the precipitation from weakly acidic to weakly alkaline. Enrichment factor analysis confirmed that SO(4)(2-) and NO(3)(-) were produced from anthropogenic sources, Ca(2+), K(+), and 80% Mg(2+) were from crustal sources, and Na(+), Cl(-), and &sim;20% of Mg(2+) were from marine sources. The annual wet depositions were estimated to be 3.5 t km(-2) per year for sulfur; 2.3 t km(-2) per year for nitrogen, of which 0.8 t km(-2) per year was oxidized nitrogen and 1.5 t km(-2) per year was reduced nitrogen; and 3.0 t km(-2) per year for Ca(2+).</p

Comparison and implications of PM2.5 carbon fractionsin different environments

The concentrations of PM2.5 carbon fractions in rural, urban, tunnel and remote environments were measured using the IMPROVE thermal optical reflectance (TOR) method. The highest OC1 and EC1 concentrations were found for tunnel samples, while the highest OC2, OC3, and OC4 concentrations were observed for urban winter samples, respectively. The lowest levels of most carbon fractions were found for remote samples. The percentage contributions of carbon fractions to total carbon (TC) were characterized by one peak (at rural and remote sites) and two peaks (at urban and tunnel sites) with different carbon fractions, respectively. The abundance of char in tunnel and urban environments was observed, which might partly be due to traffic-related tire-wear. Various percentages of optically scattering OC and absorbing EC fractions to TC were found in the four different environments. In addition, the contribution of heating carbon fractions (char and soot) indicated various warming effects per unit mass of TC. The ratios of OC/EC and char/soot at the sites were shown to be source indicators. The investigation of carbon fractions at different sites may provide some information for improving model parameters in estimating their radiative effects.</p

Physicochemical characteristics of black carbonaerosol and its radiative impact in a pollutedurban area of China

Black carbon (BC) aerosol plays an important role in the Earth&rsquo;s radiative balance. An intensive measurement campaign was conducted at Xi&rsquo;an, China, from December 2012 to January 2013 to investigate the sources and physicochemical characteristics of refractory BC (rBC) and its direct radiative forcing at the surface. The overall average rBC concentration for the campaign was 8.0 &plusmn; 7.1 &mu;g m 3. Source apportionment based on positive matrix factorization showed that traffic was the dominant rBC source (46.0%), followed by coal burning (33.9%) and biomass burning (20.1%). The rBC mass size distributions were monomodal and lognormal with larger mass median diameters for coal burning source (215 nm) compared with the traffic source (189 nm). Coal burning rBC was more strongly associated with sulfate than traffic rBC, suggesting a higher cloud condensation nuclei activity. The slope of a robust linear regression between rBC and carbon monoxide (CO) for all samples was 5.9 &mu;g m 3 ppm 1, and the slope for the coal burning source (4.5 &mu;g m 3 ppm 1) was larger than that for the traffic source (2.7 &mu;g m 3 ppm 1). The net rBC emission during winter of 2009 was estimated to be 4.5 Gg based on the relationship between rBC and CO. A Tropospheric Ultraviolet and Visible radiation model showed that the average daytime value for the clear-sky direct radiative forcing due to rBC from 23 December 2012 to 31 January 2013 was 47.7 &plusmn; 28.9 W m 2, which amounted to an average of 45.7% of the total surface atmospheric aerosol forcing.</p