Seasonal and spatial variability of the OM/OC mass ratios and high regional correlation between oxalic acid and zinc in Chinese urban organic aerosols

We calculated the organic matter to organic carbon mass ratios (OM/OC mass ratios) in PM2.5 collected from 14 Chinese cities during summer and winter of 2003 and analyzed the causes for their seasonal and spatial variability. The OM/OC mass ratios were calculated two ways. Using a mass balance method, the calculated OM/OC mass ratios averaged 1.92+/-0.39 year-round, with no significant seasonal or spatial variation. The second calculation was based on chemical species analyses of the organic compounds extracted from the PM2.5 samples using dichloromethane/methanol and water. The calculated OM/OC mass ratio in summer was relatively high (1.75+/-0.13) and spatially-invariant due to vigorous photochemistry and secondary organic aerosol (OA) production throughout the country. The calculated OM/OC mass ratio in winter (1.59+/-0.18) was significantly lower than that in summer, with lower values in northern cities (1.51+/-0.07) than in southern cities (1.65+/-0.15). This likely reflects the wider usage of coal for heating purposes in northern China in winter, in contrast to the larger contributions from biofuel and biomass burning in southern China in winter. On average, organic matter constituted 36 % and 34 % of Chinese urban PM2.5 mass in summer and winter, respectively. We report, for the first time, a high regional correlation between Zn and oxalic acid in Chinese urban aerosols in summer. This is consistent with the formation of stable Zn oxalate complex in the aerosol phase previously proposed by Furukawa and Takahashi (2011). We found that many other dicarboxylic acids were also highly correlated with Zn in the summer Chinese urban aerosol samples, suggesting that they may also form stable organic complexes with Zn. Such formation may have profound implications for the atmospheric abundance and hygroscopic properties of aerosol dicarboxylic acids.</p

Characteristics and sources of carbonaceous aerosols from Shanghai, China

An intensive investigation of carbonaceous PM2.5 and TSP (total suspended particles) from Pudong (China) was conducted as part of the MIRAGE-Shanghai (Megacities Impact on Regional and Global Environment) experiment in 2009. Data for organic and elemental carbon (OC and EC), organic species, including C17 to C40 n-alkanes and 17 polycyclic aromatic hydrocarbons (PAHs), and stable carbon isotopes OC (delta C-13(OC)) and EC (delta C-13(EC)) were used to evaluate the aerosols&#39; temporal variations and identify presumptive sources. High OC/EC ratios indicated a large fraction of secondary organic aerosol (SOA); high char/soot ratios indicated stronger contributions to EC from motor vehicles and coal combustion than biomass burning. Diagnostic ratios of PAHs indicated that much of the SOA was produced via coal combustion. Isotope abundances (delta C-13(OC) = -24.5 +/- 0.8 parts per thousand and delta C-13(EC) = -25.1 +/- 0.6 parts per thousand) indicated that fossil fuels were the most important source for carbonaceous PM2.5 (particulate matter less than 2.5 micrometers in diameter), with lesser impacts from biomass burning and natural sources. An EC tracer system and isotope mass balance calculations showed that the relative contributions to total carbon from coal combustion, motor vehicle exhaust, and SOA were 41%, 21%, and 31%; other primary sources such as marine, soil and biogenic emissions contributed 7%. Combined analyses of OC and EC, n-alkanes and PAHs, and stable carbon isotopes provide a new way to apportion the sources of carbonaceous particles.</p

Characteristics of fine particulate non-polar organic compounds in Guangzhou during the 16th Asian Games: Effectiveness of air pollution controls

The concentrations of organic compounds, including n-alkanes and polycyclic aromatic hydrocarbons (PAHs), in fine particles (PM2.5) were measured in an urban area of Guangzhou before, during, and after the 16th Asian Games (9-30 November 2010). Higher average concentrations of n-alkanes and PAHs occurred in the nighttime, presumably due to a restriction on motor vehicle operation during the day and the accumulation of pollutants due to temperature inversions at night. The carbon preference index, contributions of wax n-alkanes, and PAHs diagnostic ratios indicated that the main sources of n-alkanes and PAHs were anthropogenic, especially motor vehicle emissions. The CMAQ model was used to estimate the percentages of biogenic organic aerosol in the total organic aerosol, then the ratios of source marker PAHs to biogenic organic carbon were used to evaluate the air pollution control policies effectiveness during the Asian Games.</p

Lead concentrations in fine particulate matter after the phasing out of leaded gasoline in Xi’an, China

Daily concentrations of lead (Pb) were determined for PM2.5 samples collected from an urban location in Xi&rsquo;an, China from 2007 to 2009 to assess the effects of the phasing out of leaded gasoline in 2000. The Pb concentrations (annual average: 0.306 &mu;g m&minus;3, range: below detection limit to 2.631 &mu;g m&minus;3) have declined after the phasing out of leaded gasoline, but the concentrations were still higher than those reported in many other cities. Seasonal variations of Pb were significant, with high concentrations in winter, presumably due to the burning of coal, and low concentrations in summer, due to a deep mixed layer and scavenging of aerosols by precipitation. Correlation analyses and enrichment factor calculations both indicated that anthropogenic sources had a large influence on atmospheric Pb. The lead isotope ratios were low in winter (the average 207Pb/206Pb ratio was 0.843 &plusmn; 0.032; 208Pb/206Pb was 1.908 &plusmn; 0.058) and high in summer (207Pb/206Pb was 0.860 &plusmn; 0.032; 208Pb/206Pb was 2.039 &plusmn; 0.057), suggesting that coal combustion was the major Pb source in winter and vehicular emission was the major Pb source in summer. Positive Matrix Factorization receptor model indicated that there were five major sources for Pb in PM2.5. Coal combustion was the major contributor, accounting for 39.0% PM2.5 mass, followed by vehicular emissions (30.4%). Other contributors included 17.8% from industrial emissions, 11.6% from biomass burning, and 1.2% from fugitive dust.</p

Water-soluble ions in atmospheric aerosols measured in Xi'an, China: Seasonal variations and sources

Daily PM(2.5) and water-soluble inorganic ions (Na(+), NH(4)(+), K(+), Mg(2+), Ca(2+), Cl(-), NO(3)(-) and SO(4)(2-)) were collected in Xi&#39;an (34.23 degrees N, 108.88 degrees E), China from March 2006 to March 2007. PM(2.5) was collected using battery-powered mini-volume samplers. And the ions were determined by ion chromatography from the measured aerosol mass. The annual average mass concentration of PM(2.5) was found to be 194.1 +/- 78.6 mu g m(-3), which exceeded substantially the international guidelines for health concerns. The seasonal average mass concentration of PM(2.5) was highest in winter (266.8 mu g m(-3)) and lowest in summer (138.6 mu g m(-3)). The three highest abundant ions were SO(4)(2-), NO(3)(-), and NH(4)(-), with average concentrations of 35.6 +/- 19.5 mu g m(-3), 16.4 +/- 10.1 mu g m(-3), and 11.4 6.8 mu g m(-3), which were accounted for 18.7%, 8.0%, and 5.7% of the PM(2.5) mass, respectively. The major ions were in the species of (NH(4))(2)SO(4), NH(4)HSO(4) and NH(4)NO(3), and their concentrations were highest in winter, due to high coal combustion. The concentrations of Ca(2+) were higher in spring than other seasons, due to the higher mineral dust concentrations. Ca(2+) was strongly correlated with CO(3)(2-), which was calculated as the difference in the measured cations minus anions. Ion balance calculations indicate that the PM(2.5) was acidic, and this result is consistent with the measurement of pH values. Sulfur oxidation ratio was higher in summer and autumn, which implies that the formation of secondary sulfate-rich particles is favored by warm and relatively moist weather. Nitrogen oxidation ratio was highest in autumn.</p

Observation of atmospheric aerosols at Mt. Hua and Mt. Tai in central and east China during spring 2009-Part 2: Impact of dust storm on organic aerosol composition and size distribution

PM10 and size-resolved particles (9-stage) were simultaneously collected at Mt. Hua and Mt. Tai in central and east China during the spring of 2009 including a massive dust storm occurring on 24 April (named as DS II), and determined for organic compounds to investigate the impact of dust storm on organic aerosols. High molecular weight (HMW) n-alkanes, fatty acids, and fatty alcohols and trehalose sharply increased and almost entirely stayed in coarse particles when dust storm was present, suggesting that high level of organic aerosols in the mountain atmospheres during the event largely originated from Gobi desert plants. However, most anthropogenic aerosols (e.g. PAHs, and aromatic and dicarboxylic acids) during the event significantly decreased due to a dilution effect, indicating that anthropogenic aerosols in the mountain atmospheres during the nonevent period largely originated from local/regional sources rather than from long-range transport. Trehalose, a metabolism product enriched in biota in dry conditions, was 62 +/- 78 and 421 +/- 181 ng m(-3) at Mt. Hua and Mt. Tai during DS II, 10-30 times higher than that in the nonevent time, indicating that trehalose may be a tracer for dust emissions from Gobi desert regions. Molecular compositions of organic aerosols in the mountain samples demonstrate that domestic coal burning is still the major source of PAHs in China. n-Alkanes and fatty acids showed a bimodal size distribution during the nonevent with a major peak in fine mode (&lt; 2.1 mu m) and a small peak in coarse mode (&gt; 2.1 mu m). The coarse mode significantly increased and even dominated over the whole size range when dust was present. Glucose and trehalose were also dominant in the coarse mode especially in the DS II time. PAHs and levoglucosan concentrated in fine particles with no significant changes in size distribution when dust storm occurred. However, phthalic and succinic acids showed bimodal size distribution pattern with an increase in coarse mode during the event, because both are formed via a gas phase oxidation and a subsequent condensation/adsorption onto aerosol phase. In contrast, terephthalic and malic acids are mostly emitted from combustion process as fine particles, thus both showed a fine mode pattern during the whole campaign with a minor peak in coarse mode caused by an increased coagulation with dust during the event. Geometric mean diameters (GMDs) of the organic aerosols above are in general larger at Mt.similar to Hua than at Mt. Tai during the nonevent period. We found that during the event GMD of the fine mode organics that derived mostly from the local/regional sources rather than Gobi desert became smaller while GMD of them in coarse mode became larger. Such a splitting in sizes during the event is most likely caused by decreased fine particle coagulation due to dilution and increased adsorption/coagulation with dust.</p

Evolution of aerosol chemistry in Xi'an, inland China, during the dust storm period of 2013-Part 1: Sources, chemical forms and formation mechanisms of nitrate and sulfate

A total suspended particulate (TSP) sample was collected hourly in Xi&#39;an, an inland megacity of China near the Loess Plateau, during a dust storm event of 2013 (9 March 18: 00-12 March 10: 00 LT), along with a size-resolved aerosol sampling and an online measurement of PM2.5. The TSP and size-resolved samples were determined for elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC) and nitrogen (WSON), inorganic ions and elements to investigate chemistry evolution of dust particles. Hourly concentrations of Cl-, NO3-, SO42-, Na+ and Ca2+ in the TSP samples reached up to 34, 12, 180, 72 and 28 mu g m(-3), respectively, when dust peak arrived over Xi&#39;an. Chemical compositions of the TSP samples showed that during the whole observation period NH4+ and NO3- were linearly correlated with each other (r(2) = 0.76) with a molar ratio of 1 : 1, while SO42- and Cl- were well correlated with Na+, Ca2+, Mg2+ and K+ (r(2) &gt; 0.85). Size distributions of NH4+ and NO3- presented a same pattern, which dominated in the coarse mode (&gt;2.1 mu m) during the event and predominated in the fine mode (&lt;2.1 mu m) during the non-event. SO42- and Cl- also dominated in the coarse mode during the event hours, but both exhibited two equivalent peaks in both the fine and the coarse modes during the non-event, due to the fine-mode accumulations of secondarily produced SO42- and biomass-burning-emitted Cl- and the coarse-mode enrichments of urban soil-derived SO42- and Cl-. Linear fit regression analysis further indicated that SO42- and Cl- in the dust samples possibly exist as Na2SO4, CaSO4 and NaCl, which directly originated from Gobi desert surface soil, while NH4+ and NO3- in the dust samples exist as NH4NO3. We propose a mechanism to explain these observations in which aqueous phase of dust particle surface is formed via uptake of water vapor by hygroscopic salts such as Na2SO4 and NaCl, followed by heterogeneous formation of nitrate on the liquid phase and subsequent absorption of ammonia. Our data indicate that 54 +/- 20% and 60 +/- 23% of NH4+ and NO3- during the dust period were secondarily produced via this pathway, with the remaining derived from the Gobi desert and Loess Plateau, while SO42- in the event almost entirely originated from the desert regions. Such cases are different from those in the East Asian continental outflow region, where during Asia dust storm events SO42- is secondarily produced and concentrates in sub-micrometer particles as (NH4)(2)SO4 and/or NH4HSO4. To the best of our knowledge, the current work for the first time revealed an infant state of the East Asian dust ageing process in the regions near the source, which is helpful for researchers to understand the panorama of East Asian dust ageing process from the desert area to the downwind region.</p