Gehui Wang

Formation of Urban Fine Particulate Matter

Urban air pollution represents one of the greatest environmental challenges facing mankind in the 21st century. Noticeably, many developing countries, such as China and India, have experienced severe air pollution because of their fast-developing economy and urbanization. Globally, the urbanization trend is projected to continue: 70% of the world population will reside in urban centers by 2050, and there will exist 41 megacities (with more than 10 million inhabitants) by 2030. Air pollutants consist of a complex combination of gases and particulate matter (PM). In particular, fine PM (particles with the aerodynamic diameter smaller than 2.5 μm or PM_(2.5)) profoundly impacts nhuman health, visibility, the ecosystem, the weather, and the climate, and these PM effects are largely dependent on the aerosol properties, including the number concentration, size, and chemical composition. PM is emitted directly into the atmosphere (primary) or formed in the atmosphere through ngas-to-particle conversion (secondary) (Figure 1). Also, nprimary and secondary PM undergoes chemical and physical ntransformations and is subjected to transport, cloud processing, and removal from the atmosphere.

Persistent sulfate formation from London Fog to Chinese haze

Significance Exceedingly high levels of fine particulate matter (PM) occur frequently in China, but the mechanism of severe haze formation remains unclear. From atmospheric measurements in two Chinese megacities and laboratory experiments, we show that the oxidation of SO2 by NO2 occurs efficiently in aqueous media under two polluted conditions: first, during the formation of the 1952 London Fog via in-cloud oxidation; and second, on fine PM with NH3 neutralization during severe haze in China. We suggest that effective haze mitigation is achievable by intervening in the sulfate formation process with NH3 and NO2 emission control measures. Hence, our results explain the outstanding sulfur problem during the historic London Fog formation and elucidate the chemical mechanism of severe haze in China. Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

Winter and Summer PM2.5 Chemical Compositions in Fourteen Chinese Cities

PM2.5 in 14 of Chinas large cities achieves high concentrations in both winter and summer with averages >100 μg m−3 being common occurrences. A grand average of 115 μg m−3 was found for all cities, with a minimum of 27 μg m−3 measured at Qingdao during summer and a maximum of 356 μg m−3 at Xian during winter. Both primary and secondary PM2.5 are important contributors at all of the cities and during both winter and summer. While ammonium sulfate is a large contributor during both seasons, ammonium nitrate contributions are much larger during winter. Lead levels are still high in several cities, reaching an average of 1.68 μg m−3 in Xian. High correlations of lead with arsenic and sulfate concentrations indicate that much of it derives from coal combustion, rather than leaded fuels, which were phased out by calendar year 2000. Although limited fugitive dust markers were available, scaling of iron by its ratios in source profiles shows ∼20% of PM2.5 deriving from fugitive dust in most of the cities. Multipollutant control strategies will be needed that address incomplete combustion of coal and biomass, engine exhaust, and fugitive dust, as well as sulfur dioxide, oxides of nitrogen, and ammonia gaseous precursors for ammonium sulfate and ammonium nitrate. Implications: PM2.5 mass and chemical composition show large contributions from carbon, sulfate, nitrate, ammonium, and fugitive dust during winter and summer and across fourteen large cities. Multipollutant control strategies will be needed that address both primary PM2.5 emissions and gaseous precursors to attain Chinas recently adopted PM2.5 national air quality standards.

Chemical characterization of water-soluble components of PM10 and PM2.5 atmospheric aerosols in five locations of Nanjing, China

Abstract A comprehensive survey for atmospheric pollution was carried out from February to December 2001 in Nanjing city in order to better control the problem. As part of the work, the current study mainly aims at the chemical characterization of water-soluble species of PM10 and PM2.5 atmospheric aerosols, together with their spatial variations. Much heavier particle loadings is observed in Nanjing city, which is more than 4–6 times of NAAQS of the USA, especially fine particles. During the sampling time, 63–77% of PM10 mass is in the PM2.5 fraction. Water-soluble fractions of PM10 and PM2.5 are acidic, and the acidity of PM2.5 is stronger than that of PM10. Water-soluble organic carbon, accounting for about 10% of particle mass, is the most significant component of the water-soluble fraction and shows no clear spatial variations. NO3−, SO42−, NHx (ammonia and ammonium), Ca2+, K+ and Na+ are also abundant in PM10 and PM2.5 aerosols. Among the detected water-soluble inorganic chemicals SO42− presented the first highest level of concentration in PM10 (16.70–23.51xa0μg/m3, 3.3–10.9% of the PM10 mass) and in PM2.5 (13.19–20.24xa0μg/m3, 3.8–11.2% of the PM2.5 mass), NO3− and NHx are the second highest level of chemicals followed by Ca2+, K+ and Na+. Nearly all those water-soluble inorganic components displayed the higher level of concentrations in the traffic center (SY) and the downtown area (FZ), and presented the lower level of concentrations in the scenery district and the residential area. The total mass of water-soluble fraction, including inorganic species and organic chemicals, also presented the higher concentrations (78.97–108.68xa0μg/m3, 15.7–42.9% of particle mass) at both sites of SY and FZ.

Organic molecular compositions and temporal variations of summertime mountain aerosols over Mt. Tai, North China Plain

[1]xa0Total suspended particles (TSP) were collected at the summit of Mt. Tai (1534 m above sea level) on a daytime and nighttime basis during a summertime campaign (May–June 2006) and were characterized for organic molecular compositions using solvent extraction/derivatization and gas chromatography/mass spectrometry technique. The n-Alkanes, fatty acids, fatty alcohols, sugars, glycerol and polyacids, and phthalate esters were found as major organic compound classes, whereas lignin and resin products, sterols, aromatic acids, hopanes, and polycyclic aromatic hydrocarbons (PAHs) were detected as minor classes. Sugars (49.8–2115 ng m−3, average 640 ng m−3 in daytime; 18.1–4348 ng m−3, 799 ng m−3 in nighttime) were found to be the dominant compound class. Levoglucosan, a specific cellulose pyrolysis product, was detected as the most abundant single compound, followed by C28 fatty alcohol, diisobutyl and di-n-butyl phthalates, C29n-alkane, C16 and C28 fatty acids, and malic acid. By grouping organic compounds based on their sources, we found that emission of terrestrial plant waxes was the most significant source (30–34%) of the TSP, followed by biomass burning products (25–27%) (e.g., levoglucosan and lignin and resin products), soil resuspension (15–18%) due to agricultural activities, secondary oxidation products (8–10%), plastic emission (3–10%), marine/microbial sources (6%), and urban/industrial emissions from fossil fuel use (4%). However, low molecular weight dicarboxylic acids (such as oxalic acid) of photochemical origin were not included in this study. Malic acid was found to be much higher than those reported in the ground level, suggesting an enhanced photochemical production in the free troposphere over mountain areas. Temporal variations of biomass burning tracers (e.g., levoglucosan, galactosan, mannosan) and some higher plant wax derived compound classes suggested that there were two major (E1 and E2) and one minor (E3) biomass-burning events during this campaign. Most of the compound classes showed higher concentrations in nighttime samples when organic aerosols can be long-range transported from different source regions to the summit of Mt. Tai above the planetary boundary layer (PBL). This study also demonstrates that the free troposphere over Mt. Tai is heavily influenced by field burning of agricultural wastes such as wheat straws in the North China Plain during the harvest season in early summer.

Characterization of water-soluble species of PM10 and PM2.5 aerosols in urban area in Nanjing, China

The characterization for water-soluble species of PM10 (particle matter with aerodynamical diameter <10xa0μm) and PM2.5 (particle matter with aerodynamical diameter <2.5xa0μm) in five sites of Nanjing, China was carried out during February–May 2001.The pH and conductivity K of water-soluble matters of PM10 and PM2.5 were determined, and the water-soluble fraction of the sample was followed to identify the total carbon (TC), total organic carbon (TOC), inorganic carbon (IC), elements, NO3−, SO42− and NH3-N.The experimental results show that water-soluble matters of PM10 and PM2.5 in Nanjing are acidic, and the pH of PM2.5 is lower than PM10. Conductivity of water-soluble species of PM10 and PM2.5 aerosols varied over a wide range from 1087 to 225xa0μs/cm. Conductivity between PM10 and PM2.5 has a linear correlationship, and the equation is Y=0.8459X+44.74, r2=0.9376 (Y: conductivity of PM2.5, X: conductivity of PM10). TOC make up the majority of TC and accounts for 3.17–14.13% of PM10 and/or PM2.5 loadings, while IC only accounts for 0.12–0.47% of PM10 and/or PM2.5 mass. Al, As, Ba, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Ti, V and Zn, 17 elements were detected in water-soluble matters of PM10 and/or PM2.5. Ca, K and Na are the most abundant chemical components, which account for more than 95% of total water-soluble elements (TWSE). Of all the five sites, TWSE accounts for 1.80–6.13% of the particle mass and 61.28–72.73% of TWSE of PM10 is enriched in fine particles (<2.5xa0μm in diameter). Nitrate (NO3−), sulfate (SO42−), ammonia and ammonium (NH3-N) were determined. The highest level of nitrate was 15.49xa0μg/m3 for PM10 and 12.66xa0μg/m3 for PM2.5 at site FZ. As was the case for nitrate, the highest level of sulfate was also presented at the same site, which was 28.22xa0μg/m3 for PM10 and 21.48xa0μg/m3 for PM2.5. However, a higher level of ammonia and ammonium was presented at site ZS, which was 36.05xa0μg/m3 for PM10 and 22.06xa0μg/m3 for PM2.5.

Identification of dicarboxylic acids and aldehydes of PM10 and PM2.5 aerosols in Nanjing, China

Abstract In this study aerosol samples of PM10 and PM2.5 collected from 18 February 2001 to 1 May 2001 in Nanjing, China were analyzed for their water-soluble organic compounds. A series of homologous dicarboxylic acids (C 2–10 ) and two kinds of aldehydes (methylglyoxal and 2-oxo-malonaldehyde) were detected by GC and GC/MS. Among the identified compounds, the concentration of oxalic acid was the highest at all the five sites, which ranged from 178 to 1423xa0ng/m 3 . The second highest concentration of dicarboxylic acids were malonic and succinic acids, which ranged from 26.9 to 243xa0ng/m 3 . Higher level of azelaic acid was also observed, of which the maximum was 301xa0ng/m 3 . As the highest fraction of dicarboxylic acids, oxalic acid comprised from 28% to 86% of total dicarboxylic acids in PM10 and from 41% to 65% of total dicarboxylic acids in PM2.5. The dicarboxylic acids (C 2 , C 3 , C 4 ) together accounted for 38–95% of total dicarboxylic acids in PM10 and 59–87% of dicarboxylic acids in PM2.5. In this study, the total dicarboxylic acids accounted for 2.8–7.9% of total organic carbon (TOC) of water-soluble matters for PM10 and 3.4–11.8% of TOC for PM2.5. All dicarboxylic acids detected in this study together accounted for about 1% of particle mass. The concentration of azelaic acid was higher at one site than others, which may be resulted from higher level of volatile fat used for cooking. The amounts of dicarboxyic acids (C 2,3,4,9 ) and 2-oxo-malonaldehyde of PM2.5 were higher in winter and lower in spring. Compared with other major metropolitans in the world, the level of oxalic acid concentration of Nanjing is much higher, which may be contributed to higher level of particle loadings, especially for fine particles.

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

Size-resolved airborne particles (9-stages) in urban Xian, China, during summer and winter were measured for molecular distributions and stable carbon isotopic compositions of dicarboxylic acids, ketocarboxylic acids, and α-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 (ωC(2), 235 ± 134 ng m(-3)) in Xian 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 (<2.1 μm) in both seasons peaking at 0.7-2.1 μm. However, summertime concentrations of malonic (C(3)), succinic (C(4)), azelaic (C(9)), phthalic (Ph), pyruvic (Pyr), 4-oxobutanoic (ωC(4)), and 9-oxononanoic (ωC(9)) acids, and glyoxal (Gly) in the coarse mode (>2.1 μm) are comparable to and even higher than those in the fine mode (<2.1 μm). Stable carbon isotopic compositions of the major organics are higher in winter than in summer, except oxalic acid (C(2)), ωC(4), and Ph. δ(13)C of C(2) showed a clear difference in sizes during summer, with higher values in fine mode (ranging from -22.8‰ to -21.9‰) and lower values in coarse mode (-27.1‰ to -23.6‰). The lower δ(13)C 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 ωC(2) and their δ(13)C values suggest that ωC(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).

Mixing State of Black Carbon Aerosol in a Heavily Polluted Urban Area of China: Implications for Light Absorption Enhancement

Black carbon (BC) is important for climate forcing, and its effects on the Earths radiative balance remain a major uncertainty in climate models. In this study, we investigated the mixing state of refractory black carbon (rBC) and aerosol optical properties in a polluted atmosphere at Xi’an, western China. The average rBC mass concentration was 9.9 μg m−3 during polluted periods, 7.6 times higher than that in clean periods. About 48.6% of the rBC was internally-mixed or coated with nonrefractory materials during polluted periods; this was 27% higher than in clean periods. Correlation analysis between the number fraction of thickly-coated rBC particles (fBC) and the major particulate species indicate that organics may be the primary contributor to rBC coatings during polluted periods. The average mass absorption cross section of rBC (MACBC) particles at λ = 870 nm was 7.6 ± 0.02 m2 g−1 for the entire campaign. The MACBC showed a positive correlation with fBC, and the enhancement of MACBC due to internal mixing was 1.8 times. These observations suggest that an enhancement of BC absorption by a factor of ∼2 could be appropriate for climate models associated with high PM2.5 levels. Copyright 2014 American Association for Aerosol Research

The Campaign on Atmospheric Aerosol Research Network of China: CARE-China

AbstractBased on a network of field stations belonging to the Chinese Academy of Sciences (CAS), the Campaign on Atmospheric Aerosol Research network of China (CARE-China) was recently established as the country’s first monitoring network for the study of the spatiotemporal distribution of aerosol physical characteristics, chemical components, and optical properties, as well as aerosol gaseous precursors. The network comprises 36 stations in total and adopts a unified approach in terms of the instrumentation, experimental standards, and data specifications. This ongoing project is intended to provide an integrated research platform to monitor online PM2.5 concentrations, nine-size aerosol concentrations and chemical component distributions, nine-size secondary organic aerosol (SOA) component distributions, gaseous precursor concentrations (including SO2, NOx, CO, O3, and VOCs), and aerosol optical properties. The data will be used to identify the sources of regional aerosols, the relative contributions fr...