Aerosol optical properties based on ground measurements over the Chinese Yangtze Delta Region

The characteristics of Aerosol Optical Depth (AOD) and Angstrom exponent were analyzed and compared using Cimel sunphotometer data from 2007 to 2008 at five sites located in the Yangtze River Delta region of China. The simultaneous measurements between Lin’an and ZFU showed a very high consistency of AOD at all wavelengths. The differences are less than 0.02 for Angstrom exponent and AOD at all wavelengths. The mean values of AOD at 440 nm at the Pudong, Taihu and Lin’an were about 0.74 ± 0.43, 0.85 ± 0.46, and 0.89 ± 0.46, respectively. The mean values of Angstrom exponents were about 1.27 ± 0.30, 1.20 ± 0.28 and 1.32 ± 0.35, respectively. The variation of monthly averaged AOD over Pudong showed a single peak distribution, with the maximum value occurring in July (AOD_440nm 1.26 ± 0.61) and minimum in January (AOD_440nm 0.50 ± 0.27). However, the variations of monthly averaged AOD at Taihu and Lin’an showed a bi-modal distribution. There were peak values of AOD occurring in July (AOD_440nm 1.41 ± 0.49) and September (AOD_440nm 1.22 ± 0.52) for Taihu. For Lin’an, the two peak values of AOD occurred in June (AOD_440nm 1.17 ± 0.69) and September (AOD_440nm 1.28 ± 0.46). The AOD accumulated mainly between 0.30–0.90(68%), 0.30–1.20(75%) and 0.30–1.20 (～75%) at Pudong, Taihu, and Lin’an, respectively. The Angstrom exponent accumulated mainly between 1.10–1.60 (75%), 1.10–1.50 (63%) and 1.20–1.60, 50% (50%) at Pudong, Taihu, and Lin’an, respectively.The synchronized observation showed that the AOD at Pudong was larger than those at Dongtan by 0.03, 0.03, 0.04, 0.07, and 0.08 at wavelengths of 1020 nm, 870 nm, 670 nm, 500 nm and 440 nm, respectively. The synchronized observations at Pudong, Taihu and Lin’an showed that the three stations had high level AOD with means at 440 nm about 0.68, 0.73, and 0.78, respectively. The relationship between MODIS retrieved and ground-based measured AOD shows good agreement with R² ranging from 0.68 to 0.79 at Pudong, Taihu, Lin’an and Dongtan. The MODIS results were overestimated comparing the ground measurements at Pudong, Taihu, and Dongtan but exceptional at Lin’an.The analysis results between aerosol optical properties and wind measurement at Pudong showed that the wind speed from the east correlates with the lower observed AOD. The back trajectory analysis indicates that more than 50% airmasses were from the marine area at Pudong, while back trajectories distribution is relatively homogeneous at Lin’an.     

Optical and microphysical properties of severe haze and smoke aerosol measured by integrated remote sensing techniques in Gwangju,Korea

Aerosol optical and microphysical parameters from severe haze events observed in October 2005 at Gwangju, Korea (35.10°N, 126.53°E) were determined from the ground using a multi-wavelength Raman lidar, a sunphotometer, and a real-time carbon particle analyzer and from space using satellite retrievals. Two different aerosol types were identified based on the variability of optical characteristics for different air mass conditions. Retrievals of microphysical properties of the haze from the Raman lidar indicated distinct light-absorbing characteristics for different haze aerosols originating from eastern and northern China (haze) and eastern Siberia (forest-fire smoke). The haze transported from the west showed moderately higher absorbing characteristics (SSA = 0.90 ± 0.03, 532 nm) than from the northern direction (SSA = 0.96 ± 0.02). The organic/elemental carbon (OC/EC) ratio varied between 2.5 ± 0.4 and 4.1 ± 0.7.     

Aerosol radiative forcing over the Indo-Gangetic plains during major dust storms

Indo-Gangetic (IG) alluvial plains, one of the largest river basins in the world, suffers from the long range transport of mineral dust from the western arid and desert regions of Africa, Arabia and Rajasthan during the summer (pre-monsoon season, April–June). These dust storms influence the aerosol optical depth (AOD) across the IG plains. The Kanpur AERONET (Aerosol Robotic Network) station and Moderate Resolution Imaging Spectro-radiometer (MODIS) data show pronounced effect on the aerosol optical properties and aerosol size distribution during major dust storm events over the IG plains that have significant effect on the aerosol radiative forcing (ARF). The multi-band AOD, from AERONET and MODIS, show contrasting changes in wavelength dependency over dust affected regions. A time collocated (±30 min) validation of AERONET AOD with MODIS Terra (level 2 swath product) over Kanpur, at a common wavelength of 550 nm for the period 2001–2005 show moderate correlation (R²∼0.6) during the summer season. The average surface forcing is found to change by −23 W m⁻² during dust events and the top of the atmosphere (TOA) forcing change by −11 W m⁻² as compared to the non-dusty clear-sky days. A strong correlation is found between AOD at 500 nm and the ARF. At surface, the correlation coefficient between AOD and ARF is found to be high (R²=0.925) and is found to be moderate (R²=0.628) at the TOA. The slope of the regression line gives the aerosol forcing efficiency at 500 nm of about −46±2.6 W m⁻² and −17±2.5 W m⁻² at the surface and the TOA, respectively. The ARF is found to increase with the advance of the dry season in conjunction with the gradual rise in AOD (at 500 nm) from April (0.4–0.5) to June (0.6–0.7) over the IG plains.     

Atmospheric turbidity over Kathmandu valley

The atmosphere of Kathmandu Valley has been investigated by using Sunphotometer and Nephelometer during the pre-monsoon period of 1999. The atmospheric turbidity parameters (extinction coefficient for 500 nm wavelength τ_AG and Angstrom coefficient β) are found high in the morning and show decreasing trends from morning to late afternoon on average. Vertical dispersion of pollutants and increasing pollutant flushing rate by increasing wind speed from morning to late afternoon is the cause for this decreasing trend of turbidity over the valley. Being surrounded by high hills all around the valley, horizontal exit of pollutants without vertical dispersion is not possible. The scattering coefficient b_scat of aerosols in ground level troposphere is also found high in the morning, which decreases and becomes minimum during afternoon. During late afternoon, b_scat again shows a slightly increasing trend. The reason is the increasing vehicular emission during late afternoon rush period. The average values of Angstrom exponent α, β, τ_AG and b_scat are found to be 0.624±0.023, 0.299±0.009, 0.602±0.022 and 0.353±0.014 km⁻¹, respectively. About 76.8% of the observed values of β lie above 0.2 indicating heavy particulate pollution in the valley. A comparison of observed values of turbidity parameters with other major cities of the world shows that Kathmandu is as polluted as cities like Jakarta, Kansas, Beijing, Vienna, etc.     

Aerosol scattering properties in northern China

The aerosol scattering properties were investigated at two continental sites in northern China in 2004. Aerosol light scattering coefficient (σ_sp) at 525 nm, PM₁₀, and aerosol mass scattering efficiencies (α) at Dunhuang had a mean value of 165.1±148.8 M m⁻¹, 157.6±270.0 μg m⁻³, and 2.30±3.41 m² g⁻¹, respectively, while these values at Dongsheng were, respectively, 180.2±151.9 M m⁻¹, 119.0±112.9 μg m⁻³, and 1.87±1.41 m² g⁻¹. There existed a seasonal variability of aerosol scattering properties. In spring, at Dunhuang PM₁₀, σ_sp, and α were 184.1±211.548 μg m⁻³, 126.3±89.6 M m⁻¹, and 1.05±0.97 m² g⁻¹, respectively, and these values at Dongsheng were 146.4±142.1 μg m⁻³, 183.4±81.7 M m⁻¹, and 1.98±1.52 m² g⁻¹, respectively. However, in winter at Dunhuang PM₁₀, σ_sp, and α were 158.1±261.4 μg m⁻³, 303.3±165.2 M m⁻¹, and 3.17±1.93 m² g⁻¹, respectively, and these values at Dongsheng were 155.7±170.1 μg m⁻³, 304.4±158.1 M m⁻¹, and 2.90±1.72 m² g⁻¹, respectively. σ_sp and α in winter were higher than that in spring at both the sites, which coincides with the characteristics of dust aerosol and pollution aerosol. Overall, the dominant aerosol types in spring and winter at both sites in northern China are dust aerosol and pollution aerosol, respectively.     

Ambient sulfur dioxide,nitrogen dioxide,and ammonia at ten background and rural sites in China during 2007–2008

We present two years (January 2007–December 2008) of atmospheric SO₂, NO₂ and NH₃ measurements from ten background or rural sites in nine provinces in China. The measurements were made on a monthly basis using passive samplers under careful quality control. The results show large geographical and seasonal variations in the concentrations of these gases. The mean SO₂ concentration varied from 0.7 ± 0.4 ppb at Waliguan on Qinghai Plateau to 67.3 ± 31.1 ppb at Kaili in Guizhou province. The mean NO₂ concentration ranged from 0.6 ± 0.4 ppb at Waliguan to 23.9 ± 6.9 ppb at Houma in southern Shanxi. The mean NH₃ concentration ranged from 2.8 ± 3.0 ppb at Shangdianzi in northeastern Beijing to 13.7 ± 8.4 ppb at Houma. At most sites, SO₂ and NO₂ peaked in winter and reached minima in summer, while NH₃ showed maximum values in summer and lower values in cold seasons. On the whole, the geographical distributions of the observed gas concentrations are consistent with those of emissions. The ground measurements of SO₂ and NO₂ are contrasted to the SCIAMACHY SO₂ and OMI NO₂ tropospheric columns, respectively. Although the satellite data can capture the main features of emissions and concentrations of SO₂, they do not reflect the variations of SO₂ in the surface layer. The situation is better for the case of NO₂. The OMI NO₂ columns capture the geographical differences in the ground NO₂ and correlate fairly well with the ground levels of NO₂ at six of the ten sites.     

Arctic haze and clouds observed by lidar during four winter seasons of 1993–1997, at Eureka,Canada

A Mie-scattering-polarized Haze Lidar was installed at Eureka, Canada (80°N, 86°W), in February 1993 and has been used to observe the arctic haze during winter seasons from 1993–94 to 1996–97. Although it is difficult to distinguish between the arctic haze and other scattering objects (mainly clouds) under the specific conditions, two methods were used to classify those objects into two groups. First the vertical profiles of the arctic haze and clouds were compared with meteorological data observed by a radiosonde, and the relations between the scattering and depolarization ratios and the meteorological data were investigated. Then, since the arctic haze had very stable layers, the time dependency of the correlation coefficient between the different vertical profiles for the arctic haze and clouds was investigated. After the scattering objects were classified into two groups (the arctic haze and the clouds), the scattering ratio, the depolarization ratio, and the occurrence probabilities of them were investigated statistically. The statistical results, from the observations over the last four winter seasons, indicate that the average values of the scattering and depolarization ratios of the arctic haze were respectively 1.27 and 1.34%. The average humidity over ice of the layer containing the arctic haze was 42±19% (cloud: 92±13%). The arctic haze was observed at altitudes less than 3 km frequently and at altitudes of 3–5 km occasionally.     

Variability in ultraviolet total optical depth during the Southern California Ozone Study (SCOS97)

Formation of photochemical air pollution is governed in part by the solar ultraviolet actinic radiation flux, but wavelength-resolved measurements of UV radiation in polluted urban atmospheres are rarely available. As part of the 1997 Southern California Ozone Study, cosine weighted solar irradiance was measured continuously at seven UV wavelengths (300, 306, 312, 318, 326, 333 and 368 nm) at two sites during the period 1 July to 1 November 1997. The first site was at Riverside (260 m a.s.l.) in the Los Angeles metropolitan area, which frequently experiences severe air pollution episodes. The second site was at Mt Wilson (1725 m a.s.l.), approximately 70 km northwest of Riverside, and located above much of the urban haze layer. Measurements of direct (i.e., total minus diffuse) solar irradiance were used to compute total atmospheric optical depths. At 300 nm, optical depths (mean±1 S.D.) measured over the entire study period were 4.3±0.3 at Riverside and 3.7±0.2 at Mt Wilson. Optical depth decreased with increasing wavelength, falling at 368 nm to values of 0.8±0.2 at Riverside and 0.5±0.1 at Mt Wilson. At all wavelengths, both the mean and the relative standard deviation of optical depths were larger at Riverside than at Mt Wilson. At 300 nm, the difference between the smallest and largest observed optical depths corresponds to over a factor 2 increase in the direct beam irradiance for overhead sun, and over a factor 7 increase for a solar zenith angle of 60°. Principal component analysis was used to reveal underlying factors contributing to variability in optical depths. PCA showed that a single factor (component) was responsible for the major part of the variability. At Riverside, the first component was responsible for 97% of the variability and the second component for 2%. At Mt Wilson, 89% of the variability could be attributed to the first component and 10% to the second. Dependence of the component contributions on wavelength allowed identification of probable physical causes: the first component is linked to light scattering and absorption by atmospheric aerosols, and the second component is linked to light absorption by ozone. These factors are expected to contribute to temporal and spatial variability in solar actinic flux and photodissociation rates of species including ozone, nitrogen dioxide, and formaldehyde.     

Temporal variations of atmospheric carbonyls in urban ambient air and street canyons of a Mountainous city in Southwest China

Carbonyl compounds in urban ambient air and street canyons were measured from December 2008 to August 2009 in a mountainous city in southwest China (Guiyang). The formaldehyde yield from the photo-oxidation of isoprene emitted by vegetation was estimated to be in the range of 0.63–3.62 μg m^?3 from May to August, which accounted for 28.8–33.4% of ambient formaldehyde. Based on the calculation of photolysis rates and rates of reaction with the OH radical, it was found that photolysis was the predominant sink for formaldehyde and acetone in both summer and winter. For acetaldehyde, photo-oxidation by OH radicals and photolysis were the major sinks in summer while photo-oxidation by OH radicals was the dominant sink in winter. Wet precipitation was found to be an important removal process for the atmospheric carbonyls. In the urban ambient air, the average concentrations of formaldehyde, acetaldehyde, acetone and all carbonyls were 4.8 ± 2.1, 5.7 ± 3.3, 5.1 ± 2.5, and 25.1 ± 9.2 μg m^?3 (n = 139), respectively. The average concentrations of these species in street canyons were 18.8 ± 6.5, 9.4 ± 3.2, 10.9 ± 2.1, and 64.1 ± 16.3 μg m^?3 (n = 62), respectively. The significantly higher carbonyl levels on weekdays (compared to weekends) highlight the contribution of vehicle emissions to carbonyls in the street canyons.     

Atmospheric nitrogen fluxes at the Belgian coast: 2004–2006

Daily and seasonal variations in dry and wet atmospheric nitrogen fluxes have been studied during four campaigns between 2004 and 2006 at a coastal site of the Southern North Sea at De Haan (Belgium) located at coordinates of 51.1723° N and 3.0369° E. Concentrations of inorganic N-compounds were determined in the gaseous phase, size-segregated aerosol (coarse, medium, and fine), and rainwater samples. Dissolved organic nitrogen (DON) was quantified in rainwater. The daily variations in N-fluxes of compounds were evaluated with air-mass backward trajectories, classified into the main air-masses arriving at the sampling site (i.e., continental, North Sea, and Atlantic/UK/Channel).The three, non-episodic campaigns showed broadly consistent fluxes, but during the late summer campaign exceptionally high episodic N-deposition was observed. The average dry and wet fluxes for non-episodic campaigns amounted to 2.6 and 4.0 mg N m^?2 d^?1, respectively, whereas during the episodic late summer period these fluxes were as high as 5.2 and 6.2 mg N m^?2 d^?1, respectively.Non-episodic seasons/campaigns experienced average aerosol fluxes of 0.9–1.4 mg N m^?2 d^?1. Generally, the contribution of aerosol NH₄⁺ was more significant in the medium and fine particulate fractions than that of aerosol NO₃^?, whereas the latter contributed more in the coarse fraction, especially in continental air-masses. During the dry mid-summer campaign, the DON contributed considerably (～15%) to the total N-budget.Exceptionally high episodic aerosol-N inputs have been observed for the late summer campaign, with especially high deposition rates of 3.6 and 2.9 mg N m^?2 d^?1 for Atlantic/UK/Channel and North Sea-continental (mixed) air-masses, respectively. During this pollution episode, the flux of NH₄⁺ was dominating in each aerosol fraction/air-mass, except for coarse continental aerosols. High deposition of gaseous-N was also observed in this campaign with an average total N-flux of 2–2.5-times higher than in other campaigns.     

Surface ozone background in the United States: Canadian and Mexican pollution influences

We use a global chemical transport model (GEOS-Chem) with 1° × 1° horizontal resolution to quantify the effects of anthropogenic emissions from Canada, Mexico, and outside North America on daily maximum 8-hour average ozone concentrations in US surface air. Simulations for summer 2001 indicate mean North American and US background concentrations of 26 ± 8 ppb and 30 ± 8 ppb, as obtained by eliminating anthropogenic emissions in North America vs. in the US only. The US background never exceeds 60 ppb in the model. The Canadian and Mexican pollution enhancement averages 3 ± 4 ppb in the US in summer but can be occasionally much higher in downwind regions of the northeast and southwest, peaking at 33 ppb in upstate New York (on a day with 75 ppb total ozone) and 18 ppb in southern California (on a day with 68 ppb total ozone). The model is successful in reproducing the observed variability of ozone in these regions, including the occurrence and magnitude of high-ozone episodes influenced by transboundary pollution. We find that exceedances of the 75 ppb US air quality standard in eastern Michigan, western New York, New Jersey, and southern California are often associated with Canadian and Mexican pollution enhancements in excess of 10 ppb. Sensitivity simulations with 2020 emission projections suggest that Canadian pollution influence in the Northeast US will become comparable in magnitude to that from domestic power plants.     

Ozone and UV-B variations at Ispra from 1993 to 1997

An analysis of the variability of the total ozone column at Ispra (Italy) has been performed to ascertain if, even in a short-time interval of 5 years (1993–1997), a decline of the monthly mean ozone values could be demonstrated. A linear fit of the data displays a decrease of 0.21% per year with a mean value equal to 319±2 D.U. and an amplitude of the annual cycle of about 10% of the mean. A linear regression of the surface monthly mean ozone values has also been performed showing a decreasing trend (−1% per year) that could contribute, even if for a very small amount, to the decline of the total ozone values. Ispra monthly mean total ozone data have been compared with those of three stations located within 2° latitude and 3° longitude from Ispra (Haute Provence, Hohenpeissenberg and Arosa). A linear fit of the data shows some discrepancies in the ozone changes, which can be attributed to the limited length of the observational period.An analysis has been performed to verify if the variation of ozone at Ispra is in agreement with that of the solar UV measured at a wavelength (305 nm) where the ozone absorption is still remarkable. The results, taken at a fixed solar zenith angle of 68°, show a clear anticorrelation between the monthly mean values of UV and the corresponding values of the total ozone column; the linear fit of the UV data displays an increase of 2.0% per year, much higher than expected from the ozone decrease, and a mean value of 1.4±0.1 mW m^-2 nm^-1.     

Optical properties of Asian dusts in the free atmosphere measured by Raman lidar at Taipei,Taiwan

The optical properties (extinction-to-backscatter ratio, backscattering, depolarization, and backscatter-related Angstrom exponent) and height distribution of Asian dusts were measured using a two-wavelength Raman/depolarization lidar at Taipei, Taiwan, during the Asian dust seasons in 2004 and 2005. Dust layers were frequently observed in the free atmosphere (1–6 km). Dust optical thickness ranged from 0.01 to 0.55; backscatter-related Angstrom exponents ranged from 0.42 to 1.47; and lidar ratios (extinction-to-backscatter ratio) for 355 nm ranged from 32 to 72 sr (steradian). The mean values of dust particle depolarization and extinction coefficient are $14\pm 6\%$ and $0.16{\mathrm{km}}^{-1}$, respectively, which are close to the moderate dust depolarizations and extinctions observed in free atmosphere in China and Japan. Backscatter-related Angstrom exponents were found correlated positively with lidar ratio and negatively with particle depolarization, indicating that the dust optical characteristics are predominated by size distribution. Dusts were found to tend to exhibit unusual low depolarization properties under moist conditions (relative humidity $\mathrm{RH}>70\%$), and the possible explanations are discussed.     

A quantitative comparison of α-Å turbidity parameter retrieved in different spectral ranges based on spectroradiometer solar radiation measurements

Spectroradiometric direct irradiance measurements in the 300–1100 nm wavelength range with a spectral resolution of 6.2 nm have been used in a study of the variation in the Ångström turbidity parameter α and its dependence on the spectral range used in its determination. The measurements have been carried out under clear sky conditions at two different climate stations in Spain. Least-square fits of the experimental spectral aerosol optical depth (AOD) to the Ångström formula in different spectral ranges, selected for convenience depending on the objective or application (e.g., UV–VIS (350–400 nm), VIS (400–670 nm), VIS–NIR (370–870 nm), etc.), result in different sets for the α parameter. Due to this dependence on the spectral range, where the α-values are determined, a quantitative comparative analysis is carried out in order to assess the differences for a given data-base covering very different atmospheric conditions. The study reveals the necessity of a ‘standard spectral range’ to achieve confident data comparisons. We show some applications that are relevant for aerosol studies, from UV absorption by aerosols to satellite remote sensing.     

Comparison of organic compositions in dust storm and normal aerosol samples collected at Gosan,Jeju Island,during spring 2005

To better understand the current physical and chemical properties of East Asian aerosols, an intensive observation of atmospheric particles was conducted at Gosan site, Jeju Island, South Korea during 2005 spring. Total suspended particle (TSP) samples were collected using pre-combusted quartz filters and a high-volume air sampler with the time intervals ranging from 3 h to 48 h. The kinds and amount of various organic compounds were measured in the samples using gas chromatography–mass spectrometry. Among the 99 target compounds detected, saccharides (average, 130 ± 14 ng m^?3), fatty acids (73 ± 7 ng m^?3), alcohols (41 ± 4 ng m^?3), n-alkanes (32 ± 3 ng m^?3), and phthalates (21 ± 2 ng m^?3) were found to be major compound classes with polyols/polyacids, lignin and resin products, PAHs, sterols and aromatic acids being minor. Compared to the previous results reported for 2001 late spring samples, no significant changes were found in the levels of their concentrations and compositions for 4 years, although the economy in East Asia, especially in China, has sharply expanded from 2001 to 2005. During the campaign at Gosan site, we encountered two distinct dust storm episodes with high TSP concentrations. The first dust event occurred on March 28, which was characterized by a predominance of secondary organic aerosols. The second event that occurred on the next day (March 29) was found to be characterized by primary organic aerosols associated with forest fires in Siberia/northeastern China. A significant variation in the molecular compositions, which was found within a day, suggests that the compositions of East Asian aerosols are heterogeneous due to multi-contributions from different source regions together with different pathways of long-range atmospheric transport of particles.     

Mobile sources of atmospheric polycyclic aromatic hydrocarbons in a roadway tunnel

Amounts of polycyclic aromatic hydrocarbons (PAHs) and oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) in samples collected from the air, from the dust on a guardrail, and from the soils on a tunnel roadway at five sampling sites in a regular roadway tunnel were chemically analyzed in order to determine their sources. Among the 23 PAHs found in the air samples, pyrene was found in the highest concentration (43±7.2 ng/m³), followed by fluoranthene (26±4.3 ng/m³). Among 20 oxy-PAHs found in the air samples, anthraquinone was found in the greatest amount (56±3.9 ng/m³). The average concentration of the major PAHs found in the guardrail dust samples were 6.9±0.77 μg/g for pyrene, 5.5±0.76 μg/g for fluoranthene, and 2.6±0.30 μg/g for phenanthrene. The average concentration of the major oxy-PAHs found in the guardrail dust samples were 9.2±3.5 μg/g for anthraquinone and 1.4±0.50 μg/g for 2-methylanthraquinone. The average concentration of the major PAHs found in the soil samples were 1.1±0.31 μg/g for fluoranthene, 0.92±0.21 μg/g for pyrene, and 0.72±0.16 μg/g for phenanthrene. The average concentration of the major oxy-PAHs found in the soil samples were 1.2±0.88 μg/g for anthraquinone, 0.18±0.04 μg/g for 4-biphenylcarboxaldehyde, and 0.13±0.08 μg/g for 2-methylanthraquinone. The BeP ratios calculated from the results suggest that most PAHs found in the samples collected from the roadway tunnel were from automobile exhaust gases.     

Sorption of perfluorooctane sulfonate to carbon nanotubes in aquatic sediments

To date, sorption of organic compounds to nanomaterials has mainly been studied for the nanomaterial in its pristine state. However, sorption may be different when nanomaterials are buried in sediments. Here, we studied sorption of Perfluorooctane sulfonate (PFOS) to sediment and to sediment with 4% multiwalled carbon nanotubes (MWCNTs), as a function of factors affecting PFOS sorption; aqueous concentration, pH and Ca²⁺ concentration. Sorption to MWCNT in the sediment–MWCNT mixtures was assessed by subtracting the contribution of PFOS sorption to sediment-only from PFOS sorption to the total sediment–MWCNT mixture. PFOS Log K_D values ranged 0.52–1.62 L kg^?1 for sediment and 1.91–2.90 L kg^?1 for MWCNT present in the sediment. The latter values are relatively low, which is attributed to fouling of MWCNT by sediment organic matter. PFOS sorption was near-linear for sediment (Freundlich exponent of 0.92 ± 0.063) but non-linear for MWCNT (Freundlich exponent of 0.66 ± 0.03). Consequently, the impact of MWCNT on sorption in the mixture was larger at low PFOS aqueous concentration. Effects of pH and Ca²⁺ on PFOS sorption to MWCNT were statistically significant. We conclude that MWCNT fouling and PFOS concentration dependency are important factors affecting PFOS–MWCNT interactions in sediments.     

Characteristics of regional nucleation events in urban East St. Louis

Continuous measurements of aerosol size distributions (3 nm–2 μm) were carried out over a 26 month period (1 April 2001–31 May 2003; 650 days with valid data) in urban East St. Louis, IL, as a part of the US Environmental Protection Agency's Supersite program. This paper analyzes data for the 155 days on which “regional nucleation events” were observed during this study. Such events were observed during every month of the study except January 2003. We observed some differences, however, between events in the summer (defined here as April–September) and winter (December–February). Regional nucleation events were observed more frequently in summer months (36±13% of days) than in winter (8±7%), and nucleated particles grew faster in the summer (6.7±4.8 nm h⁻¹) than in winter (1.8±1.9 nm h⁻¹). The daily maximum in the number concentration of nanoparticles formed by nucleation (4.8±3.5×10⁴ cm⁻³) was highly variable and showed no clear seasonal dependence. Particle formation increased particle concentrations by an average factor of 3.1±2.8. Maximum daily rates of 3 nm particle production (17±20 cm⁻³ s⁻¹) were also highly variable and without a clear seasonal dependence. During these events, particle formation rates were typically near their maxima at 08:00–09:00 a.m., but particle production sometimes persisted at diminishing rates until late in the afternoon (15:00–16:00 p.m.).     

Generation and growth of aerosols over Pune,India

The measured physical size distributions of sub-micron particles during cold season at Pune, India are analyzed to explore the characteristics of nucleation and growth properties. Preliminary analysis of aerosol size distribution in time-series shows large increase in number concentration due to nucleation events between 0800 h and 1030 h at this location. The observable quantities such as condensable vapor concentration (C), its source rate (Q), growth rate (GR) and condensable sink (CS) are estimated from the time-series evolutions of aerosol size distributions. The concentration of vapor and its source rate were about 19.8 ± 2.15 × 10⁷ molecules cm^?3 and 1.28 ± 0.084 × 10⁷ cm^?3 s^?1 respectively. The average condensation sink and growth rate were 7.1 ± 0.4 × 10^?2 s^?1 and 16.95 ± 1.86 nm h^?1 respectively during the growth period. The values are high enough to trigger the nucleation bursts and enhance subsequent growth rates of nucleation mode particles at this location. The magnitudes are in the range of those observed at New Delhi, India and much higher than those of European cities. The ratio of apparent to real nucleation rate is found to be a measure of number concentration of freshly produced particles by photo-chemical nucleation. The predicted number concentrations corresponding to measured distributions of mid-point diameter increases with the size for both 1 nm nucleated clusters and 3 nm particles. The database of all the possible event days and the event characteristics forms the basis for future works into the causes and implications of atmospheric particle formation at this location.     

The rate and mechanism of the gas-phase oxidation of hydroxyacetone

The rate and mechanism for gas-phase destruction of hydroxyacetone, CH₃C(O)CH₂OH, by reaction with OH, Cl-atoms, and by photolysis have been determined. The first quantitative UV absorption spectrum of hydroxyacetone is reported over the wavelength range 235 to 340 nm; the spectrum is blue-shifted by about 15 nm relative to that of acetone and peaks at 266 nm, with a maximum absorption cross section of (6.7±0.6) ×10^-20 cm² molecule^-1. Measurable absorption extends out to about 330 nm. The quantum yield for photolysis of hydroxyacetone in the region relevant to the troposphere (λ>290 nm) was found to be significantly less than unity. Rate coefficients for the reaction of hydroxyacetone with OH radicals and Cl-atoms were determined at 298 K using the relative rate technique. The rate coefficient for reaction with OH was found to be (3.0±0.7)×10^-12 cm³ molecule^-1 s^-1, while the rate coefficient for reaction with Cl-atoms was found to be (5.6±0.7)×10^-11 cm³ molecule^-1 s^-1. Both values agree well with previous studies. The data were used to determine the lifetime of hydroxyacetone in the troposphere. Reaction with OH is the major gas-phase destruction mechanism for this compound, limiting its lifetime to about 4 days, while photolysis is found to be only of minor importance.