Wavelength Exponent Research Articles

Synoptic climatological influences on the spatial and temporal variability of aerosols over North America

AbstractThe spatial and temporal variability of atmospheric aerosols is not well understood, as most studies have been constrained to data sets that include few stations and are of short duration. Furthermore, all methods for quantifying atmospheric turbidity suffer from a major constraint in that they require cloudless sky conditions. This restriction produces gaps in the turbidity record and sampling bias, which has led to questionable inferences about the variability of aerosols.In this research, we address these concerns via analyses at scales broader than all previous studies. We analyzed the spectral aerosol optical depth at 500 nm (τa5) and Ångström's wavelength exponent (α), which represents the relative size distribution of aerosols. A total of 27 sites, with a mean period of record of 7.3 years, are included. Beyond seasonal and spatial summaries of aerosol variability, we have divided observations by synoptic condition, utilizing the Spatial Synoptic Classification (SSC).Our results show that atmospheric turbidity across North America is greatest over the east. Seasonality of both parameters was shown, most notably a greater τa5 during summertime. Utilizing the SSC, we have uncovered significant differences across weather types. Moist weather types, especially moist tropical, display considerably higher turbidity, while the colder, drier dry polar weather type is associated with low aerosol optical depth. Certain weather types show considerable seasonal variability; the dry tropical weather type is associated with relatively low values in winter, but high values in summer, when convection is significant. Cluster analyses of stations yielded three general regions, each with similar synoptic variability: a western cluster with low aerosol optical depth and minimal synoptic variability, an eastern cluster with higher turbidity and variability, and a cluster located on the periphery of the eastern cluster, associated with moderate levels of turbidity but very high variability, suggesting a varied influence of nearby industrial areas. Copyright © 2006 Royal Meteorological Society.

Turbidity as a probe of tubulin polymerization kinetics: A theoretical and experimental re-examination

We report here an examination of the validity of the experimental practice of using solution turbidity to study the polymerization kinetics of microtubule formation. The investigative approach proceeds via numerical solution of model rate equations to yield the time dependence of each microtubule species, followed by the calculation of the time- and wavelength-dependent turbidity generated by the calculated distribution of rod lengths. The wavelength dependence of the turbidity along the time course is analyzed to search for generalized kinetic regimes that satisfy a constant proportionality relationship between the observed turbidity and the weight concentration of polymerized tubulin. An empirical analysis, which permits valid interpretation of turbidity data for distributions of microtubules that are not long relative to the wavelength of incident light, is proposed. The basic correctness of the simulation work is shown by the analysis of the experimental time dependence of the turbidity wavelength exponent for microtubule formation in taxol-supplemented 0.1 M Pipes buffer (1 mM GTP, 1 mM EGTA, 1 mM MgSO 4, pH 6.4). We believe that the general findings and principles outlined here are applicable to studies of other fibril-forming systems that use turbidity as a marker of polymerization progress.

Influence of local waste burning on atmospheric aerosol properties in urban environment

Aerosols affect the radiative energy budget on both the regional and global scales. The wavelength-dependent aerosol optical depth (AOD) is a fundamental determinant of the amount by which extra-terrestrial incoming sunlight and outgoing terrestrial radiation are being attenuated in the atmosphere. The present study addresses the influence of local waste burning on aerosol characteristics, black carbon (BC) aerosol mass concentration and spectral solar irradiance using ground-based measurements over the tropical urban environment of Hyderabad, India. AOD has been observed to be maximum during burning days compared to normal days. Aerosol size spectra suggest bimodal distributions during pre-and post-burning periods and trimodal distributions during burning periods. Angstrom wavelength exponent estimated from spectral variation of AOD suggested dominance of accumulation mode particle loading during burning days compared to normal days. Diurnal variation of BC on normal days showed a broad nocturnal peak during ∼20:00 to ∼24:00 h with a maximum value of BC aerosol concentration of ∼14,000 ng m −3 whereas on local waste burning days enormous increases in BC concentrations have been observed with a peak at ∼60,000 ng m −3. Relative attenuation of global solar irradiance during burning days has been found to be of the order of 30% in the visible and 28% in the near-infrared regions. The results are discussed in detail in this paper.

In situ ship cruise measurements of mass concentration and size distribution of aerosols over Bay of Bengal and their radiative impacts

Simultaneous measurements of surface level aerosol mass concentrations, their size distribution, and aerosol optical depth (AOD) were made during a ship cruise study conducted over the Bay of Bengal (BoB) between 19 and 28 February 2003, when the prevailing surface level wind flow is predominantly from the continent toward the ocean, using a ten‐stage QCM cascade impactor and Microtops Sun photometer. On all cruise days, air parcels at different altitude levels were coming either from west or from northwest directions, crossing a significant portion of the Indian subcontinent before finally reaching over BoB. Average value of surface level aerosol mass concentration is found to be around 50, 37, and 13 μg/m3 for coarse mode (>1 μm), accumulation mode (between 1 μm and 0.1 μm), and nucleation mode (<0.1 μm) particles, respectively. Size distribution of aerosols measured during the cruise showed the presence of four distinct modes, all of which could be fitted using lognormal distribution. Mode radii for the distributions lie in the range of 0.025–0.036 μm for mode 1, between 0.15 and 0.165 for mode 2, between 0.39 and 0.55 for mode 3, and between 2.2 and 3.5 for mode 4. Over the study region, daily mean AOD values at 380 nm were in the range of 0.34 to 0.75 while those at 1020 nm varied from 0.09 to 0.25. The mean value of Angstrom wavelength exponent α is found to be 1.19 ± 0.12. Regression analysis for the scatterplots between AOD values and surface mass concentrations showed good correlation between them over the entire cruise region. Aerosol optical depths, as well as extinction coefficients calculated from surface level aerosol number concentrations, show higher values over northern and coastal areas of BoB. An estimate of aerosol scale height has been made from the ratio of columnar AOD values and surface extinction coefficients. Columnar aerosol size distributions were derived using King's inversion technique, and the results are found to be less sensitive to the choice of aerosol refractive index values used. Independently, aerosol scale height is also estimated by taking the ratio of derived columnar number distribution and measured surface level aerosol number concentrations. AOD values observed over BoB are used in estimating the aerosol radiative forcing over this region. The Average value of the net direct radiative forcing for the region is found to be in the range of −30.6 to −26.6 W/m2 at the surface and −8.4 to −10.6 W/m2 at the TOA. Magnitude of LW forcing is found to be about 10% of its SW counterpart, at both surface and TOA. Results of a sensitivity test showed that the magnitude of LW aerosol forcing at the surface level decreases with the increase in column water vapor.

Columnar aerosol optical properties at AERONET sites in central eastern Asia and aerosol transport to the tropical mid‐Pacific

The column‐integrated optical properties of aerosol in the central eastern region of Asia and midtropical Pacific were investigated based on Sun/sky radiometer measurements made at Aerosol Robotic Network (AERONET) sites in these regions. Characterization of aerosol properties in the Asian region is important due to the rapid growth of both population and economic activity, with associated increases in fossil fuel combustion, and the possible regional and global climatic impacts of related aerosol emissions. Multiyear monitoring over the complete annual cycle at sites in China, Mongolia, South Korea, and Japan suggest spring and/or summer maximum in aerosol optical depth (τa) and a winter minimum; however, more monitoring is needed to establish accurate climatologies. The annual cycle of Angstrom wavelength exponent (α) showed a springtime minimum associated with dust storm activity; however, the monthly mean α440–870 was >0.8 even for the peak dust season at eastern Asian sites suggesting that fine mode pollution aerosol emitted from population centers in eastern Asia dominates the monthly aerosol optical influence even in spring as pollution aerosol mixes with coarse mode dust originating in western source regions. Aerosol optical depth peaks in spring in the tropical mid‐Pacific Ocean associated with seasonal shifts in atmospheric transport from Asia, and ∼35% of the springtime τa500 enhancement occurs at altitudes above 3.4 km. For predominately fine mode aerosol pollution cases, the average midvisible (∼550 nm) single scattering albedo (ω0) at two continental urban sites in China averaged ∼0.89, while it was significantly higher, ∼0.93, at two relatively rural coastal sites in South Korea and Japan. Differences in fine mode absorption between these regions may result from a combination of factors including aerosol aging during transport, relative humidity differences, sea salt at coastal sites, and fuel type and combustion differences in the two regions. For cases where τa was predominately coarse mode dust aerosol in the spring of 2001, the absorption was greater in eastern Asia compared to the source regions, with ω0 at Dunhuang, China (near to the major Taklamakan dust source), ∼0.04 higher than at Beijing at all wavelengths, and Anmyon, South Korea, showing an intermediate level of absorption. Possible reasons for differences in dust absorption magnitude include interactions between dust and fine mode pollution aerosol and also variability of dust optical properties from different source regions in China and Mongolia.

A turbidimetric method for the rapid evaluation of MWF emulsion stability

The stability of metalworking emulsions is a critical characteristic impacting its usefulness as a lubricant and rust inhibitor. Loss of stability usually results in replacement of the fluid leading to both economic loss and environmental discharge. In this paper, the loss of stability caused by accumulation of hardness ions is examined by a turbidimetric method. The method is based on following the change in the wavelength exponent of the emulsions caused by changing particle size. The stability as determined by this method is compared to conventional testing based on phase separation over 24 h. The turbidimetric method is shown to be a more conservative estimate of emulsion stability. The simplicity of the method makes it useful for both product formulation and process control.

Aerosol properties and their spatial and temporal variations over North China in spring 2001

Aerosol properties and their spatial and temporal variations over North China were analysed based on ground-based radiometer data for spring 2001. On the basis of the retrievals from sun/sky radiance and broad-band radiation measurements at four AERONET stations and eight first-class radiation stations over North China, the analysis comprised a detailed description of aerosol loading, size and absorption in this period. The impact of dust events on aerosol properties over the downwind region was emphasized.Heavy aerosol loading and notable temporal variation over North China were revealed by both datasets. The average aerosol optical depth at 750 nm at the eight radiation stations ranged from 0.32 in Ejinaqi to 0.68 in Beijing, with the averaged coefficient of variation being 70%. Aerosol optical depth was dominantly contributed to by dust over western China, with 68% (±5%) of aerosol optical depth at 550 nm being attributed to large dust aerosols. A dramatic increase in aerosol optical depth associated with the remarkable decrease in the Ångstrom wavelength exponent was observed in Beijing and Xianghe during the dust episode. This indicated that a huge number of large particles were emitted into the atmosphere during the dust period, with the result that the contribution to aerosol optical depth from coarse particles approached the value observed in the dust source region.Anthropogenic pollution also frequently contributed to the high aerosol optical depth in Beijing and Xianghe, but this was characterized by fine particles, with more than 70% of aerosol optical depth at 550 nm being attributed to fine particles. Pure desert aerosol in Chinese dust source regions absorbs much less solar radiation than predicted by known aerosol models. The retrieved single-scattering albedo was around 0.98 (±0.01) and had little wavelength dependence, which is in agreement with the ground-based and satellite retrievals in other dust source regions. Contrarily, anthropogenic aerosol exhibits much stronger absorption in the urban region, with the single-scattering albedo ranging from 0.89 ± 0.04 (at 440 nm) to 0.83 ± 0.05 (at 1020 nm). Due to the large difference in the absorption between dust and anthropogenic aerosol, the consequence of the input of a large volume of dust aerosols is not only to enhance the aerosol loading but also to reduce the aerosol absorption. Retrievals in Beijing showed that the single-scattering albedo increased to about 0.90 and had little spectral dependence when anthropogenic pollution and a dust event together affected Beijing; as for the pure dust period, it ranged from 0.92 at 440 nm to 0.97 at 1020 nm. The significant decrease in the aerosol absorption due to the dust outbreak can result in a decrease in aerosol atmospheric heating efficiency; this warrants further research since the increasing trend of aerosol loading with strong absorption in China is supposed to play an active role in regional and global climate change and the hydrological cycle.

Aerosol properties and their spatial and temporal variations over North China in spring 2001

Aerosol properties and their spatial and temporal variations over North China were analysed based on ground-based radiometer data for spring 2001. On the basis of the retrievals from sun/sky radiance and broad-band radiation measurements at four AERONET stations and eight first-class radiation stations over North China, the analysis comprised a detailed description of aerosol loading, size and absorption in this period. The impact of dust events on aerosol properties over the downwind region was emphasized.Heavy aerosol loading and notable temporal variation over North China were revealed by both datasets. The average aerosol optical depth at 750 nm at the eight radiation stations ranged from 0.32 in Ejinaqi to 0.68 in Beijing, with the averaged coefficient of variation being 70%. Aerosol optical depth was dominantly contributed to by dust over western China, with 68% (±5%) of aerosol optical depth at 550 nm being attributed to large dust aerosols. A dramatic increase in aerosol optical depth associated with the remarkable decrease in the Ångstrom wavelength exponent was observed in Beijing and Xianghe during the dust episode. This indicated that a huge number of large particles were emitted into the atmosphere during the dust period, with the result that the contribution to aerosol optical depth from coarse particles approached the value observed in the dust source region.Anthropogenic pollution also frequently contributed to the high aerosol optical depth in Beijing and Xianghe, but this was characterized by fine particles, with more than 70% of aerosol optical depth at 550 nm being attributed to fine particles. Pure desert aerosol in Chinese dust source regions absorbs much less solar radiation than predicted by known aerosol models. The retrieved single-scattering albedo was around 0.98 (±0.01) and had little wavelength dependence, which is in agreement with the ground-based and satellite retrievals in other dust source regions. Contrarily, anthropogenic aerosol exhibits much stronger absorption in the urban region, with the single-scattering albedo ranging from 0.89 ± 0.04 (at 440 nm) to 0.83 ± 0.05 (at 1020 nm). Due to the large difference in the absorption between dust and anthropogenic aerosol, the consequence of the input of a large volume of dust aerosols is not only to enhance the aerosol loading but also to reduce the aerosol absorption. Retrievals in Beijing showed that the single-scattering albedo increased to about 0.90 and had little spectral dependence when anthropogenic pollution and a dust event together affected Beijing; as for the pure dust period, it ranged from 0.92 at 440 nm to 0.97 at 1020 nm. The significant decrease in the aerosol absorption due to the dust outbreak can result in a decrease in aerosol atmospheric heating efficiency; this warrants further research since the increasing trend of aerosol loading with strong absorption in China is supposed to play an active role in regional and global climate change and the hydrological cycle.

Comparisons of aerosol optical properties derived from Sun photometry to estimates inferred from surface measurements in Big Bend National Park, Texas

Abstract As a part of the Big Bend Regional Aerosol and Visibility Observational Study (BRAVO, July–October 1999), aerosol physical, optical and chemical properties were measured continuously in an effort to characterize visibility in Big Bend National Park. Surface-based estimates of aerosol extinction coefficients derived from size distribution data were compared to aerosol optical properties retrieved independently from ground-based remote sensing measurements from the United States Department of Agriculture (USDA) UVB radiometer network site. Comparisons suggested that for the majority of the study, surface visibility was a good indicator of the column aerosol optical depth, with a correlation coefficient of r 2 = 0.77 . The average (and one standard deviation) aerosol optical depth from the USDA network was 0.13±0.06 at a wavelength of 500 nm. Angstrom wavelength exponents were computed for both data sets to characterize the spectral variations of aerosol optical properties over the wavelength range of 415–860 nm. Variations in Angstrom exponents corresponded to changing aerosol properties as seen in the ground-based composition and size distribution data sets. The average Angstrom exponent from in situ measurements was 1.5±0.4 compared to column estimates of 1.3±0.4. The root mean square of the difference between the estimates of Angstrom exponents was 0.20 (∼12%) and on average the estimates were highly correlated ( r 2 = 0.89 ). Aerosol optical depths and ambient surface aerosol extinction coefficients were positively correlated with PM2.5 sulfate mass concentrations ( r 2 = 0.68 and r 2 = 0.79 , respectively), suggesting that sulfate aerosols were a major contributor to visibility degradation and column aerosol loading during the study.

Size distribution of near-surface aerosols and its relation to the columnar aerosol optical depths

Abstract. Simultaneous data on Aerosol Optical Depth (AOD) and size segregated, near-surface, aerosol mass concentration was obtained from a Multi wavelength Solar Radiometer (MWR) and Quartz Crystal Microbalance Impactor (QCM), respectively. These were used to examine the association between near-surface aerosol properties and columnar AOD. The spectral AODs were approximated to the Ångström relation τp=βλ-α, and the wavelength exponent α and turbidity coefficient β have been obtained. In general, α was found to be well associated with the relative abundance of accumulation mode aerosols (estimated from the simultaneous QCM data) while β followed the variations of the coarse mode aerosol mass concentration; the association being closer during periods of continental airmass.

Spectral aerosol optical characteristics during the northeast monsoon over the Arabian Sea and the tropical Indian Ocean: 2. Ångström parameters and anthropogenic influence

Ångström parameters are estimated from the spectral aerosol optical depths (AODs) measured over the Arabian Sea (AS) and tropical Indian Ocean (TIO) during the northeast winter monsoon seasons of 1996–2000, and their yearly and monthly mean variabilities are investigated. The 5‐year mean Ångström wavelength exponent α is found to be 1.5 over coastal India (CI), AS, and TIO. The 5‐year mean β, turbidity parameter, is found to be about 0.14 over CI, 0.10 over AS, and about 0.05 over TIO indicating the less polluted nature of TIO. The α and β values display year‐to‐year and monthly mean variations. The 5‐year mean α and β values over two land locations (Trivandrum (TVM), Vishakapatnam (VSK)) in peninsular India and marine locations (Minicoy (MCY), Kaashidhoo (KCO)) are found to be around 1 and 0.2. An examination of the daily mean wind speeds and the 0.5 μm AODs obtained during 1996–2000 yielded an index of wind dependence 0.05 for AS and 0.11 over TIO. The scaling distance, the distance up to which the effects of continental aerosols are noticeable over the oceanic regions, is estimated to be 1500 km over the Indian Ocean region. The 5‐year mean anthropogenic contribution to the 0.5 μm AODs over CI, AS, and TIO estimated by comparing with the wind speed dependent sea salt aerosol is found to be above 90% over CI and AS, while it is 74% over TIO. The 5‐year mean anthropogenic contribution over TVM and VSK is in excess of 80%. Over MCY/KCO the anthropogenic contribution is lower at 70%.

Large aerosol optical depths observed at an urban location in southern India associated with rain-deficit summer monsoon season

Abstract. Aerosol spectral optical depth (AOD) measurements were made covering three years (2001, 2002 and 2003) at an urban continental location, Bangalore (13°N, 77.6°E) in India. These ground-based observations have shown that AODs reach a maximum during April (~0.5 at 500nm) and minimum during the November to January period (~0.2). The Angstrom wavelength exponent (α) was ~1.1 during the dry season (December to April), which, in conjunction with the high optical depth indicates significant anthropogenic influence. Seasonal variations in AODs appear to have an association with monsoon rainfall. Large AODs (α~1.4) were observed during the rain-deficit summer monsoon season (SMS) of 2002, which persisted for more than six months. Enhancement in AODs during SMS 2002 was ~0.15 (at 500nm), compared to 2001 and 2003.

Measurements of aerosol optical depths and black carbon over Bay of Bengal during post‐monsoon season

Extensive and collocated measurements of aerosol optical depths (AODs), mass concentration of composite aerosols and aerosol black carbon (BC) were made over the Bay of Bengal (onboard R/V Sagar Kanya during its cruise #197) region during post‐Indian summer monsoon period. These data along with MODIS (onboard TERRA satellite) data shows that aerosol visible AODs were in the range of 0.2 to 0.7 (with mean value of 0.43 ± 0.12) and Angstrom wavelength exponent of ∼0.93 ± 0.21 which in conjunction with a single scattering albedo (ω) of ∼0.94 indicates the presence of weakly absorbing aerosols. The BC mass fraction was ∼2.9 ± 1.1% during post‐monsoon season compared to 5.8 ± 0.6% (with ω of ∼0.88) during pre‐monsoon season. The presence of aerosols over Bay of Bengal during post‐monsoon season decreases the short wave radiation reaching the surface by as much as 25 W m−2 and increases top of the atmosphere reflected radiation by 14 W m−2.

Aerosol optical depth in the UVB and visible wavelength range from Brewer spectrophotometer direct irradiance measurements: 1991–2002

Direct irradiance measurements from Brewer spectrophotometer #66 at Ispra, Italy, were reanalyzed for the period 1991–2002. Two calibration methodologies, one adapted to the ultraviolet (UV) and one adapted to the visible wavelength range, have been developed to retrieve the aerosol optical depth (AOD) from these measurements. The measurements in the UV were calibrated in absolute units using laboratory lamp calibrations of the global entrance port of the same instrument, while in the visible wavelength range a zero air mass Langley extrapolation methodology was implemented. The retrieved aerosol optical depths were validated with a colocated CIMEL Sun photometer operating within the Aerosol Robotic Network (AERONET) since 1997. AOD values at Ispra show a strong short‐term variability between 0.05 and 2 at 320 nm as well as a pronounced seasonal variability. The highest monthly mean values of around 0.6 (at 320 nm) are usually found in spring and summer while the lowest values, of about 0.3, are found in the winter months. An aerosol characterization by means of the Ångström power law yields a mean wavelength exponent of 1.6. The influence of Föhn events on the aerosol climatology at Ispra was investigated. The AOD measurements at Ispra show the strong influence of the Mount Pinatubo eruption in June 1991 and the following decay to background levels in late 1997.

Characteristics of aerosol spectral optical depths over Manora Peak: A high‐altitude station in the central Himalayas

We present, for the first time, spectral behavior of aerosol optical depths (AODs) over Manora Peak, Nainital, located at an altitude of ∼2 km in the Shivalik ranges of the central Himalayas. The observations were carried out using a multiwavelength solar radiometer during January to December 2002. The main results of the study are extremely low AODs during winter, a remarkable increase to high values in summer, and a distinct change in the spectral dependencies of AODs from relatively steeper spectra during winter to shallower ones in summer. A comparison of the total optical depths of the nighttime measurements taken during the 1970s with the daytime values from the current study underlines the fact that loading of larger size particles during summer also occurred at that time, though less severely than it does today. During transparent days the AOD values usually lie below 0.08, while during dusty (turbid) days they lie between 0.08 and 0.69. The average AOD value during the winter, particularly in January and February, is ∼0.03 ± 0.01 at 0.5 μm. The mean aerosol extinction law at Manora Peak during 2002 is best represented by 0.10λ−0.61. However, during transparent days, which covers almost 40% of the time, it is represented by 0.02λ−0.97. This value of wavelength exponent, representing reduced coarse concentration and the presence of fine aerosols, indicates that the station measures aerosol in the free troposphere at least during part of the year.

Radiative forcing by aerosols over the Bay of Bengal region derived from shipborne, island‐based, and satellite (Moderate‐Resolution Imaging Spectroradiometer) observations

Measurements of spectral aerosol optical depths (AODs) were made over the Bay of Bengal region (adjacent to the Indian landmass) on board the oceanographic research vessel Sagar Kanya during February 2003. Simultaneous measurements of spectral AODs and mass concentrations of the composite aerosols and aerosol black carbon (BC) were made at an island location, Port Blair (11.63°N, 92.71°E), also in the Bay of Bengal. At the cruise locations the AODs were in the range of ∼0.3–0.6 at 500 nm (with a mean value of 0.41 ± 0.14) and Angstrom wavelength exponent of ∼1.1 ± 0.1; while at Port Blair the AODs were in the range of 0.11–0.48 at 500 nm and Angstrom wavelength exponent of 0.98 ± 0.07. Aerosol BC constituted 5.8 ± 0.6% of the composite aerosol mass concentration with a single‐scattering albedo of ∼0.88, indicating the presence of a significant amount of submicron absorbing aerosols. Comparisons of AODs measured at Port Blair during cruise 188 and an earlier cruise (cruise 161B) during March 2001 (over the Bay of Bengal, Arabian Sea, and Indian Ocean) with those derived from Moderate‐Resolution Imaging Spectroradiometer (MODIS) (on board the TERRA platform) showed excellent agreement with a mean difference of ∼0.01 and a root‐mean‐square difference of ∼0.03. Regionally averaged aerosol (net) forcing over the Bay of Bengal was in the range −15 to −24 W m−2 at the surface and −2 to −4 W m−2 at the top of the atmosphere in February 2003; these values were smaller in magnitude than those observed over this region during March 2001 and larger than that observed over the Arabian Sea and the Indian Ocean. The resulting atmospheric heating due to aerosol absorption was ∼0.5°K/d.

Extinction and Sky Brightness at Two Solar Observatories

The Advanced Technology Solar Telescope site survey Sky Brightness Monitor simultaneously images the solar disk and the sky to about 8 solar radii in four wavelengths at 450, 530, 890 and 940 nm. One day of data from Mees Solar Observatory on Haleakala and from the National Solar Observatory at Sacramento Peak (Sunspot, New Mexico) are analyzed. Both sites show strong Rayleigh extinction, but while Haleakala shows a larger aerosol component, Sunspot shows a large variation in the aerosol component. Overall the Haleakala extinction varies as λ−2 whereas the Sunspot extinction changes from about λ−3.5 to about λ−2, suggesting an increasing aerosol component during the day. Water vapor absorption measurements from both sites are similar, though Sunspot shows larger time variations than Haleakala. The instrument-corrected sky brightness from both sites show comparable values, and again the Sunspot data show more variations. The sky brightness values show a radial dependence of sky brightness of r−0.1 at Haleakala, but a dependence of r−1.0 at Sunspot. The wavelength variation of the sky brightness at Haleakala is relatively constant at λ−1.5 but varies at Sunspot from λ−1.5 to λ−0.1 again suggesting an increasing aerosol contribution during the day at Sunspot. Finally, dust measurements near the ground are compared with the extinction wavelength exponent for data taken at Haleakala on 24 Feb. 2003. The measurements suggest more large dust particles are present near the ground than averaged over the whole air column.

A method for studying insoluble immune complexes.

Two variants of a method for determining the average composition of insoluble immune complex particles (IICP) are described. The first variant is based on measuring the specific turbidity (the turbidity per unit mass concentration of the dispersed substance) and the average size of IICP determined from dynamic light scattering (DLS). In the second variant, the slope of the logarithmic turbidity spectrum (wavelength exponent) is used instead of DLS particle size. Both variants allow the average biopolymer volume fraction to be determined in terms of the average refractive index of IICP. The method is exemplified by two experimental antigen+antibody systems: (i) lipopolysaccharide-protein complex (LPPC) of Azospirillum brasilense Sp245+rabbit anti-LPPC; and (ii) human IgG (hIgG)+sheep anti-hIgG. We have found that IICP can be modeled by incompact porous particles that contain about 30% of biopolymer substance and 70% of buffer.

Airborne Sun photometer measurements of aerosol optical depth and columnar water vapor during the Puerto Rico Dust Experiment and comparison with land, aircraft, and satellite measurements

Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV) measurements obtained with the six‐channel NASA Ames Airborne Tracking Sunphotometer (AATS‐6) mounted on a twin‐engine aircraft during the summer 2000 Puerto Rico Dust Experiment are presented. In general, aerosol extinction values calculated from AATS‐6 AOD measurements acquired during aircraft profiles up to 5 km above sea level (asl) reproduce the vertical structure measured by coincident aircraft in situ measurements of total aerosol number concentration. AATS‐6 extinction retrievals also agree with corresponding values derived from ground‐based lidar measurements for altitudes above the trade inversion. The spectral behavior of AOD within specific layers beneath the top of the aircraft profile is consistent with attenuation of incoming solar radiation by large dust particles or by dust plus sea salt, with mean Ångström wavelength exponents of ∼0.20. Values of CWV calculated from profile measurements by AATS‐6 at 941.9 nm and from aircraft in situ measurements agree to within ∼4% (0.13 g/cm2). AATS‐6 AOD values measured on the ground at Roosevelt Roads Naval Air Station and during low‐altitude aircraft runs over the adjacent Cabras Island aerosol/radiation ground site agree to within 0.004–0.030 with coincident data obtained with an AERONET Sun/sky radiometer located on Cabras Island. For the same observation times, AERONET retrievals of CWV exceed AATS‐6 values by ∼21%. AATS‐6 AOD values measured during low‐altitude aircraft traverses over the ocean are compared with corresponding AOD values retrieved over water from upwelling radiance measurements by the Moderate‐Resolution Imaging Spectroradiometer (MODIS), Total Ozone Mapping Spectrometer (TOMS), and GOES 8 Imager satellite sensors, with mixed results.

Variability of aerosol optical thickness and atmospheric turbidity in Tunisia

The aerosol optical thickness (AOT) τ a computed from the spectral sun photometer in Thala (Tunisia) exhibited variability ranging from approximately 0.03 to greater than 2.0 at 870 nm for March–October 2001. These measurements are compared to the aerosol optical thickness computed in Ouagadougou (Burkina-Faso), Banizoumbou (Niger), IMC Oristano (Sardinia) and Rome Tor Vergata (Italy). Analysis of τ a data from this observation network suggests that there is a high temporal and spatial variability of τ a in the different sites. The Angström wavelength exponent α was found to vary with the magnitude of the aerosol optical thickness, with values as high as 1.5 for very low τ a, and values of −0.1 for high τ a situations. The relationship between the two parameters τ a and α is investigated. Values of the turbidity coefficient β have been determined in Thala (Tunisia) for 8 months in 2001 based on a direct fitting method of the Angström power law expression using sun photometer data. The monthly averaged values of the turbidity coefficient β vary between 0.15 and 0.33. The months of July and October experienced the highest turbidity, while April experienced the lowest aerosol loading on average. The turbidity shows a maximum and minimum values for the Southwest and the Northwest wind directions, respectively. The single scattering albedo ω o for the 870 nm wavelength obtained from solar aureole data in Thala is analysed according to the particles' origin.