Low Aerosol Optical Depth Research Articles

Instrument calibration and aerosol optical depth validation of the China Aerosol Remote Sensing Network

This paper introduced the calibration of the CE‐318 sunphotometer of the China Aerosol Remote Sensing Network (CARSNET) and the validation of aerosol optical depth (AOD) by AOD module of ASTPWin software compared with the simultaneous measurements of the Aerosol Robotic Network (AERONET)/Photométrie pour le Traitement Opérationnel de Normalization Satellitaire (PHOTONS) and PREDE skyradiometer. The results show that the CARSNET AOD measurements have the same accuracy as the AERONET/PHOTONS. On the basis of a comparison between CARSNET and AERONET, the AODs from CARSNET at 1020, 870, 670, and 440 nm are about 0.03, 0.01, 0.01, and 0.01 larger than those from AERONET, respectively. The aerosol optical properties over Beijing acquired through the CE‐318 sunphotometers of one AERONET/PHOTONS site and two CARSNET sites were analyzed on the basis of 4‐year measurements. It was obvious that the AOD of the Shangdianzi site (rural site) was lower than that of the two urban sites (the Institute of Atmospheric Physics (IAP) site (north urban site) and the Beijing Meteorological Observatory (BJO) site (south urban site)). The AOD of BJO was about 0.05, 0.04, 0.05, and 0.06 larger than that of IAP at 1020, 870, 670, and 440 nm, respectively, indicating that there is more local pollution in the south part of Beijing. The highest AOD was found in summer because of the stagnation planetary boundary layer and transport of pollutants from large pollution centers south of Beijing. The high temperature and relative humidity in summer also favor the production of aerosol precursor and the hygroscopic growth of the existing particles locally, which results in high AOD. In contrast, the lowest AOD at the two urban sites and one rural site in Beijing occurred in winter as the frequent cold air masses help pollutants diffuse easily.

Diurnal and Seasonal Variation of Spectral Properties of Aerosols over Dehradun, India

The diurnal variation of Aerosol Optical Depth (AOD) and its spectral properties over non-cloudy days in summer and winter seasons of two years are studied. Spectral dependence of AOD shows higher values at shorter wavelengths which decreases gradually towards longer wavelengths for the summer as well as the winter period. Results suggest that during high relative humidity and stable conditions, the AOD is found to be relatively low (clear day) as compared to unstable meteorological conditions (turbid). Air mass back trajectory for 5 days of high and low AOD in March indicate that the sources of loading come from far to the west in central Asia, whereas during winter, the sources are from east and south-east direction in the Indo-Gangetic plain. The diurnal variation of Ǻngström exponent () has lowest values in the morning hours with a variation of around 5-17% for clear day and around 11-20% for turbid day, signifying the presence of larger particles. Due to inversion more fine particles lead to an increase in the  value. The intraseasonal variability in March (summer) is high and interseasonal variation in January (winter) is not observed and is also confirmed by ‘’ values. Size distribution of aerosols showed peak loading ranging around a 0.07 m radius. Aerosol loading on turbid days was due to fine mode particles and the simultaneous presence of fine- and coarse-mode particles on clear days.

Spatial and temporal characteristics of aerosol optical depth over East Asia and their association with wind fields

AbstractRotated principal component analysis (PC) was performed on monthly Aerosol Optical Depth (AOD) from Moderate Resolution Imaging Spectroradiometer (MODIS) data over East Asia for the period of March 2000–January 2006. The first PC mode is a south–north dipole with a strong positive loading between 30–35°N and 130–150°E over middle latitude East Asia and a weaker negative loading in southern parts over East Asia. The first PC mode explains 32.44% of the total variance. By analysing time series of the PC1‐3, results show the PC‐1 and PC‐3 modes are important and both have distinct changes from the period 2000–2003 to 2004–2006; the former pattern (in middle latitudes over East Asia) is characterized in a decreasing linear trend and the latter (in eastern China) is in an increasing linear trend in the full 71 month evolution. Furthermore, based on the wavelet power spectrum analysis, it is found that the distinct changes of the monthly AOD in middle latitudes over East Asia (the PC‐1 pattern) are connected to the multi‐scale variations at intra‐seasonal (3–6 month) and annual (12 month) scales. While calculating the correlation of the PC‐1 with wind fields at 850 hPa from the NCEP/NCAR reanalysis dataset, it is shown that the PC‐1 is closely related to the U and V (horizontal) components over subtropical East Asia and western north Pacific, which well represents East Asian monsoon features. Finally, by comparing the PC‐1 pattern in May between 2003 and 2004, results show that the higher (lower) AOD in 2003 (2004) is associated with weaker (stronger) U component at 850 hPa in middle latitudes over East Asia. Copyright © 2008 Royal Meteorological Society

Analysis of the weekly cycle of aerosol optical depth using AERONET and MODIS data

Multi‐year Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) data are used to study AOD weekly variations at the global scale. A clear weekly cycle of AOD is observed in the United States (U.S.) and Central Europe. AOD during the weekday is larger than that during the weekend in 36 out of 43 AERONET sites in the U.S. The average U.S. weekend effect (the percent difference in AOD during the weekday and the weekend) is 3.8%. A weekly periodicity with lower AODs on Sunday and Monday and higher AODs from Wednesday until Saturday is revealed over Central Europe and the average weekend effect there is 4.0%. The weekly cycle in urban sites is greater than that in rural sites. AOD during the weekday is also significantly larger than that during the weekend in urban AERONET sites in South America and South Korea. However, a reversed AOD weekly cycle is observed in the Middle East and India. AODs on Thursday and Friday, the “weekend” for Middle East cultures, are relatively lower than AODs on other days. There is no clear weekly variation of AOD over eastern China. The striking feature in this region is the occurrence of much higher AOD on Sunday and this phenomenon is independent of season. The analysis of MODIS aerosol data is in good agreement with that of AERONET data.

An observational study of the relationship between cloud, aerosol and meteorology in broken low‐level cloud conditions

Global satellite analyses showing strong correlations between aerosol optical depth and cloud cover have stirred much debate recently. While it is tempting to interpret the results as evidence of aerosol enhancement of cloud cover, other factors such as the influence of meteorology on both the aerosol and cloud distributions can also play a role, as both aerosols and clouds depend upon local meteorology. This study uses satellite observations to examine aerosol‐cloud relationships for broken low‐level cloud regions off the coast of Africa. The analysis approach minimizes the influence of large‐scale meteorology by restricting the spatial and temporal domains in which the aerosol and cloud properties are compared. While distributions of several meteorological variables within 5° × 5° latitude‐longitude regions are nearly identical under low and high aerosol optical depth, the corresponding distributions of single‐layer low cloud properties and top‐of‐atmosphere radiative fluxes differ markedly, consistent with earlier studies showing increased cloud cover with aerosol optical depth. Furthermore, fine‐mode fraction and Angstrom Exponent are also larger in conditions of higher aerosol optical depth, even though no evidence of systematic latitudinal or longitudinal gradients between the low and high aerosol optical depth populations are observed. When the analysis is repeated for all 5° × 5° latitude‐longitude regions over the global oceans (after removing cases in which significant meteorological differences are found between the low and high aerosol populations), results are qualitatively similar to those off the coast of Africa.

Characteristics of spectral aerosol optical depths over India during ICARB

Spectral aerosol optical depth (AOD) measurements, carried out regularly from a network of observatories spread over the Indian mainland and adjoining islands in the Bay of Bengal and Arabian Sea, are used to examine the spatio-temporal and spectral variations during the period of ICARB (March to May 2006). The AODs and the derived Ångström parameters showed considerable variations across India during the above period. While at the southern peninsular stations the AODs decreased towards May after a peak in April, in the north Indian regions they increased continuously from March to May. The Ångström coefficients suggested enhanced coarse mode loading in the north Indian regions, compared to southern India. Nevertheless, as months progressed from March to May, the dominance of coarse mode aerosols increased in the columnar aerosol size spectrum over the entire Indian mainland, maintaining the regional distinctiveness. Compared to the above, the island stations showed considerably low AODs, so too the northeastern station Dibrugarh, indicating the prevalence of cleaner environment. Long-range transport of aerosols from tshe adjoining regions leads to remarkable changes in the magnitude of the AODs and their wavelength dependencies during March to May. HYSPLIT back-trajectory analysis shows that enhanced long-range transport of aerosols, particularly from the west Asia and northwest coastal India, contributed significantly to the enhancement of AOD and in the flattening of the spectra over entire regions; if it is the peninsular regions and the island Minicoy are more impacted in April, the north Indian regions including the Indo Gangetic Plain get affected the most during May, with the AODs soaring as high as 1.0 at 500 nm. Over the islands, the Ångström exponent (α) remained significantly lower (∼1) over the Arabian Sea compared to Bay of Bengal (BoB) (∼1.4) as revealed by the data respectively from Minicoy and Port Blair. Occurrences of higher values of α, showing dominance of accumulation mode aerosols, over BoB are associated well with the advection, above the boundary layer, of fine particles from the east Asian region during March and April. The change in the airmass to marine in May results in a rapid decrease in α over the BoB.

Physical and optical characteristics of atmospheric aerosols during ICARB at Manora Peak, Nainital: A sparsely inhabited, high-altitude location in the Himalayas

Collocated measurements of the optical and physical properties of columnar and near-surface aerosols were carried out from Manora Peak, Nainital (a sparsely inhabited, high altitude location, ∼2 km above mean sea level, in the Himalayas), during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) under the Geosphere Biosphere Programme of the Indian Space Research Organization (ISRO-GBP). Under this, observational data of spectral aerosol optical depths (AOD), mass concentration of aerosol black carbon (MB), mass concentration (MT) and number concentration (Nt) of composite (total) aerosols near the surface and meteorological parameters were collected during the period February 15 to April 30, 2006. Though very low (<0.1 at 500 nm) AODs were observed during clear days, as much as a four-fold increase was seen on hazy days. The Angstrom exponent (α), deduced from the spectral AODs, revealed high values during clear days, while on hazy days α was low; with an overall mean value of 0.69 ± 0.06 for the campaign period. BC mass concentration varied between 0.36 and 2.87 μg m−3 and contributed in the range 0.7 to 1.8% to the total aerosol mass. Total aerosol number concentration and BC mass concentration showed diurnal variation with a midnight and early morning minimum and a late afternoon maximum; a pattern quite opposite to that seen in low altitude stations. These are attributed to the dynamics of the atmospheric boundary layer.

Regional and seasonal variations in aerosol optical characteristics and their frequency distributions over India during 2001–2005

Regional and temporal variations in aerosol characteristics in 35 locations spread over seven different regions in India are studied during 2001–2005 from the daily mean MODIS Terra aerosol optical depth (AOD) and fine mode fraction (FMF) data. Northeast India has the lowest annual mean AOD of 0.28 while south comes next with 0.35. In the other regions AODs are higher than 0.35. Annual mean variations in AOD and FMFs in different regions do not show any noticeable increase or decrease during the 5‐year period. High altitude locations are found to have lower AODs while densely populated, urban and industrialized locations have high AODs. Many locations show a winter low and summer high in AODs. Locations/regions dominated by pollution are found to have high FMF and high AODs, while regions in which natural (biogenic) aerosols are dominant had high FMF and range of AODs. The abundance of mechanically generated aerosols over a region results in low FMF and range of AODs. These features suggest that in addition to AOD variations knowledge on sources over a region are essential in understanding the FMF variations. Frequency distribution histograms of AODs and FMFs are consistent with the fact that aerosol sources exhibit seasonal and spatial variations over India. Dust activity peaks over north and west India during March‐May which results in low FMFs as the aerosol distributions are influenced by large size dust aerosols. In Northeast FMFs are found to be higher than 0.8 throughout the year indicating the dominance of fine mode aerosols.

Comment on “Surface changes in solar irradiance due to aerosols over central Himalayas” by U. C. Dumka et al.

Comment on “Surface changes in solar irradiance due to aerosols over central Himalayas” by U. C. Dumka et al.

A further analysis of the spatio‐temporal variability in aerosols across North America: Incorporation of lower tropospheric (850‐hPa) flow

AbstractIn our previous research (Power et al., 2006), we analysed the variability in spectral aerosol optical depth (τa), as well as Ångström's wavelength exponent (α), which represents the relative size distribution of aerosols, at 27 sites across North America. The variability was assessed by incorporating the Spatial Synoptic Classification (SSC) surface weather type across each of these sites. In this research, we further our analysis by examining the effect of different 850‐hPa flow patterns within the lower troposphere on variability in τa and α. To accomplish this, an 850‐hPa flow classification was first developed from principal components analysis (PCA) and cluster analysis on National Centers for Environmental Prediction (NCEP)/The National Center for Atmospheric Research (NCAR) reanalysis geopotential‐height data, normalized by month of year. Nine flow types were identified.Cluster analyses yielded five clusters for both τa and α. For τa, spatially cohesive regions were identified, with a western cluster with low aerosol optical depth and minimal synoptic variability, and eastern clusters that showed higher turbidity as well as synoptic variability. In these clusters, higher turbidity was associated with southerly advection. For α, results were less cohesive, with clusters largely representing the different seasonal patterns of the variable, with synoptic variability difficult to interpret.Jackknifing was also utilized to evaluate whether τa and α could be predicted based on SSC weather type and 850‐hPa flow type. Results varied at the three stations evaluated, with the method yielding potentially useful results for τa across the two eastern sites, where synoptic variability at the surface and 850‐hPa is greater. At the western site, τa could not be successfully predicted. Synoptic variability is too weak for α to be predicted effectively at any of the sites examined. Copyright © 2007 Royal Meteorological Society

Aerosol-cloud interaction inferred from MODIS satellite data and global aerosol models

Abstract. We have used the MODIS satellite data and two global aerosol models to investigate the relationships between aerosol optical depth (AOD) and cloud parameters that may be affected by the aerosol concentration. The relationships that are studied are mainly between AOD, on the one hand, and cloud cover, cloud liquid water path, and water vapour, on the other. Additionally, cloud droplet effective radius, cloud optical depth, cloud top pressure and aerosol Ångström exponent, have been analysed in a few cases. In the MODIS data we found, as in earlier studies, an enhancement in the cloud cover with increasing AOD. We find it likely that most of the strong increase in cloud cover with AOD, at least for AOD&lt;0.2, is a result of aerosol-cloud interactions and a prolonged cloud lifetime. Large and mesoscale weather systems seem not to be a cause for the increase in cloud cover with AOD in this range. Sensitivity simulations show that when water uptake of the aerosols is not taken into account in the models the modelled cloud cover mostly decreases with AOD. Part of the relationship found in the MODIS data for AOD&gt;0.2 can be explained by larger water uptake close to the clouds since relative humidity is higher in regions with higher cloud cover. The efficiency of the hygroscopic growth depends on aerosol type, the hygroscopic nature of the aerosol, the relative humidity, and to some extent the cloud screening. By analysing the Ångström exponent we find that the hygroscopic growth of the aerosol is not likely to be a main contributor to the cloud cover increase with AOD. Since the largest increase in cloud cover with AOD is for low AOD (~0.2) and thus also for low cloud cover, we argue that cloud contamination is not likely to play a large role. However, interpretation of the complex relationships between AOD and cloud parameters should be made with great care and further work is clearly needed.

40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells

An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP∕GaInAs∕Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0W∕cm2, AM1.5D, low aerosol optical depth, 25°C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.

Aerosol characteristics at a high‐altitude location in central Himalayas: Optical properties and radiative forcing

Collocated measurements of the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high‐altitude station, Manora Peak in central Himalayas, during a comprehensive aerosol field campaign in December 2004. Despite being a pristine location in the Shivalik Ranges of central Himalayas and having a monthly mean AOD (at 500 nm) of 0.059 ± 0.033 (typical to this site), total suspended particulate (TSP) concentration was in the range 15–40 μg m−3 (mean value 27.1 ± 8.3 μg m−3). Interestingly, aerosol BC had a mean concentration of 1.36 ± 0.99 μg m−3 and contributed ∼5.0 ± 1.3% to the composite aerosol mass. This large abundance of BC is found to have linkages to the human activities in the adjoining valley and to the boundary layer dynamics. Consequently, the inferred single scattering albedo lies in the range of 0.87 to 0.94 (mean value 0.90 ± 0.03), indicating significant aerosol absorption. The estimated aerosol radiative forcing was as low as −4.2 W m−2 at the surface, +0.7 W m−2 at the top of the atmosphere, implying an atmospheric forcing of +4.9 W m−2. Though absolute value of the atmospheric forcing is quite small, which arises primarily from the very low AOD (or the column abundance of aerosols), the forcing efficiency (forcing per unit optical depth) was ∼88 W m−2, which is attributed to the high BC mass fraction.

Lattice-Matched III–V Dual-Junction Solar Cells for Concentrations Around 1000 Suns

Concentration photovoltaic (PV) based on III–V solar cells is one of the most promising technologies for dramatically reducing the cost of PV electricity. In order to reduce costs, a high efficiency is usually pursued. This is the main reason for the huge development of multijunction cells (MJCs) which are able to achieve very high efficiencies thanks to their more efficient use of the solar spectrum. In the first stage, our approach to reduce the cost of photovoltaic electricity consists of a further development of the lattice matched GaInP∕GaAs dual junction solar cell in order to achieve efficiencies of over 30% at 1000 suns (AM1.5D low aerosol optical depth (AOD)). In the second stage, this approach will allow us to develop lattice matched GaInP∕Ga(In)As∕Ge triple junction solar cells with higher efficiency and lower cost. In this technical brief, we have set out the philosophy, including a brief incursion into economics, and our first results of dual-junction solar cells for high concentrator applications. Our best result is an efficiency of 27.6% at 180 suns while at 1000 suns the efficiency is 26% (AM1.5D low AOD). The price of a PV installation based on our best solar cell to date (efficiency of 26% operating at 1000 suns) would be 3.6$∕Wp. For solar cells with efficiencies of 30% at 1000 suns, the price after a cumulated production of 10MWp of a PV installation would be 3.3$∕Wp. The efficiencies attained (∼26%) at 1000 suns although still far from our objective of 30%, establish a reasonable starting point for future developments. It is evident that the conservative design implemented has much room for improvement which is now under development in our lab.

Saharan dust particle properties over the central Mediterranean

AERONET sun photometer measurements performed at five different sites of the Central Mediterranean during strong Saharan dust outbreaks are used to characterize optical and physical properties of dust particles. Sun photometer retrievals are combined at two of the five sites with lidar observations. It is shown that at low aerosol optical depths (AODs) the dust particle properties are quite dependent on dust load and monitoring site location. Differences on retrieved particle properties reduce with increasing dust load. AERONET retrievals at AOD (440 nm) ≥ 0.6 are then used in this paper to characterize dust particles over the central Mediterranean basin leading to columnar averaged values of the real refractive index < n > = 1.5 ± 0.1, the imaginary refractive index < k > = 0.004 ± 0.002, the single scattering albedo < SSA > = 0.89 ± 0.03, and the Angstrom exponent < Å > = 0.2 ± 0.1. It is shown that Å represents the best marker to trace the temporal evolution of dust events. Volume size distributions show a dominant coarse mode peaking at 1.7–3 μm. In particular, the average coarse mode that is centred at ≅ 2.2 μm at Lampedusa, which is ∼200 km away from the northwest Africa coast, gets peaked at ≅ 1.7 μm at Lecce, which is ∼800 km away. Lidar retrievals are used to characterize the vertical distribution of dust particles by the vertical profiles of the backscatter and extinction coefficients, the lidar ratio, and the depolarization ratio. Lidar retrievals show that over the Mediterranean basin, dust layers generally extend from 1 up to 6 km and that their vertical distribution can significantly change within a few hours. It is also shown that at high values of the AODs dust particles are characterized by lidar ratios spanning the 50–70 sr range and depolarization ratios larger than 30%.

Measurements of nitrogen dioxide total column amounts using a Brewer double spectrophotometer in direct Sun mode

NO2 column amounts were measured for the past 2 years at Goddard Space Flight Center, Greenbelt, Maryland, using a Brewer spectrometer in direct Sun mode. A new “bootstrap” method to calibrate the instrument is introduced and described. This technique selects the cleanest days from the database to obtain the solar reference spectrum. The main advantage for direct Sun measurements is that the conversion uncertainty from slant column to vertical column is negligible compared to the standard scattered light observations where it is typically on the order of 100% (2σ) at polluted sites. The total 2σ errors of the direct Sun retrieved column amounts decrease with solar zenith angle and are estimated at 0.2 to 0.6 Dobson units (DU, 1 DU ≈ 2.7 × 1016 molecules cm−2), which is more accurate than scattered light measurements for high NO2 amounts. Measured NO2 column amounts, ranging from 0 to 3 DU with a mean of 0.7 DU, show a pronounced daily course and a strong variability from day to day. The NO2 concentration typically increases from sunrise to noon. In the afternoon it decreases in summer and stays constant in winter. As expected from the anthropogenic nature of its source, NO2 amounts on weekends are significantly reduced. The measurements were compared to satellite retrievals from Scanning Image Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY). Satellite data give the same average NO2 column and show a seasonal cycle that is similar to the ground data in the afternoon. We show that NO2 must be considered when retrieving aerosol absorption properties, especially for situations with low aerosol optical depth.

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.

Comparison of ground‐based and Total Ozone Mapping Spectrometer erythemal UV doses at the island of Lampedusa in the period 1998–2003: Role of tropospheric aerosols

The Total Ozone Mapping Spectrometer (TOMS) has the longest time series of globally distributed estimates of UV irradiance at the Earth's surface. The proper interpretation of TOMS estimated irradiances relies on well‐calibrated and well‐maintained spectrometers at the Earth's surface. In this study, daily erythemal irradiances measured by a Brewer spectrophotometer at the island of Lampedusa (35.5°N, 12.6°E), in the Mediterranean are compared with TOMS observations in the period January 1998 to August 2003. The comparison, also because of the peculiar conditions at Lampedusa, a very good site for ground‐based validation of satellite observations, allows us to recognize how the space‐borne observations are influenced by the presence of atmospheric aerosols. Two TOMS data sets, derived applying different algorithms to retrieve ozone and UV irradiance from the backscattered radiance, are used in this study: Version 7 (V7) and the recently developed version 8 (V8), which uses new climatologies for ozone and temperature profiles and accounts for the attenuation by tropospheric aerosols through the aerosol index (AI). As shown in previous studies performed with V7 TOMS data, satellite‐derived erythemal doses systematically overestimate ground‐based measurements, mainly because of uncorrected absorption by aerosols in the troposphere. The bias between the TOMS and Brewer doses for all‐sky conditions is (9.4 ± 19.8)% for V7 and (7.3 ± 20.0)% for V8 and decreases to (5.6 ± 8.0)% for V7 and (3.4 ± 8.4)% for V8 for the cloud‐free cases. The large standard deviations for all‐sky conditions are due to nonhomogeneity in the cloud cover within the sensor field of view, while those for cloud‐free days are caused by the large aerosol variability occurring at Lampedusa. The biases for cloud‐free days have been related to differences in the TOMS AI UV attenuation algorithm and to the aerosol optical depth (AOD) at 415.6 nm measured with a Sun photometer at Lampedusa since 2001. The mean bias between the V7 TOMS and Brewer doses progressively increases with AI and AOD at 415.6 nm, from ± 3% for low AI and AOD up to 21% for 1.5 ≤ AI &lt; 2.5 and 0.5 ≤ AOD &lt; 0.6. The bias calculated with V8 data set varies between +6% for 0 ≤ AI &lt; 1 and about −8% for 4 ≤ AI &lt; 5, well within the respective uncertainties of the Brewer and TOMS measurements. TOMS V8 data show a smaller dependency on the aerosol absorption, indicating that the implemented corrections produce more reliable estimated doses. For very low aerosol loading (AOD at 415.6 nm below 0.2), the TOMS‐to‐Brewer erythemal dose ratio, both for V7 and V8, is approximately 1, indicating that the radiometric calibration of the Brewer instrument is consistent with the TOMS estimated irradiances from derived ozone and Rayleigh scattering attenuation.

Trends in aerosol optical depth in the Russian Arctic and their links with synoptic climatology

Temporal and spatial variability of aerosol optical depth (AOD) are examined using observations of direct solar radiation in the Eurasian Arctic for 1940–1990. AOD is estimated using empirical methods for 14 stations located between 66.2°N and 80.6°N, from the Kara Sea to the Chukchi Sea. While AOD exhibits a well-known springtime maximum and summertime minimum at all stations, atmospheric turbidity is higher in spring in the western (Kara–Laptev) part of the Eurasian Arctic. Between June and August, the eastern (East Siberian–Chukchi) sector experiences higher transparency than the western part. A statistically significant positive trend in AOD was observed in the Kara–Laptev sector between the late 1950s and the early 1980s predominantly in spring when pollution-derived aerosol dominates the Arctic atmosphere but not in the eastern sector. Although all stations are remote, those with positive trends are located closer to the anthropogenic sources of air pollution. By contrast, a widespread decline in AOD was observed between 1982 and 1990 in the eastern Arctic in spring but was limited to two sites in the western Arctic. These results suggest that the post-1982 decline in anthropogenic emissions in Europe and the former Soviet Union has had a limited effect on aerosol load in the Arctic. The post-1982 negative trends in AOD in summer, when marine aerosol is present in the atmosphere, were more common in the west. The relationships between AOD and atmospheric circulation are examined using a synoptic climatology approach. In spring, AOD depends primarily on the strength and direction of air flow. Thus strong westerly and northerly flows result in low AOD values in the East Siberian–Chukchi sector. By contrast, strong southerly flow associated with the passage of depressions results in high AOD in the Kara–Laptev sector and trajectory analysis points to the contribution of industrial regions of the sub-Arctic. In summer, low pressure gradient or anticyclonic conditions result in high atmospheric turbidity. The frequency of this weather type has declined significantly since the early 1980s in the Kara–Laptev sector, which partly explains the decline in summer AOD values.

Direct observations of the effects of aerosol loading on net ecosystem CO2 exchanges over different landscapes

We present the first direct, multisite observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. The daytime growing season (summer) CO2 flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for high and low diffuse radiation; effect of cloud cover; and effect of high and low aerosol optical depths (AOD). Results indicate that, aerosols exert a significant impact on net CO2 exchange, and their effect may be even more significant than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthetic pathway. The CO2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The cause for the difference in response between vegetation types is hypothesized to be canopy architecture.