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Delhi experiences frequent dust storms during the pre-monsoon season. We studied one such event that occurred on May 30th, 2014 at New Delhi to understand the changes in the optical properties of aerosols. The present study utilizes the aerosol optical depth (AOD) from the ground and satellite measurements along with reanalysis data. The aerosol characteristics were examined during the pre-week and post-week of the dust storm event. The results show that AOD values reach ~ 1.0 in all the modes of measurements (i.e., sun photometer, MODIS, and MERRA-2) on the dust event day. MODIS-derived Aerosol Angstrom Exponent (AAE), which gradually decreases from 1.7 ± 0.3 to 1.1 ± 0.3, shows the presence of coarse mode particles during the event. The aerosol layer at an elevation from 1 to 2 km was observed using indigenously developed dual-polarization micro-pulse lidar (MPL) system during the dust event. The source of the dust during the storm was identified by the air mass back-trajectories analysis. The dust column mass density averaged 24 hourly at 850 hPa is 780 mg m−2 with a prevailing wind direction of west to east with wind speed of 11–13 ms−1. Shortwave aerosol direct radiative forcing (SWADRF) diurnally averaged at the top of the atmosphere (TOA), the atmosphere (ATM), and surface (SUR) are found to be − 10 W m−2, 27 W m−2, and − 37 W m−2, respectively. The heating rate was enhanced by 56% (from 0.16 to 0.25 K day−1) during the event in the whole atmospheric column. Visibility decreased by 31% during the dust storm day compared with its average value of non-dust storm days.

We analyzed data measured by a Sun-photometer of the RIMA-AERONET network with the purpose to characterize the aerosol properties in the atmosphere over Natal, state capital of Rio Grande do Norte, at the coast of Northeast Brazil. Aerosol Optical Depth, Ångström Exponent, Volume Size Distribution, Single Scattering Albedo, Complex Refractive Index, Asymmetry Factor, and Precipitable Water were analyzed from August 2017 to March 2018. In addition, MODIS and CALIOP observations, local Lidar measurements, and modeled backward trajectories were analyzed in a case study on February 9, 2018, that consistently confirmed the identification of a persistent aerosol layer below 4 km agl. Aerosols present in the atmosphere of Natal showed monthly mean Aerosol Optical Depth at 500 nm below 0.15 (~ 75%), monthly means of the Ångström Exponent at 440-670 nm between 0.30 and 0.70 (~ 69%), bimodal Volume Size Distribution is dominantly coarse mode, Single Scattering Albedo at 440 nm is 0.80, Refractive Index - Real Part around 1.50, Refractive Index - Imaginary Part ranging from 0.01 to 0.04, and the Asymmetry Factor ranged from 0.73 to 0.80. The aerosol typing during the measurement period showed that atmospheric aerosol over Natal is mostly composed of mixed aerosol (58.10%), marine aerosol (34.80%), mineral dust (6.30%), and biomass burning aerosols (0.80%). Backward trajectories identified that 51% of the analyzed air masses over Natal originated from the African continent.

In this paper, we report first-time observations of total column ozone (TCO), aerosol optical thickness (AOT) and precipitable water content (PWC) at five different wavelengths using compact, handheld, multichannel sun photometer (Microtops II ozonometer) at Atigre village (16.74°N latitude, 74.37°E longitude, 604 ms altitude above sea level). The three optical filters at wavelengths 305 nm, 312 nm and 320 nm are used to detect the TCO, while two optical filters 936 nm and 1020 nm are used to detect the PWC and AOT, respectively. We have been collecting data from September 1, 2017, and 122 days are selected from September 1, 2017, to December 31, 2017, to present as first observations of TCO, AOD and PWC. The mean values of TCO, AOT and PWC are found to be 261.2 (± 9.22) DU, 0.25 (± 0.04) and 1.98 (± 0.79) cm, respectively, during this period. The TCO measurements taken by the Microtops II ozonometer are compared with the ozone monitoring instrument satellite data over Atigre. We found that PWC and AOT values are negatively correlated (− 0.54) during monsoon season, while they show positive correlation (0.63) during other seasons with 90% significance level. The diurnal variation of TCO, PWC and AOT differ in monsoon and the post-monsoon season.

The geographical and climatic conditions of West Africa make the region an essential area for the description of atmospheric aerosol optical properties across the globe. This study provides an intercomparison of aerosol optical depth (AOD440nm) retrievals from satellite-based CALIPSO, MISR, MODIS and OMI sensors against ground-based sun photometer (AERONET) measurements between 2004 and 2014 from six West African sites, which are Agoufou, Banizoumbou, Ouagadougou, Dakar, Djougou and Ilorin during the period 2004–2014. The analysis revealed that MODISSTD performed better with a high degree of correlation for the six study sites, the CALIPSO and MISR–AERONET comparisons showed strong correlations, and the MODISDB also indicated better correlations, as did the OMI–AERONET comparisons. The root mean square error, mean absolute error and root mean bias error were also computed. The CALIPSO instrument has the lowest RMSE and MAE values over Dakar, while the highest RMSE and MAE values were indicated by the CALIPSO and MODISDB sensors, respectively, over Ilorin. The MISR instrument showed good agreement over Dakar than the other instruments, while CALIPSO AOD retrievals were better than those from the other sensors in Banizoumbou and Ouagadogou. The expected error bounds computed for both MODIS retrievals showed that MODISSTD consistently outperformed MODISDB in all the study sites. High AOD values were averagely observed by the satellite sensors during the local dry months (December–February), due to high concentrations of dust aerosols. High AOD values were observed during March–May, due to the condensation of water vapor on aerosol leading to increase in size and optical depth.

Investigations into the influence of variations in relative humidity, mixed-layer height, and mesoscale vertical velocity on column and surface aerosol characteristics over urban regions are quite rare. Here we report on a comprehensive investigation that was conducted over Ahmedabad, an urban location in western India, during December 2006. In this campaign columnar and surface aerosol characteristics were measured and compared to relative humidity, mixed-layer height, and mesoscale vertical velocity to examine their influence on urban aerosol characteristics. The 500-nm aerosol optical depth was found to be approximately 0.8 between 24 and 26 December while on a cleaner day such as 7 and 18 December the aerosol optical depth was 0.1. Aerosol optical depths based on Moderate Resolution Imaging Spectroradiometer (MODIS) level-2 and level-3 data, and on in situ Sun photometer measurements, show good agreement. The scattering coefficient ( $$\beta _{\mathrm{sca}}$$ ) and absorption coefficient ( $$\beta _{\mathrm{abs}}$$ ) increased by a factor of 5–10 on 26 December compared to 7 December (a normal day). This latter date was characterized by a clear atmosphere, a lower mixed-layer height ( $$\approx $$ 1650 m), positive vertical velocity and higher aerosol scale height (>3 km), while 26 December was marked with hazy and smoky conditions, a larger mixed-layer height ( $$\approx $$ 2500 m), negative vertical velocity and smaller aerosol scale height ( $$\approx $$ 1 km). These atmospheric conditions lead to lower and higher values of surface and columnar aerosol concentrations on 7 and 26 December respectively. A measure of spectral aerosol absorption, $$\alpha '_{\mathrm{abs}}$$ > 2, indicating the dominance of carbonaceous aerosols from biomass/biofuel emissions (open biomass burning), is rather surprising as fossil-fuel emissions that produce strongly light absorbing carbonaceous particles usually dominate urban regions. The single-scattering albedo on both days is 0.56 and 0.67 respectively, while monthly mean hemispheric backscatter fraction b and asymmetry parameter g values are 0.16 and 0.53 respectively. Higher b and lower g values on 7 December, and lower b and higher g values on 26 December, provide the relative scale of variation in the amount of sub-micron aerosols that dominate on a normal/clear day vis-a-vis a smoky/perturbed day. Lower single-scattering albedo indicates the dominance of absorbing aerosols above Ahmedabad, and higher b and lower g values suggest the abundance of fine mode particles in aerosol size distribution. The in-depth results serve as representative inputs to modelling the column and surface characteristics, and the resultant radiative forcing of urban aerosols influenced by variations in relative humidity, mixed-layer height, and mesoscale vertical velocity.

We consider an algorithm for processing lidar signals, which for a lidar path with constant qualitative aerosol characteristics (particle size distribution in each aerosol mode, complex refractive index of the particles) lets us obtain a concentration profile for the fine aerosol component without using additional a priori information. The algorithm was verified based on lidar data from the Institute of Physics (National Academy of Sciences of Belarus). The vertical volume concentration profile for fine particles obtained by our algorithm is quite consistent with the results of its independent retrieval, based on integrated processing of lidar and sun photometer data.

In the present work a hybrid approach is implemented to retrieve aerosol optical depth (AOD) from ocean colour monitor sensor on board Oceansat-2. The retrieved AOD (490 nm) has been validated with the ground observed AOD (500 nm) made at different locations of the study area for different months observed by AERONET sites and hand held sun photometer. The Oceasat 2 retrieved AOD (490 nm) is also compared with the MODIS retrieved AOD (470 nm). It is observed that Oceansat 2 retrieved AOD is in good agreement with ground observed AOD with a significant correlation of 0.839 and with a RMSE value of 0.273. The comparison between Oceansat 2 retrieved AOD and MODIS retried AOD are also found to be encouraging with a correlation coefficient of 0.852 with RMSE 0.1243.

The detailed analysis of columnar optical and microphysical properties of aerosols obtained from the AErosol RObotic NETwork (AERONET) Cimel sun photometer operated at Skukuza (24.98° S, 31.60° E, 150 m above sea level), South Africa was carried out using the level 2.0 direct sun and inversion products measured during 1999–2010. The observed aerosol optical depth (AOD) was generally low over the region, with high values noted in late winter (August) and mid-spring (September and October) seasons. The major aerosol types found during the study period were made of 3.74, 69.63, 9.34, 8.83, and 8.41% for polluted dust (PD), polluted continental (PC), non-absorbing (NA), slightly absorbing (SA), and moderately absorbing (MA) aerosols, respectively. Much attention was given to the aerosol fine- and coarse-modes deduced from the particle volume concentration, effective radius, and fine-mode volume fraction. The aerosol volume size distribution pattern was found to be bimodal with the fine-mode showing predominance relative to coarse-mode during the winter and spring seasons, owing to the onset of the biomass burning season. The mean values of total, fine-, and coarse-mode volume particle concentrations were 0.07 ± 0.04, 0.03 ± 0.03, and 0.04 ± 0.02 μm3 μm−2, respectively, whereas the mean respective effective radii observed at Skukuza for the abovementioned modes were 0.35 ± 0.17, 0.14 ± 0.02, and 2.08 ± 0.02 μm. The averaged shortwave direct aerosol radiative forcing (ARF) observed within the atmosphere was found to be positive (absorption or heating effect), whereas the negative forcing in the surface and TOA depicted significant cooling effect due to more scattering type particles.

Sun-photometer measurements at Hefei, an urban site located in central East China, were examined to investigate the variations of aerosol loading and optical properties. It is found that aerosol optical thickness (AOT) keeps higher in winter/spring and gets relatively lower in summer/autumn. The large AOT in winter is caused by anthropogenic sulfate/nitrate aerosols, while in spring dust particles elevate the background aerosol loading and the excessive fine-mode particles eventually lead to severe pollution. There is a dramatic decline of AOT during summer, with monthly averaged AOT reaching the maximum in June and soon the minimum in August. Meanwhile, aerosol size decreases consistently and single scattering albedo (SSA) reaches its minimum in July. During summertime large-sized particles play a key role to change the air from clean to mild-pollution situation, while the presence of massive small-sized particles makes the air being even more polluted. These complicated summer patterns are possibly related to the three key processes that are active in the high temperature/humidity environment concentrating on sulfate/nitrate aerosols, i.e., gas-to-particle transformation, hygroscopic growth, and wet scavenging. Regardless of season, the increase of SSA with increasing AOT occurs across the visible and near-infrared bands, suggesting the dominant negative/cooling effect with the elevated aerosol loading. The SSA spectra under varying AOT monotonically decrease with wavelength. The relatively large slope arises in summer, reinforcing the dominance of sulfate/nitrate aerosols that induce severe pollution in summer season around this city.

Aerosols influence the radiation budget of the Earth’s atmospheric system. Aerosol particle size distribution is one of the major parameters used for characterizing aerosol influence on radiative forcing. The optical and microphysical properties of aerosol particles over Yinchuan, China, were measured with a multiwavelength lidar developed at Beifang University of Nationalities using backscatter and extinction coefficients at wavelengths of 1064, 532, and 355 nm. These data were used to retrieve particle size distributions. Given the disadvantages of the traditional regularization method, the innovative multipopulation genetic algorithm (MPGA) was used to retrieve the particle size distribution from the lidar data. To verify the feasibility of using the MPGA on multiwavelength lidar data, experiments were carried out under different atmospheric conditions, including a background sunny day, a cloudy day, and a foggy day. The particle size distributions obtained from the multiwavelength lidar data were compared with results retrieved from direct irradiance data from a sun photometer. Results showed that the MPGA is suitable for retrieving particle size distributions from multiwavelength lidar data.

Lidar is an effective remote sensing method for obtaining the optical properties of aerosols, such as the aerosol extinction coefficient (AEC), the aerosol optical depth (AOD), and the related atmospheric visibility. However, improving the accuracy and efficiency of lidar data retrieval remains challenging due to the uncertainties associated in determining the AEC boundary value (AEC-BV) and the aerosol extinction-to-backscatter ratio (AEBR), as well as the complex and time-consuming calculations required. In this paper, we propose a novel method, a feedback radial basis function (RBF-FB), for retrieving high-precision AEC profiles based on a radial basis function neural network. First, using the secant method, we determine accurate values for AEC-BV and AEBR, and generate the AEC profiles by the Fernald method. We then choose a set of lidar signals and their corresponding AEC profiles as learning samples for network training and establish an RBF network model for AEC retrieval. Next, we correct the network output by introducing a feedback mechanism that uses the AOD measured by a sun photometer as the error criterion. Tests on measured signals confirm that the outputs of the proposed RBF-FB model are consistent with the Fernald method and have the advantages of speed and robustness.

Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) provides three-dimensional information on aerosol optical properties across the globe. However, the performance of CALIPSO aerosol optical depth (AOD) products under different air quality conditions remains unclear. In this research, three years of CALIPSO level 2 AOD data (November 2013 to December 2017) were employed to compare with the Moderate Resolution Imaging Spectroradiometer (MODIS) level 2 columnar AOD products and ground-based sun photometer measurements for the same time period. To investigate the effect of air quality on AODs retrieved from CALIPSO, the AODs obtained from CALIPSO, MODIS, and sun photometer were inter-compared under different air quality conditions over Wuhan and Dengfeng. The average absolute bias of AOD between CALIPSO and sun photometer was 0.22 ± 0.21, 0.11 ± 0.07, and 0.14 ± 0.13 under clean, moderate, and polluted weather, respectively. The result indicates that the CALIPSO AOD were more reliable under moderate and polluted days. Moreover, the deviation of AOD between CALIPSO and sun photometer was largest (0.23 ± 0.21) in the autumn season, and lowest (0.13 ± 0.12) in the winter season. The results show that CALIPSO AOD products were more applicable to regions and seasons with high aerosol concentrations.

Particle size distribution is essential for describing direct and indirect radiation of aerosols. Because the relationship between the aerosol size distribution and optical thickness (AOT) is an ill-posed Fredholm integral equation of the first type, the traditional techniques for determining such size distributions, such as the Phillips–Twomey regularization method, are often ambiguous. Here, we use an approach based on an improved particle swarm optimization algorithm (IPSO) to retrieve aerosol size distribution. Using AOT data measured by a CE318 sun photometer in Yinchuan, we compared the aerosol size distributions retrieved using a simple genetic algorithm, a basic particle swarm optimization algorithm and the IPSO. Aerosol size distributions for different weather conditions were analyzed, including sunny, dusty and hazy conditions. Our results show that the IPSO-based inversion method retrieved aerosol size distributions under all weather conditions, showing great potential for similar size distribution inversions.

An improved method of calibration of sun photometers in the case of different optical air masses is proposed. An analytic condition for assurance of stable performance of periodic calibration of such photometers, taking into account random temporal variations of atmospheric aerosol, is obtained. An example of a calibration performed on the basis of fuzzy sets is demonstrated.

An algorithm to retrieve aerosol optical properties using multi-angular, multi-spectral, and polarized data without a priori knowledge of the land surface was developed. In the algorithm, the surface polarized reflectance was estimated by eliminating the atmospheric scattering from measured polarized reflectance at 1640 nm. A lookup table (LUT) and an iterative method were adopted in the algorithm to retrieve the aerosol optical thickness (AOT, at 665 nm) and the Angstrom exponent (computed between the AOTs at 665 and 865 nm). Experiments were performed in Tianjin to verify the algorithm. Data were provided by a newly developed airborne instrument, the Advanced Atmosphere Multi-angle Polarization Radiometer (AMPR). The AMPR measurements over the target field agreed well with the nearby ground-based sun photometer. An algorithm based on Research Scanning Polarimeter (RSP) measurements was introduced to validate the observational measurements along a flight path over Tianjin. The retrievals were consistent between the two algorithms. The AMPR algorithm shows potential in retrieving aerosol optical properties over a vegetation surface.

Solar irradiance is attenuated spectrally when passing through the earth’s atmosphere and it is strongly dependent on sky conditions, cleanliness of the atmosphere, composition of aerosols and gaseous constituents. In this paper, aerosol optical properties including aerosol optical depth (AOD), Angstrom exponent (α) and Angstrom turbidity coefficient (β) have been investigated during December 2009 to October 2010, in a suburban area of Zanjan (36°N, 43°E, 1700 m), in the north–west of Iran, using meteorological and sun photometric data. Results show that turbidity varies on all time scales, from the seasonal to hourly, because of changes in the atmospheric meteorological parameters. The values of α range from near zero to 1.67. The diurnal variation of AOD in Zanjan is about 15%. The diurnal variability of AOD, showed a similar variation pattern in spring (including March, April, May) and winter (December, January, February) and had a different variation pattern in summer (June, July, August) and autumn (September and October). During February, spring and early summer winds transport continental aerosols mostly from the Iraq (dust events) and cause the increase of beta and turbidity of atmosphere of Zanjan.

Measurements were obtained at two sites in northern Tien-Shan in Central Asia during a 1-year period beginning July 2008 to examine the statistical relationship between aerosol optical depth (AOD) and of fine [PM2.5, particles less than 2.5 μm aerodynamic diameter (AD)] and coarse (PMCoarse, particles between 2.5 and 10 μm AD) mass concentrations and composition. The measurements represent the first extended particulate matter measurements in the northern Tien-Shan region of Central Asia. A sun photometer (Microtops II) was used to measure AOD from the surface, which is a widely used aerosol monitoring technique that is used in the AERONET network. In parallel, less routine measurements of continuous hourly PM2.5 data were obtained with the TEOM/FDMS whereas daily average PM2.5 and PM10 were obtained using URG-3000ABC samplers. Daily samples were collected on an every-other-day basis throughout the year. Since clouds interfere with the AOD measurement, a cloud screening procedure based on LIDAR measurements was applied to the AOD data and cloud impacted days were removed from the AOD data set. Depending on the season, the correlation coefficient (r) varied from 0.56 up to 0.87. Higher correlation coefficients between PM2.5 mass and AOD were observed during the spring and autumn periods and appeared to result from the transport of Asian dust (desert crustal material) particles from outside the area. One of the main source areas was the Taklimakan desert located in northwestern China. Linear regression results between AOD and PM2.5 are presented that allow for an estimate of PM2.5 mass concentrations at the surface based on the AOD data, which can be used to help interpret AOD measurements made in Central Asia and potentially other regions of the world.

The UV attenuation due to atmospheric aerosols in Guangzhou was quantitatively assessed using surface ultraviolet radiation (UV, 295–385 nm) observation, sun photometer and radiation models. Observations showed that the annual average value of aerosol optical depth (AOD) was 1.19 in UV spectral region of 340 nm, the annual average occurrence frequency of aerosol optical depth AOD340 nm>1.0 was 55%, and the annual average attenuation rate of surface UV direct radiation of 340 nm was 68%. It was proven in the observation of surface UV radiation and model evaluation that annual average attenuation of UV due to atmosphere was 75%, and that reached 72% in the dry season (October, November, December and January); while average attenuation of UV due to atmospheric aerosols reached 62% in the dry season. It was indicated that very significant UV attenuations due to atmospheric aerosols existed in Guangzhou urban agglomeration, and at least half of UV radiation was attenuated due to atmospheric aerosols. Such large-amplitude attenuation will have a significant impact on urban ecosystem and species chemical cycles, especially photochemical reaction processes.

Total column ozone (TCO) measured by the portable, handheld, multichannel Sun photometer (ozonometer-Microtops-II) at Visakhapatnam (17.43°N, 83.14°E; 51 m above mean sea level), India, which is a tropical urban station has been analysed in this study. The results showed the estimate of the annual and seasonal trends of TCO from the period February 2005 to December 2008. The linear regression technique was applied in Microtops II data to study the trend of TCO during 2005–2008. It also provides statistics of mean and variability on temporal scale. The results showed increasing trend at Visakhapatnam an average of 1.88 DU per year (or 0.61% per year). However the maximum deviation in diurnal variability of TCO were 28 DU (9.19%) while the minimum deviation were −36 DU (−11.8%) respectively, which is one of the unique result of this study. The range of the seasonal change represents 9.2% of the mean TCO value with maximum/minimum during March/December.

Aerosol optical properties have been studied for spring seasons when increased values of PM10 are registered. Measurements of aerosol optical properties were taken by collocated lidar and sun-photometers at Belsk, Poland, and Minsk, Belarus. A significant increase of registered aerosol optical thickness (AOT) was found during episodes with elevated PM10 concentrations. An increase of AOT at 1020 nm amounted to 50% in the case of Minsk and 18% in the case of Belsk, while an increase of AOT at 400 nm was 66% and 33%, respectively. We noted an increase of Angstrom exponent by 6% at both stations and no significant increase of single scattering albedo. The LIDAR measurements together with NAAPS model results and backtrajectory analysis suggest that both the biomass burning products and the Saharan dust are responsible for increased PM10 concentrations and large AOT values during spring time. The smoke aerosol is transported over Central Europe mainly in the boundary layer, increasing both PM10 concentration and AOT. The dust aerosol transported in the free troposphere slightly affects the AOT values only. Statistically significant correlation between PM10 concentration and AOT was found during reporting period.

This paper presents the results of the studies of aerosol optical properties measured using lidars and sun photometers. We describe two case studies of the combined measurements made in two coastal zones in Crete in 2006 and in Rozewie on the Baltic Sea in 2009. The combination of lidar and sun photometer measurements provides comprehensive information on both the total aerosol optical thickness in the entire atmosphere as well as the vertical structure of aerosol optical properties. Combination of such information with air mass back-trajectories and data collected at stations located on the route of air masses provides complete picture of the aerosol variations in the study area both vertically and horizontally. We show that such combined studies are especially important in the coastal areas where depending on air mass advection directions and altitudes the influence of fine or coarse mode (in this case possibly sea-salt) particles on the vertical structure of aerosol optical properties is an important issue to consider.

This paper presents the measurements of a vertical structure of aerosol optical properties performed during the MACRON (Maritime Aerosol, Clouds and Radiation Observation in Norway) campaign, which took place in July and August 2007 at ALOMAR observatory on Andoya island (69.279°N, 16.009°E, elevation 380 m a.s.l.). The mean value of the aerosol optical thickness (AOT) at 500 nm during campaign was 0.12. Significant increase of the AOT above longtime mean value was observed on 7 and 8 August 2007 when the AOT exceeded 0.4 at 500 nm. Analyses of back trajectories show the aerosol transported from over Africa and Central Europe. The aerosol extinction coefficient obtained from the synergy of ceilometer and sun photometer observations reached 0.05-0.08 km –1 (at 1064 nm) in the dust layer. The single scattering

The relationship between the radiance ratio (radiance at wavelength 450 nm to 650 nm) and aerosol optical depth (AOD) is analyzed in this paper by numerical simulation and a “LUT” (look-up table) approach is then presented for the retrieval of AOD from the radiance ratio. In this LUT approach, the AOD retrieval error depends mainly on the assumption of aerosol types. From the preliminary simulation, a typical error of 15%–20% in AOD obtained from the radiance ratio is estimated, due to neglecting changes in the ground albedo and background atmosphere. At its worst, the AOD error reached a maximum of around 50%, which will be refined in the future. In the latter part of the paper, comparisons are made between AOD from the imager and from the CE-318 sun photometer, both located at Xianghe observatory in Hebei Province (39.75°N, 116.96°E). This field experiment shows that AOD from the imager is highly correlated with AOD from the sun photometer, with a correlation coefficient of 0.95 and an average retrieval error of around 7%. A contrast experiment confirms the feasibility of retrieving AOD from all-sky images, but more analysis and future research are required to improve the accuracy.

The lidar ratios at 500 and 1020 nm were derived from POM 01L sun-sky scanning photometer measurements taken at Belsk Geophysical Observatory (long. 20°47′, lat. 51°50′) in the period from 2002 to 2006. The most frequently occurring lidar ratio values for the study period are 50 sr and 30 sr at 500 nm and 1020 nm, respectively. Calculations of lidar ratios for summer and winter seasons have been made as well. Back trajectory analysis was also performed to final aerosol source of origin.

In this paper, optical measurements of aerosol properties made during a ship cruise from Poland to Antarctic Station in September and October 2006, and during the cruise back to Gdynia in April and May 2007 are described. A large gradient of pollution between the clear South Atlantic and the dusty North Atlantic was observed. The maximum of aerosol optical thickness at a wavelength of 500 nm reached 0.4 at 20°N in September 2006 and 0.3 at 40°N in May 2007, respectively. Strong Saharan dust transport is suggested as an explanation for the small values of Angstrom exponent observed (values of 0.2 and 0.4 on these respective dates). On the Southern Hemisphere the aerosol optical thickness at 500 nm ranged from 0.05 to 0.2. Significant increases of the aerosol optical thickness were associated with strong wind and sea salt production. Good agreement was found when the in situ measurements of aerosol optical thickness were compared to satellite retrievals and modelling results.

Spatial variations in aerosol optical properties as function of latitude and longitude are analysed over the Bay of Bengal and Arabian Sea during ICARB cruise period of March–May 2006 from in situ sun photometer and MODIS (Terra, Aqua) satellite measurements. Monthly mean 550 nm aerosol optical depths (AODs) over the Bay of Bengal and Arabian Sea show an increase from March to May both in spatial extent and magnitude. AODs are found to increase with latitude from 4°N to 20°N over the Bay of Bengal while over Arabian Sea, variations are not significant. Sun photometer and MODIS AODs agree well within ±1σ variation. Bay of Bengal AOD (0.28) is higher than the Arabian Sea (0.24) latitudinally. Aerosol fine mode fraction (FMF) is higher than 0.6 over Bay of Bengal, while FMF in the Arabian Sea is about 0.5. Bay of Bengal α(∼1) is higher than the Arabian Sea value of 0.7, suggesting the dominance of fine mode aerosols over Bay of Bengal which is corroborated by higher FMF values over Bay of Bengal. Air back trajectory analyses suggest that aerosols from different source regions contribute differently to the optical characteristics over the Bay of Bengal and Arabian Sea.

Measurements of total ozone column and solar UV radiation under different atmospheric conditions are needed to define variations of both UV and ozone and to study the impact of ozone depletion at the Earth’s surface. In this study, spectral and broadband measurements of UV-B irradiance were obtained along with total ozone observations and aerosol optical depth measurements in the tropical urban region of Hyderabad, south India. We specifically used an Ultra-Violet Multifilter Rotating Shadow band Radiometer (UVMFR-SR), to measure UV irradiance in time and space. To assess the aerosol and O3 effects on ground-reaching UV irradiance, we used measurements from a Microtops II sun photometer in addition to the Tropospheric Ultraviolet Visible radiation (TUV) model. We also assessed the Defense Meteorological Satellite Program – Operational Line Scanner (DMSP-OLS) night time satellite data for inferring biomass burning fires during the study period. Results clearly suggested a negative correlation between the DMSP-OLS satellite derived fire count data and UVMFR-SR data suggesting that aerosols from biomass burning are directly attenuating UV irradiance in the study region. Also, correlation analysis between UV index and ozone measurements from sun photometer and TOMS-Ozone Mapping Instrument (OMI) indicated a clear decrease in ground reaching UV-B irradiance during higher ozone conditions. The higher levels are attributed to photochemical production of O3 during the oxidation of trace gases emitted from biomass burning. Results also suggested a relatively high attenuation in UV irradiance (~6% higher) from smoke particles than dust. We also found a relatively good agreement between the modeled (TUV) and measured UV irradiance spectra for different atmospheric conditions. Our results highlight the factors affecting UV irradiance in a tropical urban environment, south India.

In the paper application of lidars for investigation of aerosol particle size distribution and for observation of aerosol consisting of solid state particles is presented. For size distribution the multiwavelength lidar and original method of data analysis was applied. For registration of dust transported to Central Europe from Sahara and Middle East deserts analysis of depolarization of the backscattered signals was used. In order to solve the lidar equation measurements of total atmospheric optical depth by means of Microtops sun photometer was done. Mean size and the aspect ratio of dust particles were determined by comparing of lidar observations with data from T-matrix calculations.

We present measurements of the vertical aerosol structure and the aerosol optical depth in the lower troposphere performed above the city of Sofia (an urban area situated in a mountain valley), western Bulgaria by means of a ground-based aerosol lidar operating continuously for a number of years. The lidar measurements were accompanied by measurements of the aerosol optical depth (AOD) in the visible and near infrared regions of the spectrum performed in October 2004 using Microtops II radiometers. The maximum values of the AOD were found to occur 1-2 h before the complete development of the atmospheric boundary layer, i.e. during the residual layer destruction, which confirms our hypothesis concerning the slope circulation effect on the processes taking place in the atmospheric boundary layer. The AOD values obtained by the lidar are lower than those taken by the sun photometer. Further, the AOD exhibits two different types of behaviour. In the case of a 'clear atmosphere' (i.e. in the absence of volcanic eruptions and/or dust transport from the Sahara) most of the aerosol accumulated within the atmospheric boundary layer over the urban area considered. The combined use of the two instruments allows the comparison between the optical characteristics of the atmospheric aerosol (e.g. aerosol extinction coefficient, etc.) obtained by the lidar and through an independent method (sun photometer).

The atmospheric correction bands 7 and 8 (765nm and 865nm respectively) of the Indian Remote Sensing Satellite IRS P4-0CM (Ocean Colour Monitor) can be used for deriving aerosol optical depth (AOD) over the oceans. A retrieval algorithm has been developed which computes the AOD using band 7 data by treating the ocean surface as a dark background after removing the Rayleigh path radiance in the sensor-detected radiances. This algorithm has been used to detect marine aerosol distributions at different coastal and offshore locations around India. A comparison between OCM derived AOD and the NOAA operational AOD shows a correlation ∼0.92 while that between OCM derived AOD and the ground-based sun photometer measurements near the coast of Trivandrum shows a correlation of ∼0.90.

The uncertainty in aerosol size distributions is a main source of errors in aerosol optical thickness determined from satellite measurements. To reduce the errors resulting from the uncertainty in aerosol size distributions, we have performed sensitivity analyses. It is found the errors resulting from the uncertainty in aerosol size distribution can be considerably reduced by using the Junge power law to approximate the aerosol size distribution in an actual atmosphere, if the exponent value is determined at the same time. An iterative algorithm is then developed for the simultaneous determination of aerosol optical thickness and the exponent of the Junge power law over ocean areas from the upwelling radiances measured in AVHRR visible and near infrared channels. A number of numerical experiments are carried out to investigate the validity of the Junge power law approximation by assuming the aerosol size distributions in an actual atmosphere are bimodal with different mode parameters, and by using the actual aerosol size distributions determined at several places by Kaufman et al. (1994). The results show that the errors in determined aerosol optical thickness resulting from the Junge power law approach are significantly reduced. The iterative algorithm is investigated further by comparing the aerosol optical thickness deduced from satellite measurement with that observed by a sun photometer.

The multi-wavelength aerosol optical depths and the Angstrom exponent between 450 nm and 900 nm in four locations of China (Miyun, Beijing; Xinfeng, Guangdong; Waliguan, Qinghai and Damxung, Tibet) have been observed by sun-photometers. This paper analyzes their characteristics for a one-year period from February 1998 to January 1999. The results show that in the arid and semi-arid locations like Miyun (117.12°E, 40.65°N) and Waliguan (100.90°E, 36.29°N), there is a maximum of aerosol optical depth in the spring, it is about twice as large as in any other season. In a humid region like Xinfeng (114.2“E, 24.5°N), the aerosol optical depth also has a maximum in the spring, however it is only slightly larger than in other seasons. The Angstrom exponent shows a significant minimum at Waliguan in spring, about 0.15, indicating relatively large dust aerosol particles. Large variability of the monthly mean Angstrom exponent is also found in Miyun and Xinfeng, but there is no tendency with seasons. It means that the source of aerosol in these locations is complicated.

A sun photometer fitted with 9 narrowband interference filters from 368 nm to 1024 nm was used to determine the aerosol optical depth at 8 measuring stations in Europe. Of the 9 filters, 4 were not included in the standard equipment of the sun photometer. Given the temporal instabilities of the calibration factors of these 4 filters, meaningful results for the aerosol optical depth can be achieved only if calibration and measurements follow each other closely. Calibration was performed using the Langley plot method. For wavelengths < 500 nm, the measured aerosol optical depths were compared with the results of a high resolution spectrometer. Broadband aerosol optical depths, centred at 427 nm, were determined with an actinometer and were also used for comparison. Generally, there was good agreement of the results. To characterize the aerosol optical depths, the Angstrom parametersα andβ were used. The individual measuring stations showed a clustered distribution ofα andβ values.

An infrared Sun photometer has been designed, developed and set up at NPL, New Delhi. The IR Sun photometer has been used successfully to study the absorption spectrum of the Earth’s atmosphere in the 2.5 μm-14.5 μm wavelength range. The observed atmospheric spectrum in the 2.5 μm-14.5 μm range showed absorption by water vapour, carbon dioxide and ozone. The monthly mean variation of the absorption of solar radiation at 9.6 urn ozone band has been analyzed. The monthly mean variation of the absorption at 9.6 urn ozone band compares well with that of the total ozone values measured over Delhi. In the present paper the salient features of the IR Sun photometer and results obtained are discussed.

Measurements of direct solar irradiance were taken at the San Pietro Capofiume station in the centre of the Po Valley using two examples of the multispectral sun photometer, model UVISIR, on several clear-sky days of late spring and early summer in the years 1989 and 1990. Realistic evaluations of aerosol optical thickness at several sun-photometric wavelengths were obtained from these measurements by i) using values of the calibration constants carefully found by following both the Langley plot method and the spectral-correction procedure, which had previously been applied with satisfactory results to the sun-photometric measurements carried out in Antarctica and ii) taking into account the extinction effects produced by Rayleigh scattering and absorption due to ozone, water vapour and nitrogen dioxide on the solar radiation passing through the atmosphere. By examining a wide set of spectral series of the aerosol optical thickness in terms of the well-known Angstrom formula, a large number of pairs of atmospheric-turbidity parameters α and β was obtained, presenting a range of the mean daily values of α from 1.0 to 1.7 (with the median of 1.48) and a range of the mean daily values of β from 0.10 to 0.30 (with the median of 0.15). Using the particulate extinction models CR (for continental particles) and TR (for small tropospheric particles and large rural particles), the aerosol mass loading of the atmosphere calculated for these values of α and β was estimated to vary between 0.09 and 0.31 g m−2 (with the median of 0.21 g m−2) during the measurement period of 1989 and between 0.09 and 0.41 g m−2 (with the median of 0.13 g m−2 for model CR and of 0.21 g m−2 for model TR) during June and July 1990.

The measurements of solar radiation in the IR spectral region is of great significance in understanding physics, chemistry and the radiation budget of the atmosphere. The occurrence of a total solar eclipse on October 24, 1995 at New Delhi provided a unique opportunity to take some special solar radiation measurements in an infrared spectral region with the use of an IR photometer, the experiments were conducted during, after and before the total eclipse at Neem Ka Thana (75° 47'E, 27° 47'N) and New Delhi (77° l3'E, 28° 39'N) in the spectral ranges of 0.75 to 1.6μm and 2.5 to 14.5μm respectively. The measurements were used to estimate the water vapour (0.94μm and 1.14μm), Ozone (9.6μm) etc. A significant variation in IR solar irradiance at various window wavelengths during the solar eclipse was observed. Also measurements of temperature and humidity were made. In the present paper, the experimental set-up and results are discussed in detail.

Accurate evaluations of the optical thicknesses produced by tropospheric aerosol particles at visible and near-infra-red wave-lengths can be obtained from Sun photometer measurements taken at high mountain stations, provided that the attenuation due to other atmospheric constituents is calculated with high precision. For this purpose, particulate extinction models based on fourteen Junge-type size distribution curves of tropospheric particles were determined to represent the average spectral features of aerosol particle populations in the upper troposphere. Moreover, procedures suitable for determining the optical thicknesses produced by Rayleigh scattering, water vapour, ozone and nitrogen dioxide are herein proposed. In particular, use is made of a method which determines the atmospheric content of nitrogen dioxide by separation of gaseous absorption from particulate extinction. Sun-photometric measurements have been examined by following the above procedures and by using the particulate extinction models to obtain realistic estimates of both the total particle number and particulate mass loading in the atmospheric column.

Measuring, with the aid of two filters, the instantaneous intensity of the solar radiation in two wave lengths (λB = 0.44 μ, λR = 0.64 μ) by means of a sun photometer designed byVolz, we carried out determinations of the decadic turbidity coefficientB (λ=0.5 μ) and the wave length exponent α of the haze extinction for Mexico City. Observations were made for almost two and a half years (1960 to 1962 period). A seasonal size distribution in both parameters was found. Although the data thus obtained are provenient of a contaminated atmosphere, comparison of our data is made with those found for higher latitudes ofAngstrom, Schuepp andVolz. The height of the homogeneous haze layerHD was calculated showing pronounced variations for a given wind direction. The maximum and minimum values ofB enable us to get, by the first approximation, the aerosol size distribution ofJunge for our latitudes. However, for exceptional very clear days having maximum actinometric intensity of the solar radiation the sensitivity of the microamperimeter in theVolz sun photometer fails.