Aerosol Optical Depth Series Research Articles

Remote sensing of aerosol properties during intense smog events over Lahore (Pakistan)

In recent years, smog has been one of the main concerns in heavily populated urban areas like Lahore (Pakistan) and Kanpur (India). Atmospheric pollutants like aerosols play an important role in smog. In this paper, aerosol types are identified in smog episodes, based on Aerosol Robotic Network (AERONET) data, during 4-year period i.e., 2015-2018. Aerosols are classified based on fine mode fraction (FMF) and single scattering albedo (SSA). One of the main aerosol types which are abundant in every smog episode is Black Carbon (BC) aerosol while dust is present throughout the year. BC is responsible for radiation imbalance and is considered the main component in climate changes at regional and global levels. Furthermore, time series of aerosol optical depth (AOD) during smog episodes is used to identify the variability of aerosol burden in the atmosphere. Backward trajectories from the HYSPLIT model are used to trace the origin of aerosols during the days of maximum AOD.

Reducing cloud contamination in aerosol optical depth (AOD) measurements

Abstract. We propose a new cloud screening method for sun photometry that is designed to effectively filter thin clouds. Our method is based on a k-nearest-neighbour algorithm instead of scanning time series of aerosol optical depth. Using 10 years of data from a precision filter radiometer in Innsbruck, we compare our new method and the currently employed screening technique. We exemplify the performance of the two routines in different cloud conditions. While both algorithms agree on the classification of a data point as clear or cloudy in a majority of the cases, the new routine is found to be more effective in flagging thin clouds. We conclude that this simple method can serve as a valid alternative for cloud detection, and we discuss the generalizability to other observation sites.

A Dual-Channel Aerosol Optical Depth Retrieval Algorithm Incorporating the BRDF Effect from AVHRR over Eastern Asia

A NOAA/AVHRR dual-channel method over land is proposed to simultaneously retrieve aerosol optical depth (AOD) at 0.55 μm, and surface reflectance at 0.63 and 0.85 μm. Compared with previous well-established one-channel retrieval algorithms, this algorithm takes advantage of the surface reflectance ratio between 0.63 and 0.85 μm in an attempt to account for the effect induced by the surface bidirectional reflectance distribution function (BRDF). This effect cannot be negligible due to the orbit drift and phasing running of NOAA satellites, both of which potentially cause large solar angular variation. Meanwhile, the observation posture change of AVHRR would cause large sensor angular variation in time series measurements. The used surface reflectance ratio based on dual channels at 0.63 and 0.85 μm is found more reasonable to be assumed as unchanged during a certain period of time, compared to the traditional ratio when addressing the BRDF issue. AOD retrievals have been carried out over Eastern Asia. Validation against aerosol robotic network (AERONET) measurements shows that up to 83% of AOD validation collocations are within error lines (±0.15 ± 0.15τ, τ is AOD) with an R of 0.88 and an root mean square error (RMSE) of 0.15. The dual-channel algorithm taking into account the surface BRDF effect is proved outperforming the conventional 0.63 μm-channel method. It indicates that our algorithm has the potential to be applied to the retrieval of long series of AOD, especially to the AOD retrieval of the sensors which lack a shortwave infrared channel required in the MODerate resolution Imaging Spectroradiometer (MODIS) dark target AOD algorithm.

Comparison of MODIS- and CALIPSO-Derived Temporal Aerosol Optical Depth over Yellow River Basin (China) from 2007 to 2015

In this study, Collection 6.1 (C6.1) of different aerosol optical depth (AOD) products of different spatial resolutions were used from the aqua moderate resolution imaging spectroradiometer (MODIS) including dark target (DT), deep blue (DB), deep blue (DB), and DT-DB (DTB). These products were compared with cloud-aerosol lidar, and infrared pathfinder satellite observation (CALIPSO) AOD retrievals over the Yellow River Basin (YERB), China from 2003 to 2017. The YERB was divided into three sub-regions, namely YERB1 (the mountainous terrain in the upper reaches of the YERB), YERB2 (the Loess Plateau region in the middle reaches of the YERB), and YERB3 (the plain region downstream of the YERB). Errors and agreement between MODIS and CALIPSO data were reported using Pearson’s correlation (R) and relative mean bias (RMB). Results showed that the CALIPSO whole layers AOD (AODS) were better matched with MODIS AOD than the CALIPSO lowest layer AOD (AOD1). The time series of AOD shows higher values in spring and summer, and a small difference in AOD products was observed in autumn. The overall average value of CALIPSO AOD and MODIS AOD both fitted the order: YERB3 > YERB2 > YERB1. The CALIPSO AOD retrievals have the best consistency with the DTB10K and the lowest consistency with DT3K. Overall, the regional distributions of the CALIPSO AOD and MODIS AOD are significantly different over the YERB, and the difference is closely related to the season, region, and topography. This study can help researchers understand the difference of aerosol temporal and spatial distribution utilizing different satellite products over YERB, and also can provide data and technical support for the government in atmospheric environmental governance over YERB.

Sun photometer retrievals of Saharan dust properties over Barbados during SALTRACE

Abstract. The Saharan Aerosol Long-Range Transport and Aerosol–Cloud-Interaction Experiment (SALTRACE) was devoted to the investigation of Saharan dust properties over the Caribbean. The campaign took place in June–July 2013. A wide set of ground-based and airborne aerosol instrumentation was deployed at the island of Barbados for a comprehensive experiment. Several sun photometers performed measurements during this campaign: two AERONET (Aerosol Robotic Network) Cimel sun photometers and the Sun and Sky Automatic Radiometer (SSARA). The sun photometers were co-located with the ground-based multi-wavelength lidars BERTHA (Backscatter Extinction lidar Ratio Temperature Humidity profiling Apparatus) and POLIS (Portable Lidar System). Aerosol properties derived from direct sun and sky radiance observations are analyzed, and a comparison with the co-located lidar and in situ data is provided. The time series of aerosol optical depth (AOD) allows identifying successive dust events with short periods in between in which the marine background conditions were observed. The moderate aerosol optical depth in the range of 0.3 to 0.6 was found during the dust periods. The sun photometer infrared channel at the 1640 nm wavelength was used in the retrieval to investigate possible improvements to aerosol size retrievals, and it was expected to have a larger sensitivity to coarse particles. The comparison between column (aerosol optical depth) and surface (dust concentration) data demonstrates the connection between the Saharan Air Layer and the boundary layer in the Caribbean region, as is shown by the synchronized detection of the successive dust events in both datasets. However the differences of size distributions derived from sun photometer data and in situ observations reveal the difficulties in carrying out a column closure study.

Validation, Stability, and Consistency of MODIS Collection 6.1 and VIIRS Version 1 Deep Blue Aerosol Data Over Land

AbstractThe Deep Blue (DB) algorithm has been used to retrieve aerosol optical depth (AOD) and Ångström exponent (AE) over land from multiple satellite instruments, including the Moderate Resolution Imaging Spectroradiometers (MODIS) aboard the Terra and Aqua platforms and the Visible Infrared Imaging Radiometer Suite (VIIRS). This study first validates the latest MODIS (Collection 6.1) and VIIRS (Version 1) DB data products against Aerosol Robotic Network observations. On global average, the typical level of uncertainty in AOD is slightly better than ±(0.05 + 20%) relative to Aerosol Robotic Network. AE is quantitatively more uncertain but qualitatively shows skill at distinguishing between fine‐mode and coarse‐mode dominated aerosol columns. Results are also compared with the previous MODIS Collection 6. The stability of the three DB data sets ranges from 0.005–0.01 AOD per decade. Second, spatial and temporal patterns in AOD and AE are compared between the three data sets. It is found that they all show similar patterns of spatial coverage, which is predominantly linked to cloud cover, snow, and polar night. Regional time series of AOD also show highly consistent seasonal and interannual variations and are strongly correlated, although have offsets in some regions due to a combination of algorithmic and sensor‐related differences.

Comparison of aerosol optical depth from satellite (MODIS), sun photometer and broadband pyrheliometer ground-based observations in Cuba

Abstract. In the present study, we report the first comparison between the aerosol optical depth (AOD) and Ångström exponent (AE) of the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on the Terra (AODt) and Aqua (AODa) satellites and those measured using a sun photometer (AODSP) at Camagüey, Cuba, for the period 2008 to 2014. The comparison of Terra and Aqua data includes AOD derived with both deep blue (DB) and dark target (DT) algorithms from MODIS Collection 6. Combined Terra and Aqua (AODta) data were also considered. Assuming an interval of ±30 min around the overpass time and an area of 25 km around the sun photometer site, two coincidence criteria were considered: individual pairs of observations and both spatial and temporal mean values, which we call collocated daily means. The usual statistics (root mean square error, RMSE; mean absolute error, MAE; median bias, BIAS), together with linear regression analysis, are used for this comparison. Results show very similar values for both coincidence criteria: the DT algorithm generally displays better statistics and higher homogeneity than the DB algorithm in the behaviour of AODt, AODa, AODta compared to AODSP. For collocated daily means, (a) RMSEs of 0.060 and 0.062 were obtained for Terra and Aqua with the DT algorithm and 0.084 and 0.065 for the DB algorithm, (b) MAE follows the same patterns, (c) BIAS for both Terra and Aqua presents positive and negative values but its absolute values are lower for the DT algorithm; (d) combined AODta data also give lower values of these three statistical indicators for the DT algorithm; (e) both algorithms present good correlations for comparing AODt, AODa, AODta vs. AODSP, with a slight overestimation of satellite data compared to AODSP, (f). The DT algorithm yields better figures with slopes of 0.96 (Terra), 0.96 (Aqua) and 0.96 (Terra + Aqua) compared to the DB algorithm (1.07, 0.90, 0.99), which displays greater variability. Multi-annual monthly means of AODta establish a first climatology that is more comparable to that given by the sun photometer and their statistical evaluation reveals better agreement with AODSP for the DT algorithm. Results of the AE comparison showed similar results to those reported in the literature concerning the two algorithms' capacity for retrieval. A comparison between broadband aerosol optical depth (BAOD), derived from broadband pyrheliometer observations at the Camagüey site and three other meteorological stations in Cuba, and AOD observations from MODIS on board Terra and Aqua show a poor correlation with slopes below 0.4 for both algorithms. Aqua (Terra) showed RMSE values of 0.073 (0.080) and 0.088 (0.087) for the DB and DT algorithms. As expected, RMSE values are higher than those from the MODIS–sun photometer comparison, but within the same order of magnitude. Results from the BAOD derived from solar radiation measurements demonstrate its reliability in describing climatological AOD series estimates.

On the association of aerosol optical depth and total ozone fluctuations with recent earthquakes in Greece

A number of recent studies suggest that atmospheric changes that precede an earthquake might offer the hope of providing early warning. This study attempts to examine seismo-atmospheric anomalies around the time and the location of some of the major earthquakes in Greece in the period 2001–2015. Daily values of aerosol optical depth (AOD) and total ozone column (TOZ) obtained from satellite observations are used, in the time window between 25 days before and 14 days after each of these events, in a 1° × 1° area centred on the epicentre of each earthquake. For some of these earthquake events, abnormal increases in time series of AOD and TOZ data were detected before and after the occurrence of the earthquake. Nevertheless, in other cases, no clear anomalies were observed around the earthquake date. In addition, examining the statistics of AOD and TOZ daily values, there were many cases of prominent abnormal variations, without, however, being associated with an earthquake event. Therefore, no clear association was found between AOD and TOZ fluctuations and recent earthquakes in Greece.

Reconstruction of long‐term aerosol optical depth series with sunshine duration records

AbstractWe report the suitability of sunshine duration (SD) records as a proxy for the reconstruction of atmospheric aerosol content, for which little information exists, especially prior to the 1980s. Specifically, we have treated cloudless summer days in 16 stations throughout Spain. For almost all sites we find statistically significant relationships between aerosol optical depth (AOD) and daily SD. The correlation coefficient presents a mean value of −0.72, and slope values of the linear regressions are within the range [−0.11, −0.36]. The relationships are used to generate AOD series back to the 1960s (to the 1920s for Madrid). These reconstructed series show an increase in AOD from the mid‐1960s to the 1980s, followed by a decrease until the present, in agreement with changes in anthropogenic aerosol emissions and with opposite trends of solar irradiance. The method can be used to reconstruct AOD from the late nineteenth century at many stations worldwide.

Recovering long-term aerosol optical depth series (1976–2012) from an astronomical potassium-based resonance scattering spectrometer

Abstract. A 37-year long-term series of monochromatic aerosol optical depth (AOD) has been recovered from solar irradiance measurements performed with the solar spectrometer Mark-I, deployed at Izaña mountain since 1976. The instrument operation is based on the method of resonant scattering, which affords wavelength absolute reference and stability (long-term stability and high precision) in comparison to other instruments based purely on interference filters. However, it has been specifically designed as a reference instrument for helioseismology, and its ability to determine AOD from transmitted and scattered monochromatic radiation at 769.9 nm inside a potassium vapour cell in the presence of a permanent magnetic field is evaluated in this paper. Particularly, the use of an exposed mirror arrangement to collect sunlight as well as the Sun–laboratory velocity dependence of the scattered component introduces some important inconveniences to overcome when we perform the instrument's calibration. We have solved this problem using a quasi-continuous Langley calibration technique and a refinement procedure to correct for calibration errors as well as for the fictitious diurnal cycle on AOD data. Our results showed similar calibration errors retrieved by means of this quasi-continuous Langley technique applied in different aerosol load events (from 0.04 to 0.3), provided aerosol concentration remains constant throughout the calibration interval. It assures the validity of this technique when it is applied in those periods with relatively high aerosol content. The comparative analysis between the recovered AOD data set from the Mark-I and collocated quasi-simultaneous data from the Cimel-AErosol RObotic NETwork (AERONET) and Precision Filter Radiometer (PFR) instruments showed an absolute mean bias ≤ 0.01 in the 10- and 12-year comparison, respectively. High correlation coefficients between AERONET and Mark-I and PFR/Mark-I pairs confirmed a very good linear relationship between instruments, proving that recovered AOD data series from Mark-I can be used together with PFR and AERONET AOD data to build a long-term AOD data series at the Izaña site (1976–now), suitable for future analysis of aerosols trends and inter-annual variability. Finally, the AOD preliminary trend analysis in the 29-year period from 1984 to 2012 with Mark-I AOD revealed no significant trends.

Marine and continental aerosol effects on the upwelling solar radiation flux in Southern Portugal during the ACE-2 experiment

An overall number of 447 spectral series of aerosol optical depth were determined in the 0.4-3.7 mm wavelength range by examining the IR-RAD sun-radiometer measurements carried out at Sagres (Portugal) on six clear-sky days, during the CLEARCOLUMN (ACE-2) experiment in June and July 1997. These spectral series were then analysed with the King inversion method to defi ne the size-distribution curves of columnar aerosol particle total number and volume, assuming values of both real and imaginary parts of the particulate refractive index obtained on the six days by combining our measurements with simultaneous sky-brightness measurements taken by the Leipzig University group. For these results, we then calculated the daily time-patterns of the average single scattering albedo of the columnar aerosols, fi nding instantaneous values ranging between 0.70 and 0.96 on those days, with daily mean values varying from 0.83 to 0.95. Furthermore, for each spectral series of aerosol optical depth, we determined the instantaneous change DF^ induced by the columnar aerosols on the upwelling solar radiation fl ux leaving the atmosphere, over oceanic areas presenting low surface albedo. The 24-h average values of DF^ obtained on the six days were found to increase as a function of the daily mean value of aerosol optical depth at the 0.55 mm wavelength, following relationship curves whose positive slope coeffi cients decrease gradually with the single scattering albedo of the columnar aerosols. The said curves can be used to achieve reliable estimates of change DF^ directly from daily ground-based multispectral measurements of aerosol optical depth and skybrightness at different angular distances from the Sun.An overall number of 447 spectral series of aerosol optical depth were determined in the 0.4-3.7 mm wavelength range by examining the IR-RAD sun-radiometer measurements carried out at Sagres (Portugal) on six clear-sky days, during the CLEARCOLUMN (ACE-2) experiment in June and July 1997. These spectral series were then analysed with the King inversion method to defi ne the size-distribution curves of columnar aerosol particle total number and volume, assuming values of both real and imaginary parts of the particulate refractive index obtained on the six days by combining our measurements with simultaneous sky-brightness measurements taken by the Leipzig University group. For these results, we then calculated the daily time-patterns of the average single scattering albedo of the columnar aerosols, fi nding instantaneous values ranging between 0.70 and 0.96 on those days, with daily mean values varying from 0.83 to 0.95. Furthermore, for each spectral series of aerosol optical depth, we determined the instantaneous change DF^ induced by the columnar aerosols on the upwelling solar radiation flux leaving the atmosphere, over oceanic areas presenting low surface albedo. The 24-h average values of DF^ obtained on the six days were found to increase as a function of the daily mean value of aerosol optical depth at the 0.55 mm wavelength, following relationship curves whose positive slope coeffi cients decrease gradually with the single scattering albedo of the columnar aerosols. The said curves can be used to achieve reliable estimates of change DF^ directly from daily ground-based multispectral measurements of aerosol optical depth and skybrightness at different angular distances from the Sun.

Extreme Saharan dust event over the southern Iberian Peninsula in september 2007: active and passive remote sensing from surface and satellite

Abstract. This study investigates aerosol optical properties during the extreme Saharan dust event detected from 3 to 7 September 2007 over Granada, southern Iberian Peninsula, with both active and passive remote sensing instrumentation from surface and satellite. The intensity of the event was visualized on the aerosol optical depth series obtained by the sun-photometer Cimel CE 318-4 operated at Granada in the framework of AERONET from August 2004 until December 2008 (level 2 data). A combination of large aerosol optical depth (0.86–1.50) at 500 nm, and reduced Angström exponent (0.1–0.25) in the range 440–870 nm, was detected on 6 September during daytime. This Saharan dust event also affected other Iberian Peninsula stations included in AERONET (El Arenosillo and Évora stations), and it was monitored by MODIS instrument on board Aqua satellite. Vertically resolved measurements were performed by a ground-based Raman Lidar and by CALIPSO satellite. During the most intense stage, on 6 September, maximum aerosol backscatter values were a factor of 8 higher than other maxima during this Saharan dust event. Values up to 1.5×10−2 km−1 sr−1 at 355 and 532 nm were detected in the layer with the greatest aerosol load between 3–4 km a.s.l., although aerosol particles were also detected up to 5.5 km a.s.l. In this stage of the event, dust particles at these altitudes showed a backscatter-related Angström exponent between –0.44 and 0.53 for the two spectral intervals considered. The results from different measurements (active/passive and ground-based/satellite) reveal the importance of performing multi-instrumental measurements to properly characterize the contribution of different aerosol types from different sources during extreme events. The atmospheric stabilization effect of the aerosol particles has been characterized by computing the solar heating rates using SBDART code.

Characterization of the optical properties of atmospheric aerosols in Amazônia from long‐term AERONET monitoring (1993–1995 and 1999–2006)

We present a new climatology of atmospheric aerosols (primarily pyrogenic and biogenic) for the Brazilian tropics on the basis of a high‐quality data set of spectral aerosol optical depth and directional sky radiance measurements from Aerosol Robotic Network (AERONET) Cimel Sun‐sky radiometers at more than 15 sites distributed across the Amazon basin and adjacent Cerrado region. This network is the only long‐term project (with a record including observations from more than 11 years at some locations) ever to have provided ground‐based remotely‐sensed column aerosol properties for this critical region. Distinctive features of the Amazonian area aerosol are presented by partitioning the region into three aerosol regimes: southern Amazonian forest, Cerrado, and northern Amazonian forest. The monitoring sites generally include measurements from the interval 1999–2006, but some sites have measurement records that date back to the initial days of the AERONET program in 1993. Seasonal time series of aerosol optical depth (AOD), Ångström exponent, and columnar‐averaged microphysical properties of the aerosol derived from sky radiance inversion techniques (single‐scattering albedo, volume size distribution, fine mode fraction of AOD, etc.) are described and contrasted for the defined regions. During the wet season, occurrences of mineral dust penetrating deep into the interior were observed.

Atmospheric effects of volcanic eruptions as seen by famous artists and depicted in their paintings

Abstract. Paintings created by famous artists, representing sunsets throughout the period 1500–1900, provide proxy information on the aerosol optical depth following major volcanic eruptions. This is supported by a statistically significant correlation coefficient (0.8) between the measured red-to-green ratios of a few hundred paintings and the dust veil index. A radiative transfer model was used to compile an independent time series of aerosol optical depth at 550 nm corresponding to Northern Hemisphere middle latitudes during the period 1500–1900. The estimated aerosol optical depths range from 0.05 for background aerosol conditions, to about 0.6 following the Tambora and Krakatau eruptions and cover a period practically outside of the instrumentation era.

Convective transport of particulate matter in an Apennine valley: 2. Time variations in the columnar aerosol mass content and vertical profiles of aerosol mass concentration

Multispectral sun-photometric measurements of aerosol optical depth were performed at five stations located at different altitudes from about 500 to 2165 m amsl in the Leo valley in the Apennines (Italy), on some clear-sky autumn days of 1981 and 1982. These measurements showed that the aerosol optical depth greatly increased during the morning at all stations due to the convective transport of particulate matter induced by the local upslope breezes. To evaluate the corresponding variations of the vertical mass loading of particulate matter, two continental and rural aerosol extinction models were determined: the first provides the spectral series of both volume and mass aerosol extinction coefficients at nine visible and near-infrared wavelengths for 27 aerosol particle size-distribution curves of Junge-type with values of the Ångström exponent α varying between 0.15 and 1.99; the second is based on linear combinations of three monomodal particle size-distribution curves determined to give form to a nuclei mode, an accumulation mode and a coarse particle mode, respectively, whose particle number concentrations were defined to fit the values of α characterising the spectral series of aerosol optical depth. These aerosol extinction models were used to obtain estimates of the columnar aerosol mass content at the five stations, finding values mostly ranging between 0.03 and 0.12 g m −2 at the two lower stations, 0.02 and 0.08 g m −2 at the two intermediate stations and between 0.01 and 0.03 g m −2 at the high-altitude Mt. Cimone station. The corresponding hourly mean percentage variations of the vertical aerosol mass loading were found to vary between a few percentages and nearly 100%, with median values ranging from 14% to 29% at the various stations. From these estimates, evaluations of the aerosol mass concentration were obtained within the four layers, varying from 15 to 160 μg m −3 in the first layer, from 15 to 95 μg m −3 in the second, from 10 to 60 μg m −3 in the third and from 5 to 20 μg m −3 in the fourth. Correspondingly, the hourly mean percentage variations were found to range between −26% and +124% within the four layers, with median values gradually increasing from +4% to +20%, passing from the first to the fourth layer.

Spectral measurements of aerosol particle extinction in the 0.4-3.7 mum wavelength range, performed at Sagres with the IR-RAD sun-radiometer

During the CLEARCOLUMN campaign which took place at Sagres (Portugal) from 16 June to 25 July 1997, more than 2000 spectral measurements of direct solar irradiance were performed at thirteen window-wavelengths in the 0.4–3.7 μm range, on 27 days. The measurements were performed using the IR-RAD sun-radiometer designed and manufactured at the Institute ISAO (FISBAT), Bologna (Italy), and carefully calibrated by applying the Langley plot method to the measurements performed on 24 October 1996, at the Schneefernerhaus Observatory on the Zugspitze (Germany). From these measurements, the values of the total atmospheric optical depth were obtained at the various wavelengths, from which the corresponding spectral values of aerosol optical depth were determined through accurate corrections for Rayleigh scattering and gaseous absorption. Such values were found to be mostly smaller than 0.1 during June and early July, presenting almost neutral spectral dependence characteristics, closely related to the prevailing oceanic origins of particulate matter. Higher values of aerosol optical thickness, mainly ranging between 0.1 and 0.5 at visible wavelengths and sharply decreasing with wavelength, were found during the rest of July in the presence of predominant contents of continental and anthropogenic aerosol particles arriving from polluted European regions. The King inversion method was applied to the spectral series of aerosol optical depth to determine the particle size-distribution curves in the 0.2–20 μm diameter range. For 2 of these cases (one chosen for a low atmospheric loading of marine aerosols and the other for a mean content of continental/anthropogenic aerosols), the changes in the outgoing solar radiation flux produced by aerosol particles were calculated, considering different surface albedo conditions. The results obtained in the second case show that changes of opposite sign can be caused by the same atmospheric aerosol loading when passing from oceanic to continental areas.

Retrieval of columnar aerosol size distributions and radiative-forcing evaluations from sun-photometric measurements taken during the CLEARCOLUMN (ACE 2) experiment

Spectral measurements of direct solar irradiance were taken within 13 narrow-band channels in the 401–3676 nm wavelength range, using the IR–RAD sun-radiometer at the Sagres station in southern Portugal during the CLEARCOLUMN (ACE 2) experiment, from June 16 to 25 July 1997. The measurements performed on 21 clear-sky days were examined in terms of the Bouguer–Lambert–Beer law, following realistic correction procedures for Rayleigh scattering and absorption by minor atmospheric gases, to determine more than 2100 spectral series of aerosol optical depth. All these spectral series were examined with the King inversion method to retrieve the columnar aerosol particle size distributions over the 0.07–10 μm radius range, for values of the real part of the particulate refractive index ranging between 1.43 and 1.50 and values of the imaginary part between 0.003 and 0.010. Using the well-known 6S computer code for all the columnar aerosol size-distribution curves determined on ten “golden days” and both refractive index parts varying with wavelength, evaluations of the change Δ F↑ caused by aerosol particles in the outgoing solar radiation flux were made at solar zenith angles θ ranging between 15 and 76° and for spectral albedo features of both clear water and green vegetation surfaces. The results show that the radiative forcing Δ F↑ assumes positive values (associated with cooling effects) for the clear water surface on all the measurement days and at all the solar zenith angles, and mostly negative values (warming effects) for the green vegetation surface in the range θ<60°. The present evaluations also give clear evidence of the close dependence of Δ F↑ not only on the surface albedo, solar zenith angle and aerosol optical depth but also on the mean single scattering albedo of the columnar aerosols.