Spectral Optical Depth Research Articles

Aerosol size properties at Armilla, Granada (Spain)

AbstractThe size characteristics of atmospheric aerosol over a suburban environment on the outskirts of Granada, a non‐industrialized medium sized city in south‐eastern Spain, were investigated using five years of continuous spectral measurements in the range 368–778 nm. From these the aerosol optical depths at the corresponding wavelengths were derived. Aerosol columnar size distributions were then retrieved from the spectral optical depths. These, and a set of derived aerosol properties including columnar mass, columnar surface or columnar number of particles, were then examined. For the analysed series two different periods were observed, one with a prevalence towards Junge type distributions and a second where bimodal log–normal distributions were favoured; these coincided with periods of atypical severe drought and typical meteorological conditions, respectively, that affected the measurement area. The bimodal distributions present a clearly defined mode in the range 0.4–0.8 µm radius, with a less well‐defined mode for smaller radii. For periods typifying normal conditions the behaviour of the lower and higher modes were analysed. The higher mode is slightly sharper with a consistent position compared to the lower mode. The relationships among the different aerosol size properties and the aerosol optical properties were also analysed. A good correlation was found between the Angstr öm exponent, that measures the aerosol optical‐depth's spectral dependence, and the ratio of columnar particle density for large and small particle sizes in the accumulation mode. It is also shown that the aerosol optical depth at 500 nm is a good predictor of columnar aerosol properties such as the aerosol mass or surface area. Copyright © 2003 Royal Meteorological Society

Aerosol spectral optical depths over the Bay of Bengal: Role of transport

Recent experiments have shown the potential role of air masses in transporting aerosols to locations far away from source regions. Despite the importance of the Bay of Bengal to Indian climate and monsoon, no serious aerosol observations are available for this region. Extensive aerosol optical depth estimates, made for the first time from an island location, Port Blair (11.63°N; 92.71°E) in the Bay of Bengal, during the Indian winter of 2002, are used to examine the impact of air trajectories in modifying the optical depths and their spectral dependences. The results are examined for their distinctiveness with respect to the origin as well as transport. It is seen that the trajectories arriving from the regions east of the station (South China, Thailand, Laos, Cambodia, Vietnam, Burma) are richer in aerosol abundance, more in the sub micron size range, than those arriving from the west, across the Indian landmass.

Measurements of aerosol optical depth over Arabian Sea during summer monsoon season

Aerosol spectral optical depths were measured over Arabian Sea as a part of the Arabian Sea Monsoon Experiment (ARMEX). These are the first measurements of aerosols over northern and central Arabian Sea during Indian summer monsoon season. Aerosol visible optical depths were in the range of 0.4 to 0.7 with Angstrom wavelength exponent of ∼0.35 and single scattering albedo of ∼0.97 indicating the dominance of large non‐absorbing aerosols. Aerosol optical depth increases exponentially with sea‐surface wind speed. Our estimates show that sea‐salt contributes ∼60% to the composite aerosol optical depth. The presence of aerosols over Arabian Sea during summer monsoon season decreases the short wave radiation arriving at the surface by as much as 21 W m−2 and increases top of the atmosphere reflected radiation by 18 W m−2.

Estimates of the spectral aerosol single scattering albedo and aerosol radiative effects during SAFARI 2000

Using measurements of the spectral solar radiative flux and optical depth for 2 days (24 August and 6 September 2000) during the SAFARI 2000 intensive field experiment and a detailed radiative transfer model, we estimate the spectral single scattering albedo of the aerosol layer. The single scattering albedo is similar on the 2 days even though the optical depth for the aerosol layer was quite different. The aerosol single scattering albedo was between 0.85 and 0.90 at 350 nm, decreasing to 0.6 in the near infrared. The magnitude and decrease with wavelength of the single scattering albedo are consistent with the absorption properties of small black carbon particles. We estimate the uncertainty in the single scattering albedo due to the uncertainty in the measured fractional absorption and optical depths. The uncertainty in the single scattering albedo is significantly less on the high‐optical‐depth day (6 September) than on the low‐optical‐depth day (24 August). On the high‐optical‐depth day, the uncertainty in the single scattering albedo is 0.02 in the midvisible whereas on the low‐optical‐depth day the uncertainty is 0.08 in the midvisible. On both days, the uncertainty becomes larger in the near infrared. We compute the radiative effect of the aerosol by comparing calculations with and without the aerosol. The effect at the top of the atmosphere (TOA) is to cool the atmosphere by 13 W m−2 on 24 August and 17 W m−2 on 6 September. The effect on the downward flux at the surface is a reduction of 57 W m−2 on 24 August and 200 W m−2 on 6 September. The aerosol effect on the downward flux at the surface is in good agreement with the results reported from the Indian Ocean Experiment (INDOEX).

Point and column aerosol radiative closure during ACE 1: Effects of particle shape and size

We used data collected during the First Aerosol Characterization Experiment (ACE 1) to study point and column aerosol radiative closure over the remote ocean. To test point closure, total and hemispheric backscattering coefficients calculated with a Mie single‐scattering model were compared with measurements made by ship and aircraft at three wavelengths (400, 550, and 700 nm). On the ship, assuming spherical particles, calculated total scattering was usually within 10% of measurements (closure obtained in >80% of the cases) but calculated backscattering was usually 15–25% lower than measurements (closure obtained in <50% of the cases). When a model for particle nonsphericity was applied to the dried sea spray, assuming the particles to be ideal cubes or irregular convex and concave crystals resulted in overestimation of backscattering. However, when nonsphericity parameters were fit to the measurements, calculated backscattering was also usually within 10% of measurements (closure obtained in >80% of the cases). On the aircraft, however, calculated scattering and backscattering were usually lower than measurements by 20–45% regardless of assumed particle shape (closure obtained in <50% of the cases), likely owing to differences in the aerosol inlet penetration efficiencies to each instrument or unidentified uncertainties in the measured number size distributions or scattering coefficients. To test column closure, aerosol extinction profiles calculated from in situ observations (below 5.5 km) and satellite observations (above 5.5 km) were vertically integrated, and the resulting aerosol optical depth was compared with measurements made on the ship during two clear‐sky days at three wavelengths (500, 778, and 862 nm). Calculated spectral optical depths were usually within 25% of measurements (closure obtained at one or more wavelengths on both days), and agreement at longer wavelengths was improved when satellite measurements were spectrally scaled using in situ model results. On both days, large sea salt particles produced a spectrally neutral aerosol optical depth in the marine boundary layer whereas smaller ammonium sulfate particles contributed to greater optical depth at shorter wavelengths in the overlying upper atmosphere.

Optical closure for an aerosol column: Method, accuracy, and inferable properties applied to a biomass‐burning aerosol and its radiative forcing

During the Lindenberg Aerosol Characterization Experiment (LACE 98), airborne measurements of aerosol size distribution, fine‐particle concentration, particle absorption coefficient, backscatter coefficient, depolarization, and chemical composition as well as ground‐based measurements of spectral particle optical depth and of spectral backscatter and extinction coefficients were performed in the aerosol column above Lindenberg, Germany. We compare the measured optical parameters with calculations from the size distributions, which assume the aerosol to consist of sulfuric acid near the tropopause and mixtures of ammonium sulfate and soot in the remaining column. We obtain closure to within 25% for the optical depth of a column, which includes a biomass‐burning aerosol of North American origin, and infer a soot volume fraction of 35% for this aerosol. Assuming spheroidal particles of prolate shape and the average aspect ratio of the particles to be 1.3 in the biomass‐burning aerosol layer, the calculated depolarization agrees with the lidar measurement, whereas comparing the spectral backscatter coefficient shows the soot to be externally mixed with the nonabsorbing particles. With the two‐stream approximation, we estimate the local, instantaneous, cloud‐free radiative forcing of the biomass‐burning aerosol at the tropopause to −5.8 W/m2 with a corresponding optical depth of 0.09 at 710 nm wavelength and solar zenith angle of 56°. The radiative forcing for the biomass‐burning aerosol is as sensitive to a change in state of mixture, either external or internal, as to a change in surface albedo, ocean to coniferous forest.

Aerosol radiative forcing due to enhanced black carbon at an urban site in India

During a comprehensive aerosol field campaign, simultaneous measurements were made of aerosol spectral optical depths, black carbon mass concentration (Mb), total (Mt) and size segregated aerosol mass concentrations over an urban continental location, Bangalore (13° N, 77°E, 960 m msl), in India. Large amounts of BC were observed; both in absolute terms and fraction of total mass (∼11%) and sub‐micron mass (∼23%) implying a significantly low single scatter albedo. The aerosol visible optical depth (τp) was in the range 0.24 to 0.45. Estimated surface forcing is as high as −23 W m−2 and top of the atmosphere (TOA) forcing is +5 W m−2 during relatively cleaner periods (τp ∼ 0.24). The net atmospheric absorption translates to an atmospheric heating of ∼0.8 K day−1 for cleaner periods and ∼1.5 K day−1 for less cleaner periods (τp ∼ 0.45). Our observations raise several issues, which may have impacts to regional climate and monsoon.

Aerosol optical depth over a remote semi-arid region of South Africa from spectral measurements of the daytime solar extinction and the nighttime stellar extinction

Spectral daytime aerosol optical depths have been measured at Sutherland, South Africa (32°22′S, 20°48′E), from January 1998 to November 1999. Sutherland is located in the semi-arid Karoo desert, approximately 400-km northeast from Cape Town. The site, remote from major sources of aerosols, hosts the South African Astronomical Observatory (SAAO), where nighttime stellar extinction is being measured. The comparison of daytime and nighttime measurements for the years 1998–1999 makes it possible to validate the astronomical dataset of aerosol optical depth ( τ a) dating back to 1991. The 1998 and 1999 annually averaged daytime τ a at 500 nm are 0.04±0.04 and 0.06±0.06, respectively. Half-day averages vary between 0.03 and 0.44, with peak values in August–September. This pronounced seasonality is linked to the biomass-burning season in the Southern Hemisphere. Smoke haze layers transported to Sutherland originated primarily on the African landmass at latitudes between 10° and 20°S and passed over Namibia and Angola. On one occasion, aerosols from fires in Brazil transported across the Atlantic Ocean were likely detected. The haze layers reaching Sutherland are therefore at least 2–3 days old. The spectral dependence of the aerosol optical depth for the smoke layers supports the bimodality of the volume size distribution for biomass burning aerosols. The accumulation mode has a volume modal diameter of 0.32 μm, consistent with the hypothesis of aged haze. The stellar measurements (1991–2001) show that, due to the eruption of Mt. Pinatubo, the atmospheric extinction depth at 550 nm in the years 1991–1993 increased by 33% with respect to the average value (0.14±0.03) for the period 1994–2001. Outside the Pinatubo event, extinction is largest in the period 1997–1999.

Photometric observations of Mt. Etna's different aerosol plumes

We have investigated Mt. Etna's summit crater's plumes using a visible and NIR sun-photometer during eight days in July 1999. After removal of the background optical depth, we have applied the Ångstrom equation and a King-type inversion to the volcanic aerosol spectral optical depths to retrieve (1) the Ångstrom coefficients α and β, (2) the particle size, surface area and volume spectra and (3) the effective radius. Plumes supposed to contain ash have a larger effective radius (1.48±0.28 μm vs. 0.68±0.18 μm), a smaller Ångstrom exponent α (−0.02 vs. 1.63) and a more uni-modal Junge-type size distribution. Inversions of the spectral optical depth to size distributions are observed to be little sensitive to small changes in the assumed value of the refractive index. Plumes measured further downwind seem to have higher effective radii (1.68 vs. 1.48 μm) and a more distinct bi-modal type distribution, but the difference lies within experimental error.

Mountain polar stratospheric cloud measurements by Ground Based FTIR Solar Absorption Spectroscopy

An analysis of mountain polar stratospheric cloud observations by solar absorption FTIR from Kiruna/Sweden was performed and compared with co‐located airborne lidar measurements. The infrared cloud spectral signature and optical depth indicated water ice particles compatible with the high backscatter and depolarization ratio of the lidar signal. Retrieval of mean particle properties from the FTIR spectrum depend on the assumed width of the particle size distribution and range from 2.0 to 1.1 µm radius and from 1.9 to 5.1 cm−3 number density. The volume density is well determined with 63.8–60.2 µm³/cm³ equivalent to about 2.5 ppmv of condensed water at 20 hPa. Simulations of lidar backscatter ratio based on these results are consistent with the lidar observations.

An observational approach for determining aerosol surface radiative forcing: Results from the first field phase of INDOEX

This paper presents one of the few quantitative estimates of surface aerosol forcing made directly from surface irradiance observations. The method described within yields estimates of the forcing accurate to 20%. The study was conducted from February to March 1998 at the Kaashidhoo Climate Observatory (KCO) during the First Field Phase of the Indian Ocean Experiment (INDOEX‐FFP). For the 400–700 nm region studied here, the forcing is −7.6±1.5 W m−2. The data are obtained from two photodiode radiometers measuring global and diffuse irradiance in five channels in the visible and ultraviolet. The instruments were chosen, calibrated, and deployed specifically for a precise measurement of aerosol forcing. The angular, spectral, and absolute response characteristics of the instruments are determined in the laboratory and used to calibrate the data, as described here. The accuracy in the calibrated data is 2.4% for the global irradiance and 1.8% for the diffuse irradiance. Direct aerosol forcing is obtained from the measured aerosol forcing efficiency, which is determined by two methods: hybrid and differential. The hybrid method uses a radiative transfer model to subtract out the contribution from the aerosol‐free atmosphere. The differential method assumes that changes in 400–700 nm solar flux are forced by changes in aerosol optical depth. By using flux changes, the differential method is not sensitive to the small calibration uncertainties, and is independent of model assumptions about the single‐scatter properties of the aerosol. For this soot‐laden marine region south of India, a 0.1 change in aerosol optical depth produces a −4.0±0.8 W m−2; change in the 400–700 nm surface flux; 55% of this forcing is observed in the 400–540 nm region. The global and diffuse data agree to within 5 W m−2 of results calculated by a Monte Carlo radiative transfer model. The model assumes an aerosol consistent with the spectral optical depth, lidar vertical profiles, and surface optical properties measured simultaneously at KCO.

Accuracy assessments of aerosol optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and sky radiance measurements

Sensitivity studies are conducted regarding aerosol optical property retrieval from radiances measured by ground‐based Sun‐sky scanning radiometers of the Aerosol Robotic Network (AERONET). These studies focus on testing a new inversion concept for simultaneously retrieving aerosol size distribution, complex refractive index, and single‐scattering albedo from spectral measurements of direct and diffuse radiation. The perturbations of the inversion resulting from random errors, instrumental offsets, and known uncertainties in the atmospheric radiation model are analyzed. Sun or sky channel miscalibration, inaccurate azimuth angle pointing during sky radiance measurements, and inaccuracy in accounting for surface reflectance are considered as error sources. The effects of these errors on the characterization of three typical and optically distinct aerosols with bimodal size distributions (weakly absorbing water‐soluble aerosol, absorbing biomass‐burning aerosol, and desert dust) are considered. The aerosol particles are assumed in the retrieval to be polydispersed homogeneous spheres with the same complex refractive index. Therefore we also examined how inversions with such an assumption bias the retrievals in the case of nonspherical dust aerosols and in the case of externally or internally mixed spherical particles with different refractive indices. The analysis shows successful retrieval of all aerosol characteristics (size distribution, complex refractive index, and single‐scattering albedo), provided the inversion includes the data combination of spectral optical depth together with sky radiances in the full solar almucantar (with angular coverage of scattering angles up to 100° or more). The retrieval accuracy is acceptable for most remote sensing applications even in the presence of rather strong systematic or random uncertainties in the measurements. The major limitations relate to the characterization of low optical depth situations for all aerosol types, where high relative errors may occur in the direct radiation measurements of aerosol optical depth. Also, the results of tests indicate that a decrease of angular coverage of scattering (scattering angles of 75° or less) in the sky radiance results in the loss of practical information about refractive index. Accurate azimuth angle pointing is critical for the characterization of dust. Scattering by nonspherical dust particles requires special analysis, whereby approximation of the aerosol by spheres allows us to derive single‐scattering albedo by inverting spectral optical depth together with sky radiances in the full solar almucantar. Inverting sky radiances measured in the first 40° scattering angle only, where nonspherical effects are minor, results in accurate retrievals of aerosol size distributions of nonspherical particles.

Temporal Characteristics of Aerosol Optical Depths and Size Distribution at Visakhapatnam, India

Aerosol spectral optical depths and size distribution derived from the direct solar flux measurements at nine discrete wavelengths in the visible and near IR region for a period of about 10 years exhibit a gradual increase in the aerosol spectral optical depths from 1994-1997. There was a very large increase in the optical depths during 1991 after the volcanic eruption of Mt. Pinatubo and its effect persisted until 1993-94. The size distribution during 1988-1996 (excluding 1991, 1992, and 1993) remained bimodal in nature with an increase in the small particle number density with year. The aerosol optical depths and size distribu tions at Visakhapatnam also show some short period changes, characterizing the high sensitivity of the coastal urban aerosol system to the mesoscale weather. The observed features have been explained on the basis of the aerosol genesis based on local sources, sinks, and the prevailing weather.

Particle size distributions of Mount Etna's aerosol plume constrained by Sun photometry

Near simultaneous instrument calibration and volcanic aerosol measurement were performed near the summit of Mount Etna, Sicily, in October 1997 using an eight channel Sun‐tracking photometer. Three objectives were achieved using this methodology: calibration of the instrument, measurement of the background atmosphere, and measurement of the volcanic aerosol plume. Out‐of‐plume calibration measurements were used to extrapolate extraterrestrial solar voltages for the five nonpolarized visible and near‐infrared channels using a Langley calibration routine. The Langley coefficients were then used to calculate the optical depth of the background atmosphere and subsequently the volcanic plume using the Beer‐Bouger‐Lambert law. Coefficients were calculated from the Ångström equation with means of α = 1.67 (0.13 < α < 2.42) and β = 0.06 (0.001 < β < 0.14). A constrained linear inversion of the spectral optical depth of the plume was performed using Mie code to produce particle number, surface area and volume spectra, and the effective scattering radius. The volume spectra indicate a trimodal distribution consisting of particles of radius < 0.1 μm (nucleation mode), a possible second fine mode (< 1.0 μm), and large particles of radius >1.0 μm (acid droplets) with minima at 0.5 and 1.5 μm. The mean effective radius was determined to be at 0.83 μm within the range 0.35 < r < 1.6 μm, and the total aerosol mass flux was estimated as 4.5–8.0 kg s−1 with the smaller radius mode contributing 6–18% of the total mass flux.

Characteristics of aerosols over a remote island, Minicoy in the Arabian Sea: Optical properties and retrieved size characteristics

AbstractChanges in the characteristics of atmospheric aerosol over a marine environment are investigated by making regular spectral extinction measurements in the visible and near‐infrared region from a tiny island location. Minicoy (8.3°N, 73.04°E), situated in the Arabian Sea about 400 km due west of the southern tip of the Indian peninsula. The role of seasonally changing air‐mass type in causing a regular annual variation in the spectral optical depths is delineated. The association between aerosol optical depths, surface wind speed and rainfall is examined. An increase in wind speed causes an increase in optical depths, the effect is predominant when a marine air mass prevails. The impact of changes in wind speed on optical depths (due to sea‐spray production over the sea) is parametrized in the case that the island is influenced by a marine air mass. Columnar size distributions, retrieved from the spectral optical depths, in general, show a bimodal log‐normal distribution in the optically active size range. The accumulation mode is more sensitive to continental air‐mass types, while the coarse mode is influenced by the marine conditions. The coarse mode is sharper but its position is variable. Increase in wind speed leads to a remarkable enhancement in the concentration and relative abundance of coarse particles, particularly during the monsoon season. The mass loading and effective radius are well associated and depend on wind speed histories. The findings are discussed.

Observations of the spectral clear‐sky aerosol forcing over the tropical Indian Ocean

During the first field phase (FFP) of the Indian Ocean Experiment (INDOEX) in February and March, 1998, the spectral global and direct beam irradiance have been measured between 350 and 1050 nm wavelengths using a 512‐channel, fixed grating, photodiode array spectroradiometer. A detailed analysis of the instrument's reliability, the absolute calibration, and the corrections for deviation from the ideal cosine response are presented. For most of the spectral region the total uncertainty is shown to be <2%. The spectral optical depth, the spectral aerosol forcing, and the aerosol forcing for the photosynthetically active radiation have been derived from direct beam measurements and global irradiance measurements. The optical depth at 500 nm wavelength decreases from ∼0.5 in the northern Arabian Sea to as low as 0.05 south of the Intertropical Convergence Zone (ITCZ) near ∼15°S latitude. The surface aerosol forcing efficiency is defined as the rate of change of net irradiance at the surface due to an increase by 1 in optical depth at 500 nm. The normalization procedure we adopt to determine the aerosol forcing efficiency with respect to a reference pristine day in the Southern Hemisphere eliminates most of the radiometric calibration uncertainties. The continental aerosol south of the ITCZ shifts the peak in the direct solar radiation from 470 nm (for pristine conditions) to ∼580 nm for the polluted region. The spectral aerosol forcing efficiency peaks around 460 nm, with −1.2, −0.6, and +0.6 W m−2 nm−1 for the direct, global, and diffuse irradiance, dropping for the lower and higher wavelengths to about −0.3, −0.25, and 0.05 W m−2 nm−1 at 350 nm and −0.3, −0.1, and +0.2 at 1050 nm. Integrated over 400–700 nm, the aerosols decrease the noontime solar flux by as much as −38 W m−2 in the Arabian Sea to as little as −2 W m−2 south of the ITCZ. This introduces a strong north to south gradient in the climate forcing of the ocean. In addition, the strong aerosol modulation of the photosynthetically active radiation (400–700 nm) and its north to south gradient have important implications for biomass production of the ocean.

Pinatubo volcanic aerosol characteristics as observed from a low latitude location in india using a ground-based multiwavelength solar radiometer

Aerosol spectral optical depths and size distributions evaluated from multispectral solar radiation extinction measurements from 1987 to 1996 were used to study the Pinatubo aerosol characteristics at a low latitude location, Visakhapatnam (17.7°N, 83.3°E) in India. The optical depths from June 1991 showed significant increase after the Mt. Pinatubo volcanic eruption. The volcanic aerosol optical depths have been obtained by subtracting the pre-volcanic mean aerosol optical depths from the corresponding monthly mean values during the post-eruption period. The optical depths of the Pinatubo aerosols have shown typical temporal and spectral variations, which were found to be significant for about two to three years after the eruption. The inversion of the spectral optical depths yielded a well-defined bi-modal size distribution for the first year after the eruption and it showed a broad monomodal form in the second year. The Pinatubo aerosol optical depths became very small by the end of 1993. The effective radius increased from 0.38 in June 1991 to 0.6 in November 1991 and it decreased from January 1992 onwards. The aerosol columnar mass loading decreased steadily from July 1991 onwards with an e-folding time of about 16.75 months.

On the optical properties of the polluted Athens atmosphere, during May 1995

Using ground-based spectral solar extinction data taken over the Athens atmosphere, reduced (total minus Rayleigh) and aerosol (Angstrom) spectral optical depths of the atmosphere, have been retrieved, under different polluted conditions. The results suggest that the optical depth, on days with relatively low pollution, exhibits slight variation with wavelength denoting that aerosols deplete all wavelengths almost equally. In contrast, under dense pollution, small particles scattering and trace gases absorption, are the dominant processes, resulting in steeper optical depth’s slopes, mainly in the ultraviolet domain. The Angstrom’s parameters β and α were determined through a least-squares fitting method. The turbidity β coefficient always shows a temporal pattern with high values in the morning and afternoon and low values midday.

A new algorithm to determine the spectral aerosol optical depth from satellite radiometer measurements

A new aerosol retrieval algorithm is presented which computes the spectral optical depth over the ocean from spaceborne radiometers. It includes both multiple scattering and the bi-directional reflectance of the ocean surface. The algorithm is applied to data from the Along Track Scanning Radiometer 2 (ATSR-2). This radiometer aboard the ERS-2 satellite has 4 bands in the visible and near-infrared. The ATSR-2 has a dual view capability: the reflectance is measured both at nadir and at a forward angle of approximately 55° along track. This feature is used to test the algorithm by comparing independent retrievals from the forward and the nadir view, applied to Southern Hemisphere data from 23 July 1995. The retrieved aerosol optical depths compare favorably. The retrieved aerosol optical depths and spectral behavior are in agreement with expected values in clean marine environments.

Airborne spectral measurements of surface anisotropy during SCAR‐B

During the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) deployment, angular distributions of spectral reflectance for vegetated surfaces and smoke layers were measured using the scanning cloud absorption radiometer (CAR) mounted on the University of Washington C‐131A research aircraft. The CAR contains 13 narrowband spectral channels between 0.3 and 2.3 μm with a 190° scan aperture (5° before zenith to 5° past nadir) and 1° instantaneous field of view. The bidirectional reflectance is obtained by flying a clockwise circular orbit above the surface, resulting in a ground track ∼3 km in diameter within about 2 min. Although the CAR measurements are contaminated by minor atmospheric effects, results show distinct spectral characteristics for various types of surfaces. Spectral bidirectional reflectances of three simple and well‐defined surfaces are presented: cerrado (August 18, 1995) and dense forest (August 25, 1995), both measured in Brazil under nearly clear‐sky conditions, and thick smoke layers over dense forest (September 6 and 11, 1995). The bidirectional reflectances of cerrado and dense forest revealed fairly symmetric patterns along the principal plane, with varying maximal strengths and widths spectrally in the backscattering direction. In the shortwave‐infrared region the aerosol effect is very small due to low spectral optical depth. Also, these backscattering maxima can be seen on the bidirectional reflectance of smoke layer over dense forest. These detailed measurements of the angular distribution of spectral reflectance can be parameterized by a few independent variables and utilized to retrieve either surface characteristics or aerosol microphysical and optical properties (e.g., size distribution and single‐scattering parameters), if proper physical and radiation models are used. The spectral‐hemispherical albedo of these surfaces is obtained directly by integrating all angular measurements and is compared with the measured nadir reflectance. Using CAR nadir reflectance as a surrogate for spectral‐hemispherical albedo can cause albedos to be underestimated by 10–60%, depending on solar zenith angle.