Tropospheric Aerosol Radiative Forcing Observational Experiment Research Articles

Regional aerosol properties: Comparisons of boundary layer measurements from ACE 1, ACE 2, Aerosols99, INDOEX, ACE Asia, TARFOX, and NEAQS

Means and variability of aerosol chemical composition and optical properties are compared for the first and second Aerosol Characterization Experiments (ACE 1 and ACE 2), a cruise across the Atlantic (Aerosols99), the Indian Ocean Experiment (INDOEX), the Asian Aerosol Characterization Experiment (ACE Asia), the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), and the New England Air Quality Study (NEAQS). These experiments were focused either on the remote marine atmosphere (ACE 1) or areas downwind of continental aerosol source regions including western Europe, North America, Africa, India, and Asia. Presented here are size‐segregated concentrations of aerosol mass, sea salt, non‐sea‐salt (nss) SO4=, NH4+, NO3−, dust, organic carbon (OC), elemental carbon (EC), and nss K+, as well as mass ratios that are commonly used to identify aerosol sources and to assess aerosol processing (Cl− to Na+, OC to nss SO4=, EC to total carbon (TC), EC to nss SO4=, nss K+ to EC, Fe to Al, and Si to Al). Optical properties that are compared include size‐segregated scattering, backscattering, and absorption coefficients, and single‐scattering albedo at 550 nm. Size‐segregated mass scattering and mass absorption efficiencies for the total aerosol and mass extinction efficiencies for the dominant chemical components also are compared. In addition, we present the contribution to light extinction by the dominant chemical components for each region. All data are based on shipboard measurements performed at a relative humidity of 55 ± 5%. Scattering coefficients and single‐scattering albedos also are reported at ambient relative humidity (RH) using published values of f(RH). Finally, aerosol optical depths from each region are compared. Identical sampling protocols were used in all experiments in order to eliminate sampling biases and to make the data directly comparable. Major findings include (1) nss SO4= makes up only 16 to 46% of the submicron aerosol mass, which means there is a large and variable fraction of the aerosol that is not sulfate, (2) particulate organic matter (POM) makes up 1 to 51% of the submicron mass, with highest POM mass fractions observed downwind of the NE United States, (3) highest submicron mass fractions of EC and lowest single‐scattering albedos were observed in biomass‐burning plumes from Africa and downwind of the Indian subcontinent, (4) NO3− was found predominantly in the supermicron size range due to the interaction of gas phase oxidized nitrogen species with sea salt aerosol, and (5) mass extinction efficiencies for the individual chemical components were consistent between regions. All data presented are available as auxiliary material.

Airborne measurements of atmospheric carbonaceous aerosols during ACE‐Asia

Airborne levels of carbonaceous aerosols were measured using the Twin Otter aircraft during the Aerosol Characterization Experiment (ACE)‐Asia. Particles were collected using a newly developed honeycomb denuder sampler and organic carbon (OC), elemental carbon (EC), and carbonate (CC) carbon levels determined using a thermal–optical carbon analyzer. During some flights, atmospheric layers could be identified as marine boundary, pollution dominated, or mineral dust dominated. Ångstrom exponent (å) values, calculated based on data from an onboard three‐wavelength nephelometer, were used to discern the nature of some individual layers. Values of å for individual layers ranged from 0.2 to 2, corresponding to dust‐ and pollution‐dominated layers, respectively. OC and EC concentrations below 3 km ranged from 0.58 to 29 μg C m−3 and from 0.20 to 1.8 μg C m−3, respectively. In general, for a given type of atmospheric layer, higher levels of total carbon (TC) were observed during ACE‐Asia than those observed during ACE‐2, Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and Indian Ocean Experiment (INDOEX). Mixed layers of dust and pollution were found on some occasions. CC was detected in samples taken from layers in which å = 1.6, indicating that significant amounts of dust can be present even though å > 0.2. A linear regression of light absorption coefficient σap (Mm−1) versus EC concentration had an r2 of only 0.50, indicating that parameters other than the mass of EC significantly affected the value of σap. The mass absorption coefficient Eabs (m2 g−1) varied by as much as a factor of 8 between sampling events, and the average value of 11 m2 g−1 (±5.0) agrees well with previous published values.

Comparison of Aerosol Single Scattering Albedos Derived by Diverse Techniques in Two North Atlantic Experiments

Aerosol single scattering albedo v (the ratio of scattering to extinction) is important in determining aerosol climatic effects, in explaining relationships between calculated and measured radiative fluxes, and in retrieving aerosol optical depths from satellite radiances. Recently, two experiments in the North Atlantic region, the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and the Second Aerosol Characterization Experiment (ACE-2), determined aerosol v by a variety of techniques. The techniques included fitting of calculated to measured radiative fluxes; retrievals of v from skylight radiances; best fits of complex refractive index to profiles of backscatter, extinction, and size distribution; and in situ measurements of scattering and absorption at the surface and aloft. Both TARFOX and ACE-2 found a fairly wide range of values for v at midvisible wavelengths (;550 nm), with 0.85 # vmidvis # 0.99 for the marine aerosol impacted by continental pollution. Frequency distributions of v could usually be approximated by lognormals in vmax 2 v, with some occurrence of bimodality, suggesting the influence of different aerosol sources or processing. In both TARFOX and ACE-2, closure tests between measured and calculated radiative fluxes yielded best-fit values of vmidvis of 0.90 6 0.04 for the polluted boundary layer. Although these results have the virtue of describing the column aerosol unperturbed by sampling, they are subject to questions about representativeness and other uncertainties (e.g., thermal offsets, unknown gas absorption). The other techniques gave larger values for vmidvis for the polluted boundary layer, with a typical result of vmidvis 5 0.95 6 0.04. Current uncertainties in v are large in terms of climate effects. More tests are needed of the consistency among different methods and of humidification effects on v.

An aerosol model and aerosol‐induced changes in the clear‐sky albedo off the east coast of the United States

Changes in the cloud‐free albedo of the Earth, and therefore direct aerosol radiative forcing, due to aerosol can be parameterized as a function of one extensive quantity that depends on the amount of aerosol present (e.g., the aerosol optical depth, τ) and two intensive quantities that depend on the internal characteristics of the aerosol (the aerosol single‐scattering albedo, ωo, and the aerosol asymmetry parameter, g). Airborne measurements of the dry aerosol scattering and absorption coefficients and dry particle size distributions made aboard the University of Washington's research aircraft during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) field campaign off the east coast of the United States in July 1996 are used in conjunction with measurements of the effects of humidity on aerosol light scattering to determine an optically equivalent model for the physical and optical properties of the ambient aerosol. This model is validated and used to derive values of ωo and g for the ambient (wet) aerosol. The average and standard deviation of the column‐mean, ambient values of ωo and g at a wavelength of 550 nm, determined from 12 vertical profiles, are 0.958±0.050 and 0.703±0.092, respectively. A pronounced increase in g with increasing relative humidity is observed. Using the aerosol model, the mean instantaneous change in the local albedo induced by the aerosol is found to be 0.027±0.018 at a wavelength of 550 nm (compared to an albedo of about 0.05 for the ocean).

Remote sensing of total precipitable water vapor in the near‐IR over ocean glint

A method for remote sensing of total precipitable water vapor using water vapor absorption band at 0.94 µm was previously developed for continental regions. Here we apply a similar technique for ocean areas over the glint region. The glint, or oceanic specular reflection, has a high value of surface reflectance and thus, can work as well as or better than applications over land regions. The method is applied for glint regions measured by the Moderate Resolution Imaging Spectro‐radiometer (MODIS) simulator, an imager flown on the NASA ER‐2 research aircraft and simulating the expected measurements from the MODIS instrument on board the Earth Observing System (EOS)‐Terra satellite. The measurements are made for the Atlantic coast of the United States during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). The remote sensing technique is compared with measurements of water vapor column by the Lidar Atmospheric Sensing Experiment (LASE) Differential Infrared Absorption Lidar (DIAL) lidar system also on board the ER‐2. Water vapor was derived with an error of ±5 mm PW (precipitable water vapor). Most of the errors are associated with the limitations of an experiment that was not originally designed for this purpose. Much better performance is expected from the actual MODIS instrument.

LASE measurements of aerosol and water vapor profiles during TARFOX

The Lidar Atmospheric Sensing Experiment (LASE) was operated autonomously from the NASA high‐altitude ER‐2 aircraft on nine flights during July 10–26, 1996, as part of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). LASE measured high‐resolution profiles of water vapor and aerosols in regions of urban haze plumes over the U.S. eastern seaboard. Real‐time LASE aerosol measurements were used to guide the in situ aircraft to sample haze layers. In this paper the vertical and horizontal distributions of aerosol backscatter measured by LASE are presented along with the temporal evolution of the haze layers. The aerosol backscatter data also identify the presence of gradients in the aerosol plumes, the presence of low‐altitude clouds, and optically thin cirrus. This information is useful for many of the radiometeric observations made during TARFOX and can help explain observational differences among ground, airborne, and satellite observations. An iterative procedure is discussed which was used to invert lidar data to retrieve aerosol scattering ratios, extinction, and total optical depths from the LASE measurements. The sensitivity of these retrievals to assumed parameters is discussed and the results of retrievals are also compared to the well‐known Bernoulli method. LASE water vapor measurements were made across the entire troposphere using a three “line pair” method to cover the range of water vapor mixing ratio from < 0.01 g/kg near the tropopause to ∼ 20 g/kg near the surface in a single aircraft pass over the experiment region. These measurements also show two‐dimensional distributions of large spatial gradients in water vapor in the lower and upper troposphere. These observations are useful in the calculation of IR radiation fields and relative humidity fields, since relative humidity has a strong influence on the growth of aerosols and their scattering properties. Water vapor profiles, aerosol scattering ratios, aerosol extinction coefficients and aerosol optical depths were derived using the methodology presented in this paper from LASE measurements during TARFOX. These measurements are compared with other in situ and remote measurements during TARFOX in the companion papers [Ferrare et al., this issue (a, b)]

Case studies of the vertical structure of the direct shortwave aerosol radiative forcing during TARFOX

The vertical structure of aerosol‐induced radiative flux changes in the Earth's troposphere affects local heating rates and thereby convective processes, the formation and lifetime of clouds, and hence the distribution of chemical constituents. We present observationally based estimates of the vertical structure of direct shortwave aerosol radiative forcing for two case studies from the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) which took place on the U.S. east coast in July 1996. The aerosol radiative forcings are computed using the Fu‐Liou broadband radiative transfer model. The aerosol optical properties used in the radiative transfer simulations are calculated from independent vertically resolved estimates of the complex aerosol indices of refraction in two to three distinct vertical layers, using profiles of in situ particle size distributions measured aboard the University of Washington research aircraft. Aerosol single‐scattering albedos at 450 nm thus determined range from 0.9 to 0.985, while the asymmetry factor varies from 0.6 to 0.8. The instantaneous shortwave aerosol radiative forcings derived from the optical properties of the aerosols are of the order of −36 W m−2 at the top of the atmosphere and about −56 W m−2 at the surface for both case studies.

Measurement of atmospheric optical parameters on U.S. Atlantic coast sites, ships, and Bermuda during TARFOX

The Aerosol Robotic Network (AERONET) of automatic Sun/sky radiometers collected data on U.S. Atlantic coast sites, ships, and Bermuda in 1996 during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Spatial and temporal analysis of Sun photometry data was supported by synoptic analysis of air mass evolution. The spatial distribution of aerosol optical depth is presented. In several cases the aerosol size distributions deduced from sky almucantar measurements and solar disk attenuation measurements at the various coastal sites yielded similar results within the same air masses. Ship‐based measurements in the Atlantic Ocean showed significant maritime aerosol optical property variations which for the most part could be attributed to the influence of continental sources and Saharan dust events. The Bermuda data (optical depths and Ängström parameter values) illustrated changes in atmospheric optical properties for various air masses and trajectories. Almost no correlation was observed between aerosol optical depth and water vapor content when the data from all stations and ship measurements were considered together. In the case of individual stations or ship transects, different degrees of correlation could be observed. In continental conditions on the east coast, optical depth and water vapor are well correlated, while in a maritime environment, optical depth can be relatively small despite high water vapor contents.

Retrieving the vertical structure of the effective aerosol complex index of refraction from a combination of aerosol in situ and remote sensing measurements during TARFOX

The largest uncertainty in estimates of the effects of atmospheric aerosols on climate stems from uncertainties in the determination of their microphysical properties, including the aerosol complex index of refraction, which in turn determines their optical properties. A novel technique is used to estimate the aerosol complex index of refraction in distinct vertical layers from a combination of aerosol in situ size distribution and remote sensing measurements during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). In particular, aerosol backscatter measurements using the NASA Langley LASE (Lidar Atmospheric Sensing Experiment) instrument and in situ aerosol size distribution data are utilized to derive vertical profiles of the “effective” aerosol complex index of refraction at 815 nm (i.e., the refractive index that would provide the same backscatter signal in a forward calculation on the basis of the measured in situ particle size distributions for homogeneous, spherical aerosols). A sensitivity study shows that this method yields small errors in the retrieved aerosol refractive indices, provided the errors in the lidar‐derived aerosol backscatter are less than 30% and random in nature. Absolute errors in the estimated aerosol refractive indices are generally less than 0.04 for the real part and can be as much as 0.042 for the imaginary part in the case of a 30% error in the lidar‐derived aerosol backscatter. The measurements of aerosol optical depth from the NASA Ames Airborne Tracking Sunphotometer (AATS‐6) are successfully incorporated into the new technique and help constrain the retrieved aerosol refractive indices. An application of the technique to two TARFOX case studies yields the occurrence of vertical layers of distinct aerosol refractive indices. Values of the estimated complex aerosol refractive index range from 1.33 to 1.45 for the real part and 0.001 to 0.008 for the imaginary part. The methodology devised in this study provides, for the first time, a complete set of vertically resolved aerosol size distribution and refractive index data, yielding the vertical distribution of aerosol optical properties required for the determination of aerosol‐induced radiative flux changes.

Comparisons of LASE, aircraft, and satellite measurements of aerosol optical properties and water vapor during TARFOX

We examine aerosol extinction and optical thickness from the Lidar Atmospheric Sensing Experiment (LASE), the in situ nephelometer and absorption photometer on the University of Washington C‐131A aircraft, and the NASA Ames Airborne Tracking Sun Photometer (AATS‐6) on the C‐131 A measured during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) over the east coast of the United States in July 1996. On July 17 and 24 the LASE profiles of aerosol extinction and aerosol optical thickness (AOT) had a bias difference of 0.0055 km−1 (10%) and a root‐mean‐square difference of 0.026 km−1 (42%) when compared to corresponding profiles derived from the airborne in situ data when the nephelometer measurements are adjusted to ambient relative humidities. Larger differences for two other days were associated with much smaller aerosol optical thicknesses (July 20) and differences in the locations sampled by the two aircraft (July 26). LASE profiles of AOT are about 10% higher than those derived from the airborne Sun photometer, which in turn are about 10–15% higher than those derived from the airborne in situ measurements. These differences are generally within the error estimates of the various measurements. The LASE measurements of AOT generally agree with AOT derived from both the Along‐Track and Scanning Radiometer 2 (ATSR 2) sensor flown on the European Remote Sensing Satellite 2 (ERS‐2) and from the Moderate‐Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS) which flew with LASE on the NASA ER‐2 aircraft. Effective particle sizes derived from the MAS data indicate that the LASE retrievals of AOT are valid for effective particle radii less than 0.4 μm. Variations in the relative humidity derived from the LASE water vapor measurements on July 26 are found to be highly correlated with variations in the effective particle size derived from the MAS.

Aerosol properties and radiative effects in the United States East Coast haze plume: An overview of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX)

Aerosol effects on atmospheric radiation are a leading source of uncertainty in predicting climate change. The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) was designed to reduce this uncertainty by measuring and analyzing aerosol properties and effects on the United States eastern seaboard, where one of the world's major plumes of urban/industrial haze moves from the continent over the Atlantic Ocean. The TARFOX intensive field campaign was conducted July 10–31, 1996. It included coordinated measurements from four satellites (GOES‐8, NOAA‐14, ERS‐2, Landsat), four aircraft (ER‐2, C‐130, C‐131A, and a modified Cessna), land sites, and ships. A variety of aerosol conditions was sampled, ranging from relatively clean, behind frontal passages, to moderately polluted, with aerosol optical depths exceeding 0.5 at midvisible wavelengths. Gradients of aerosol optical thickness were sampled to aid in separating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals. Early results from TARFOX include demonstration of the unexpected importance of carbonaceous compounds and water condensed on aerosol in the United States East Coast haze plume, chemical apportionment of the aerosol optical depth, measurements of aerosol‐induced changes in upwelling and downwelling shortwave radiative fluxes, and generally good agreement between measured flux changes and those calculated from measured aerosol properties. This overview presents the TARFOX objectives, rationale, overall experimental approach, and key initial findings as a guide to the more complete results reported in this special section and elsewhere.

Retrieval of aerosol optical thickness and size distribution over ocean from the MODIS airborne simulator during TARFOX

Radiation and in‐situ measurements collected during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) are used to test the method for remote sensing of aerosol properties and loading from the MODIS instrument. MODIS, a Moderate Resolution Imaging Spectroradiometer, will be launched in 1999 aboard the first EOS (Earth Observing System). Following the MODIS procedure [Tanré et al, 1997], the spectral radiance at the top of the atmosphere (TOA) measured over the ocean in a wide spectral range (0.55–2.13 μm) is used to derive the aerosol optical thickness (proportional to the aerosol total loading) and the aerosol size distribution (integrated over the vertical column) of the ambient (undisturbed) aerosol by comparing measured radiances with values in look‐up table (LUT). The LUT includes the gas‐phase oxidation accumulation mode, cloud‐phase accumulation mode, and a coarse mode that represents maritime particles (salt) and dust. In each inversion, one accumulation and one coarse mode can be retrieved. The inversion retrieves the ratio of the contribution to the optical thicknesses of the two particle modes and the mean particle size that best fits the measurements. This algorithm is successfully applied to the data sets acquired during TARFOX. The MODIS airborne simulator (MAS) aboard the NASA ER‐2 aircraft flew several times during the experiment above the University of Washington C‐131A research aircraft on which the six‐channel Ames Airborne Tracking Sun Photometer (AATS‐6) was mounted. It flew also above surface‐based Sun photometers. Optical thicknesses (at λ = 550 nm) as well as the spectral dependence from the various data sets compare very well.

Aerosol optical depth retrieval using ATSR‐2 and AVHRR data during TARFOX

Satellite retrieved aerosol optical properties are compared to aircraft measurements for a case study during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Two satellite instruments are used: the Along Track Scanning Radiometer 2 (ATSR‐2) and the advanced very high resolution radiometer (AVHRR). The aerosol optical depth in the mid‐visible (0.555 μm) retrieved from the ATSR‐2 data agrees within 0.03 with colocated sunphotometer measurements. Also, the spectral behavior of the aerosol optical depth is retrieved accurately. Good correlation is found between aerosol optical depths for AVHRR channel 1 (0.64 μm) and sunphotometer derived values, but the satellite retrieved values are 0.05 to 0.15 lower. The Ångström wavelength exponent is determined both from the ATSR‐2 and the AVHRR data. The ATSR‐2 derived Ångström exponents are in good agreement with the values computed from the sunphotometer data. The Angstrom exponents determined from AVHRR data show very large variations. Both the ATSR‐2 and the AVHRR aerosol optical depth images show a large gradient. Vertical profile data of temperature, relative humidity, and particle scattering indicate that this gradient is probably caused by changes in the dry aerosol properties, rather than a change in the relative humidity.

An overview of the University of Washington airborne measurements and results from the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX)

During the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) conducted off the East Coast of the United States in July 1996, the University of Washington research aircraft facility was used to obtain comprehensive in situ and remote sensing measurements of the physical and chemical nature of the aerosol and its effects on direct radiative forcing. This paper describes the airborne instrumentation used for this purpose, the sampling strategy, the times and locations of the 18 research flights, and a summary of the highlights of the results obtained to date.

Aerosol‐induced radiative flux changes off the United States mid‐Atlantic coast: Comparison of values calculated from sunphotometer and in situ data with those measured by airborne pyranometer

The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) measured a variety of aerosol radiative effects (including flux changes) while simultaneously measuring the chemical, physical, and optical properties of the responsible aerosol particles. Here we use TARFOX‐determined aerosol and surface properties to compute shortwave radiative flux changes for a variety of aerosol situations, with midvisible optical depths ranging from 0.06 to 0.55. We calculate flux changes by several techniques with varying degrees of sophistication, in part to investigate the sensitivity of results to computational approach. We then compare computed flux changes to those determined from aircraft measurements. Calculations using several approaches yield downward and upward flux changes that agree with measurements. The agreement demonstrates closure (i.e., consistency) among the TARFOX‐derived aerosol properties, modeling techniques, and radiative flux measurements. Agreement between calculated and measured downward flux changes is best when the aerosols are modeled as moderately absorbing (midvisible single‐scattering albedos between about 0.89 and 0.93), in accord with independent measurements of the TARFOX aerosol. The calculated values for instantaneous daytime upwelling flux changes are in the range +14 to +48 W m−2 for midvisible optical depths between 0.2 and 0.55. These values are about 30 to 100 times the global‐average direct forcing expected for the global‐average sulfate aerosol optical depth of 0.04. The reasons for the larger flux changes in TARFOX include the relatively large optical depths and the focus on cloud‐free, daytime conditions over the dark ocean surface. These are the conditions that produce major aerosol radiative forcing events and contribute to any global‐average climate effect.

Comparison of observed and modeled direct aerosol forcing during TARFOX

Aircraft measurements have been made of the downward and upward solar irradiance under cloud‐free conditions over a range of aerosol loadings in the summer haze plume off the East Coast of the United States during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Optical properties calculated from measured aerosol microphysical and chemical properties have been used as input to a shortwave radiative transfer model. This model was used to calculate the diurnally averaged direct aerosol forcing and to compare with values deduced from the aircraft radiative measurements. The modeled and observed forcings agree well when the aerosol has a significant absorbing component.

Optical properties of Saharan dust during ACE 2

The Aerosol Robotic Network (AERONET) of automatic Sun/sky radiometers collected data on Tenerife, Canary Islands, in June‐July 1997 during the second Aerosol Characterization Experiment (ACE 2). Initially, two instruments were deployed at Izana observatory (2360 m above sea level) and one at a mountain station Teide (3570 m above sea level). Repeatability of the calibration constants (Langley method) for all instruments was less than 0.5%. Aerosol optical depths measured by colocated sunphotometers and column size distributions, retrieved from spectral sky radiance data, were in good agreement. Later, one of the instruments was relocated at sea level. On July 8,17, and 25, Saharan dust outbreaks were observed. Diurnal variations of spectral aerosol optical depth are presented. Relative diurnal stability of Saharan dust optical properties has been observed. Volume size distributions at various heights (sea level and 2360 m above sea level) show that the main portion of coarse particles is situated above 2360 m level. Measurements on July 25 showed how incoming dust has changed the magnitude and spectral dependence of aerosol optical depth and volume spectra of columnar aerosol. Mean optical depth and Angstrom parameter values for Saharan dust outbreaks during the ACE 2 experiment agree well with the Atlantic Ocean and Bermuda data obtained during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) in July 1996, as well as with previously reported Atlantic Ocean results. Also, there is a good agreement between ACE 2 data for Saharan air masses and data obtained on certain sites of the AERONET network.

Retrieval of aerosol optical depth over land using two‐angle view satellite radiometry during TARFOX

A new aerosol optical depth retrieval algorithm is presented that uses the two‐angle view capability of the Along Track Scanning Radiometer 2 (ATSR‐2). By combining the two‐angle view and the spectral information this so‐called dual view algorithm separates between aerosol and surface contributions to the top of the atmosphere radiance. First validation of the dual view algorithm was performed during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), which was conducted at the mid‐Atlantic coast of the United States in July 1996. The satellite retrieved spectral aerosol optical depth is in good agreement with the aerosol optical depth from ground‐based Sun/sky radiometers in three out of four cases. This shows the potential of aerosol retrieval over land using two‐angle view satellite radiometry.