Christophe Pietras

An emerging ground‐based aerosol climatology: Aerosol optical depth from AERONET

Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.

Optical Properties of Atmospheric Aerosol in Maritime Environments

Systematic characterization of aerosol over the oceans is needed to understand the aerosol effect on climate and on transport of pollutants between continents. Reported are the results of a comprehensive optical and physical characterization of ambient aerosol in five key island locations of the Aerosol Robotic Network (AERONET) of sun and sky radiometers, spanning over 2‐5 yr. The results are compared with aerosol optical depths and size distributions reported in the literature over the last 30 yr. Aerosol found over the tropical Pacific Ocean (at three sites between 208S and 208N) still resembles mostly clean background conditions dominated by maritime aerosol. The optical thickness is remarkably stable with mean value of ta(500 nm) 5 0.07, mode value at tam

Atmospheric Aerosol Optical Properties in the Persian Gulf

Aerosol optical depth measurements over Bahrain acquired through the ground-based Aerosol Robotic Network (AERONET) are analyzed. Optical depths obtained from ground-based sun/sky radiometers showed a pronounced temporal trend, with a maximum dust aerosol loading observed during the March-July period. The aerosol optical depth probability distribution is rather narrow with a modal value of about 0.25. The Angstrom parameter frequency distribution has two peaks. One peak around 0.7 characterizes a situation when dust aerosol is more dominant, the second peak around 1.2 corresponds to relatively dust-free cases. The correlation between aerosol optical depth and water vapor content in the total atmospheric column is strong (correlation coefficient of 0.82) when dust aerosol is almost absent (Angstrom parameter is greater than 0.7), suggesting possible hygroscopic growth of fine mode particles or source region correlation, and much weaker (correlation coefficient of 0.45) in the presence of dust (Angstrom parameter is less than 0.7). Diurnal variations of the aerosol optical depth and precipitable water were insignificant. Angstrom parameter diurnal variability (;20%-25%) is evident during the April-May period, when dust dominated the atmospheric optical conditions. Variations in the aerosol volume size distributions retrieved from spectral sun and sky radiance data are mainly associated with the changes in the concentration of the coarse aerosol fraction (variation coefficient of 61%). Geometric mean radii for the fine and coarse aerosol fractions are 0.14 mm (std dev 5 0.02) and 2.57 mm (std dev 5 0.27), respectively. The geometric standard deviation of each fraction is 0.41 and 0.73, respectively. In dust-free conditions the single scattering albedo (SSA) decreases with wavelength, while in the presence of dust the SSA either stays neutral or increases slightly with wavelength. The changes in the Angstrom parameter derived from a ground-based nephelometer and a collocated sun photometer during the initial checkout period were quite similar.

Ship-Based Sun Photometer Measurements Using Microtops Sun Photometers

Abstract The use of hand-held Microtops II sun photometers (built by Solar Light Inc.) on ship platforms is discussed. Their calibration, filter stability, and temperature effects are also described. It is found that under rough conditions, the ship motion causes the largest error, which can result in a bias toward higher optical depths. In order to minimize this bias, a large number of sun photometer measurements (∼25) should be taken in a short period of time, and the higher values should be discarded. Under rough ocean conditions, it is also best to shorten the Microtops sun photometer sampling period (less than 5 s) and save only a single value (no averaging) and remove the high optical depths in postprocessing. It is found that the Microtops should be turned off frequently to correct for zero drift caused by temperature effects. Calibration is maintained by routine Langley plot calibrations at the Mauna Loa Observatory for each unit or through cross calibration.

A High-Accuracy Multiwavelength Radiometer for In Situ Measurements in the Thermal Infrared. Part I: Characterization of the Instrument

Abstract The new infrared radiometer (conveyable low-noise infrared radiometer for measurements of atmosphere and ground surface targets, or CLIMAT) is a highly sensitive field instrument designed to measure brightness temperatures or radiances in the infrared, from the ground level, or from an aircraft. It can be equipped with up to six channels in the 8–14-μm range. This instrument is characterized by its portability (total mass less than 5 kg), its self-sufficiency, and its automated operation. It can be operated either manually or automatically. The optical head of the instrument contains an objective lens and a condenser mounted according to the Kohler design, providing a uniform irradiation on the detector and a well-delimited field of view. The radiation is measured by a low-noise fast thermopile whose responsivity is slightly temperature dependent. The radiometric noise expressed as an equivalent brightness temperature is on the order of 50 mK for a 1-μm bandwidth at room temperature. The applicat...

A High-Accuracy Multiwavelength Radiometer for In Situ Measurements in the Thermal Infrared. Part II: Behavior in Field Experiments

The performances of the new conveyable low-noise infrared radiometer for measurements of atmosphere and ground surface targets, or CLIMAT, are presented for in situ measurements. For this, quantitative analyses were carried out on measurements performed with a prototype during various field experiments. The accuracy of the radiometric measurements controlled by using a field blackbody is estimated for severe environmental conditions. Two modes of operation and two types of targets are described. Ground-based measurements of the sky radiance are compared to radiative transfer calculations that use atmospheric profiles from radiosoundings as input parameters. Sea surface temperatures estimated from airborne CLIMAT measurements are compared to satellite retrievals. These experiments constitute a first set of quantitative tests of the CLIMAT radiometer for groundbased and airborne remote sensing applications. They demonstrate that CLIMAT can be considered for future studies on clouds and aerosols, sea water, and surface such as ice, vegetation, bare soil, and rocks.

Thermal-infrared field radiometer for vicarious cross-calibration: characterization and comparisons with other field instruments

A four-band (8.2 to 9.2, 10.5 to 11.5, 11.5 to 12.5, and 8 to 14 mm), prototype, thermal-IR radiometer, model CE 312 (CE 312 is the company model number. In previous papers, the CE 312 was called the CLIMAT (conveyable low-noise IR radiometer for measurements of at- mosphere and ground-surface targets)), with a built-in radiance refer- ence is been fabricated by CIMEL Electronique (Paris, France) for use as a field instrument. The instrument is briefly described, laboratory char- acterization is detailed, and its field measurements are compared with those from three other radiometers. The CE 312s main characteristics are linearity of better than 0.8%, field of view of 9.5 deg; noise-equivalent temperature difference of 0.06 to 0.2 K (depending on the band) for brightness temperatures of 0 to 75°C; SNR greater than 1100 for the broadband and greater than 400 for the other bands for brightness tem- peratures between 10 and 80°C; and repeatability of the measured radi- ance smaller than 0.35% after four field campaigns, corresponding to 0.2 K in terms of brightness temperature. Field measurements were con- ducted over different periods during 1996 at Jornada Experimental Range, New Mexico, Lunar Lake and Railroad Valley, Nevada, and Lake Tahoe, California. The CE 312 compares quite favorably with the other instruments: the brightness temperature at two different sites compared to within 0.3 K with two instruments. These measurements show that the CE 312 thermal-IR radiometer is very stable for ambient temperatures varying between 15 and 60°C and that the availability of several filters in the thermal-IR region can help tremendously to improve the accuracy of the radiance determination.

Sun-Pointing-Error Correction for Sea Deployment of the MICROTOPS II Handheld Sun Photometer

Handheld sun photometers, such as the MICROTOPS II (manufactured by Solar Light, Inc.), provide a simple and inexpensive way to measure in situ aerosol optical thickness (AOT), ozone content, and water vapor. Handheld sun photometers require that the user manually point the instrument at the sun. Unstable platforms, such as a ship at sea, can make this difficult. A poorly pointed instrument mistakenly records less than the full direct solar radiance, so the computed AOT is much higher than reality. The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has been collecting maritime AOT data since 1997. As the dataset grew, a bias of the MICROTOPS II data with respect to other instrument data was noticed. This bias was attributed to the MICROTOPS II measurement protocol, which is intended for land-based measurements and does not remove pointing errors when used at sea. Based upon suggestions in previous literature, two steps were taken to reduce pointing errors. First, the measurement protocol was changed to keep the maximum (rather than average) voltage of a sequence of measurements. Once on shore, a second screening algorithm was utilized to iteratively reject outliers that represent sun-pointing errors. Several versions of this method were tested on a recent California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise, and were compared to concurrent measurements using the manufacturer-supplied protocol. Finally, a separate postprocessing algorithm was created for data previously gathered with the manufacturer-supplied protocol, based on statistics calculated by the instrument at the time of capture.

Non-supervised classification of aerosol mixtures for ocean color remote sensing

Satellite ocean-color algorithms generally use aerosol-mixture models to estimate and remove the atmospheric contribution to the measured signal. These models, based on aerosol samples, may or may not be realistic. In atmospheric correction, we are more interested in the optical behavior of the aerosols through the entire atmosphere. Comparisons of SeaWiFS-derived and measured aerosol optical thickness have revealed a systematic underestimation of the Angstrom coefficient, suggesting that the reference models may not be representative of actual conditions. To investigate the adequacy of the models and ultimately to improve atmospheric correction, we analyze atmospheric optics data collected by the AERONET project under a wide range of aerosol conditions at coastal and island sites. Using non-supervised classification techniques (self-organized mapping, hierarchical clustering), we determine the natural distribution of retreived aerosol properties of the total atmospheric column, i.e., the volume size distribution function and the refractive index, and more importantly identify clusters in this distribution. These clusters may be used as new aerosols mixtures in radiative transfer algorithms. We compare the clusters with the SeaWiFS reference models and, through application examples, conclude about their potential to improve atmospheric correction of satellite ocean color.

Analysis of shipboard aerosol optical thickness measurements from multiple sunphotometers aboard the R/V Ronald H. Brown during the Aerosol Characterization Experiment - Asia

Marine sunphotometer measurements collected aboard the R/V Ronald H. Brown during the Aerosol Characterization Experiment-Asia (ACE-Asia) are used to evaluate the ability of complementary instrumentation to obtain the best possible estimates of aerosol optical thickness and Angstrom exponent from ships at sea. A wide range of aerosol conditions, including clean maritime conditions and highly polluted coastal environments, were encountered during the ACE-Asia cruise. The results of this study suggest that shipboard hand-held sunphotometers and fast-rotating shadow-band radiometers (FRSRs) yield similar measurements and uncertainties if proper measurement protocols are used and if the instruments are properly calibrated. The automated FRSR has significantly better temporal resolution (2 min) than the hand-held sunphotometers when standard measurement protocols are used, so it more faithfully represents the variability of the local aerosol structure in polluted regions. Conversely, results suggest that the hand-held sunphotometers may perform better in clean, maritime air masses for unknown reasons. Results also show that the statistical distribution of the Angstrom exponent measurements is different when the distributions from hand-held sunphotometers are compared with those from the FRSR and that the differences may arise from a combination of factors.