G. Pavese

EAQUATE: An International Experiment For Hyperspectral Atmospheric Sounding Validation

The international experiment called the European Aqua Thermodynamic Experiment (EAQUATE) was held in September 2004 in Italy and the United Kingdom to validate Aqua satellite Atmospheric Infrared Sounder (AIRS) radiance measurements and derived products with certain groundbased and airborne systems useful for validating hyperspectral satellite sounding observations. A range of flights over land and marine surfaces were conducted to coincide with overpasses of the AIRS instrument on the Earth Observing System Aqua platform. Direct radiance evaluation of AIRS using National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder Testbed-Interferometer (NAST-I) and the Scanning High-Resolution Infrared Sounder has shown excellent agreement. Comparisons of level-2 retrievals of temperature and water vapor from AIRS and NAST-I validated against high-quality lidar and dropsonde data show that the 1-K/l-km and 10%/1-km requirements for temperature and water vapor (respectively) are ge...

Retrieval of foreign-broadened water vapor continuum coefficients from emitted spectral radiance in the H2O rotational band from 240 to 590 cm −1

The paper presents a novel methodology to retrieve the foreign-broadened water vapor continuum absorption coefficients in the spectral range 240 to 590 cm(-1) and is the first estimation of the continuum coefficient at wave numbers smaller than 400 cm(-1) under atmospheric conditions. The derivation has been accomplished by processing a suitable set of atmospheric emitted spectral radiance observations obtained during the March 2007 Alps campaign of the ECOWAR project (Earth Cooling by WAter vapor Radiation). It is shown that, in the range 450 to 600 cm(-1), our findings are in good agreement with the widely used Mlawer, Tobin-Clough, Kneizys-Davies (MT CKD) continuum. Below 450 cm(-1) however the MT CKD model overestimates the magnitude of the continuum coefficient.

Interferometer for ground-based observations of emitted spectral radiance from the troposphere: evaluation and retrieval performance

We evaluate the spectral quality, radiometric noise, and retrieval performance of a Fourier transform infrared spectrometer, which has been developed for recording spectrally resolved observations in a region of the spectrum which is important both for the science of Earths climate and applications, such as the remote sensing of temperature and atmospheric gas species. This spectral region extends from 100 to 1600 cm(-1) and encompasses the two fundamental, rotation and vibration, absorption bands of water vapor. The instrument is a customized version of a Bomem AERI (Atmospheric Emitted Radiance Interferometer) spectrometer, whose spectral coverage has been extended in the far infrared with the use of uncooled pyroelectric detectors. Retrieval examples for water vapor and temperature profiles are shown, which also allow us to intercompare the retrieval performance of both H(2)O vibration and rotation bands.

Inverting High Spectral Resolution Aerosol Optical Depth to Determine the Size Distribution of Atmospheric Aerosol

Measurements of aerosol optical depth are presented in the spectral interval 0.40–1.10 μm, which includes the most transparent region of the solar spectrum and the near infrared. The measurements were obtained by a grating spectrometer with a resolution ≈ 0.5 nm during the 1994 summer season at a mountain site (about 850 m above sea level) in the South of Italy. Spectral regions free from gas absorption features have been singled out and used to retrieve the aerosol columnar size distribution. Inversions have been performed by using the Phillips-Twomey inversion method along with a χ2 criterion which allows one to choose a suitable value for the regularization parameter. The result of the inversions are presented in the particle radius range 0.1 ÷ 3 μm and indicate the presence of a bimodal aerosol with the second mode radius at about 1.0 μm undergoing transformations which are well correlated with relative humidity.

Determining Ångström's Turbidity Coefficients: An Analysis with a Wide-Range Grating Spectrometer

The dependence of Angstroms turbidity parameters on the wavelength has been analyzed in the wavelength interval 0.45–0.70 μm. Measurements of aerosol optical thicknesses in this range were obtained by a wide-range grating spectrometer with a resolution of ∼ 2.5 nm. Based on such measurements, two different methods for obtaining turbidity parameters have been validated and intercompared: (a) the ratio method based on processing couples of aerosol optical depths, and (b) a linear best-fitting method based on log-log plot of aerosol optical thicknesses vs. wavelength. We found that Angstrms turbidity relation is quite effective in characterizing atmospheric aerosol in low-medium turbidity cases. However, the usual method of computing the turbidity parameters on the basis of a few measurements (< 10) does not produce adequate results. Accurate estimates can be only obtained using a larger number of data points

Effects of Saharan Dust Advection on Atmospheric Aerosol Properties in the West-Mediterranean Area

Eight measurement campaigns for the characterization of atmospheric aerosol properties were conducted from 2001 until 2008 at five sites located in the Western Mediterranean basin. Radiometric measurements were used to obtain Aerosol Optical Depth, Ångström parameters, and aerosol size distributions, allowing differentiation of background conditions from anthropogenic, marine, or Saharan dust aerosol advection. The analysis was focused on the study of optical and physical properties variation of atmospheric aerosols under Saharan outbreaks. Dust-affected data were analysed all together, independently from the measurements site, thus allowing the highlighting of similarities and differences among them. The scatter-plot Ångström exponent versus AOD at 780 nm shows a correlation among all dust data, while an overlapping region with no-dust data reveals the simultaneous presence of mineral, anthropogenic, and marine particles. Daily averaged volume size distributions can be unimodal or bimodal functions and one three-modal distribution, with a coarse mode generally prevailing. Finally, considering the ratio of small/large particles 𝑛𝑠/𝑛𝑙 and plotting the corresponding histogram for all dust data, a sharp frequency distribution is obtained with 89% of data in the range 5–65, while 89% of no-dust data extend from 5 to 135, in spite of different sources, pathways, and arrival sites.

Diurnal and Nocturnal Measurements of Aerosol Optical Depth at a Desert Site in Namibia

The Namibian desert is a candidate site for astrophysical observations with ground-based instrumentation, such as the High Energy Stereoscopic System experiment. For this kind of application, the characterization of the atmospheric transmittance is mandatory. In this context, a first campaign of both solar and lunar direct irradiance measurements has been performed from 7 July 1998 to 10 July 1998 on a desert plateau in Namibia. The aerosol optical depth in the visible range (330-700 nm) has been measured; we found daily values (at 483 nm) ranging from 0.10 to 0.26, while the nightly ones range from 2.5 2 10 m 3 up to 0.86. By means of a least square fitting procedure, the Ångström turbidity parameters have been estimated. ( f daily values 0.92 1 3.64, f nightly values 0.008 1 4.2; g daily values 0.06 1 0.56, g nightly values 0.001 1 0.68). Furthermore, a relationship between Ångström parameters and meteorological variables such as relative humidity and wind speed has been investigated. The results do not highlight any correlation, except for two diurnal data sets that show a negative correlation between the optical thickness and the wind speed. Although preliminary, our measurements allow us to have a first insight into characterizing the aerosol optical properties of the Namibian background aerosol. On the other hand, an extended campaign of measurements is needed for a full characterization of the site.

Regional transport of anthropogenic pollution and dust aerosols in spring to Tianjin — A coastal megacity in China

Simultaneous measurements of columnar aerosol microphysical and optical properties, as well as PM2.5 chemical compositions, were made during two types of spring pollution episodes in Tianjin, a coastal megacity of China. The events were investigated using field observations, satellite data, model simulations, and meteorological fields. The lower Ångström Exponent and the higher aerosol optical depth on 29 March, compared with the earlier event on 26 March, implied a dominance of coarse mode particles - this was consistent with the differences in volume-size distributions. Based on the single scattering spectra, the dominant absorber (at blue wavelength) changed from black carbon during less polluted days to brown carbon on 26 March and dust on 29 March. The concentrations of major PM2.5 species for these two episodes also differed, with the earlier event enriched in pollution-derived substances and the later with mineral dust elements. The formation mechanisms of these two pollution episodes were also examined. The 26 March episode was attributed to the accumulation of both local emissions and anthropogenic pollutants transported from the southwest of Tianjin under the control of high pressure system. While the high aerosol loading on 29 March was caused by the mixing of transported dust from northwest source region with local urban pollution. The mixing of transported anthropogenic pollutants and dust with local emissions demonstrated the complexity of springtime pollution in Tianjin. The synergy of multi-scale observations showed excellent potential for air pollution study.

Radiometric performances of the Fourier transform spectrometer for the Radiation Explorer in the Far-Infrared (REFIR) space mission

In the framework of the Radiation Explorer in the Far InfraRed space mission for the characterization in the far infrared of the Earth outgoing emission, a breadboard version of the Fourier transform spectrometer, which is the core instrument of the payload package, has been developed. The Fourier transform spectrometer operates in the spectral range of 100-1100 cm-1 with a resolution of 0.5 cm-1, 6.5 s acquisition time, and signal-to-noise ratio better than 100. It is a compact prototype designed both for laboratory applications and for field campaigns, in particular for operations in high-altitude ground-based sites and on-board of stratospheric balloon platforms. This paper describes the instrument characterization performed in laboratory conditions and under vacuum. The study has allowed to study the trade-off among all the instrument parameters and to test the new optical design of the interferometer, with particular attention to the photolithographic beam splitters and the room-temperature pyroelectric detectors. The instrument was operated for the first time in a field campaign in June 2004 from 1247~m altitude ground-based site located in the South of Italy. The results of this test and the comparison with the measurements taken with a BOMEM spectrometer, that partially overlaps the REFIR bandwidth in the higher spectral region, are reported. Info can be found at http://radiation.ifac.cnr.it.

A differential absorption technique in the near infra-red to determine precipitable water

Abstract The analysis of an improved version of a radiometric method is described by which the total water amount over the entire atmospheric column may be determined by the use of a moderate resolution grating spectrometer. The method requires transmission measurements of solar spectral irradiance at differentially absorbing channels located in the wing of a water vapour band. Two water vapour bands are investigated: the 0.9- and l.4-μm H z0 bands. Since the method is based on the establishment of ratios of optical signals at given channels, there is no need for absolute calibration. Using an extensive set of rawinsonde temperature and water vapour mixing ratio profiles along with LOWTRAN 7 code, empirical curves between precipitable water and ratios of solar direct irradiance at differentially absorbing channels have been derived. The interrelationship with atmospheric aerosol has been particularly analysed. The method which has been proposed is based on observing the extinction of solar beam irradiance at four adjacent wavelengths rather than at two as it is habit. The result is a good independence of the method on the aerosol atmospheric load.