Ultraviolet spectral irradiances are systematically recorded in Briancon (at an altitude of 1310 m), in the French Southern Alps. Up to now, aerosols optical depth (AOD) had never been routinely measured in Briancon, so it has appeared of great interest to take advantage of the many spectra recorded to learn more about aerosols over the spectral range: 320 nm - 448 nm. To retrieve AOD, one only has to remove the contribution of ozone and molecule from the total optical depth, obtained from the direct irradiance. The method has been implemented for every spectra acquired during clear sky days selected among the period from July 2004 to March 2005. Data recorded show that at a given wavelength, AOD is roughly constant during the day. Lowest values for AOD at 12:00 U. T. are found to be around 0.05 at 440 nm and 0.15 at 320 nm, and may increase threefold for some days. AOD range between 320 and 448 nm is larger in summer than in winter. This agrees with the way α, the Angstrom parameter, varies from one season to the other. Unfortunately, one can only make a restricted use of α, as its determination over the ultraviolet range is subject to high AOD uncertainties. The main source of AOD error rests in the 10% of uncertainty on direct irradiance measured. This gives rises to relatively high uncertainties on AOD, which increase all the more when aerosols load in the atmosphere is low. In Briancon, renowned for its air goodness, true AOD values are likely to be reached within 45% - 50% as the best in a majority of cases.
Albedo inversion techniques are investigated in this work. Several methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. One first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow- covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present sensitivity analysis, the differences and the correlations between the various methods as well as the results for the different locations.
The occultation experiment SAGE III (Stratospheric Aerosol and Gas Experiment III) is expected to be launched in Autumn 1999. The instrument will be on board the satellite 'Meteor 3M' on a polar orbit. This experiment is based on the same principle than SAGE II. However some channels are divided in some subchannels. For example it is the case of nitrogen dioxide channel at 430 - 450 nm, ozone at 560 - 624 nm and water vapor at 933 - 960 nm. On the other hand the lunar occultation is added in order to detect minor components: OClO, NO3. A simulation of the transmissions for only the channel devoted to the water vapor has been done, from a water vapor profile obtained for a SAGE II event. The wavelength range 933 - 960 nm is planned to be divided in thirty subchannels. The inverted profiles of water vapor obtained in each subchannel are compared to the input profile. The relative difference between the mean retrieved profile and the data is smaller than 15%.
A detailed discussion on the inversion algorithm developed by the Laboratoire d'Optique Atmospherique, University of Lille, France, for the analysis of the Stratospheric Aerosol and Gas Experiment III (SAGE III) solar occultation data is presented. The scope of the paper is limited to nitrogen dioxide, ozone and aerosol retrieval. The forward model algorithm for calculating atmospheric transmittances in the two SAGE III solar channels, at 440 nm and at 600 nm, is presented. Then the inversion algorithm is introduced, accomplished in two sequential steps: the first one is the spatial inversion of the simulated slant optical thickness profile to obtain the extinction coefficient profile and the second is the spectral inversion of the extinction coefficient at each altitude to separate gas and aerosol contributions by using a least squares method over the spectral signatures. Error analysis is also discussed: the results of this analysis indicate regression error of 1% or less in the aerosol retrieval, of 0.1% or less for the ozone retrieval and of about 0.1% for the nitrogen dioxide retrieval. The paper also discuss on the correlation between the relative differences and the relative contribution of the considered constituents in the total extinction coefficient and then can determine the inversion algorithm's performances.
The Stratospheric Aerosol and Gas Experiment (SAGE) III is planned to be launched in summer 1998. It will provide aerosol extinction coefficients from solar occultation measurements at more wavelengths than SAGE II. This paper describes two methods used for the retrieval of some aerosol characteristics of great interest for the climate modeling and for the study of heterogeneous chemistry: effective radius, effective variance, surface area density and volume density. The first technique consists in a King inversion scheme and the second relies on a least squares fit on the extinction measurements. The two methods are applied to simulated extinction measurements at SAGE III wavelengths to investigate the ability of retrieving the aerosol characteristics in case of unimodal or bimodal log-normal size distributions. The contribution of channel 1.550 micrometer is estimated and uncertainties are also determined. The results derived from the two techniques are consistent in unimodal and bimodal cases for the four quantities: the least squares fit method is much faster but leads to larger uncertainties, in bimodal case the King method allows a better retrieval.
A balloon borne instrument BALLAD (balloon limb aerosol detection) has been developed at the LOA (Laboratoire d'Optique Atmospherique). It scans the Earth's limb at three wavelengths (450, 600, and 850 nm) from the float altitude between 30 - 35 km when the sun is low above the horizon; polarization is also measured in the 850 nm channel. A flight has been performed from the southwest of France on October 13, 1994 during the phase II of the SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment) campaign. An analysis of the reflectances at 850 and 450 nm without polarization is presented.
The Stratospheric Aerosol and Gas Experiment II (SAGE II), that was launched in October 1984, has monitored the stratospheric aerosol layer after the Pinatubo's eruption. Two flights of the balloon-borne experiment RADIBAL (RADIometer BALloon) were performed in June 1992 and May 1993 in coincidence with SAGE II events. Because of the large aerosol loading, the inversion of the balloon measurements (consisting in radiance and polarization diagrams) was impracticable. A code taking into account the multiple scatterings has then been used to calculate theoretical diagrams for an aerosol model deduced from SAGE II data. The obtained diagrams have been compared satisfactorily to the experimental ones.
The Polar Ozone and Aerosol Measurements II (POAM II) has been developed by the Naval Research Laboratory and launched end of September 1993 on the french satellite SPOT 3. The instrument observes solar occultations at 9 wavelength channels. The inversion algorithm allows the retrieval of the extinction vertical profiles and the separation of the species contributing to this extinction: air molecules, aerosols, ozone, water vapor and nitrogen dioxide; furthermore the retrieval of aerosol extinction at 4 wavelengths provides an information on the aerosol size distribution. An inversion algorithm has been developed at the LOA and preliminary comparisons with the NRL algorithm are presented. Two balloon-borne instruments have been flown from Kiruna (Sweden) in coincidence with POAM II observations: one of the instrument (RADIBAL) measures the radiance and polarization diagrams in the near infrared during the balloon ascent; the other instrument (BALLAD) observes the earth's limb at 3 wavelengths from the balloon ceiling altitude. Both instruments are complementary and provide the aerosol profile and the aerosol size distribution. Preliminary comparisons with POAM II data are presented.
The Polar Ozone and Aerosol Measurements II (POAM II) has been developed by the Naval Research Laboratory and launched end of September 1993 on the french satellite SPOT 3. The instrument observes solar occultations at 9 wavelength channels. Two channels are devoted to the retrieval of the water vapor profile, one at 935 nm in the water vapor absorption band, and a nearby channel at 920 nm, almost free of water vapor absorption. The two channels are used in a differential mode to separate the aerosol extinction from the water vapor absorption. The major difficulty to retrieve a water vapor profile from the transmission data, is due to the line structure of the absorption spectrum. Line-by-line models are used as basic benchmarks, and the GEISA and HITRAN models are compared. However the line-by-line models are much too complex to be run at each step in an inversion algorithm; a simple parameterization has been sought, following the method used for SAGE II. Results are presented.
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