L. Blarel

Climatology of dust aerosol size distribution and optical properties derived from remotely sensed data in the solar spectrum

Simultaneous spectral remote observations of dust properties from space and from the ground create a powerful tool for the determination of ambient dust properties integrated on the entire atmospheric column. The two measurement methods have a complementary sensitivity to variety of dust properties. The methodology is demonstrated using spectral measurements (0.47-2.21 mm) from Landsat TM over the bright Senegalian coast and dark ocean, and Aerosol Robotic Network (AERONET) radiances measured in several locations. We derive (1) the dust size distribution, showing a dominant coarse mode at 1-5 mm and a secondary mode around 0.5 mm effective radius; (2) dust absorption, which is found to be substantially smaller than reported from previous measurements; (3) the real part of the refractive index which varies within the range 1.53- 1.46; and we show that (4) the effect of the dust nonspherical shape on its optical properties is not significant for scattering angles ,1208.

Dust optical properties retrieved from ground-based polarimetric measurements

We have systematically processed one year of sunphotometer measurements (recorded at five AERONET/PHOTONS sites in Africa) in order to assess mineral dust optical properties with the use of a new polarimetry-based algorithm. We consider the Cimel CE318 polarized sunphotometer version to obtain single-scattering albedo, scattering phase matrix elements F(11) and F(12) for dust aerosols selected with Angström exponents ranging from -0.05 to 0.25. Retrieved F(11) and F(12) differ significantly from those of spherical particles. The degree of linear polarization -F(12)/F(11) for single scattering of atmospheric total column dust aerosols in the case of unpolarized incident light is systematically retrieved for the first time to our knowledge from sunphotometer measurements and shows consistency with previous laboratory characterizations of nonspherical particles.

Calibration of the degree of linear polarization measurement of polarized radiometer using solar light

We establish a polarimetric reference for the degree of linear polarization (DOLP) measurement calibration, based on direct and reflected solar light, with a theoretical error of about 0.0012. This calibration source can be used to calibrate polarized radiometers instead of complex laboratory devices and can respond from UV to near infrared wavelengths. A two-step method for calibrating the DOLP measurement is proposed and applied to a ground-based polarized radiometer. The first step is correcting the transmittance difference between polarizer units using the direct solar beam, while the second step corrects possible bias in DOLP measurement using the reflected solar light as a reference. Based on instrument characterization, calibration results obtained with the new polarized sun-sky radiometer, CE-318-DP, are discussed and compared with laboratory results.

Climatology of aerosol properties and clear‐sky shortwave radiative effects using Lidar and Sun photometer observations in the Dakar site

This paper presents the analysis of nearly a decade of continuous aerosol observations performed at the Mbour site (Senegal) with Sun photometer, Lidar, and Tapered Electromagnetic Oscillating Microbalance. This site is influenced all year-round by desert dust and sporadically, in wintertime, by biomass burning particles. Different patterns are revealed for winter and summer, seasons associated to air masses of different origin. The summer (wet season) is characterized by a high aerosol loading (optical thickness, AOT, around 0.57 at 532 nm) composed of large and weakly absorbing particles (Angstrom exponent, α, of 0.23 and single-scattering albedo, ϖ0, of 0.94 at 532 nm). A lower aerosol loading (AOT = 0.32) is observed during winter (dry season) for finer and absorbing particles (α = 0.48 and ϖ0 = 0.87) revealing the presence of biomass burning aerosols and a greater proportion of local emissions. This latter anthropogenic contribution is visible at weekly and daily scales through AOT cycles. A decrease of about 30% in AOT has been featured in autumn since 2003. The derivation of the extinction profiles highlights a dust transport close to the ground during winter and in an aloft layer (up to 5 km) during summer. Accurate calculations of the daily aerosol radiative effect in clear-sky conditions are finally addressed. From spring to winter, seasonal shortwave radiative forcing averages of 14.15, 11.15, 8.92, and 12.06 W m−2 have been found respectively. Up to 38% of the solar clear-sky atmospheric heating can be attributed to the aerosols in this site.

Method to intercalibrate sunphotometer constants using an integrating sphere as a light source in the laboratory

A calibration method is introduced to transfer calibration constants from the reference to secondary sunphotometers using a laboratory integrating sphere as a light source, instead of the traditional transferring approach performed at specific calibration sites based on sunlight. The viewing solid angle and spectral response effects of the photometer are taken into account in the transfer, and thus the method can be applied to different types of sunphotometers widely used in the field of atmospheric observation. A laboratory experiment is performed to illustrate this approach for four types of CIMEL CE318 sunphotometers belonging to the aerosol robotic network (AERONET). The laboratory calibration method shows an average difference of 1.4% from the AERONET operational calibration results, while a detailed error analysis suggests that the uncertainty agrees with the estimation and could be further improved. Using this laboratory calibration approach is expected to avoid weather influences and decrease data interruption due to operationally required periodic calibration operations. It also provides a basis for establishing a network including different sunphotometers for worldwide aerosol measurements, based on a single standard calibration reference.

Description and applications of a mobile system performing on-roadaerosol remote sensing and in situ measurements

The majority of ground-based aerosols observations are limited to fixed locations, narrowing the knowledge on their spatial 15 variability. In order to overcome this issue, a compact Mobile Aerosol Monitoring System (MAMS) was developed to explore the aerosol vertical and spatial variability. This mobile laboratory is equipped with a micropulse lidar, a sunphotometer and an aerosol spectrometer. It is distinguished by other transportable platforms through its ability to perform onroad measurements and its unique feature lies in the sun-photometer capable to track the sun during motion. The system presents a great flexibility, being able to respond quickly in case of sudden aerosol events such as pollution episodes, dust, 20 fire or volcano outbreaks. On-road mapping of aerosol physical parameters such as attenuated aerosol backscatter, aerosol optical depth, particle number and mass concentration and size distribution is achieved through the MAMS. The performance of remote sensing instruments on-board has been evaluated through intercomparison with instruments in reference networks (i.e. AERONET and EARLINET), showing that the system is capable of providing high quality data. This also illustrates the application of such system for instrument intercomparison field campaigns. Applications of the 25 mobile system have been exemplified through two case studies in northern France. MODIS AOD data was compared to ground-based mobile sun-photometer data. A good correlation was observed with R of 0.76, showing the usefulness of the mobile system for validation of satellite-derived products. The performance of BSC-DREAM8b dust model has been tested by comparison of results from simulations to the lidar-sun-photometer derived extinction coefficient and mass concentration profiles. The comparison indicated that observations and model are in good agreement in describing the vertical variability 30 of dust layers. Moreover, on-road measurements of PM10 were compared with modelled PM10 concentrations and with ATMO Hauts-de-France and AIRPARIF air quality in situ measurements, presenting an excellent agreement in horizontal

Assessment of Sun photometer Langley calibration at the high-elevation sites Mauna Loa and Izaña

The aim of this paper is to analyze the suitability of the high-mountain stations Mauna Loa and Izaña for Langley plot calibration of Sun photometers. Thus the aerosol optical depth (AOD) characteristics and seasonality, as well as the cloudiness, have been investigated in order to provide a robust estimation of the calibration accuracy, as well as the number of days that are suitable for Langley calibrations. The data used for the investigations belong to AERONET and GAW-PFR networks, which maintain reference Sun photometers at these stations with long measurement records: 22 years at Mauna Loa 5 and 15 years at Izaña. In terms of clear sky and stable aerosol conditions, Mauna Loa (3397m a.s.l.) exhibits on average of 377 Langleys (243 morning and 134 afternoon) per year suitable for Langley plot calibration, whereas Izaña (2373m a.s.l.) shows 343 Langleys (187 morning and 155 afternoon) per year. The background AOD(500nm) values, on days that are favorable for Langley calibrations, are in the range 0.01-0.02 throughout the year, with well defined seasonality that exhibits a spring maximum at both stations plus a slight summer increase at Izaña. The statistical analysis of the long-term determination of 10 extraterrestrial signals yields to a calibration uncertainty of ~0.2-0.5%, being this uncertainty smaller in the near infrared and larger in the ultraviolet wavelengths. This is due to atmospheric variability that cannot be reduced based only on quality criteria of individual Langely plots.

Combined ground-based and satellite remote sensing of atmospheric aerosol and Earth surface in the Antarctic

The paper presents lecture materials given at the Nineteenth International Conference and School on Quantum Electronics “Laser Physics and Applications” (19th ICSQE) in 2016, Sozopol, Bulgaria and contains the results of the 10-year research of Belarusian Antarctic expeditions to study the atmospheric aerosol and Earth surface in Antarctica. The works focus on the studying variability and trends of aerosol, cloud and snow characteristics in the Antarctic and the links of these processes with the long range transport of atmospheric pollutants and climate changes.

Interactions aérosols-rayonnement-climat en région méditerranéenne : Impact de l'effet radiatif direct sur le cycle de l'eau

An experimental campaign, coupled with three-dimensional modeling, was conducted in the western Mediterranean during the summer of 2013 to study the impact of aerosols on the radiative balance and climate of this region. In situ observations were obtained on the ground, aboard two research aircraft and balloons to characterize the physico-chemical and optical properties of particles and their vertical stratification. This campaign was mainly characterized by moderate events of desert aerosols. During these episodes, strong vertical stratification was observed and the measurements of the optical properties reveal moderate absorbing particles in the visible spectrum. Climate simulations indicate a significant impact of aerosols in particular by changing the surface temperature of the sea, the ocean-atmosphere fluxes and consequently seasonal precipitation.

Influence of sky radiance measurement errors on inversion-retrieved aerosol properties

Remote sensing of the atmospheric aerosol is a well-established technique that is currently used for routine monitoring of this atmospheric component, both from ground-based and satellite. The AERONET program, initiated in the 90’s, is the most extended network and the data provided are currently used by a wide community of users for aerosol characterization, satellite and model validation and synergetic use with other instrumentation (lidar, in-situ, etc.). Aerosol properties are derived within the network from measurements made by ground-based Sun-sky scanning radiometers. Sky radiances are acquired in two geometries: almucantar and principal plane. Discrepancies in the products obtained following both geometries have been observed and the main aim of this work is to determine if they could be justified by measurement errors. Three systematic errors have been analyzed in order to quantify the effects on the inversion-derived aerosol properties: calibration, pointing accuracy and finite field of view. Si...