Thierry Elias

Haze layer characterization and associated meteorological controls along the eastern coastal region of southern Africa

[1] Episodes of regionally extensive haze were observed over southern African during the dry season intensive of the Southern African Regional Science Initiative (SAFARI 2000). Several case studies of southern African haze layers were examined and characterized in terms of physical structure as they exited off of the eastern coastal region of southern Africa. In situ observations of aerosols and trace gases and their physical and chemical characteristics were collected on board South African Weather Service Aerocommander research aircraft. Haze structure, based on these measurements, is examined as it varies with synoptic type. Despite strong differences in the observed ENSO regime between SAFARI 2000 and that observed during the Southern African Fire-Atmosphere Research Initiative (SAFARI-92) and their respective aerosol accumulation mechanisms (col Rrgions/weak anticyclones versus strong anticyclones), a surprising degree of consistency in the observed vertical structure of the lower troposphere was found in southern Africa. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: haze layers, synoptic circulations, ENSO

Aerosol radiative forcing efficiency in the UV region over southeastern Mediterranean: VELETA2002 campaign

[i] Atmospheric aerosol effects on spectral global UV irradiance were evaluated during the VELETA2002 field campaign between 8 and 19 July 2002 in southeast Spain. In the first stage, seven UV spectroradiometer and six CIMEL Sun photometer measurements were carried out simultaneously, allowing them to be calibrated and intercompared. The mean ratio obtained for the global irradiance between the spectroradiometers, with regards to a reference instrument, ranges from 0.98 up to 1.04 with standard deviations that oscillate between ±0.01 and ±0.17. In particular, the two spectroradiometers used to obtain the aerosol forcing efficiencies have a ratio of 1.000 ± 0.001. The aerosol optical depth (AOD) obtained with the CIMEL Sun photometers has a standard deviation of lower than ±0.01 for all the channels. Under clear sky conditions, the diurnal aerosol forcing efficiency (ΔDF e ) and fractional diurnal forcing efficiency (ΔFDF e ) was calculated for two Mediterranean stations: Armilla (691 m.a.s.l.) within the boundary layer and Sabinas (2200 m.a.s.l) on the lower limit of the free troposphere and 25 km away from the first station. The ΔDF e values obtained at Armilla range between -2.72 ± 0.45 W m-2/τ 380 and -2.88 ± 0.45 W m-2/τ 440 and between -3.22 ± 0.61 W m-2/τ 380 and -3.40 ± 0.62 W m-2/τ 440 at Sabinas station; the ΔFDF e values range from -8.0 ± 1.4%/τ 380 to -8.6 ± 1.3%/T 440 and -12.0 ± 2.3%/τ 380 to -12.6 ± 2.3%/τ 440 at the two stations, respectively. Also, an experimental aerosol transmittance factor, C T , used to obtain UV satellite derived products was found as a result of the dependence of the global irradiance with the AOD, under cloudless conditions. The average aerosol attenuation factor, η, obtained from the C T , is 6 ± 2% under weakly absorbing aerosols, with a negligible spectral dependence.

Surface optical properties measured by the airborne research scanning polarimeter during the CLAMS experiment

Multi-angle multi-spectral scans of upward total and polarized radiance were measured by the Research Scanning Polarimeter onboard a Cessna 210 aircraft during the Chesapeake Lighthouse and Aircraft Measurements for Satellites experiment. The surface contribution to the polarized radiance (PR) was evaluated for its effect on the retrieval of aerosol properties from PR over land surfaces. The atmospheric contribution to PR was negligible at 2250 nm for an aerosol having an aerosol optical thickness (AOT) at 440 nm of 0.72 and AOT at 865 nm of 0.17. The atmospheric and surface contribution to PR at 865 nm were found to be similar for this aerosol load. Thus, although the 2250 nm measurements can be used to characterize the surface on this very turbid day, this is not the case for the 865 nm measurements. The polarized and total radiances at 1590 and 2250 nm were found to be strongly correlated, suggesting that a common physical process affects reflection and polarization since this correlation was present even within a given surface type. Near the backscattering direction a simple front facet Fresnel reflection model of surface polarization breaks down and the surface polarized reflectance shows considerable spectral variability and angular structure that may be of use in evaluating the structural parameters of vegetated and soil surfaces.

Aerosols attenuating the solar radiation collected by solar tower plants: The horizontal pathway at surface level

Aerosols attenuate the solar radiation collected by solar tower plants (STP), along two pathways: 1) the atmospheric column pathway, between the top of the atmosphere and the heliostats, resulting in Direct Normal Irradiance (DNI) changes; 2) the grazing pathway close to surface level, between the heliostats and the optical receiver. The attenuation along the surface-level grazing pathway has been less studied than the aerosol impact on changes of DNI, while it becomes significant in STP of 100 MW or more. Indeed aerosols mostly lay within the surface atmospheric layer, called the boundary layer, and the attenuation increases with the distance covered by the solar radiation in the boundary layer. In STP of 100 MW or more, the distance between the heliostats and the optical receiver becomes large enough to produce a significant attenuation by aerosols. We used measured aerosol optical thickness and computed boundary layer height to estimate the attenuation of the solar radiation at surface level at Ouarzaz...

Estimation of the total uncertainty of sky-radiance measurements in field experimental conditions: implications for the aerosol single-scattering albedo.

We compare the spectral sky radiance measured by three ground-based optical radiometers during the second Aerosol Characterization Experiment (ACE-2) to estimate the total uncertainty of the radiance in field experimental conditions. The propagation of this uncertainty on the column-integrated aerosol single-scattering albedo omega0 at 868 nm is investigated. The radiance measurements are affected by a systematic gain uncertainty of less than 2% in the visible spectral region and within 6% in the near-IR region. Correcting the measured radiance by a systematic uncertainty reduces the dispersion of the omega0 from 0.07 to 0.03. Besides, the total relative uncertainty of the radiance measurements in field experimental conditions is within 4% at any wavelength. The corresponding uncertainty delta omega0 is 4% for an aerosol optical thickness of 0.2.

Aerosol extinction and absorption in Évora, Portugal, during the European 2003 summer heat wave

Aerosol optical properties are retrieved from measurements acquired during the 2003 summer at the new AERONET station of Evora, Portugal, with a sun/sky photometer, a fluxmeter and a nephelometer. Aerosol optical thickness (aot) derived at several wavelengths shows that an exceptionally long turbid event occurred in July-August. Desert dust particles transported from North Africa increased aot at 873 nm (aot873) to the value of 0.27 with an Ångstrom exponent αC=0.5. Emissions from forest fires in The Iberic peninsula affected Evora since the end of the dust episode, with aot441 reaching 0.81 and aC=1.8. The aerosol scattering coefficient measured at surface level shows that desert dust does not reach the surface level at Evora while the forest fire emissions were uniformly distributed over the atmospheric column. Sky-radiance and flux measurements agree in retrieval of the aerosol single scattering albedo (assa) at several wavelengths. A large absorption rate is found with a high spectral dependence for desert dust particles (assa441=0.86 and ass873=0.93) and with a flat spectral dependence during the forest fires emission episode (assa441=0.88 and assa873=0.87). All measurements as well as back-trajectory calculations indicate mixture of particles during the desert dust.

Solar energy incident at the receiver of a solar tower plant, derived from remote sensing: Computation of both DNI and slant path transmittance

By scattering and absorbing solar radiation, aerosols generate production losses in solar plants. Due to the specific design of solar tower plants, solar radiation is attenuated not only in the atmospheric column but also in the slant path between the heliostats and the receiver. Broadband attenuation by aerosols is estimated in both the column and the slant path for Ouarzazate, Morocco, using spectral measurements of aerosol optical thickness (AOT) collected by AERONET. The proportion of AOT below the tower’s height is computed assuming a single uniform aerosol layer of height equal to the boundary layer height computed by ECMWF for the Operational Analysis. The monthly average of the broadband attenuation by aerosols in the slant path was 6.9±3.0% in August 2012 at Ouarzazate, for 1-km distance between the heliostat and the receiver. The slant path attenuation should be added to almost 40% attenuation along the atmospheric column, with aerosols in an approximate 4.7-km aerosol layer. Also, around 1.5% a...

Estimation of the Aerosol Radiative Forcing at Ground Level, Over Land, and in Cloudless Atmosphere, From METEOSAT‐7 Observation: Validation of First Results

A method is proposed to estimate the spatial and temporal variability of the solar radiative flux reaching the surface over land (DSSF), as well as the Aerosol Radiative Forcing (ARF) at ground level, in cloud‐free atmosphere. The method relies on a selection of best correspondence between METEOSAT‐7 radiance and DSSF, computed with a radiative transfer code. The validation of DSSF is performed comparing retrievals with ground‐based measurements acquired in a continental background site located South East of France. The study is concentrated on aerosol episodes occurring around the 2003 summer heat wave, providing 24 cases of comparison for variable solar zenith angle (from 60° to 65°), variable aerosol type (biomass burning emissions and continental background), and variable aerosol optical thickness (a factor 6 in magnitude). The method reproduces measurements of DSSF within an accuracy assessment of 20 Wm−2 (5% in relative) in 70% of the situations, and within 40 Wm−2 in 90% of the situations, for the ...