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Aerosols Clouds

Last update : 2011/07/06

 

Aerosols caracterisation in Paris area using Lidar + Photometer synergism

 

Atmospheric aerosols are involved in many physical and chemical processes that play a key role in air quality and climate change. Their contribution can result in a direct effect on solar radiation i.e. a net reduction and conversion of direct flux into diffuse flux and changes in cloud droplet size and number density. The scientific community is currently deploying complementary instrumentation i.e. in situ probes and remote sensors, either for routine operation or in the framework of dedicated field campaigns. In situ probes have long time experience, but their inherent limitations in spatial sampling and representativity result into a call for application of Lidars and sun photometers. Over the past decade, the sun photometer AERONET network has expanded internationally and the data sets are currently available for atmospheric research, air quality monitoring and satellite validation. Sun photometers provide direct measurements of aerosol optical depths at several standard wavelengths from ultraviolet to near infrared. In addition, the so-called AERONET inversion can provide with aerosol size distribution and refractive index information integrated over the atmospheric column.

Fig. 1: Sunphotometer, Minilidar inside the container of the TReSS mobile station. Operation during the COPS campaign in 2007.

Concurrently, various Lidar techniques have been developed to retrieve aerosol spatial distribution and properties: i) standard elastic backscatter Lidar (SEBL) with polarization diversity capability, ii) inelastic Raman scattering Lidar and iii) high spectral resolution Lidar. In the past decade, Lidar technology has improved tremendously to foster reliable networking activity as illustrated by EARLINET and space based application as demonstrated by CALIPSO. In practice, SEBL are simpler to use and to deploy unattended and so they are affordable to a large user community. SEBL provide with range resolved calibrated attenuated backscatter coefficients (in m-1 sr-1), color ratio (for two or more transmitted wavelengths) and depolarization ratio (for two crossed polarizations). On their own, when used independently, sun photometer and SEBL techniques suffer from inherent limitations that result in ambiguities and an incomplete set of information to fully characterize the aerosol column in terms of spatial distribution, optical and microphysical properties.

Fig.2:Application of the LidAlm algorithm to a one-layer case (on September 6th 2004 in Palaiseau, France): (a) an example of the 5 AERONET-retrieved size distributions broken down in LNMs (each mode has a colour associated), (b) the time series of attenuated backscatter profiles at 532 nm, measured by Lidar [44-45], normalized at 2 km and extrapolated down to the surface, as a function of time UTC (Coordinated Universal Time) (c) and (d) the time series modal radii ri and concentrations Ci, respectively, (e) mean aerosol BER retrieved for each AERONET sounding and (f) comparison of the AOD (at 532 nm) retrieved by Lidar inversion τLid, using the AERONET retrievals τAlm and the sun photometer direct measurements τTOT (in red, orange and dotted line in orange, respectively, with bars at the AERONET soundings and a solid red line for τLid derived using the Lidar time series and interpolation of the LidAlm BER).

We present a so-called LidAlm (Lidar & Almucantar) algorithm that combines information provided by standard elastic backscatter Lidar (i.e. calibrated attenuated backscatter coefficient profile at one or two wavelengths) and sun photometer AERONET inversion of Almucantar-like measurements (i.e. column-integrated aerosol size distribution and refractive index) (Fig. 1). The purpose of the LidAlm technique is to characterize the atmospheric column into different aerosol layers. These layers may be distinct or partially mixed and they may contain different aerosol species (e.g. urban, desert or biomass burning aerosols). The LidAlm synergetic technique provides with the extinction and backscatter coefficient profiles, particle size distributions and backscatter-to-extinction ratios for each aerosol layer. [Cuesta et al. 2008] presents the LidAlm procedure and sensitivity studies. The applications are illustrated with examples of actual atmospheric conditions encountered in the Paris area (Fig. 2).

Diurnal and seasonal cycles of aerosols in the Sahara, Tamanrasset, Algeria

 

We documented the seasonal evolution of the Saharan Atmospheric Boundary Layer (SABL), in terms of vertical structure, diurnal cycle, aerosol content and cloud cover as well as the surface radiative budget, during 2006, using a mobile multi-platform atmospheric observatory implemented in Tamanrasset (Algeria).

 

Fig. 3: TReSS mobile station deployed in Tamanrasset, Algeria, during the AMMA campaign in 2006.

Ground-based remote sensing (both active and passive) and in-situ instruments were deployed in the framework of the African Monsoon Multidisciplinary Analysis field experiment and were used in synergy with satellite observations. Observations showed a marked seasonal evolution of the SABL characteristics and a large variability during the West African Monsoon onset phase. At the beginning of June, hazy conditions prevailed in a deep SABL (~5 km).

 

Fig. 4: (a) Mini-Lidar derived total attenuated backscatter coefficient profiles at 532 nm on 11 June 2006. Altitudes are AGL (left) and MSL (right) and time is UTC (Local Time=UTC+1 h). Lidar profiles were calibrated in a particle-free region of the troposphere (indicated by the blue arrow). The potential temperature profiles from radiosoundings made at 0600 UTC, 1200 UTC and 1800 UTC are superimposed (white solid lines). Each potential temperature profile is plotted between 300 K and 340 K. (b) Surface aerosol scattering coefficient at 700 nm. Panels (c), (d) and (e) show individual Mini-Lidar profiles (30 minute averages) at 0600 UTC, 1200 UTC and 1800 UTC, respectively. The dashed grey lines, the solid black lines and the thin dashed lines are the attenuated backscatter profiles b.T2(z), the total backscatter profiles b(z) and the molecular backscatter profiles bm(z), respectively. The attenuated profiles where extrapolated from 150 m AGL down to the surface (in the range of incomplete overlap between the emitter divergence and detection field of view).

Following this, reduced cloud cover induced by anomalous large-scale subsidence resulted in high surface insolation which enhanced the convective development of the SABL (~6 km deep). During that period, the proximity of the Saharan Heat Low was also favorable to the SABL deepening. In August and September, humidity advected from the South enhanced cloud cover and limited the SABL vertical development (~3.8 km deep). In the wintertime, weak dry convection and the Hadley cell-related subsidence resulted in high visibility and an extremely shallow SABL (~500 m deep). Throughout 2006, the aerosol vertical distribution within the SABL was non-uniform, with the majority of coarse particles being located near the surface. The aerosol content over Tamanrasset was influenced by dust transport from a variety of source regions after being lifted through different mechanisms (Fig. 5) (low-level jets; cold-pools or topographic flows) [Cuesta et al. 2006].

Fig.5 : SEVIRI-derived false color images of the West Africa (a) on 8 June at 2200 UTC, (b) 9 June at 2000 UTC, (c) 10 June 10 at 1800 UTC and (d) 11 June at 0200 UTC. Aerosols, clouds and land surface are identified by the purple, orange and green colors, respectively. The open circle and the square indicate Tamanrasset and Bodélé depression (18°N 17°E) in Chad, respectively. The white dotted lines indicate the position of dust fronts associated with the leading edges of cold-pools. The thick white lines indicate the position of the dust fronts associated with plumes from the Bodélé depression. The black solid lines in panels b and d indicate the CALIPSO track on 9 and 11 June. The pink dashed (solid) lines in panels b and c indicate the position of the ITD at 0600 UTC (1800 UTC) as derived from ECMWF analyses. Panels e and f show CALIOP measurements at 532 nm along the tracks shown in panels b and d for 9 June (1309 UTC) and 11 June (1252 UTC), respectively. CALIPSO passed 15 km to the West (325 km to the East) of Tamanrasset on 9 June (11 June). The CALIOP lidar data was processed with a 60-m resolution on the vertical and a 12-km resolution on the horizontal. CALIOP backscatter profiles have been normalized at 25 km of altitude by NASA Langley Atmospheric Science Data Center [http://eosweb.larc.nasa.gov]. The outline of the topography appears in dark red. ECMWF analyses derived wind fields projected onto the satellite trajectory are superimposed to the CALIOP measurements. A scale factor of 50 was used between the vertical and horizontal wind components.

 


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