Marine aerosol vertical distribution retrieval using airborne backscatter lidar measurements

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  • 1. Aemsol Sci. Vol. 29. Suppl. I, pp. S6694670, 1998 0 1998 Published by Elsevier Science Ltd. All rights reserved

    Frinted in Great Brirain 0021-8502!98 $19.00 + 0.00

    Marine aerosol vertical distribution retrieval using airborne backscatter lidar measurements

    Cyrzlle Flamantf Jacques Pelont 1 Patrzck Chazettr and Vzncent Trodlet+

    +Service dA6ronomie du CNRS. Paris. France Laboratoire des Sciences du Climat et de 1Environnement. iff-sur-Yvette, France

    Key words Marine aerosol model, lidar. extinction coefficient, vertical profile.

    During the last two decades, airborne backscatter lidar have been used t,o provide with very detailed analysis of the atmospheric scattering layers, namely aerosols and clouds layers, which are of interest for energy budget studies. Several lidar equation inversion procedures have been proposed to determine aerosol optical properties from lidar measurements. They require the knowledge of a boundary condition. t,aken in the form of a reference extinction coefficient value. and the profile of the normalized backsratter phase function (the ratio of the backscatter to the extinction coefficient) of the scatterers. For optically t,hin layers, this reference value can be taken close to the laser emission source (Klett, 1981).

    In order to determine the normalized backscatter phase function profile it, is necessary to know the vertical distribution of aerosols in the tropospheric layers remotely sounded by lidar. Over the open ocean. aerosols essentially consist of sulphate and sea-salt which concent,rations depend on surface wind speed (SWS) and/or cloud cover (Jaenicke. 1993). Relative humidity also affects the normalized backscatter phase function and should be taken into account.

    A marine aerosol model has been tested with the dat,a acquired over t,he Azores during the SOFIA and SEMAPHORE experiments. Relative humidity soundings and SWS measurements were made by the ship Le Suroit. Aerosol reflectivity at 0.532 pm in the lower troposphere was measured by the lidar LEANDRE 1. Extinction measurements at 0.55 pm, made by a nephelometer at the aircraft altitude. provided the reference extinction for the lidar inversion procedure.

    We have considered a trimodal size distribution (the first mode corresponding to sulphate originat,ing from dimethylsulfide gas-to-particle conversion, the second one to in-cloud sulphate production and the third one to aged sea-salt aerosols) as representative of the scatterer population interacting with the laser beam in the boundary layer over the open ocean (Jaenicke. 1993). All modes size spectra are approximated by log-normal distributions. The aerosol population which size is closest to the laser wavelength will have the largest contribution to t,he lidar signal. Therefore, the extinction retrieved from lidar measurements is sensitive to the number of particules in the in-cloud produced sulphate and sea-salt modes. Since, there exists a large uncertainty on the background wind conditions and on the cloud cover experienced by the air masses sampled by lidar over the Azores, we have investigated the sensitivity of the lidar inversion procedure to the aerosol model. We show that lidar-derived extinction values in the surface layer and extinct,ion profiles calculated from the aerosol model, using a Mie code for spherical particles. converge within 20% for an average concentration of 10 particlescm -3 in the sea-salt mode at low wind speed (SWS 5 3 ms-).

    We also have investigated the impact of cloudiness on the extinction profile. The sensitivity of t,he inversion procedure to the aerosol model (presence or not of a cloud-processed sulphate mode) is illus- t,rated on Figure 1. We have assumed values of 135. 65 and 10 particlescm-3 (Jaenicke. 1993), to be representative of air masses having experienced cloudy conditions for at least five days (Model I). In a second model, we have assumed t,he cloud cover to be very small and the in-cloud sulphate production t,o be insignificant. We set the concentration of the three modes to 200, 0 and 10 particles crne3 (Model II). Because of the increasing stability and sedimentation effects, sea-salt and cloud-processed sulphatc particle concentrations are assumed to decrease exponentially as a fun&on of a height scale (Jaenicke.

    S669

  • S670 Abstracts of the 5th International Aerosol Conference 1998

    Figure 1: Profile of the extinction coefficient during flight 27 of SOFIA. The solid (dashed) line represents the extinc- tion coefficient profile recovered with a forward lidar equation inversion procedure using Model I (Model II). The boundary condition is given by extinction measurementsat the aircraft level provided by a nephelometer. Error bars relate to the uncertainties on the SWS relative humidity measurements as well as the 10% uncertainty on the extinction measured by the nephelometer.

    I I I I I I I I, I,, I

    0 12 3 4 5 6 7 6 9 10 11 12

    Instantaneous sea surface wind speed (m s.)

    Figure 2: Sea surface reflectance retrieved from lidar as a function of the surface wind speed for Model II. The er- TOT bars are related to the uncertainty on the normalized backscatter phase function. The reflectance calculated using Coxs and Munks model for an incidence angle of zero (corre- sponding to nadir lidar measurements) and 2 (corresponding to off nadir lidar measurements) is given by the solid and dotted lines, respecively.

    1993) to be determined. The sensitivity of the inversion procedure to the aerosol model (presence or not of a cloud-processed sulphate mode) is illustrated on Figure 1. In the free troposphere, where the error associated with the inversion procedure is less than 15%, extinction values are found to be significantly larger when accounting for cloud-processed sulphates. In the boundary layer, the choice of the model also induces a bias. Therefore, cloud cover related sulphate production cannot be ignored. Neglecting the impact of the cloudiness and related sulphate production leads to relative errors of 20% and 25%, in the MABL and in the entire lower troposphere, respectively. The corresponding biases are 0.04 km- and 0.03 km-.

    We will show that a comparison between lidar-derived extinction profiles and extinction profiles cal- culated with the aerosol model, the vertical distribution of relative humidity and a Mie code provides an information on the aerosol size distribution and its vertical distribution in the lower troposphere.

    Further validation of the aerosol model is provided by investigating the optical properties of the air-sea interface. The relationship between the measured ocean surface reflectance and atmospheric backscatter coefficient above the surface is then used to determine the sea surface reflectance using the normalized backscatter function profiles. Results are found in excellent agreement with the reflectance values calculated as a function of SWS by the model of Cox and Munk (1954) (Figure 2).

    Acknowledgments. This research was funded by CNRS and IFREMER through the Programme Atmosphhe O&an & Moyenne Echelle and by the European Space Agency.

    References Cox, C., and W. Munk, Measurements of Roughness of the Sea Surface from Photographs of the Suns

    Glitter, J. Opt. Sot. of America, 44, 11, 832-850, 1954. Jaenicke, R., Tropospheric aerosols in Aerosol-Cloud-Climate Interactions, P. V. Hobbs Ed.. Academic

    Press, pp 237, 1993. Klett, J. D., Stable Analytical Inversion Solution for Processing Lidar return, Appl. Opt., 20, 211-220,

    1981.

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