8.23-P Lidar Profiling of Tropospheric Aerosols and Clouds over a Tropical Station

P.C.S. Devara, P. Ernest Raj, K.K. Dani, G. Pandithurai and K.M.C. Reddy

Indian Institute of Tropical Meteorology, Pashan, Pune 411 008, India

1. Introduction. Aerosols and clouds affect global and regional climates by altering the albedo of the atmosphere and modulating outgoing long-wave radiation (IPCC, 2001). Measurements of the optical properties and the distribution of naturally occurring and anthropogenic aerosols are essential to evaluate their climatic impacts (Charlson et al., 1992). Aerosols in the boundary layer are directly produced from natural and anthropogenic processes while those in the troposphere and aloft are largely due to gas-to-particle conversion processes while those in the troposphere and aloft are largely due to gas-to-particle conversion processes. Moreover, the space-time variability of inhomogeneities in aerosol content can be used as tracers of the structure and stratification of planetary boundary layer. Thus, aerosols can provide unique information on pollution transport and dispersion needed for environmental research and operational program over urban regions where a general build-up of aerosols in large geographical areas has been realized mainly due to human activities. A bi-static Argon ion lidar at the Indian Institute of Tropical Meteorology (IITM), Pune (18o32’N, 73o51’E, 559 m AMSL), India has been extensively used for more than two decades to study the vertical distributions of aerosol and macro-physical parameters of clouds in the troposphere (Devara, 1998; Devara et al., 2002). In order to enlarge the scope of these studies, a collocated dual polarization micro pulse lidar (DPMPL) has been installed. The first observations of space-time variation of boundary layer structure that have been carried out during winter 2005-06 together with some details of this newly built facility will be presented. 2. System Description and Results The system composed of a low energy (eye-safe), high repetition rate Nd-YAG laser as transmitter and Schmidt-Cassegrain telescope with focal ratio of f /10 as receiver. A photograph depicting the complete transmitter-receiver and data acquisition and processing system is shown in Fig. 1. The bore-sight mechanism of the system provides adjustment in two axes and maintains co-linearity between transmitter’s outgoing laser beam and the telescope that is used to collect the return energy. The finest range resolution that can be achieved with the system is 0.3 m. The system has built-in provision to apply corrections to the observed data due to background and dark count. Also, the system has both manual and real-time modes of operation, raw backscattered intensity profiles every minute as per the prescribed altitude range and resolution settings in the later mode. The system has been operated during clear as well as cloudy conditions. The time evolution of profiles in the boundary layer only is presented here (Figure 2). By computing the height gradients from the backscatter intensity profiles, it is possible to determine the heights of mixed layer and stable layer. Such information would be very valuable input to the models aiming at regional air pollution or air quality.

Figure 1: Inner and outer views along with cooler housing and data logger of the DPMPL.

[A] [B]

Figure 2: Back scatter intensity profiles recorded by Cross- [A] and Co-[B] polarization channels of the DPMPL in the intervening night between 30 and 31 December 2005.

References Charlson, R.J. and co-authors, 1992: Climate forcing by anthropogenic aerosols, Science, 255,

423-430. Devara, P.C.S., 1998: Remote sensing of atmospheric aerosols using from active and passive optical

techniques, Intl. J. Remote Sensing, 19, 3271-3288. Devara, P.C.S. and co-authors, 2002: Recent trends in aerosol climatology and air pollution as

inferred from multi-year lidar observations over a tropical urban station, Intl. J. Climatology, 22, 435-449.

Intergovernmental Panel on Climate Change (IPCC) (2001), Climate Change 2001: The Scientific Basis: Contribution of Working Group Ito the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by J.T. Houghton et al., 881 pp, Cambridge Univ. Press, New York.

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