Lidar-radiometer study of boundary-layer aerosols and their contribution to long-term trends and changes in columnar aerosol optical depth at a tropical station

As atmospheric aerosols show high degree of variability in space and time in their characteristics and as the anthropogenic share of total aerosol loading is quite substantial, it is essential to monitor systematically the aerosol features over longer time scale. One such parameter that plays an important role in the study of aerosol forcing on climate system is the aerosol optical depth (AOD). Even though numerous experimental and theoretical techniques have been in progress for the past more than four decades, our ability to assess the impact of aerosols on Earth's environment is incomplete due to many inherent unknown complex properties of aerosols such as physico-chemical and source-sink cycle transformation patterns.

The aerosol residence time and transport mechanisms are different in different regions of the atmosphere. Aerosols in the boundary-layer are strongly influenced by various natural and anthropogenic processes. Although the parameters, columnar AOD and the corresponding size distribution are useful for the study of seasonal and long-term trends, then radiative effects of aerosols on regional scale can be understood better only by accounting for the local influences separately. A portion of the boundary-layer aerosols will be transported to troposphere due to vertical eddy diffusion. In addition, tropospheric aerosols, especially in the sub-micron range are produced through gas-to-particle conversion processes. Even though the precursor gases responsible to these aerosols are produced from the surface, within the boundary-layer they do not form aerosols. Thus the aerosol system in the boundary-layer is different from that in the troposphere. It implies that the seasonal and long-term changes in these two regions are caused by different processes and hence will be different. In order to have clear understanding of the tropospheric aerosol system it is necessary to separate out the boundary-layer component from the total columnar AOD and examine the changes and trends. Separation of these two components provides better understanding of the aerosol physical, chemical, radiative and dynamical processes taking place in these two important regions of the atmosphere.

The computer-controlled, bistatic, Argon-ion (operating since 1986) and the PC-based spectroradiometer and multi-channel sunphotometer (both operating since 1992) at the Indian Institute of Tropical Meteorology (IITM), Pune, India have been used to investigate the above aspects of aerosols present over this semi-arid station. In this paper, the columnar AOD derived from the vertical distribution of aerosol number density, particularly in the atmospheric boundary-layer, and its contribution to the total columnar AOD up to the stratospheric attitudes are studied. For this purpose, more than 1000 (about 650 weekly-based) vertical profiles of lidar-derived aerosol number density obtained during October 1986 through December 1998, and radiometric observations of AOD collected on over 400 cloud-free days during February 1993-December 1998 have been used. The seasonal and long-term trends in the AOD have been examined. Besides a systematic seasonal variation with maximum AODs during pre-monsoon and minimum during south-west monsoon months, the results show an increasing long-term trend in the AOD during the study period of 12 years (1986-1998). This increasing trend appears to vary from year to year and have strong bearing on the surface-generated aerosols and meteorolgy over the station. Comparison between the lidar-observed AOD in the boundary layer and the total AOD obtained using the radiometer reveals that the observed increasing trend is not confined to the boundary-layer alone but extends to higher altitudes also with a contribution of about 40% from the former to the latter. The experimental, data retrieval methods and more details of the results obtained will be presented.