The monsoon aerosol extinction properties at Goa during INDOEX as measured with lidar

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Abstract
The aerosol extinction properties in the framework of the Indian Ocean Experiment (INDOEX) are documented over the Goa area (western coast of India, 15.45°N, 73.08°E) between 1 and 15 March 1999. The temporal evolution and the vertical distribution of the aerosol trapped inside the winter monsoon plume have been studied using a ground‐based lidar system (micropulse lidar (MPL) emitting at 523 nm). Both the vertical structure and the optical properties of the particles have been assessed from the inversion of the lidar signals. A statistical approach has been used to determine the backscatter‐to‐extinction ratio (Φp), a key parameter to invert the lidar signal. Sun photometer‐retrieved optical thickness at 523 nm is used to constrain the inversion in daytime and leads to a mean value of Φp close to 0.03 sr−1 with a standard deviation of ∼0.010 sr−1. Black carbon concentration (BC) is shown to be a representative tracer of the surface scattering coefficient of aerosols (αs), the relative humidity (RH) effect seeming to be of a second order. A statistical linear relationship between BC and αs has been found and then used to constrain the lidar inversion during nighttime. During nighttime, about the same mean Φp has been assessed. The lidar‐derived optical thickness is shown to be more important during nighttime over the entire measurement period (a mean value of 0.76 ± 0.15 for nighttime instead of 0.55 ± 0.09 for daytime). A significant contribution to the optical thickness is shown to be due to an aerosol layer within 0.7 km above sea level (ASL). A second aerosol layer is found between 0.7 and ≈3.5 km ASL with an important diurnal variability in both its altitude extension and its optical thickness. Such a variability seems to be due to the sea breeze cycle. In the upper aerosol layer, optical thickness is 0.49 ± 0.14 during nighttime whereas it is only about 0.25 ± 0.07 during daytime.

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