Title : Measurements of Angstrom exponent of aerosol by ozone lidar with four wavelengths
Abstract:
Simulation of the Angstrom exponent k of aerosol from synthetic ozone lidar signals at three wavelengths and measurements of the Angstrom exponent k aerosol from ozone Lidar at four wavelengths were presented in this paper. Ozone Lidar signals were simulated at three wavelengths 276nm, 287nm and 299nm, from which the Angstrom exponent k was calculated to be 0.85, with ideal value being 1. Ozone lidar measurements were carried out at four wavelengths 276nm, 287nm, 299nm and 396nm to derive the Angstrom exponent k. Aerosol extinction coefficient was calculated to be 0.825*10-3 / km, in the altitude range from 400 m to 520 m using the measured ozone lidar signal at the 396 nm wavelength. We used the measured ozone signals at 276nm, 287nm and 299nm and the aerosol Ext.co 0.825*10-3 /km to calculate the Angstrom exponent K to be 1.6 - 1.98.
Introduction: Angstrom exponent k of aerosol is a very important optic parameter. It relates to aerosol optic depths (AOD), wavelengths and aerosol size distribution, and is widely used in aerosol classification research. Untill now, Angstrom exponent k was measured by Sun photometers or Nephalometers. Few papers reported measurement of the Angstrom exponent k by Lidar.
Angstrom exponent k is associated with aerosol absorption cross section and extinction coefficient (Ext.co) and changes with time is the formula about the relationship between the Angstrom exponent k, aerosol Ext.co, and wavelength is the aerosol Ext.co at 532 nm. Generally, aerosol Ext.co can be measured by Lidar. In aerosol Lidar measurements and Ext.co inversion, the relationship between aerosol Ext.co and wavelength was not clearlly considered. Klett and Fernald method neglected the relationship between Ext.co and wavelength. In this case, aerosol Ext.co inversion is not accuracy due to lack of k consideration.
In this paper, the Angstrom exponent k was calculated in detail, according to ozone Lidar equations. Then, we simulated the ozone Lidar signals at 276nm, 287nm and 299nm to calculate the Angstrom exponent k. Finally, we presented how to measure the Angstrom exponent k using ozone lidar measurement at four wavelengths 276nm, 287nm, 299nm, and 396nm. The aerosol Angstrom exponent k was calculated to be 0.85 from the simulated ozone Lidar signals, and was measured to be 1.6-1.98 based on the measured ozone lidar signals at the four wavelengths.
Discussion: In this paper, we just discuss the Angstrom exponent k measurement and calculation in homogeneous atmosphere. In the range from 400 m-550 m, the aerosol Ext.co was constant, 0.825*10-3m-1. In other ranges, the aerosol Ext.co varied with heights, in which case the Angstrom exponent k measurements and calculation should be more complicated, we will carry out these research works in future.
