Independent Calibration of Water Vapor Raman Lidar by using Additional Elastic Measurements at Water Vapor Raman Wavelength

Decheng Wu,Dong Liu,Yingjian Wang,Zhien Wang,Chenbo Xie,B Gross,F Moshary,M Arend

doi:10.1051/epjconf/201611925007

Abstract

A new method for independent calibration of water vapor Raman lidar by using additional measurements of elastic signals at water vapor Raman wavelength was introduced. The water vapor mixing ratio calibration constant was derived from nitrogen mixing ratio calibration constant and Rayleigh to water vapor Raman backscattering cross section ratio. The nitrogen mixing ratio calibration constant with uncertainties of 1~2% can be determined from the nitrogen mixing ratio measurement in an aerosolfree zone in the upper troposphere with the additional elastic measurement and nitrogen Raman measurement, the Rayleigh to water vapor Raman backscattering cross section ratio can be calculated with uncertainties of about 8%, or can be determined experimentally with uncertainties of less than 5%. Using the experimental cross section ratio, a total uncertainty of the water vapor calibration less than 6% is possible to achieve. Because this calibration method utilizes actual lidar signals in nitrogen and water vapor detection channels, the calibration can fully capture lidar state changes.

Highlights

water vapor Raman lidar (WVRL) calibrations need to determine the difference of system detection efficiencies between the nitrogen and water vapor channels and nitrogen to water vapor Raman backscattering cross section ratio
The backscattering cross section ratio of nitrogen to water vapor is needed in the lamp independent calibration, the cross section ratio can be calculated with uncertainties of about ±10%, or can be determined experimentally with an error of less than 5% based on a well-calibrated WVRL [4, 5]
A new independent calibration method for WVRL was proposed based on additional elastic measurements at water vapor Raman wavelength

Summary

INTRODUCTION

Calibration constant, a regular calibration is needed for a WVRL [1, 3]. it is difficult to obtain long-term regularly independent water vapor measurements at most of WVRL sites. WVRL calibrations need to determine the difference of system detection efficiencies between the nitrogen and water vapor channels and nitrogen to water vapor Raman backscattering cross section ratio. Using a calibrated tungsten lamp, the ratio of lidar channel detection efficiencies could be calibrated with errors of about 1% [4]. The backscattering cross section ratio of nitrogen to water vapor is needed in the lamp independent calibration, the cross section ratio can be calculated with uncertainties of about ±10%, or can be determined experimentally with an error of less than 5% based on a well-calibrated WVRL [4, 5]. A new independent calibration method for WVRL based on additional elastic measurement at water vapor Raman wavelength was introduced and discussed. As the changes of the WVRL state, such as changes in overall system efficiency and detector sensitivity due to ageing, will influence the

METHODOLOGY

Rayleigh Backscattering Signal

Backscattering Cross Sections and Temperature Dependence

Atmospheric Transmission Correction

Findings

CONCLUSIONS