Retrieval of precipitable water vapor from sun–sky radiometer (POM-01) at 940 nm absorption band: Calibration, measurement and validation

Abstract

Sun–sky radiometer (model POM-01) is commonly used for studying aerosol optical and physical properties at selected aerosol-specific channels. Although the instrument is equipped with a precipitable water vapor (PWV) channel at 940 nm, the inbuilt software does not provide a tool for estimation of PWV. Hence, the current study adopted a new methodology to estimate PWV from three high-altitude (> 3400 m MSL) sites, Hanle, Merak, and Leh, located in Ladakh, India. The retrieval algorithm focuses on the precise estimation of the calibration constant (𝑉0) and coefficients 𝑎 and 𝑏 using modified Langley plots in two different methods. The estimated average value of 𝑏 is 0.59 ± 0.09 which is very close to those commonly used in global studies. Further, the estimated 𝑉0 and 𝑏 values from both methods are found to be similar, which may be due to the advantages of the dry and high-altitude environment, where the annual total column water vapor is typically less than 6 mm. The estimated PWV using observations at selected full clear and stable atmospheric conditions compares well with satellite, AERONET, GPS, reanalysis and empirical model data with correlation coefficient varying from 0.91 to 0.97. Further, the estimated propagated root mean square error (rmse) varies from 0.37 mm to 2.58 mm. These results indicated that sun–sky radiometer derived PWV showed good consistency with the derived PWV from independent data sources at the three sites.