Column Integrated Water Vapor and Aerosol Load Characterization with the New ZEN-R52 Radiometer

Abstract

The study shows the first results of the column-integrated water vapor retrieved by the new ZEN-R52 radiometer. This new radiometer has been specifically designed to monitor aerosols and atmospheric water vapor with a high degree of autonomy and robustness in order to allow the expansion of the observations of these parameters to remote desert areas from ground-based platforms. The ZEN-R52 device shows substantial improvements compared to the previous ZEN-R41 prototype: a smaller field of view, an increased signal-to-noise ratio, better stray light rejection, and an additional channel (940 nm) for precipitable water vapor (PWV) retrieval. PWV is inferred from the ZEN-R52 Zenith Sky Radiance (ZSR) measurements using a lookup table (LUT) methodology. The improvement of the new ZEN-R52 in terms of ZSR was verified by means of a comparison with the ZEN-R41, and with the Aerosol Robotic Network (AERONET) Cimel CE318 (CE318-AERONET) at Izaña Observatory, a Global Atmosphere Watch (GAW) high mountain station (Tenerife, Canary Islands, Spain), over a 10-month period (August 2017 to June 2018). ZEN-R52 aerosol optical depth (AOD) was extracted by means of the ZEN–AOD–LUT method with an uncertainty of ±0.01 ± 0.13*AOD. ZEN-R52 PWV extracted using a new LUT technique was compared with quasi-simultaneous (±30 s) Fourier Transform Infrared (FTIR) spectrometer measurements as reference. A good agreement was found between the two instruments (PWV means a relative difference of 9.1% and an uncertainty of ±0.089 cm or ±0.036 + 0.061*PWV for PWV <1 cm). This comparison analysis was extended using two PWV datasets from the same CE318 reference instrument at Izaña Observatory: one obtained from AERONET (CE318-AERONET), and another one using a specific calibration of the 940-nm channel performed in this work at Izaña Atmospheric Research Center Observatory (CE318-IARC), which improves the PWV product.

Water Vapor Retrievals from Spectral Direct Irradiance Measured with an EKO MS-711 Spectroradiometer—Intercomparison with Other Techniques

“…Recently, at IZO, Ref. [35] further studies corroborated these results, obtaining a mean bias of −0.006 mm (−1.3%) when comparing TCCON FTIR PWV values to those obtained from meteorological radiosondes launched on Tenerife island and processed according to the GRUAN scheme (http://www.gruan.org, last access: 28 November 2020). The Almansa's study also found a relative difference of +3.4% respect to collocated CIMEL-AERONET data (FTIR-CIMEL).…”

“…On the other hand, when analyzing the bias among techniques, the least differences are found respect to GNSS PWV with MB of 0.02 (3.3%), while the largest differences are respect to CIMEL-AERONET and FTIR PWV with MB of 0.33 mm (9.5%) and −0.57 mm (14.5%), respectively. These systematic differences can be attributed the possible calibration inaccuracy of the AERONET PWV product [35], while the differences found respect to FTIR PWV are likely due to spectroscopy inconsistences between the different spectral ranges in which the PWV is determined (930-960 nm for EKO PWV and 1500-2200 nm for FTIR PWV). The bias found respect to different techniques are within the uncertainty determined with the Monte-Carlo method (Section 5) with values ranging between 0.13 and 0.81 mm.…”

“…Ref. [35] studied the TCCON FTIR and CIMEL-AERONET PWV time series between 2007 and 2019 at IZO, observing that the wettest months occur in summer (maximum PWV in August) and the driest months correspond to winter (minimum PWV in February), with a total average PWV of 4.8 ± 2.8 mm from TCCON FTIR, and 3.8 ± 1.4 mm from CIMEL-AERONET.…”

“…Given the diversity of measurement techniques and approaches available in the literature, numerous comparison studies have been carried out in the last decades (e.g., [12][13][14]18,25,[33][34][35]). These studies show differences ranging between 0.40% and −25.4%.…”

Spectral Aerosol Optical Depth Retrievals by Ground-Based Fourier Transform Infrared Spectrometry

“…It should be noted that these two high PWV amounts are extreme values at IZO given the predominantly dry atmospheric conditions of the observatory. Considering the TCCON FTIR PWV time series in the 2007-2019 period, the annual average of PWV is only 4.8 mm (standard deviation, σ, of 2.8 mm), with maximum PWV amounts reached in summer (mean and σ of 7.1 and 3.3 mm, respectively) [77]. All remaining inputs for the simulations have been kept identical.…”