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Article

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

1
Izaña Atmospheric Research Center (IARC), State Meteorological Agency of Spain (AEMET), 38001 Santa Cruz de Tenerife, Spain
2
Cimel Electronique, 75011 Paris, France
3
Atmospheric Optics Group of Valladolid University (GOA–UVa), Valladolid University, 47001 Valladolid, Spain
4
Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK-ASF), 76021 Karlsruhe, Germany
*
Author to whom correspondence should be addressed.
Remote Sens. 2020, 12(19), 3148; https://doi.org/10.3390/rs12193148
Received: 24 August 2020 / Revised: 16 September 2020 / Accepted: 21 September 2020 / Published: 24 September 2020
(This article belongs to the Section Atmospheric Remote Sensing)
Aerosol Optical Depth (AOD) and the Ångström Exponent (AE) have been calculated in the near infrared (NIR) and short-wave infrared (SWIR) spectral regions over a period of one year (May 2019–May 2020) at the high-mountain Izaña Observatory (IZO) from Fourier Transform Infrared (FTIR) solar spectra. The high-resolution FTIR measurements were carried out coincidentally with Cimel CE318-T photometric observations in the framework of the Aerosol Robotic Network (AERONET). A spectral FTIR AOD was generated using two different approaches: by means of the selection of seven narrow FTIR micro-windows (centred at 1020.90, 1238.25, 1558.25, 1636.00, 2133.40, 2192.00, and 2314.20 nm) with negligible atmospheric gaseous absorption, and by using the CE318-AERONET’s response function in the near-coincident bands (1020 nm and 1640 nm) to degrade the high-resolution FTIR spectra. The FTIR system was absolutely calibrated by means of a continuous Langley–Plot analysis over the 1-year period. An important temporal drift of the calibration constant was observed as a result of the environmental exposure of the FTIR’s external optical mirrors (linear degradation rate up to 1.75% month1). The cross-validation of AERONET-FTIR databases documents an excellent agreement between both AOD products, with mean AOD differences below 0.004 and root-mean-squared errors below 0.006. A rather similar agreement was also found between AERONET and FTIR convolved bands, corroborating the suitability of low-resolution sunphotometers to retrieve high-quality AOD data in the NIR and SWIR domains. In addition, these results demonstrate that the methodology developed here is suitable to be applied to other FTIR spectrometers, such as portable and low-resolution FTIR instruments with a potentially higher spatial coverage. The spectral AOD dependence for the seven FTIR micro-windows have been also examined, observing a spectrally flat AOD behaviour for mineral dust particles (the typical atmospheric aerosols presented at IZO). A mean AE value of 0.53 ± 0.08 for pure mineral dust in the 1020–2314 nm spectral range was retrieved in this paper. A subsequent cross-validation with the MOPSMAP (Modeled optical properties of ensembles of aerosol particles) package has ensured the reliability of the FTIR dataset, with AE values between 0.36 to 0.60 for a typical mineral dust content at IZO of 100 cm3 and water-soluble particle (WASO) content ranging from 600 to 6000 cm3. The new database generated in this study is believed to be the first long-term time series (1-year) of aerosol properties generated consistently in the NIR and SWIR ranges from ground-based FTIR spectrometry. As a consequence, the results presented here provide a very promising tool for the validation and subsequent improvement of satellite aerosol products as well as enhance the sensitivity to large particles of the existing databases, required to improve the estimation of the aerosols’ radiative effect on climate. View Full-Text
Keywords: aerosol optical depth; Fourier transform infrared spectrometry; atmospheric aerosols; infrared remote sensing aerosol optical depth; Fourier transform infrared spectrometry; atmospheric aerosols; infrared remote sensing
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MDPI and ACS Style

Barreto, Á.; García, O.E.; Schneider, M.; García, R.D.; Hase, F.; Sepúlveda, E.; Almansa, A.F.; Cuevas, E.; Blumenstock, T. Spectral Aerosol Optical Depth Retrievals by Ground-Based Fourier Transform Infrared Spectrometry. Remote Sens. 2020, 12, 3148. https://doi.org/10.3390/rs12193148

AMA Style

Barreto Á, García OE, Schneider M, García RD, Hase F, Sepúlveda E, Almansa AF, Cuevas E, Blumenstock T. Spectral Aerosol Optical Depth Retrievals by Ground-Based Fourier Transform Infrared Spectrometry. Remote Sensing. 2020; 12(19):3148. https://doi.org/10.3390/rs12193148

Chicago/Turabian Style

Barreto, África, Omaira Elena García, Matthias Schneider, Rosa Delia García, Frank Hase, Eliezer Sepúlveda, Antonio Fernando Almansa, Emilio Cuevas, and Thomas Blumenstock. 2020. "Spectral Aerosol Optical Depth Retrievals by Ground-Based Fourier Transform Infrared Spectrometry" Remote Sensing 12, no. 19: 3148. https://doi.org/10.3390/rs12193148

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