Spectral aerosol optical depth retrievals by ground-based fourier transform infrared spectrometry

• Main Authors: Barreto, Á., García, O. E., Schneider, Matthias, García, R. D., Hase, F., Sepúlveda, E., Almansa, A. F., Cuevas, E., Blumenstock, Thomas
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI 2020
• Subjects: aerosol optical depth; Fourier transform infrared spectrometry; atmospheric aerosols; infrared remote sensing; ddc:550; Earth sciences; info:eu-repo/classification/ddc/550; Aerosol Robotic Network
• Online Access: https://publikationen.bibliothek.kit.edu/1000125666; https://publikationen.bibliothek.kit.edu/1000125666/91901213; https://doi.org/10.5445/IR/1000125666

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% month−1). 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 ± ...