A New Ground-Based Assessment of Near-Infrared Solar Spectral Irradiance between 2000-10000 cm-1

The solar spectral irradiance (SSI) is a key climate variable (Bojinski et al. 2014). The spectral distribution of the Sun’s energy drives atmospheric processes via the deposition of energy at various levels in the atmosphere. Recent measurements of the near-infrared SSI (which contains ~25% of the total solar energy) disagree by around 10%, with no agreement within the respective uncertainties of these measurements. Integrated over the spectral region (2000-10000 cm^-1), this accounts for ~20 W m^-2 of the total solar irradiance. These measurements are from a variety of ground-based and space-based sources, each with their respective advantages and disadvantages. This work presents observations of the Sun using an absolutely calibrated Fourier Transform spectrometer, taken at two field sites by the National Physical Laboratory. Atmospheric scattering and absorption is accounted for using either a line-by-line model and observations of aerosol scattering, or the use of zenith-angle dependent observations and the Langley method. Particular focus was placed on deriving a rigorous uncertainty budget using metrological principles. Our results show SSI which is ~10% lower than the community standard ATLAS3 spectrum, in line with other prior and subsequent analyses.