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. Measurements of spectral irradiance between 306 and 1020 nm were performed with a GUVis-3511 multi-channel filter radiometer at SmithRock State Park, Oregon, during the total solar eclipse of 21 August 2017.The radiometer was equipped with a shadowband, allowing the separation of theglobal (sun and sky) and direct components of solar radiation. Data wereused to study the wavelength-dependent changes in solar irradiance atEarth's surface. Results were compared with theoretical predictions usingthree different parameterizations of the solar limb darkening (LD) effect,which describes the change in the solar spectrum from the Sun's center toits limb. Results indicate that the LD parameterization that has been mostwidely used during the last 15 years underestimates the LD effect, inparticular at UV wavelengths. The two alternative parameterizations arebased on two independent sets of observations from the McMath–Pierce solartelescope. When these parameterizations are used, the observed andtheoretical LD effects agree to within 4 % for wavelengths larger than400 nm and occultation of the solar disk of up to 97.8 %. Maximumdeviations for wavelengths between 315 and 340 nm are 7 %. These somewhatlarger differences compared to the visible range may be explained withvarying aerosol conditions during the period of observations. The aerosoloptical depth (AOD) and its wavelength dependence was calculated frommeasurements of direct irradiance. When corrected for the LD effect, the AODdecreases over the period of the eclipse: from 0.41 to 0.34 at 319 nm andfrom 0.05 to 0.04 at 1018 nm. These results show that AODs can be accuratelycalculated during an eclipse if the LD effect is corrected. The total ozonecolumn (TOC) was derived from measurements of global irradiance at 306 and340 nm. Without correction for the LD effect, the retrieved TOC increases by20 DU between the first and second contact of the eclipse. With LDcorrection, the TOC remains constant to within natural variability ( ±2.6  DU or ±0.9  % between first and secondcontact and ±1.0  DU or ±0.3  % between third and fourth contact). Incontrast to results of observations from earlier solar eclipses, nofluctuations in TOC were observed that could be unambiguously attributed togravity waves, which can be triggered by the supersonic speed of the Moon'sshadow across the atmosphere. Furthermore, systematic changes in the ratioof direct and global irradiance that could be attributed to the solareclipse were not observed, in agreement with results of three-dimensional(3-D) radiative transfer (RT) models. Our results advance the understanding of theeffects of solar LD on the spectral irradiance at Earth's surface, thevariations in ozone during an eclipse, and the partitioning of solarradiation in direct and diffuse components.

atmospheric chemistry and physics

Measurements of spectral irradiance during the solar eclipse of 21 August 2017: reassessment of the effect of solar limb darkening and of changes in total ozone