The Montreal Protocol on Substances that Deplete the Ozone Layer has been hailed as the most successful environmental treaty ever ( https://www.unenvironment.org/news-and-stories/story/montreal-protocol-triumph-treaty ). Yet, although our main concern about ozone depletion is the subsequent increase in harmful solar UV radiation at the Earth's surface, no studies to date have demonstrated its effectiveness in that regard. Here we use long-term UV Index (UVI) data derived from high-quality UV spectroradiometer measurements to demonstrate its success in curbing increases in UV radiation. Without this landmark agreement, UVI values would have increased at mid-latitude locations by approximately 20% between the early 1990s and today and would approximately quadruple at mid-latitudes by 2100. In contrast, an analysis of UVI data from multiple clean-air sites shows that maximum daily UVI values have remained essentially constant over the last ~20 years in all seasons, and may even have decreased slightly in the southern hemisphere, especially in Antarctica, where effects of ozone depletion were larger. Reconstructions of the UVI from total ozone data show evidence of increasing UVI levels in the 1980s, but unfortunately, there are no high-quality UV measurements available prior to the early 1990s to confirm these increases with direct observations.

Absorption of solar radiation by water vapor in the near‐UV region is a poorly understood but important issue in atmospheric science. To better understand water vapor near‐UV absorption, we constructed a cavity ring‐down spectrometer with bandwidth of 5 cm −1 (~0.05 nm) and obtained water vapor absorption cross sections at 1‐nm increments in the 290‐ to 350‐nm region. Water vapor displays structured absorption over this range with maximum and minimum cross sections of 8.4 × 10 −25 and 1.6 × 10 −25 cm 2 /molecule. Major water vapor absorption bands were observed at 293–295, 307–313, 319, 321–322, and 325 nm, with cross‐section values higher than 4.0 × 10 −25 cm 2 /molecule. To obtain further insight into major water vapor absorption bands, we measured water vapor absorption cross sections at 0.05‐nm intervals in the 292‐ to 296‐nm, 306‐ to 314‐nm, and 317‐ to 326‐nm region. Field UV residual spectra not only exhibited increased attenuation at higher atmospheric water vapor loadings but also showed structures suggested by the laboratory water vapor absorption spectrum. Spaceborne UV radiance spectra have spectral structures resembling the differential cross‐section spectrum constructed from the laboratory wavelength‐dependent water vapor absorption cross sections presented here. Incorporating water vapor absorption cross‐section data into a radiative transfer model yielded an estimated energy budget of 0.26 W/m 2 for the standard U.S. atmosphere and 0.76 W/m 2 for the tropics. This shows that water vapor near‐UV absorption is an important contributor for climate simulation and ozone retrievals.

We use multi-year datasets of UV spectral irradiances, measured to the exacting standards required by the Network for the Detection of Atmospheric Composition Change (NDACC), to investigate the enhancement effects of clouds and their wavelength dependence at sites that span a wide range of altitudes. These enhancements are derived by comparing weighted UV irradiance measurements with corresponding model calculations at each site for clear skies. We find that the frequency, magnitude, and wavelength-dependence of cloud enhancements are insufficient to explain the repeatedly high values of UVB and UVI observed by Cabrol et al., 2014, and that ozone amounts lower than have ever been seen there would be required.

The fifth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held June 13 to 21, 2003 at Table Mountain outside of Boulder, Colorado, USA. The main purpose of the Intercomparison was to assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks. This Intercomparison was coordinated by NOAA and included participants from six national and international agencies. The UV measuring instruments included scanning spectroradiometers, spectrographs, and multi-filter radiometers. Synchronized spectral scans of the solar irradiance were performed between June 16 and 20, 2003. The spectral responsivities were determined for each instrument using the participants' lamps and calibration procedures and with NOAA/CUCF standard lamps. This paper covers the scanning spectroradiometers and the one spectrograph. The solar irradiance measurements from the different instruments were deconvolved using a high resolution extraterrestrial solar irradiance and reconvolved with a 1-nm triangular band-pass to account for differences in the bandwidths of the instruments. The measured solar irradiance from the spectroradiometers using the rivmSHIC algorithm on a clear-sky day on DOY 172 at 17.0 UTC (SZA = 30 degrees) had a relative 1-sigma standard deviation of +/-2.6 to 3.4% for 300- to 360-nm using the participants' calibration.

Combined measurements of diffuse-to-global radiation ratio and global spectral irradiances in the UV are used to derive cosine-corrected UV irradiances and aerosol optical depth (AOD). The diffuse-to-global radiation ratio is used first in the cosine correction of the global irradiance, then to calculate absolutely calibrated direct irradiances. The Beer-Lambert law is applied to derive the UV AOD using independent measurements of the extraterrestrial solar flux. The AOD can be derived with an uncertainty of about 0.03 at 60 degrees solar zenith angle. The method was applied to measurements obtained with two UV multifilter rotating shadowband radiometers (UV-MFRSRs) and a MK III Brewer spectrophotometer on the Island of Lampedusa in the Central Mediterranean during two periods of 2002 and 2004. The derived AOD at 318 and 332 nm was compared with UV AOD measured at 318, 320, and 368 nm with different techniques. The retrieved AOD, combining MFRSR and Brewer measurements, is in good agreement with the optical depth derived with the other methods.

The USDA ultraviolet radiation network currently includes four high-resolution spectroradiometers, located at Table Mountain, Colorado (deployed November 1998); the Atmospheric Radiation Measurement Climate Research Facility in Oklahoma (October 1999 ); Beltsville, Maryland (November 1999); and Fort Collins, Colorado (October 2002). These spectroradiometers contain Jobin Yvon's 1-m Czerny-Turner double additive spectrometers. The instruments measure total horizontal radiation in the 290- to 371-nm range, once every 30 min, with a nominal FWHM of 0.1 nm. We describe data quality control techniques as well as the data processing required to convert the raw data into calibrated irradiances. The radiometric calibration strategies using Central UV Calibration Facility FEL lamps that are directly NIST-traceable, portable field calibrators, and vicarious calibrations using data from UV multifilter rotating shadowband radiometers (MFRSRs) are discussed. Using direct-to-diffuse ratios from UV MFRSRs, we derive direct and diffuse high-resolution horizontal spectra from the collocated UV spectroradiometers of the USDA network. The direct-beam spectra can be used in a Langley regression that leads to spectroradiometric in situ calibration and to ozone column and aerosol optical depth retrievals. The high-resolution direct spectra are used to obtain the ozone column and aerosol optical depth in the 290- to 360-nm range at 0.1-nm resolution. A statistical summary of network performance is presented. (c) 2007 Society of Photo-Optical Instrumentation Engineers.

The data from the Rotating Shadownband Spetroradiometer UV-RSS deployed at Table Mountain, Boulder Colorado since June 2003 are used to retrieve ozone column and aerosol Angstrom coefficients in the 300 nm-380 nm range. The retrievals are performed from Langley regressions and from direct normal instantaneous irradiance measurements. The results from retrievals are used to verify assumption on ozone absorption cross-sections and ozone vertical profiles. A comparison between UV-RSS retrievals and those from the collocated instruments like the UV-MFRSR, Dobson, ozone sondes and TOMS & OMI is performed.

The Central UV Calibration Facility (CUCF) annually calibrates and characterizes 47 Ultraviolet Multi-Filter Rotating Shadow-band Radiometers (UV-MFRSR) for the USDA UV Monitoring and Research Program (UVMRP). The UV-MFRSR instrument has seven 2-nm wide channels with nominal centroids at 300, 305, 311, 317, 325, 332, and 368 nm. The first two channels 300 and 305 nm use silicon-carbide (SiC) photodiodes, and in the original design the remaining five channels used gallium-phosphide (GaP) photodiodes. Because of the high rate of failure in the channels with GaP photodiodes, channels 3 through 7 were replaced with silicon (Si) photodiodes starting in June 2000 by the manufacturer Yankee Environmental Systems, Inc. The newer design radiometers were tested for out-of-band rejection with two sources, in the laboratory using a 1000W FEL quartz tungsten halogen lamp and in the field using the sun. Out-of-band light measurements were completed in the field on all 47 radiometers and show there is no appreciable signal from out-of-band light contributing to the total solar horizontal irradiance in each of the seven wavelength bands. However, in the calibration procedure, using a 1000W FEL quartz- tungsten-halogen lamp there is significant out-of-band signal contributing to the measured signal. The out-of-band signal is measured at the time of the calibration and corrections are applied to the calibration factors of the radiometer in each channel. At the Table Mountain Test Facility, solar irradiance from a calibrated filter radiometer with and without the out-of-band correction factors are compared to filter weighted solar irradiance from the U111 reference spectroradiometer.

The National Institute of Standards and Technology (NIST) employ the 1000 watt FEL-type quartz-tungsten-halogen (QTH) lamp as its transfer device for the spectral irradiance scale. The cost of the calibrated lamps from the NIST makes it prohibitive to use them for routine calibrations. To dilute the costs and extend the working lifetime of the lamps it is customary to transfer the NIST scale to secondary lamps and then using these to transfer to tertiary (working standards) lamps. NOAA's Central UV Calibration Facility (CUCF) currently owns six of the NIST primary standards of spectral irradiance. The CUCF transfers the spectral irradiance scale from the NIST primaries to secondary standards, which in this case are also used as working standards. Careful seasoning and screening of the secondary lamps is essential to achieve the maximum benefit out of this transfer and the operation of the expensive primary standards. The process of lamp seasoning and screening techniques used by the CUCF is described here. They include visual inspection of the lamp envelope, filament and lead wires during each step of the screening process. Also, the temporal stability of the lamp irradiance, lamp current and voltage and anomalous emission and absorption lines is discussed. Some of the problems that the CUCF has found with lamps are also shown.

Horizontal standard lamps calibrate the spectral irradiance responsivity of spectroradiometers that measure solar ultraviolet irradiance. A field calibration unit and power supply developed to meet the requirements for using these standards in the field are described and their operation and associated uncertainties are detailed. Results from assessments obtained at two field instrument intercomparisons indicate that the horizontal standard, field calibration unit, and power supply operate within the design tolerances, making them suitable for performing routine calibrations in the field on most ultraviolet spectroradiometers.