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 = 30o) had a relative 1- standard deviation of +/-2.6 to 3.4% for 300- to 360-nm using the participants' calibration.
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.
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 data from the UV-RSS deployed at Table Mountain, Boulder Colorado since June 2003 are used to retrieve ozone column and aerosol Angstrom coefficients in the 290nm-380nm range. Retrievals are performed from Langley regression and from direct normal instantaneous irradiance.
From direct-to-diffuse ratios from UV Multifilter Rotating Shadowband Radiometers (MFRSR) we derive direct and diffuse high resolution total horizontal spectra from the collocated UV spectroradiometer of the USDA network. The direct beam spectra can be used in 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 ozone column, and aerosol optical depth in the 290nm-360nm range at 0.1nm resolution.
KEYWORDS: Calibration, Ultraviolet radiation, Lamps, Mercury, Free electron lasers, Data processing, Diffusers, Spectroscopy, Data conversion, Received signal strength
The USDA UV radiation network currently consists of four high resolution spectroradiometers located at Table Mountain, Colorado (deployed 11/1998); the Atmospheric Radiation Measurement testbed site at Southern Great Plains, Oklahoma (deployed 10/1999); Beltsville, Maryland (deployed 11/1999); and Fort Collins, Colorado (deployed 10/2002). These spectroradiometers contain Jobin Yvon’s one meter asymmetric Czerny-Turner double additive spectrometer. The instruments measure total horizontal radiation in the 290nm to 360nm range, once every 30 minutes, with a nominal full-width at half-maximum (FWHM) of 0.1nm. 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 NIST FEL lamps, portable field calibrators, and vicarious calibrations using UVMFRSR data are discussed and a statistical summary of network performance is presented. All results are presented in the context of data processing and analysis tools including software and database systems.
At present the United States Department of Agriculture (USDA) Reference Spectroradiometric Network consists of 4 sites: Table Mt. CO, Ft. Collins CO, Lamont OK (The ARM program SGP site), and Beltsville MD. At each site we operate a 1-meter cascaded additive-double Czerny-Turner scanning monochromator with a bi-alkali Photomultiplier and photon-counting detection. Irradiance calibrations are provided for the instrument at Table Mt CO by NOAA's Central Ultraviolet Calibration Facility (CUCF) from NIST-traceable standards. Calibrations are transferred from this instrument to others in the network (and additional stability monitoring of the primary instrument conducted) using shippable transfer calibrators we have designed. Here we describe these transfer calibrators, and our operational experience seen at the Table Mt. Site in 2002 and 2003 to date.
KEYWORDS: Atmospheric modeling, Ultraviolet radiation, Data modeling, Shortwaves, Visible radiation, Received signal strength, Aerosols, Radiometry, Absorption, Data acquisition
Broadband shortwave diffuse horizontal irradiance models overestimate measurements by between 7 and 14% using the most reliable input data for the models and the best available broadband measurements of diffuse irradiance. This paper uses spectral irradiance measurements and models as opposed to broadband measurements and models to investigate the contributions to this difference from various regions of the spectrum. The data are from the first Atmospheric Radiation Measurement (ARM) diffuse irradiance intensive observation period (IOP) held in September and October of 2001 at the Oklahoma ARM site near Ponca City. Visible and ultraviolet (UV) rotating shadowband spectroradiometers (RSS) acquired data during the IOP. Diffuse measurements with conventional broadband diffuse pyranometers and direct irradiance measurements using an absolute cavity radiometer are also available for analysis. Integrated spectral measurements are consistent with broadband measurements and, therefore, confirm the earlier results that models over predict diffuse. The wavelength dependent differences in models and measurements are illustrated and discussed.
We will measure the solar spectral irradiance by deploying a CCD-array solar spectrograph to a high altitude favorable site, as part of a self-contained autonomous system with a calibration system using a monochromator and absolute photodiode "trap detectors." Data will be reduced using Langley extrapolation (and in the stronger absorption bands methods similar to Reagan-Brugge fitting), to yield the solar output free of atmospheric absorption.
This measurement system will substantially improve the accuracy of the field measurements by making the instrument continuously self-calibrating against a local absolute standard in the range 400 - 900 nm. In the ranges 360 - 400 nm and also 900 - 1100 the trap detectors are not an absolute standard, but serve as a very reproducible transfer standard from an irradiance scale to be taken from either NIST lamps, or more recently-introduced detectors with calibrated efficiencies. We expect an absolute accuracy of 0.3% for solar-spectrum determination in the range 400 to 900 nm, not including the O2 band at 760 nm, and the H2O bands at 820 and 940 nm. In the 360 - 400 nm domain we may be able to extend trap-detector quantum efficiency to allow an accuracy better than a secondary irradiance transfer, otherwise this domain and the range 900 to 1100 nm will have an accuracy of ≈ 1 %. The extrapolations in the strong-absorption bands will have an increased uncertainty which can be estimated from the statistics of the data. We describe the instrument and self-calibration methadologies and design.
The Rotating Shadowband Spectroradiometer (RSS) is a tandem-prisms spectrograph that uses a CCD array to measure solar direct and diffuse irradiances. Two versions of the RSS were designed at the Atmospheric Sciences Research Center at the State University of New York at Albany to measure UV from 295-370 nm and VIS-NIR from 360-1050 nm. A number of prototypes have been deployed at two sites of DOE's Atmospheric Radiation Measurement program since 1996. The first commercial UV RSS built by Yankee Environmental Systems, Inc. was deployed in 2001 and the VIS-NIR RSS is slated for permanent installation at the ARM SGP site in 2002. The paper describes instrument characterization procedures, spectral and radiometric calibrations. Mathematical algorithms applied to the spectra to correct wavelength shifts, to reduce stray light effects, and to correct drifts in radiometric calibration are described.
At present the USDA Reference Spectroradiometric Network consists of 3 sites: Table Mt. CO, Lamont OK (the ARM program SGP site), and Beltsville MD. At each site we deploy and continuously operate a 1-meter cascaded additive-double Czerny-Turner scanning monochromator with a bi-alkali Photomultiplier and photon-counting detection. Lambertian fore-optic errors are less than 1% over the range of zenith angles from 0 to 80 degrees. The instruments use photon counting and make measurements not affected by stray light at 290 nm under typical conditions. The basic performance specifications of the instrument were demonstrated by a prototype at the 1997 North American UV Spectroradiometer Intercomparison. Data shown here demonstrate that these are met in routine operation.
The UV rotating shadowband spectroradiometer (UV-RSS) is capable of measuring direct, diffuse and total horizontal irradiances simultaneously with spectral resolution of 0.25- 0.45 nm in the 290-380 nm range. It is based on a two-prism spectrograph that has very high out-of-band rejection of 2*10-6) as defined by the 325 nm HeCd laser line. Without moving parts, the radiometric stability is limited by the stability of the diffuser throughput and the stability of the cooled CCD. The wavelength stability is maintained by temperature control of the fused silica prisms and air pressure in the spectrograph. The current signal-to- noise ratio allows optical depth retrievals in the 305-360 nm range at mid latitudes in summer for typical ozone loading of 300 DU. This signal-to-noise can be increased by a factor of 5 within a one-minute shadowbanding cycle by means of multiple exposures. The UV-RSS permits ozone retrieval from diffuse irradiance using the DOAS method or from direct irradiance via Langley regression. Either method is robust as the UV-RSS provides 205 pixels of data within 310-330 nm range.
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