Jerry L. Berndt

The rotating shadowband spectroradiometer (RSS) at SGP

The RSS provides continuous spectral measurements of total-horizontal, diffuse-horizontal, and direct-normal irradiance from 360 to 1100 nm using the automated shadowband technique. We show instrument performance, calibration method and accuracies, case data from the first-generation instrument operated at the Southern Great Plains (SGP) site, and comparisons with the improved design now starting operation.

Comparison of spectral irradiance standards used to calibrate shortwave radiometers and spectroradiometers

Absolute calibration of spectral shortwave radiometers is usually performed with National Institute of Standards and Technology (NIST) or NIST-traceable incandescent lamps. We compare 18 irradiance standards from NIST and three commercial vendors using the same spectrometer to assess their agreement with our working standard. The NIST procedure is followed for the 1000-W FEL lamps from NIST, Optronics, and EG&G. A modified calibration procedure developed by Li-Cor is followed for their 200-W tungsten-halogen lamps. Results are reproducible from one day to the next to approximately 0.1% using the same spectrometer. Measurements taken four months apart using two similar but different spectrometers were reproducible to 0.5%. The comparisons suggest that even NIST standards may disagree with each other beyond their stated accuracy. Some of the 1000-W commercial lamps agreed with the NIST lamps to within their stated accuracy, but not all. Surprisingly, the lowest-cost lamps from Li-Cor agreed much better with the NIST lamps than their stated accuracy of 4%, typically within 2%. An analysis of errors leads us to conclude that we can transfer the calibration from a standard lamp to a secondary standard lamp with approximately 1% added uncertainty. A field spectrometer was calibrated with a secondary standard, producing a responsivity for the spectrometer that was within 5% of the responsivity obtained by Langley calibration using routine field measurements.

The 1997 North American Interagency Intercomparison of Ultraviolet Spectroradiometers Including Narrowband Filter Radiometers

The fourth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held September 15 to 25, 1997 at Table Mountain outside of Boulder, Colorado, USA. Concern over stratospheric ozone depletion has prompted several government agencies in North America to establish networks of spectroradiometers for monitoring solar ultraviolet irradiance at the surface of the Earth. 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 NIST and NOAA, and included participants from the ASRC, EPA, NIST, NSF, SERC, USDA, and YES. The UV measuring instruments included scanning spectroradiometers, spectrographs, narrow band multi-filter radiometers, and broadband radiometers. Instruments were characterized for wavelength accuracy, bandwidth, stray-light rejection, and spectral irradiance responsivity. The spectral irradiance responsivity was determined two to three times outdoors to assess temporal stability. Synchronized spectral scans of the solar irradiance were performed over several days. Using the spectral irradiance responsivities determined with the NIST traceable standard lamp, and a simple convolution technique with a Gaussian slit-scattering function to account for the different bandwidths of the instruments, the measured solar irradiance from the spectroradiometers excluding the filter radiometers at 16.5 h UTC had a relative standard deviation of ±4 % for wavelengths greater than 305 nm. The relative standard deviation for the solar irradiance at 16.5 h UTC including the filter radiometer was ±4 % for filter functions above 300 nm.

O2‐O2 absorption band identification based on optical depth spectra of the visible and near‐infrared

The first implementation of the rotating shadowband spectroradiometer allows the routine collection of direct normal solar spectral irradiance data at 512 wavelengths. These data are used on clear days to calculate spectra in optical depth. A persistent absorption band near 477 nm in the first optical depth spectra collected led to the eventual identification of six bands in the 400 to 1080 nm range resulting from absorption by O2 collision pairs. All of these O2-O2 bands were discovered earlier using atmospheric long pathlength or laboratory high pressure techniques, however, it is notable that this methods sensitivity permitted their detection at one air mass. These data suggest that there are no unexplained narrowband absorption features in the visible/near-infrared, that excess optical depth measured near 1000 nm may now have an explanation, and that the linearity of the O2-O2 absorption may be exploited to investigate mean pathlengths in the atmosphere.

Column water vapor from diffuse irradiance

A possibility of measuring water vapor column from diffuse irradiance, and thus the extension of optical retrievals to cloudy days, was investigated. The data from the Rotating Shadowband Spectroradiometer (RSS) during its winter deployment at the North Slope of Alaska (NSA) site is used. The initial analysis covers 20 days in March 1999 that include clear, partly cloudy, and overcast days. During these days, water vapor column according to the NSA site’s Microwave Radiometer (MWR) varied between 1 and 5 mm. The diffuse irradiances in the 820 and 940-nm water vapor absorption bands are compared with water vapor column obtained from the MWR. While these irradiances do not correlate well with the water vapor column, we found that by using the diffuse transmittance in the 760-nm oxygen absorption band to obtain an effective air mass, we could derive a method that greatly increases the correlation. The end result is a correlation of 0.97 and 0.95 between MWR and diffuse RSS retrievals using 820 and 940-nm transmittances, respectively. This encouraging result is based on empirical data analysis only. It implies that the diffuse irradiance may contain sufficient information to retrieve water vapor column. However, we must analyze broader data sets to ensure that the resulting high correlation in these data is not predicated on the limited climatological conditions experienced in March 1999 at the NSA site.

Specifications and performance of UV rotating shadowband spectroradiometer (UV-RSS)

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.

United States Department of Agriculture reference ultraviolet spectroradiometer: current performance and operational experience at Table Mountain, Colorado

At present the United States Department of Agriculture (USDA) Reference Spectroradiometric Network consists of three sites: Table Mountain, Colorado, Lamont, Oklahoma (the ARM program SGP site), and Beltsville, Maryland. At each site we deploy and continuously operate a 1-m cascaded additive-double Czerny-Turner scanning monochromator with a bialkali photomultiplier and photon-counting detection. Lambertian fore-optic errors are less than 1% over the range of zenith angles from 0 to 80°. The instruments use photon counting and make measurements at 290 nm not affected by stray light 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 fundamental instrument performance specifications are: Optical resolution: 0.1 nm FWHM, triangular slit-function. Wavelength reproducibility: ′0.0025-nm 2<T with 296-nm Hg retrace-scan corrections applied, ′0.007 nm 2σ over typical diurnal variability, without correction. Wavelength accuracy: Limited by calibration systematic errors. Believed to be 0.005-nm worst case. Stray light: < 10 - 7 at 4 FWHM, 10 - 1 0 at 20 nm, slit-scattering function versus 325 nm HeCd. Angular response: less than 1% error from cosine over the range of zenith angles from 0 to 80°. Signal linearity: The instrument uses a photomultiplier with 2-ns rise-time and photon counting detection. The dual-threshold discriminator has a 700-Mhz synchronous signal counting limit. The maximum counting rates seen at the longest wavelengths are less than 10 MHz; less than 1/5 of the frequency where nonlinearity can be detected, as tested for the 1997 Intercomparison. 2000 was the first full year of operation of our instrument at the NOAA Table Mountain site (140.177 °N 105.276 °W, 1900 m asl) for which the operational and calibration frequencies justify making the data accessible to outside users for scientific application. We show performance in routine operation and issues of calibration over the period April 2000 to 31 December 2001.

Calibration, data processing, and maintenance of the United States Department of Agriculture high-resolution ultraviolet spectroradiometers

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, Mary- land November 1999; and Fort Collins, Colorado October 2002. These spectroradiometers contain Jobin Yvons 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 irra- diances. The radiometric calibration strategies using Central UV Calibra- tion Facility FEL lamps that are directly NIST-traceable, portable field calibrators, and vicarious calibrations using data from UV multifilter ro- tating 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.

Data and signal processing of rotating shadowband spectroradiometer (RSS) data

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 DOEs 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.

Transfer of UV irradiance calibration to our field spectroradiometers: current performance and operational experience at Table Mountain, Colorado

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 NOAAs 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.