2008 Report of the Spectral Measurements Working Group

1. 2008 Report of the Spectral Measurements Working Group Bruce Forgan, Ain Kallis, Alex Manes,BruceMcArthur, Richard McKenzie, Joe Michalsky, Rachel Pinker, Ken Rutledge (?), Laurent Vuilleumier • Primary Interests: • Ultraviolet (mostly broadband, some spectral) • Moderate resolution UV, visible, and near-IR • PAR (photosynthetically active radiation)

2. Alex Manes (IMS-ret) - Precision, Erythemally-Weighted Broadband UV Radiometry: 2008 • Although the inclusion of routine broadband UV-B measurements in the measurement suite of BSRN stations was adopted at the 5th BSRN meeting in Budapest in 1998, only a few Station Managers/Site Scientists have implemented the above resolution. The main reason for such reluctance was the uncertainty of characterization and calibration of the commercially-available UV-B radiometers, and, not the least, the lack of a generally recognized calibration hierarchy. • The BSRN UV-B WG has repeatedly urged and recommended in all its reports since the Budapest meeting the establishment of a World Central UV Calibration Facility.The CUCF in Boulder, Colorado, serving for many years as a Central UV Calibration Facility for the American UV networks, and the newly established (January 2008) European UV Calibration Centre ECUV/PMOD in Davos, seem to be able to serve the above purpose. However, a framework of close cooperation and coordination, including relatively frequent inter-comparison campaigns, and the maintenance of a common reference group of broadband UV radiometers, are required.

3. An international inter-comparison and calibration campaign of broadband UV radiometers was held recently at Davos, hosted by the European UV Calibration Center ECUV/PMOD, with the participation of the CUCF and six additional European UV calibration laboratories. The results of this inter-comparison provides a more accurate estimate of the precision that can be reached with erythemally-weighted broadband UV radiometers. (Publications at bottom of pagehttp://www.cost726.org/) • A Practical Guide to Operating Broadband Instruments Measuring Erythemally Weighted Irradiance by Ann Webb, Julian Gröbner, Mario Blumthaler •  Report of the PMOD/WRC-COST Calibration and Intercomparison of Erythemal radiometers Davos, Switzerland 28 July – 23 August 2006 by J. Gröbner, G. Hülsen, L. Vuilleumier, M. Blumthaler, J. M. Vilaplana, D. Walker, and J. E. Gil

4. Julian Gröbner (PMOD) - Characterisation and Calibration Campaign of COST 726 - WG-IV • 28 July to 23 August 2006 @ PMOD • 36 UV meters from 16 countries including 1 CUCF • SL (19), YES (9), K&Z (5), Scintec (2), Eldonet (1) • QASUME & U of Innsbruck spectrometers agreed to within 2% for the whole measurement campaign • After calibration and cosine correction BB UV’s agreed to within 7%; but up to 50% off w/ original cals • Compared calibrations performed by 7 cal facilites • Cosine responses within 4% • Spectral responses were worse • http://www.atmos-chem-phys.net/special_issue120.html

5. Richard McKenzie (NIWA) - Spectral UV • Primary instrument - NIWA spectrometer measures total horizontal spectral irradiance between 285 and 450 nm with 0.75 nm resolution Activities: • Validating OMI estimates of UV daily dose • Comparing UV spectrometers and TUV model • Studying the effects of pollution on UV • Investigating the balance between good and bad UV • Erythemally-weighted UV corrected for sza and ozone

6. NIWA’s Ultraviolet Spectrometer

7. Figure 1. Erythemal daily doses retrieved from AvaSpec-256 spectra versus Scintec UV-SET measured values in overcast (left) and sunshine (right) Ein Kallis - Tartu Observatory (Estonia) Overcast Mostly Clear

8. Figure 2. Covariance of the AvaSpec-256 UV-B and narrowband filter instrument CUVB1 306 nm measured spectral irradiance daily doses.

9. Bruce Forgan - Spectral • Compact, low-cost CCD spectrometers (1) Middleton ASR (absolute spectral radiometer) • U95 of 1.5% from 400 to 900 nm with 1999 technology • Hope to improve with better order-filtered CCD and temperature control (2) Another CCD spectrometer in combination with low-cost trap detectors and interference filters offers an alternate approach to irradiance standards for AOD region (3) Plan to put NIWA spectrometer in direct normal configuration on their BSRN tracker

10. Rachael Pinker - University of Maryland (“… not too much has happened in the instrument development arena and … no decisions have been made on the need for calibration of spectroradiometers …”) Scatter plot of daily mean surface downward PAR from both GMAO (Global Modeling and Assimilation Office - NASA model) and UMD/SRB_MODIS (University Maryland/Surface Radiation Budget_MODIS - satellite) against ground measurements at six SURFRAD sites for 2003-2005

11. Lower left: 0.45 used to convert SW to PAR, but not a constant

12. Net primary production of carbon (primarily a function of PAR): Blue is under- and red is overestimate based on GMAO model.

13. Rotating Shadowband Spectroradiometer Joseph Michalsky

16. Brief (and Ugly) Illustration of PAR & How It Is Measured McCree, K.J., Ag. Met., 1972

17. PAR Proposal - All Interested Parties • IOP for Fall 2009 at ARM Oklahoma USA site • Use calibrated visible RSS and LI-1800 to calculate PAR in Wm-2 and in Einsteins m-2 sec-1 • Compare as many PAR sensors as there are manufacturers • Possibly do angular and spectral responses and lamp calibrations • Graduate student (MS or maybe PhD - depends on how involved the study becomes)