1. Working Group-Ultraviolet Julian Gröbner and Mario Blumthaler

2. Scientific Objectives Open issues with regard to surface UV radiation: Reference extraterrestrial solar spectrum (ETS) with high resolution Spectral aerosol optical depth (AOD) retrieval

3. Scientific Objective : ETS - UV Modelling studies using ETS show systematic spectral features which are a clear indication of features in ETS spectra. This currently limits the interpretation of UV measurement and Modelling studies (examples surface albedo and atmospheric composition studies, …). Courtesy G. Bernhard, 2006

4. ETS from various sources in the UV 15% Courtesy C. Gueymard

5. Scientific Objective: Spectral AOD Retrieval of spectral AOD is obtained from direct sun irradiance spectra. Current limitations are: - Total column ozone is necessary to remove ozone absorption: Inconsistences in ozone x-sections and TOC retrievals by various instruments (Brewer, Dobson, Satellites) 3% TOC uncertainty -> ±0.10 at 300 nm The WMO goal for AOD determination is ~0.01 This problem is under investigation (ICAGO-O3 Activity, WMO ozone theme meeting, 2010) - Reference ETS Cannot be obtained from standard Langley technique (Even Resolution of 0.5nm leads to systematic effects due to steep absorption of ETS)

6. WG UV proposed activities Address these objectives through: -Sessions at Conferences (i.e. IAMAS 2011 in Melbourne) -Discussions and coordinated approach by the UV Community -Consider a IRC UV Newsletter: Upcoming Conferences, recent publications, new and ongoing projects, project calls, … Further activities: -Promote Instrument developments for surface UV measurements -Supporting projects

7. Project: Traceability of solar UV radiation Time frame: 2011-2013 Project Proposal to improve solar UV spectroradiometry by: -New reference standards -Improved instruments -Well characterised array spectradiometers Activity is embedded in the European Metrology Research Programme (EMRP) of EURAMET and the EU (Call 2010) To reach uncertainties of the order of 1-2% - Currently ±5% is state-of-the-art A Letter of expression of support from the IRC will strengthen the project submission and demonstrate the support from the radiation community.