An intercomparison campaign of ground‐based UV‐visible measurements of NO 2 , BrO, and OClO slant columns: Methods of analysis and results for NO 2

Within the framework of the Network for the Detection of Stratospheric Change (NDSC), an intercomparison campaign of ground‐based zenith‐sky viewing UV‐visible spectrometers was held at the Andøya Rocket Range (69°N, 16°E) at Andenes, Norway, from February 12 to March 8, 2003. The chosen site is classified as a complementary NDSC site. Eight groups from seven countries participated in the campaign which focused on the measurements of slant columns of NO 2 , BrO, and OClO. This first campaign publication concentrates on measurements of the NO 2 slant columns. Different analysis criteria were investigated during the campaign. These included the use of fitting parameters as chosen by each group to provide what they considered to be optimized retrievals. Additional sets of parameters, imposed for all the groups, were also used, including the wavelength interval, absorption cross sections, and species fitted. Each instrument's results were compared to the measurements of selected reference instruments, whose choice was based on a technique combining regression analysis and examination of the residuals with solar zenith angle. Considering the data obtained during the whole campaign for solar zenith angles between 75° and 95°, all instruments agreed within 5% in the case of NO 2 with imposed analysis parameters in the 425–450 nm region. Measurements agree less well when retrieving the NO 2 slant columns in the 400–418 nm region or when using parameters optimized by each investigator for their instrument.