Intercomparison of NO2, O4, O3 and HCHO slant column measurements by MAX-DOAS and zenith-sky UV-Visible spectrometers during the CINDI-2 campaign
Authors: Kreher Karin , Van Roozendael Michel , Hendrick Francois , Apituley Arnoud , Dimitropoulou Ermioni , Frieß Udo , Richter Andreas , Wagner Thomas , Abuhassan Nader , Ang Li , Anguas Monica , Bais Alkis , Benavent Nuria , Bösch Tim , Bognar Kristof , Borovski Alexander , Bruchkouski Ilya , Cede Alexander , Chan Ka L. , Donner Sebastian , Drosoglou Theano , Fayt Caroline , Finkenzeller Henning , Garcia-Nieto David , Gielen Clio , Gómez-Martín Laura , Hao Nan , Herman Jay R. , Hermans Christian , Hoque Syedul , Irie Hitoshi , Jin Junli , Johnston Paul , Khayyam Butt Junaid , Khokhar Fahim , Koenig Theodore K. , Kuhn Jonas , Kumar Vinod , Lampel Johannes , Liu Cheng , Ma Jianzhong , Merlaud Alexis , Mishra Abhishek K. , Müller Moritz , Navarro-Comas Monica , Ostendorf Mareike , Pazmino Andrea , Peters Enno , Pinardi Gaia , Pinharanda Manuel , Piters Ankie , Platt Ulrich , Postylyakov Oleg , Prados-Roman Cristina , Puentedura Olga , Querel Richard , Saiz-Lopez Alfonso , Schönhardt Anja , Schreier Stefan F. , Seyler Andre , Sinha Vinayak , Spinei Elena , Strong Kimberly , Tack Frederik , Tian Xin , Tiefengraber Martin , Tirpitz Jan-Lukas , van Gent Jeron , Volkamer Rainer , Vrekoussis Mihalis , Wang Shanshan , Wang Zhuoru , Wenig Mark , Wittrock Folkard , Xie Pinhua H. , Xu Jin , Yela Margarita , Zhang Chengxin , Zhao Xiaoyi 
Published: May 2019

Abstract. In September 2016, 36 spectrometers from 24 institutes measured a number of key atmospheric pollutants for a period of 17 days during the Second Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2) that took place at Cabauw, The Netherlands (51.97° N, 4.93° E). We report on the outcome of the formal semi-blind intercomparison exercise, which was held under the umbrella of the Network for the Detection of Atmospheric Composition Change (NDACC) and the European Space Agency (ESA). The three major goals of CINDI-2 were to characterise and better understand the differences between a large number of Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) and zenith-sky DOAS instruments and analysis methods, to discuss the performance of the various types of instruments and to contribute to a harmonisation of the measurement settings and retrieval methods. This, in turn, creates the capability to produce consistent high-quality ground-based data sets, which are an essential requirement to generate reliable long-term measurement time series suitable for trend analysis and satellite data validation. The data products investigated during the semi-blind intercomparison are slant columns of nitrogen dioxide (NO2), the oxygen dimer (O4) and ozone (O3) measured in the UV and visible wavelength region, formaldehyde (HCHO) in the UV spectral region and NO2 in an additional (smaller) wavelength range in the visible. The campaign design and implementation processes are discussed in detail including the measurement protocol, calibration procedures and slant column retrieval settings. Strong emphasis was put on the careful alignment and synchronisation of the measurement systems, resulting in an unprecedented set of measurements made under highly comparable air mass conditions. The CINDI-2 data sets were investigated using a regression analysis of the slant columns measured by each instrument and for each of the target data products. The slope and intercept of the regression analysis respectively quantify the mean systematic bias and offset of the individual data sets against the reference, and the RMS error provides an estimate of the measurement noise or dispersion. These three criteria are examined and for each of the parameters and each of the data products, performance thresholds are set and applied to all the measurements. The approach presented here has been developed based on heritage from previous intercomparison exercises. It introduces a quantitative assessment of the measurement performance of all the participating instruments for the MAX-DOAS and zenith-sky DOAS techniques.