Intercomparison of NO2, O4, O3 and HCHO slant column measurements by MAX-DOAS and zenith-sky UV–visible spectrometers during CINDI-2
Authors: Kreher Karin , Van Roozendael Michel , Hendrick Francois , Apituley Arnoud , Dimitropoulou Ermioni , Frieß Udo , Richter Andreas , Wagner Thomas , Lampel Johannes , Abuhassan Nader , Ang Li , Anguas Monica , Bais Alkis , Benavent Nuria , Bösch Tim , Bognar Kristof , Borovski Alexander , Bruchkouski Ilya , Cede Alexander , Chan Ka Lok , Donner Sebastian , Drosoglou Theano , Fayt Caroline , Finkenzeller Henning , Garcia-Nieto David , Gielen Clio , Gómez-Martín Laura , Hao Nan , Henzing Bas , 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 , 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 André , Sinha Vinayak , Spinei Elena , Strong Kimberly , Tack Frederik , Tian Xin , Tiefengraber Martin , Tirpitz Jan-Lukas , van Gent Jeroen , 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 2020

Abstract. In September 2016, 36 spectrometers from 24 institutes measured a
number of key atmospheric pollutants for a period of 17 d 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 (1) 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, (2) to define a robust methodology for performance assessment of
all participating instruments, and (3) 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 collision complex
(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 region. 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 a unique 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 selected reference (which is obtained from the median of either
all data sets or a subset), 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 consistency between all the
participating instruments for the MAX-DOAS and zenith-sky DOAS techniques.