Published by Copernicus Publications on behalf of the European Geosciences Union. 458 A. J. M. Piters et al.: The CINDI campaign: design, execution and early resultsAbstract. From June to July 2009 more than thirty different in-situ and remote sensing instruments from all over the world participated in the Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI). The campaign took place at KNMI's Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Its main objectives were to determine the accuracy of state-ofthe-art ground-based measurement techniques for the detection of atmospheric nitrogen dioxide (both in-situ and remote sensing), and to investigate their usability in satellite data validation. The expected outcomes are recommendations regarding the operation and calibration of such instruments, retrieval settings, and observation strategies for the use in ground-based networks for air quality monitoring and satellite data validation. Twenty-four optical spectrometers participated in the campaign, of which twenty-one had the capability to scan different elevation angles consecutively, the so-called Multi-axis DOAS systems, thereby collecting vertical profile information, in particular for nitrogen dioxide and aerosol. Various in-situ samplers and lidar instruments simultaneously characterized the variability of atmospheric trace gases and the physical properties of aerosol particles. A large data set of continuous measurements of these atmospheric constituents has been collected under various meteorological conditions and air pollution levels. Together with the permanent measurement capability at the CE-SAR site characterizing the meteorological state of the atmosphere, the CINDI campaign provided a comprehensive observational data set of atmospheric constituents in a highly polluted region of the world during summertime. First detailed comparisons performed with the CINDI data show that slant column measurements of NO 2 , O 4 and HCHO with MAX-DOAS agree within 5 to 15 %, vertical profiles of NO 2 derived from several independent instruments agree within 25 % of one another, and MAX-DOAS aerosol optical thickness agrees within 20-30 % with AERONET data. For the in-situ NO 2 instrument using a molybdenum converter, a bias was found as large as 5 ppbv during day time, when compared to the other in-situ instruments using photolytic converters.

Abstract.A growing number of space-borne instruments measures nitrogen dioxide (NO 2 ) concentrations in the troposphere, but validation of these instruments is hampered by the lack of ground-based and in situ profile measurements.The Royal Netherlands Meteorological Institute (KNMI) has developed a working NO 2 sonde. The sonde is attached to a small meteorological balloon and measures a tropospheric NO 2 profile. The NO 2 sonde has a vertical resolution of 5 m and a measurement range between 1 and 100 ppbv. The instrument is light in weight (0.7 kg), cheap (disposable), energy efficient and not harmful to the environment or the person who finds the package after use. The sonde uses the chemiluminescent reaction of NO 2 in an aqueous luminol solution. The NO 2 -luminol reaction produces faint blue/purple light (at about 425 nm), which is detected by an array of silicon photodiodes. The luminol solution is optimised to be specific to NO 2 .

From June to July 2009 more than thirty different in-situ and remote sensing instruments from all over the world participated in the Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI). The campaign took place at KNMI's Cabauw Experimental Site for Atmospheric Research in the Netherlands. Its main objectives were to determine the accuracy of state-of-the-art ground-based measurement techniques for the detection of atmospheric nitrogen dioxide (both in-situ and remote sensing), and to investigate their usability in satellite data validation. The expected outcomes are recommendations regarding the operation and calibration of such instruments, retrieval settings, and observation strategies for the use in ground-based networks for air quality monitoring and satellite data validation. Twenty-four optical spectrometers participated in the campaign, of which twenty-one had the capability to scan different elevation angles consecutively, the so-called Multi-axis DOAS systems, thereby collecting vertical profile information, in particular for nitrogen dioxide and aerosol. Various in-situ samplers simultaneously characterized the variability of atmospheric trace gases and the physical properties of aerosol particles. A large data set of continuous measurements of these atmospheric constituents has been collected under various meteorological conditions and air pollution levels. Together with the permanent measurement capability at the Cabauw site characterizing the meteorological state of the atmosphere, the CINDI campaign provided a comprehensive observational data set of atmospheric constituents in a highly polluted region of the world during summertime. First detailed comparisons performed with the CINDI data show that slant column measurements of NO₂, O₄ and HCHO with MAX-DOAS agree within 5 to 15%, vertical profiles of NO₂ derived from several independent instruments agree within 25%, and MAX-DOAS aerosol optical thickness agrees within 20–30% with AERONET data. For the in-situ NO₂ instrument using a molybdenum converter, a bias was found as large as 5 ppbv during day time, when compared to the other in-situ instruments using photolytic converters

A growing number of space-borne instruments measures nitrogen dioxide (NO₂) concentrations in the troposphere, but validation of these instruments is hampered by lack of ground-based and in-situ profile measurements. <br><br> The Royal Netherlands Meteorological Institute (KNMI) has developed a working NO₂ sonde. The sonde is attached to a small meteorological balloon and measures a tropospheric NO₂ profile. The NO₂ sonde has a vertical resolution of 5 m, and a measurement range between 1 and 100 ppbv. The instrument is light in weight (±700 g), cheap (disposable), energy efficient and not harmful to the environment or the person who finds the package after use. The sonde uses the chemiluminescent reaction of NO₂ in an aqueous luminol solution. The NO₂–luminol reaction produces faint blue/purple light (at about 425 nm), which is detected by an array of silicon photodiodes. The luminol solution is optimised to be specific to NO₂. <br><br> An on-ground comparison with measurements from a Photolytic Analyzer of RIVM shows that both instruments measure similar NO₂ variations in ambient air. <br><br> During the Cabauw Intercomparison campaign of Nitrogen Dioxide measuring Instruments (CINDI) in June/July 2009 six vertical profiles of NO₂ from the ground to 5 km altitude were measured, which clearly show that the largest amount of NO₂ is measured in the boundary layer. The measured boundary layer heights of the NO₂ sonde are in good agreement with boundary layer heights determined by a LD40 Ceilometer at Cabauw