The latest generation of nadir-looking air pollution monitoring instruments (e.g. Sentinel-5P) deliver daily global maps of tropospheric NO2 columns with an unprecedented spatial resolution and accuracy. Still, the ground pixel size makes the product unusable for applications targeting the assessment of personal exposure, or other street-level effects. While a lot of research is dedicated to upscaling the satellite products by the synergistic use of air quality monitoring stations and pollution dispersion models, there is a need for innovative instrumentation capable of bridging the street-level needs with the city-wide sampling of satellite NO2 maps. An innovative passive optical instrument has been developed, which is capable of imaging the field of NO2 above the skyline of a city. The measurement principle consists in recording a multispectral cube of the scene at several wavelengths providing a good sensitivity to NO2. A DOAS-like retrieval algorithm fits the abundance of NO2 seen by every pixel of the image. The product is a high resolution map of the NO2 above a city, with a spatial and temporal resolution unseen so far. We present the instrumental concept, and measurement approach. Data acquired above Brussels, Belgium, have been validated with a reference instrument, a MAX-DOAS spectrometer driven in an unusual way in order to sample the NO2 camera field of view. Excellent consistency between the two instruments was found, opening the way for further applications of this concept.@en

A Sentinel-5p/TROPOMI validation campaign was held in the Netherlands based at the Cabauw Experimental Site for Atmospheric Research during September 2019. The TROpomi vaLIdation eXperiment (TROLIX) consisted of active and passive remote sensing platforms in conjunction with several balloon-borne, airborne and surface chemical measurements. The goal of this geophysical validation campaign was to make intensive observations to establish the quality of TROPOMI L2 main data products (UVAI, Aerosol Layer Height, NO2, O3, HCHO, Clouds) under realistic non-idealized conditions with varying cloud cover and a wide range of atmospheric conditions. Since TROPOMI is a hyperspectral imager with a very high spatial resolution of 3.5x7 km2, understanding local effects such as inhomogeneous sources of pollution, sub-pixel clouds and variations in ground albedo is important to interpret TROPOMI results. Therefore, the campaign included sub-pixel resolution local networks of sensors, involving Pandora and MAXDOAS instruments, around Cabauw (51.97° N, 4.93° E) and within the city of Rotterdam. Cabauw is considered rural while Rotterdam is densely populated and industrialized. These focal areas were connected through airborne as well as ground based mobile observations. Cabauw, using its comprehensive in-situ and remote sensing observation program in and around the 213 m meteorological tower, was the main site of the campaign with focus on vertical profiling using lidar instruments for aerosols, clouds, water vapor, tropospheric and stratospheric ozone, as well as balloon-borne sensors for NO2 and ozone. The data set collected can be directly compared to the TROPOMI L2 data products, while measurements of parameters related to a-priori data and auxiliary parameters that influence the quality of the L2 products such as aerosol and cloud profiles and in-situ aerosol and atmospheric chemistry were also collected. This paper gives an overview of the campaign, and an overview of the participating main and ancillary instrumentation. Furthermore, an overview of the meteorological and atmospheric conditions observed during the campaign is given from the respective perspectives of the participating instruments, including satellite observations and the support by atmospheric modeling (CAMS). @en