Full-azimuthal imaging-DOAS observations of NO2 and O4 during CINDI-2

A novel imaging-DOAS (differential optical absorption spectroscopy) instrument IMPACT (Imaging MaP-per for AtmospheriC observaTions) is presented combining full-azimuthal pointing (360 degrees) with a large vertical coverage (similar to 41 degrees). Complete panoramic scans are acquired at a temporal resolution of similar to 15 min, enabling the retrieval of NO2 vertical profiles over the entire panorama around the measurement site. IMPACT showed excellent agreement (correlation > 99 %) with coincident multiaxis DOAS (MAX-DOAS) measurements during the Second Cabauw Intercomparison of Nitrogen Dioxide measuring Instruments (CINDI-2) campaign. The temporal variability of NO2 slant columns within a typical MAX-DOAS vertical scanning sequence could be resolved and was as large as 20 % in a case study under good viewing conditions. The variation of corresponding profiles and surface concentrations was even larger (40 %). This variability is missed when retrieving trace gas profiles based on standard MAX-DOAS measurements. The azimuthal distribution of NO2 around the measurement site showed inhomogeneities (relative differences) up to 120 % (on average 35 %) on short timescales (individual panoramic scans). This is more than expected for the semirural location. We explain this behavior by the transport of pollution. Exploiting the instrument's advantages, the plume's trajectory during a prominent transport event could be reconstructed. Finally, the potential for retrieving information about the aerosol phase function from O-4 slant columns along multiple almucantar scans of IMPACT is demonstrated, with promising results for future studies.