The Retrieval of Tropospheric NO2 Vertical Column Density from Spectrolite Measurements over Berlin

Abstract

Air pollution has become one of the most serious problems societies worldwide are facing, thus there is an emergent need for air quality monitoring to quantify pollution level and supervise pollution sources. One of the main pollutants, nitrogen dioxide, mostly comes from human activities such as the burning of fossil fuels, vehicle exhaust and electricity generation by power stations. In recent years, methods have been successfully developed and widely applied to monitor trace gases by measuring the vertical column density from space-borne satellites or ground-base stations. Recently, airborne observation for tropospheric trace gases column densities has become more and more popular, providing unique high spatial resolution observations that can be used for emission monitoring and for validation of satellite or ground-based observations. The Netherlands Organization for Applied Scientific Research (TNO) has developed Spectrolite, a compact, low cost hyperspectral imaging spectrometer based on the technological heritage fromTROPOsphericMonitoring Instrument (TROPOMI). In this project, an algorithm was developed to retrieve tropospheric NO2 vertical column densities from Spectrolite spectral observations during the AROMAPEX campaign in Berlin on 21 April, 2016. We applyDifferentialOptical Absorption Spectroscopy (DOAS) approach to obtain differential slant column densities (dSCDs) from spectral measurements. Afterwards, a look-up table which contains radiances output as function of various parameters was derived from radiative transfer model to compute air mass factors (AMFs). However, since we do not know the surface reflectance during the measurements, Landsat observations over a homogeneous region are utilized for vicarious calibration of radiances and this allows us to retrieve surface reflectance needed for AMF calculations. Subsequently, OMI data is used in order to determine tropospheric background and to correct for effects related to stratospheric NO2. Results of the dSCD retrieval show a pronounced NO2 plume over Berlin stretching out from West to East over the city. Several hot spots can be observed and related to emission sources on the ground. They also acquire a good correspondence with the dSCDs retrieved by other instruments (AirMap, SWING) at the same time during the campaign and therefore give much confidence for the future development at TNO. In order to obtain VCDs, AMFs were derived using complementary observations for some parameters (e.g. aerosol optical). Sensitivity studies were performed to assess the impact on the retrieval accuracy of other parameters. It can come to a conclusion that aerosol and NO2 vertical profiles are very essential to the VCD retrieval and need to be more well-defined in order to provide precise VCD results in absolutemagnitude.