Comparisons of ground-based tropospheric NO<sub>2</sub> MAX-DOAS measurements to satellite observations with the aid of an air quality model over the Thessaloniki area, Greece

Abstract

Abstract. One of the main issues arising from the comparison of ground-based and satellite measurements is the difference in spatial representativeness, which for locations with inhomogeneous spatial distribution of pollutants may lead to significant differences between the two data sets. In order to investigate the spatial variability of tropospheric NO 2 within a sub-satellite pixel, a campaign which lasted for about 6 months was held in the greater area of Thessaloniki, Greece. Three multi-axial differential optical absorption spectroscopy (MAX-DOAS) systems performed measurements of tropospheric NO 2 columns at different sites representative of urban, suburban and rural conditions. The direct comparison of these ground-based measurements with corresponding products from the Ozone Monitoring Instrument onboard NASA's Aura satellite (OMI/Aura) showed good agreement over the rural and suburban areas, while the comparison with the Global Ozone Monitoring Experiment-2 (GOME-2) onboard EUMETSAT's Meteorological Operational satellites' (MetOp-A and MetOp-B) observations is good only over the rural area. GOME-2A and GOME-2B sensors show an average underestimation of tropospheric NO 2 over the urban area of about 10.51 ± 8.32 × 10 15 and 10.21 ± 8.87 × 10 15 molecules cm −2 , respectively.The mean difference between groundbased and OMI observations is significantly lower (6.60 ± 5.71 × 10 15 molecules cm −2 ). The differences found in the comparisons of MAX-DOAS data with the different satellite sensors can be attributed to the higher spatial resolution of OMI, as well as the different overpass times and NO 2 retrieval algorithms of the satellites. OMI data were adjusted using factors calculated by an air quality modeling tool, consisting of the Weather Research and Forecasting (WRF) mesoscale meteorological model and the Comprehensive Air Quality Model with Extensions (CAMx) multiscale photochemical transport model. This approach resulted in significant improvement of the comparisons over the urban monitoring site. The average difference of OMI observations from MAX-DOAS measurements was reduced to −1.68 ± 5.01 × 10 15 molecules cm −2 .

An improved total and tropospheric NO<sub>2</sub> column retrieval for GOME-2

“…Several validation studies show a significant underestimation of tropospheric trace gases, such as NO 2 , from satellite observations over regions with strong spatial gradients in tropospheric pollution (e.g. Celarier et al, 2008;Kramer et al, 2008;Chen et al, 2009;Irie et al, 2012;Ma et al, 2013;Wu et al, 2013;Kanaya et al, 2014;Wang et al, 2017;Drosoglou et al, 2017Drosoglou et al, , 2018. Other possible explanations include the uncertainties in the applied satellite retrieval assumptions, such as the choices of surface albedo, a priori NO 2 profiles, or cloud and aerosol treatment (Boersma et al, 2004Leitão et al, 2010;Heckel et al, 2011;Lin et al, 2014Lin et al, , 2015.…”

MAX-DOAS NO<sub>2</sub> observations over Guangzhou, China; ground-based and satellite comparisons

“…Currently, there are three MAX-DOAS systems routinely operating in the greater area of Thessaloniki, Greece. Their operation and their capability in retrieving the tropospheric NO 2 have been tested successfully under different air pollution conditions and NO 2 loadings (Drosoglou et al, 2017).…”

“…The tropospheric NO 2 VCDs retrieved from the groundbased radiance spectra measured at 15 and 30 • elevation viewing angles and at all available azimuth viewing angles were used in the comparison with corresponding space-borne observations. The system had been proven to be able to retrieve NO 2 with a spectral fitting residual of the order of 10 −3 , typical residual values of mini MAX-DOAS systems (Drosoglou et al, 2017). The value of 1 × 10 −2 has been used as a threshold to filter out disturbed retrievals under variable conditions, such as when fast moving clouds of mist emerge from the nearby river in the Guangzhou area.…”

“…3. The method used in this study to derive the vertical column density (VCD) of NO 2 is similar to the one applied in Drosoglou et al (2017). For the conversion of dSCD into VCD a look-up table (LUT) of differential air mass factors (dAMFs) was constructed by simulations performed with the uvspec radiative transfer model (RTM), libRadtran version 1.7 (Mayer and Kylling, 2005), using a pseudo-spherical discrete ordinates radiative transfer method (Buras et al, 2011).…”

“…Moreover, Kramer et al (2008) have calculated a negative difference of OMI tropospheric NO 2 columns from CMAX-DOAS corresponding data of 1.78 × 10 15 molecules cm −2 over Leicester, UK. Drosoglou et al (2017) have calculated an underestimation of 6.60 ± 5.71 × 10 15 molecules cm −2 for OMI and of about 10 ± 8 × 10 15 molecules cm −2 for both GOME-2A and GOME-2B tropospheric NO 2 observations over Thessaloniki, Greece. Considering that the NO 2 is distributed mainly in the planetary boundary layer (PBL), wellestablished ground-based measurements of tropospheric vertical columns and profiles of NO 2 are essential for the validation and, subsequently, the improvement of satellite retrievals.…”