Near-surface and path-averaged mixing ratios of NO<sub>2</sub> derived from car DOAS zenith-sky and tower DOAS off-axis measurements in Vienna: a case study

Abstract

Abstract. Nitrogen dioxide (NO2), produced as a result of fossil fuel combustion, biomass burning, lightning, and soil emissions, is a key urban and rural tropospheric pollutant. In this case study, ground-based remote sensing has been coupled with the in situ network in Vienna, Austria, to investigate NO2 distributions in the planetary boundary layer. Near-surface and path-averaged NO2 mixing ratios within the metropolitan area of Vienna are estimated from car DOAS (differential optical absorption spectroscopy) zenith-sky and tower DOAS horizon observations. The latter configuration is innovative in the sense that it obtains horizontal measurements at more than a hundred different azimuthal angles – within a 360∘ rotation taking less than half an hour. Spectral measurements were made with a DOAS instrument on nine days in April, September, October, and November 2015 in the zenith-sky mode and on five days in April and May 2016 in the off-axis mode. The analysis of tropospheric NO2 columns from the car measurements and O4 normalized NO2 path averages from the tower observations provide interesting insights into the spatial and temporal NO2 distribution over Vienna. Integrated column amounts of NO2 from both DOAS-type measurements are converted into mixing ratios by different methods. The estimation of near-surface NO2 mixing ratios from car DOAS tropospheric NO2 vertical columns is based on a linear regression analysis including mixing height and other meteorological parameters that affect the dilution and reactivity in the planetary boundary layer – a new approach for such conversion. Path-averaged NO2 mixing ratios are calculated from tower DOAS NO2 slant column densities by taking into account topography and geometry. Overall, lap averages of near-surface NO2 mixing ratios obtained from car DOAS zenith-sky measurements, around a circuit in Vienna, are in the range of 3.8 to 26.1 ppb and in good agreement with values obtained from in situ NO2 measurements for days with wind from the southeast. Path-averaged NO2 mixing ratios at 160 m above the ground as derived from the tower DOAS measurements are between 2.5 and 9 ppb on two selected days with different wind conditions and pollution levels and show similar spatial distribution as seen in the car DOAS zenith-sky observations. We conclude that the application of the two methods to obtain near-surface and path-averaged NO2 mixing ratios is promising for this case study.

Validation of tropospheric NO<sub>2</sub> column measurements of GOME-2A and OMI using MAX-DOAS and direct sun network observations

“…Several studies have discussed in detail the impact of algorithmic differences on the NO 2 column uncertainty, which can reach 42 %, mainly due to tropospheric AMF uncertainties (Lorente et al, 2017). The underestimation of the NO 2 satellite products identified here at a large number of stations confirms what was obtained in previous validation exercises using fewer sites and different satellite products (Celarier et al, 2008;Brinksma et al, 2008;Vlemmix et al, 2010;Irie et al, 2008aIrie et al, , 2012Lin et al, 2014;Halla et al, 2011;Shaiganfar et al, 2011;Ma et al, 2013;Kanaya et al, 2014;Wang et al, 2017b;Mendolia et al, 2013;Tzortziou et al, 2014;Lamsal et al, 2014;Drosoglou et al, 2017;Herman et al, 2019;Judd et al, 2019;Compernolle et al, 2020). These studies generally reported small negative or positive biases over rural (unpolluted) measurement sites and stronger (systematic) negative biases over urban polluted sites.…”

Validation of TROPOMI tropospheric NO<sub>2</sub> columns using dual-scan MAX-DOAS measurements in Uccle, Brussels

“…The in-situ telemetric air quality network (Bruxelles Environne ment/Leefmilieu Brussel) of the Brussels region is used for verifying the retrieved near-surface NO2 VMR. Previous studies (e.g., Kramer et al, 2008;Schreier et al, 2019a)) have compared NO2 MAX-DOAS measurements with in-situ concentrations, concluding to a considerable underestimation of NO2…”

Observations of Atmospheric NO2 Using a New Low-Cost MAX-DOAS System

“…Car-DOAS observations have been used for quantification of total emissions from urban or industrial emission sources [25,26], comparisons with satellite observations [27][28][29], validation of airborne DOAS measurements [30,31] and for comparisons with model simulations [29,32]. Also, car-DOAS measurements have recently been used for comparison with in situ measurements [33].…”