Alexis Merlaud

Measurements of Tropospheric NO2 in Romania Using a Zenith-Sky Mobile DOAS System and Comparisons with Satellite Observations

In this paper we present a new method for retrieving tropospheric NO2 Vertical Column Density (VCD) from zenith-sky Differential Optical Absorption Spectroscopy (DOAS) measurements using mobile observations. This method was used during three days in the summer of 2011 in Romania, being to our knowledge the first mobile DOAS measurements peformed in this country. The measurements were carried out over large and different areas using a mobile DOAS system installed in a car. We present here a step-by-step retrieval of tropospheric VCD using complementary observations from ground and space which take into account the stratospheric contribution, which is a step forward compared to other similar studies. The detailed error budget indicates that the typical uncertainty on the retrieved NO2tropospheric VCD is less than 25%. The resulting ground-based data set is compared to satellite measurements from the Ozone Monitoring Instrument (OMI) and the Global Ozone Monitoring Experiment-2 (GOME-2). For instance, on 18 July 2011, in an industrial area located at 47.03°N, 22.45°E, GOME-2 observes a tropospheric VCD value of (3.4 ± 1.9) × 1015 molec./cm2, while average mobile measurements in the same area give a value of (3.4 ± 0.7) × 1015 molec./cm2. On 22 August 2011, around Ploiesti city (44.99°N, 26.1°E), the tropospheric VCD observed by satellites is (3.3 ± 1.9) × 1015 molec./cm2 (GOME-2) and (3.2 ± 3.2) × 1015 molec./cm2 (OMI), while average mobile measurements give (3.8 ± 0.8) × 1015 molec./cm2. Average ground measurements over “clean areas”, on 18 July 2011, give (2.5 ± 0.6) × 1015 molec./cm2 while the satellite observes a value of (1.8 ± 1.3) × 1015 molec./cm2.

Modeling results of atmospheric dispersion of NO2 in an urban area using METI-LIS and comparison with coincident mobile DOAS measurements

Synergetic use of in–situ measurements, remote sensing observations and model simulations can provide valuable information about atmospheric chemistry and air quality. In this work we present for the first time a qualitative comparison between modeled NO2 concentrations at ground level using dispersion model METI–LIS and tropospheric NO2 columns obtained by mobile DOAS technique. Experimental and modeling results are presented for a Romanian city, Braila (45.26 ° N, 27.95 ° E). In–situ observations of NO2 and meteorological data from four ground stations belonging to the local environmental agency were used to predict the concentration of NO2 at ground level by atmospheric dispersion modeling on two days when mobile DOAS measurements were available. The mobile DOAS observations were carried out using a UV–VIS spectrometer mounted on board a car. The tropospheric Vertical Column Density (VCD) of NO2 is deduced from DOAS observations. The VCD was obtained using complementary ground and space observations. The correlation between model and DOAS observations is described by a correlation coefficient of 0.33. Also, model results based on averaged in–situ measurements for a period of 5 years (2008–2012) are used for an overview of the background NO2 evolution in time and space for the selected urban area.

Intercomparison of four airborne imaging DOAS systems fortropospheric NO 2 mapping – The AROMAPEX campaign

We present an intercomparison study of four airborne imaging DOAS instruments, dedicated to the retrieval and high-resolution mapping of tropospheric nitrogen dioxide (NO2) vertical column densities (VCDs). The AROMAPEX campaign took place in Berlin, Germany, in April 2016 with the primary objective to test and intercompare the performance of experimental airborne imagers. The imaging DOAS instruments were operated simultaneously from two manned aircraft, performing synchronised flights: APEX (VITO–BIRA-IASB) was operated from DLR’s DO228 D-CFFU aircraft at 6.2 km in altitude, while AirMAP (IUP-Bremen), SWING (BIRA-IASB), and SBI (TNO–TU Delft–KNMI) were operated from the FUB Cessna 207T DEAFU at 3.1 km. Two synchronised flights took place on 21 April 2016. NO2 slant columns were retrieved by applying differential optical absorption spectroscopy (DOAS) in the visible wavelength region and converted to VCDs by the computation of appropriate air mass factors (AMFs). Finally, the NO2 VCDs were georeferenced and mapped at high spatial resolution. For the sake of harmonising the different data sets, efforts were made to agree on a common set of parameter settings, AMF look-up table, and gridding algorithm. The NO2 horizontal distribution, observed by the different DOAS imagers, shows very similar spatial patterns. The NO2 field is dominated by two large plumes related to industrial compounds, crossing the city from west to east. The major highways A100 and A113 are also identified as line sources of NO2. Retrieved NO2 VCDs range between 1× 1015 molec cm−2 upwind of the city and 20× 1015 molec cm−2 in the dominant plume, with a mean of 7.3±1.8×1015 molec cm−2 for the morning flight and between 1 and 23× 1015 molec cm−2 with a mean of 6.0± 1.4× 1015 molec cm−2 for the afternoon flight. The mean NO2 VCD retrieval errors are in the range of 22 % to 36 % for all sensors. The four data sets are in good agreement with Pearson correlation coefficients better than 0.9, while the linear regression analyses show slopes close to unity and generally small intercepts. Published by Copernicus Publications on behalf of the European Geosciences Union. 212 F. Tack et al.: The AROMAPEX campaign

Can Aircraft-Based Remote-Sensing NO 2 Measurements Combined with High Resolution Model Data Improve NO 2 Exposure Estimates over Urban Areas?

As part of the STEREO-III BUMBA (Belgian Urban NO2 Monitoring Based on APEX hyperspectral data) project, we try to exploit the synergy between NO2 column measurements derived from the aircraft-based APEX hyperspectral imaging system and high-resolution model data from a combined land use regression-Gaussian plume model system for the complex city-port region of Antwerp (Belgium). The resulting model maps are then used to determine the NO2-exposure of the population of the region.