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

Abstract

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.

High-resolution airborne imaging DOAS measurements of NO<sub>2</sub> above Bucharest during AROMAT

“…More detailed information about the MPIC and UGAL VCD retrieval algorithms used can be found in Wagner et al (2010) and Shaiganfar et al (2011) as well as Constantin et al (2013), respectively.…”

“…They can thus also be used for validation of satellite data and chemical models (Eskes et al, 2015). Mobile DOAS measurements are also performed, mostly from cars, offering the advantage of covering a large area at comparably low costs Ibrahim et al, 2010;Constantin et al, 2013;Shaiganfar et al, 2015). However, practical issues such as having to follow the pattern of roads or traffic jams limit the spatial extent of these measurements.…”

High-resolution mapping of the NO<sub>2</sub> spatial distribution over Belgian urban areas based on airborne APEX remote sensing

“…Nitrogen dioxide (NO 2 ) is an atmospheric trace gas and a key pollutant that attracts considerable attention due to several reasons: (1) NO 2 is a proxy for air pollution in general, as its abundance mostly coincides with a range of other pollutants; (2) recent studies by the World Health Organization (WHO, 2013) have shown that NO 2 exposure can have a direct health impact; (3) it is a precursor in the formation of aerosols (Chan et al, 2010) and tropospheric ozone (Crutzen, 1970) and therefore it contributes locally to radiative forcing (Solomon et al, 1999), through which it indirectly affects the climate system. In an urban environment, NO 2 mainly originates from anthropogenic sources such as the burning of fossil fuels, related to industrial activities and traffic.…”

Tropospheric nitrogen dioxide column retrieval from ground-based zenith–sky DOAS observations

“…Furthermore, some tropospheric contamination could still be observed, even at high SZA. Constantin et al (2013) reported similar issues with the chemically modified Langley plot method, when applied on observations from a polluted site. For the selection of the SZA interval, a trade-off was discussed between having a sufficiently large set of observations while avoiding tropospheric contamination at lower SZA.…”

“…Richter and Burrows, 2002;Beirle et al, 2010;Boersma et al, 2011;Valks et al, 2011;Bucsela et al, 2013;Hilboll et al, 2013). Other experiments have been published, presenting approaches to monitor tropospheric NO 2 from car (Johansson et al, 2009;Wagner et al, 2010;Constantin et al, 2013) and airborne platforms (Berg et al, 2012;Popp et al, 2012). …”