SAOZ measurements of NO2 at Aberystwyth

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.