Airborne DOAS observations of tropospheric NO2 using an UltraLight Trike and flux calculation

In this paper we present airborne DOAS observations of tropospheric NO2 using an Ultralight Trike (ULT) and associated flux calculation. The instrument onboard the ULT was developed for measuring the tropospheric NO2 Vertical Column Density (VCD). Measurements were performed for several days during 2011-2014, in a region SE of Romania, over the cities of Galati (45.43°N, 28.03°E) and Braila (45.26°N, 27.95°E). Measurements of the NO2 column in the same area were performed using car-DOAS observations. The correlation between the tropospheric NO2 VCD from airborne and mobile ground-based DOAS observations was used to validate the airborne observations. A specific AMF for each case was calculated using the radiative transfer model (RTM) UVspec/DISORT. We present also a comparison between SCDstrato derived from DOMINO (Dutch OMI NO2) and the SCDstrato obtained from ground and airborne measurements. Due to the mobility and flexibility of the ULT flights, this aerial platform provides a promising tool for satellite validation, especially for space observations by high resolution sensors such as the future TROPOMI instrument. A key added value of the ULT-DOAS, illustrated in this work, is the capacity to investigate the spatial variability of NO2 inside the horizontal extent of satellite pixels, e.g. above plant exhaust plumes.