Measurements of tropospheric NO2 with an airborne multi-axis DOAS instrument

The AMAXDOAS instrument is an airborne multi-axis DOAS instrument covering the spectral range from 300 to 600 nm. During one flight of the SCIAVALUE campaign on 19 March 2003, the AMAXDOAS onboard the DLR Falcon detected tropospheric NO2 over Europe under both cloudy and cloud free conditions. By combining the measurements in nadir and zenith direction, and analysing the spectra in the UV and the visible spectral region, information was derived on where the bulk of the observed NO2 was located. Vertical columns of up to 5.7×1016 molec cm−2 were observed close to Frankfurt, with a distribution peaking in the boundary layer and in good agreement with surface measurements. On several occasions, strong tropospheric NO2 signals were also detected when flying above clouds. The ratio of zenith and nadir measurements indicates that the NO2 observed was located within the cloud, and assuming the same profile as for the cloud free situation the NO2 vertical column was estimated to be 5.0 × 1016 molec cm−2. The results are relevant for the retrieval of tropospheric NO2 columns from space-borne instruments in cloudy situations.