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

Abstract. 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 5.7x10¹⁶ 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 5.0x10¹⁶ molec cm^-2. The results are relevant for the retrieval of tropospheric NO₂ columns from space-borne instruments in cloudy situations.