Intercomparison of four airborne imaging DOAS systems for tropospheric NO2 mapping - the AROMAPEX campaign
We present an intercomparison study of four airborne imaging DOAS instruments, dedicated to the retrieval and high-resolution mapping of tropospheric nitrogen dioxide (NO2) vertical column densities (VCDs). The AROMAPEX campaign took place in Berlin, Germany, in April 2016 with the primary objective to test and intercompare the performance of experimental airborne imagers. The imaging DOAS instruments were operated simultaneously from two manned aircraft, performing synchronised ﬂights: APEX (VITO–BIRA-IASB) was operated from DLR’s DO228 D-CFFU aircraft at 6.2km in altitude, while AirMAP (IUP-Bremen), SWING (BIRA-IASB), and SBI (TNO–TU Delft–KNMI) were operated from the FUB Cessna 207T DEAFU at 3.1km. Two synchronised ﬂights took place on 21 April 2016. NO2 slant columns were retrieved by applying differential optical absorption spectroscopy (DOAS) in the visible wavelength region and converted to VCDs by the computation of appropriate air mass factors (AMFs). Finally, the NO2 VCDs were georeferenced and mapped at high spatial resolution. For the sake of harmonising the different data sets, efforts were made to agree on a common set of parameter settings, AMF look-up table, and gridding algorithm. The NO2 horizontal distribution, observed by the different DOAS imagers, shows very similar spatial patterns. The NO2 ﬁeld is dominated by two large plumes related to industrial compounds, crossing the city from west to east. The major highways A100 and A113 are also identiﬁed as line sources of NO2. Retrieved NO2 VCDs range between 1×1015 moleccm−2 upwind of the city and 20×1015 moleccm−2 in the dominant plume, with ameanof7.3±1.8×1015 moleccm−2 forthemorningﬂight and between 1 and 23×1015 moleccm−2 with a mean of 6.0±1.4×1015 moleccm−2 for the afternoon ﬂight. The mean NO2 VCD retrieval errors are in the range of 22% to 36%forallsensors.Thefourdatasetsareingoodagreement with Pearson correlation coefﬁcients better than 0.9, while the linear regression analyses show slopes close to unity and generally small intercepts.