Observing the two- and three- dimensional distributions of trace gases with the Heidelberger Airborne Imaging DOAS Instrument (HAIDI) on board HALO during the phase II mission EMerGe

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The focus of this project is on the application of a new remote sensing 2D and 3D imaging DOAS instrument HAIDI to investigate the chemical composition of plumes emitted from major urban pollution centers (MPC) (megacities) in Europe and Asia. The activities will be part of the EMerGe mission for the German research airplane HALO, with an additional participation in the CAFE mission being currently under investigation. Airborne observations by HAIDI allow for measurements of key parameters for the characterization of these MPC outflow plumes, including trace gas distributions of NO2, HCHO, SO2, CHOCHO, BrO, IO, OClO, H2O, O4 and O3 as well as aerosol optical properties. The resulting 2D maps have a resolution better than 100 m with a width of ~10 km and thus cover large areas at once. Using tomographic techniques, 3D maps including information on the vertical distribution can be retrieved. HAIDI observes some of the most relevant gases to study the chemical composition and transformation of the MPC plumes in the outflow. We will study the size, distribution and spatial change of these plumes in dependence of topography and meteorology. Especially emission, transformation, and loss of NO2 as a key component will be investigated. In combination with further observed components on HALO, the chemical change of the trace gases and aerosol composition is of main interest to answer questions of the local, regional, and global influence of MPC emission plumes. HAIDI can also derive aerosol optical properties, which will be used in combination with in-situ measurements of aerosol characteristics on HALO to quantify the effect of these MPC plumes on the atmospheric radiation budget. Furthermore, the derived trace gas distributions are key quantities for the validation and improvement of model data within the EMerGe project. A further validation of satellite data is envisaged. With HAIDI, a system with 3D capability, allowing to derive full 3D trace gas (and aerosol optical property) distributions, will be deployed for the first time. For this purpose, HAIDI uses three imaging DOAS instruments pointing at different directions. In this project we will - based on our previous work - develop algorithms to combine these dataset in a radiative transfer model with tomographic approaches. Since this is the first time HAIDI operates on HALO, it has to undergo the certification and adaptation process within this project.