High-resolution NO 2 remote sensing from the Airborne Prism EXperiment (APEX) imaging spectrometer
We present and evaluate the retrieval of high spatial resolution maps of NO 2 vertical column densities (VCD) from the Airborne Prism EXperiment (APEX) imaging spectrometer. APEX is a novel instrument providing airborne measurements of unique spectral and spatial resolution and coverage as well as high signal stability. In this study, we use spectrometer data acquired over Zurich, Switzerland, in the morning and late afternoon during a flight campaign on a cloud-free summer day in June 2010. NO 2 VCD are derived with a two-step approach usually applied to satellite NO 2 retrievals, i.e. a DOAS analysis followed by air mass factor calculations based on radiative transfer computations. Our analysis demonstrates that APEX is clearly sensitive to NO 2 VCD above typical European tropospheric background abundances (>1 × 10 15 molec cm −2). The two-dimensional maps of NO 2 VCD reveal a very convincing spatial distribution with strong gradients around major NO x sources (e.g. Zurich airport, waste incinerator, motorways) and low NO 2 in remote areas. The morning overflights resulted in generally higher NO 2 VCD and a more distinct pattern than the afternoon overflights which can be attributed to the meteorological conditions prevailing during that day with stronger winds and hence larger dilution in the afternoon. The remotely sensed NO 2 VCD are also in reasonably good agreement with ground-based in-situ measurements from air quality networks considering the limitations of comparing column integrals with point measurements. Airborne NO 2 remote sensing using APEX will be valuable to detect NO 2 emission sources, to provide input for NO 2 emission modelling, and to establish links between in-situ measurements, air quality models, and satellite NO 2 products.