GOME-2A retrievals of tropospheric NO₂ in different spectral ranges – influence of penetration depth

Abstract. In this study, we present a novel nitrogen dioxide (NO₂) differential optical absorption spectroscopy (DOAS) retrieval in the ultraviolet (UV) spectral range for observations from the Global Ozone Monitoring Instrument 2 on board EUMETSAT's MetOp-A (GOME-2A) satellite. We compare the results to those from an established NO₂ retrieval in the visible (vis) spectral range from the same instrument and investigate how differences between the two are linked to the NO₂ vertical profile shape in the troposphere.

As expected, radiative transfer calculations for satellite geometries show that the sensitivity close to the ground is higher in the vis than in the UV spectral range. Consequently, NO₂ slant column densities (SCDs) in the vis are usually higher than in the UV if the NO₂ is close to the surface. Therefore, these differences in NO₂ SCDs between the two spectral ranges contain information on the vertical distribution of NO₂ in the troposphere. We combine these results with radiative transfer calculations and simulated NO₂ fields from the TM5-MP chemistry transport model to evaluate the simulated NO₂ vertical distribution.

We investigate regions representative of both anthropogenic and biomass burning NO₂ pollution. Anthropogenic air pollution is mostly located in the boundary layer close to the surface, which is reflected by large differences between UV and vis SCDs of  ∼  60 %. Biomass burning NO₂ in contrast is often uplifted into elevated layers above the boundary layer. This is best seen in tropical Africa south of the Equator, where the biomass burning NO₂ is well observed in the UV, and the SCD difference between the two spectral ranges is only  ∼  36 %. In tropical Africa north of the Equator, however, the biomass burning NO₂ is located closer to the ground, reducing its visibility in the UV.

While not enabling a full retrieval of the vertical NO₂ profile shape in the troposphere, our results can help to constrain the vertical profile of NO₂ in the lower troposphere and, when analysed together with simulated NO₂ fields, can help to better interpret the model output.