MAX-DOAS observations of spatial variability of NO2 in Athens

Nitrogen dioxide is a key pollutant in the troposphere. Most of its sources are anthropogenic and linked to the burning of fossil fuels, but wildfires, lightning and soil emissions also contribute to the overall NOx (NO + NO₂) loading.

Using passive remote sensing in the UV and visible spectral range, NO₂ columns can be retrieved with both ground-based MAX-DOAS and satellite instruments. As a result of the multitude of sources and the short atmospheric lifetime of NOx, spatial and temporal variability of NO₂ in the troposphere is large. This variability complicates the interpretation of satellite measurements, which integrate over relatively large areas, and also has to be considered in satellite validation.

The University of Bremen, in collaboration with the National Observatory of Athens (NOA), has been operating a MAX-DOAS instrument at the NOA premises on Penteli Hill in the northeast of Athens since 2012. The measurement location allows for several viewing directions over the city of Athens and towards less polluted background regions. The measurements are, therefore, ideal to investigate an inhomogeneous NO₂ distribution and its impact on satellite validation.

Using the data from the MAX-DOAS instrument, the temporal and spatial variability of NO₂ is evaluated, and simple parametrisations are developed and tested to characterise the degree of variability. Comparing the results from variations in time and those in azimuth direction, interesting similarities and differences are found. The derived inhomogeneity parameters can be used to classify situations where the MAX-DOAS data is particularly well suited for satellite validation and which days should be excluded from such evaluations.