# WP4

For exposure analysis, the NO_{2} surface concentration, which is the value regulated by the European legislation, must be determined instead of the integrated NO_{2} column measured by the APEX sensor. So APEX observations must be transformed into equivalent NO_{2} concentrations maps representative for the ground layer. To this aim, the information concerning the **vertical distribution of NO _{2} concentrations in the total column** (up to a height of approximately 5 km) is necessary.

To achieve this transformation, the CHIMERE model was run based on emission data from EMAP, meteorological data and land use data. The CHIMERE output is a result of several nested runs with different spatial resolutions. The final CHIMERE output contains NO2 data, calculated from April 1st until June 30th with a spatial grid resolution of 0.05°. A vertical discretisation of 8 vertical layers was used based on a pressure interval between 1010 and 500 hPa, the latter corresponding with a height of 6.5 km. The thickness of the layers increases quasi-exponentially with altitude in order to provide a better vertical resolution in the lower model levels.

VITO-RMA nested its AURORA model in the output of the CHIMERE model (7 x 11 km), thus a similar approach using 2 nested models as for the CHIMERE model runs. The model nested into the CHIMERE model is a 4x4 km² horizontal resolution model with 77 x 71 compartments and 18 model layers for which the layer thickness varies from 20 m at the surface to 500 m at the top. The results of this 4x4 km² resolution model were then used as boundary conditions for models centred above Antwerp and Brussels with a spatial resolution of 1x1 km².

For the process of transforming APEX column totals, a constant vertical discretisation is more appropriate. The AURORA model was therefore rerun with a vertical discretisation consisting of 75 layers. Of these 75 layers the lower 40 layers have a constant layer thickness of 50 m and the upper 35 layers a thickness of 100 m resulting in a total model height of 5500 m. These results were then used to calculate the APEX surface concentrations proportional to the total column based on the local AURORA profile via the following formula:

The final result of this work package is are the NO2 surface concentrations derived from APEX hyperspectral data for validation of the NO2 exposure model.