| Tropospheric NO2 is hazardous to human health and can lead to tropospheric ozone
formation, eutrophication of ecosystems and acid rain production. It is therefore very
important to accurately observe and simulate tropospheric NO2 on a regional and global
scale.
In the present study, MAX-DOAS tropospheric NO2 column retrievals from three European
measurement stations are applied for validation of a regional model ensemble. In general,
there is a good agreement between simulated and retrieved NO2 column values for
individual MAX-DOAS measurements, indicating that the model ensemble does well
represent the emission and tropospheric chemistry of NOx. However, the model
ensemble tends to overestimate low and underestimate high tropospheric NO2 column
values, respectively. Pollution transport towards the stations is on average well
represented by the models. However, large differences can be found for individual
pollution plumes. Seasonal cycles are overestimated by the model ensemble, which
could point to problems in simulating photochemistry. While weekly cycles are
reproduced well by the models, model performance is rather poor for diurnal cycles. In
particular, simulated morning rush hour peaks are not confirmed by MAX-DOAS
retrievals, which may result from inappropriate hourly scaling of NOx emissions,
possibly combined with errors in chemistry. Our results demonstrate that a large
number of validation points are available from MAX-DOAS data, which should
therefore be used more extensively in future regional air quality modelling studies. |