Abstract
Abstract. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) tropospheric NO2 column retrievals from four European measurement stations are compared to simulations from five regional air quality models which contribute to the European regional ensemble forecasts and reanalyses of the operational Copernicus Atmosphere Monitoring Service (CAMS). Compared to other observational data usually applied for regional model evaluation, MAX-DOAS data are closer to the regional model data in terms of horizontal and vertical resolution, and multiple measurements are available during daylight, so that, for example, diurnal cycles of trace gases can be investigated. In general, there is good agreement between simulated and retrieved NO2 column values for individual MAX-DOAS measurements with correlations between 35 % and 70 % for individual models and 45 % to 75 % for the ensemble median for tropospheric NO2 vertical column densities (VCDs), indicating that emissions, transport and tropospheric chemistry of NOx are on average well simulated. However, large differences are found for individual pollution plumes observed by MAX-DOAS. Most of the models overestimate seasonal cycles for the majority of MAX-DOAS sites investigated. At the urban stations, weekly cycles are reproduced well, but the decrease towards the weekend is underestimated and diurnal cycles are overall not well represented. In particular, simulated morning rush hour peaks are not confirmed by MAX-DOAS retrievals, and models fail to reproduce observed changes in diurnal cycles for weekdays versus weekends. The results of this study show that future model development needs to concentrate on improving representation of diurnal cycles and associated temporal scalings.
Highlights
Given the influence of NOx on air quality and climate through effects on radiation, it is of high environmental and scientific interest to accurately observe and simulate spatial distribution and time evolution of NO2 concentrations in the troposphere
Larger NO2 values inside individual pollution plumes are generally underestimated by the model ensemble, especially at Uccle and De Bilt. This is in agreement with Shaiganfar et al (2015), who compared car MAX-DOAS measurements and Ozone Monitoring Instrument (OMI) retrievals with a regional model (CHIMERE) and found that values inside emission plumes are systematically underestimated
Out of the four MAX-DOAS sites investigated, the magnitude of NO2 vertical column densities (VCDs) is lowest at the rural station (OHP), which is sometimes affected by nearby pollution plumes that show up in the time series
Summary
Given the influence of NOx on air quality and climate through effects on radiation, it is of high environmental and scientific interest to accurately observe and simulate spatial distribution and time evolution of NO2 concentrations in the troposphere. Using observations in low-elevation angles as measurement intensity and zenith measurements as reference intensity, the total amount of molecules of a certain species along the light path difference (zenith subtracted from non-zenith measurement), so-called differential slant column densities, can be determined using Beer–Lambert law. These can be inverted to tropospheric columns and lower altitude tropospheric profiles by radiative transfer modeling and optimal estimation techniques
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