Abstract
Abstract. A 14-month data set of MAX-DOAS (Multi-Axis Differential Optical Absorption Spectroscopy) tropospheric NO2 column observations in De Bilt, the Netherlands, has been compared with the regional air quality model Lotos-Euros. The model was run on a 7×7 km2 grid, the same resolution as the emission inventory used. A study was performed to assess the effect of clouds on the retrieval accuracy of the MAX-DOAS observations. Good agreement was found between modeled and measured tropospheric NO2 columns, with an average difference of less than 1% of the average tropospheric column (14.5 · 1015 molec cm−2). The comparisons show little cloud cover dependence after cloud corrections for which ceilometer data were used. Hourly differences between observations and model show a Gaussian behavior with a standard deviation (σ) of 5.5 · 1015 molec cm−2. For daily averages of tropospheric NO2 columns, a correlation of 0.72 was found for all observations, and 0.79 for cloud free conditions. The measured and modeled tropospheric NO2 columns have an almost identical distribution over the wind direction. A significant difference between model and measurements was found for the average weekly cycle, which shows a much stronger decrease during the weekend for the observations; for the diurnal cycle, the observed range is about twice as large as the modeled range. The results of the comparison demonstrate that averaged over a long time period, the tropospheric NO2 column observations are representative for a large spatial area despite the fact that they were obtained in an urban region. This makes the MAX-DOAS technique especially suitable for validation of satellite observations and air quality models in urban regions.
Highlights
Tropospheric reactive nitrogen (NOx = NO + NO2) has an important role in atmospheric chemistry
Averaging over time (1 h) reduces the difference in representation between model and observations. This implies that the spatial representativeness of MAX-DOAS tropospheric column observations is, even for an urban site, quite comparable to that of a regional air quality model
A data set of MAX-DOAS tropospheric NO2 column observations has been compared with the Lotos-Euros regional air quality model
Summary
Averaging over time (1 h) reduces the difference in representation between model and observations This implies that the spatial representativeness of MAX-DOAS tropospheric column observations is, even for an urban site, quite comparable to that of a regional air quality model. In the present work a 14-month data set of MAX-DOAS tropospheric NO2 column observations performed in De Bilt, the Netherlands, is used This measurement site can certainly not be characterized as rural, with several highways and local roads around it and only four kilometers from the city center of Utrecht (approximately 300 000 inhabitants). The comparison of MAX-DOAS observations with the Lotos-Euros air quality model consists of four parts: a selection of individual days is presented to illustrate typical agreements and differences without averaging; a comparison of all tropospheric NO2 columns in the data set, on an hourly. Basis, as well as daily averages; a comparison of temporal cycles (season, weekly, diurnal); and a comparison of tropospheric NO2 columns as a function of meteorological parameters: temperature, wind direction, wind speed, and boundary layer height
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