Regional nitrogen dioxide (NO2)
Access to regional
nitrogen dioxide (NO2) data
ascii data from GOME and SCIAMACHY are provided together with
trajectory analysis and
auxiliary information (e.g., Meteo-wind fields, lightening, DOAS measurements) to facilitate user applications.
and some examples of applications of the provided data are given in case studies
From the provided
trajectory analysis, the potential source regions of NO2 pollution and
the potential ground contact of elevated air masses can be deduced.
Backward trajectory calculation
are calculated for every high quality GOME/SCIAMACHY column in the region
of interest (alpine area from 5°E to 14°E and 44°N to 49°N).
The retrieved columns are defined to be of high quality if the following
criteria are fulfilled:
• The flag “fltrop” in the KNMI data has to indicate a meaningful tropospheric retrieval (fltrop=0; clear sky case), or
• The cloud fraction from FRESCO (“clfrac” in the KNMI data) exceeds a critical value of 0.75 (overcast case).
Because of the complicated interpretation of intermediate cloud cover, i.e. more than about 10-15% cloud fraction (decision criterion fltrop=-1) or less than 75% cloud fraction (clfrac≤0.75), such cases were abstained from trajectory calculation.
As the NO2 distribution within the GOME/SCIAMACHY columns is unknown, the backward trajectory arrival points cover the columns both horizontally and vertically in order to account for its whole tropospheric volume (Tab. 1). In the vertical, 11 height levels between 950 hPa and 450 hPa in 50 hPa steps are used. This vertical resolution allows distinguishing cases where boundary layer air has been transported into the middle troposphere from cases where no such transport is expected and thus the tropospheric NO2 amount can be assumed to linger in the lower layers. In this trajectory analysis, the possibility of air above 450 hPa which experienced ground contact is not covered. Further, the NO2 produced by lightning is not accounted for. If NO2 production by lightning has to be considered, reference to lightning archives (e.g., http://www.wetterzentrale.de/topkarten/tkbeoblar.htm) is suggested.
Tab. 1: Distribution of the backward trajectory arrival points
in the GOME/SCIAMACHY NO2 columns
|Number of trajectory
|In the vertical||11||11|
The trajectories are calculated with analysed wind fields with a six hour temporal and 1° x 1° geographical resolution provided by the model of the European Centre for Medium-Range Weather Forecast (ECMWF). Three dimensional kinematic 4-day backward trajectories are calculated with the software package “Lagranto” (Wernli and Davies, 1997). The arrival time point is chosen to be on 9:00 UTC.
Deriving potential source regions
From the trajectories,
the potential source region maps are derived corresponding to each height
level. The trajectories are plotted point wise in the horizontal projection
and indicate the geographical region which has contributed ground near
air to the tropospheric column of interest. As the first arrival height
of the trajectories (950 hPa) is often in the planetary boundary layer anyway,
only the trajectories from the levels above (900 – 450 hPa) are investigated
for their ground contact. The ground distance of the trajectories
is colour coded. Red marks the most ground near trajectory points (ground
distance < 50 hPa), green points have a ground distance 50-100 hPa and
blue have a ground distance of 100-150 hPa. Points of trajectories with
a higher ground distance are omitted.
For sake of clarity, only every 4 hours a point of the trajectory is plotted. When the density of points is high, a rather stagnant air mass is indicated. On the other hand, scattered points indicate high wind velocities. Occasionally, very high wind velocities lead to an apparent periodical point distribution when the trajectory starting points all exhibit similar trajectories. This artefact of periodicity should be kept in mind when interpreting such maps. Another artefact of displaying the trajectory information may occur when the trajectories of the different starting points stay rather coherent. Then the points associated to the different trajectories can align thread-like. Both artefacts could be eliminated by choosing to plot the trajectory points more than every 4 hours, on the expense to obscure the image for slower velocities. With the above explanation of artefacts the current trade-off should not lead to misinterpretations. The trajectory files from which the potential source regions have been constructed can be ordered at EMPA.
Satellite observations of atmospheric composition need to be validated with independent measurements in order to be usable for air pollution monitoring. Validation, in this sense, means not only comparing numbers of homogeneous quantity (NO2 tropospheric column for example) but also give a correct interpretation to the satellite measurements. Often, the discrepancy between the two independent measurements (satellite and ground-based) are due to the fact that they are measuring different air masses. The extension of ground pixels of GOME (320 km x 60 km) and SCIAMACHY (60 km x 30 km) cause the retrieval to perform a sort of average of the NO2 column present within the field of view (see Petritoli et al., J. Geophys. Res., 2004). Thus often the comparison between the space and the ground-based measurements give a clue on the horizontal distribution of the NO2 in the ground pixels. Here we provide NO2 PBL column measurements (PBL_NO2_C) performed in Bologna between March and September 2003. The PBL_NO2_C is obtained by comparing simultaneous total column measurements of NO2 obtained with two GASCOD (Gas Analyzer Sepctrometer Correlating Optical Differences) installed in Bologna (44.3N, 11.2 E, 50 m asl) and in the Mt. Cimone research station (44.2N, 10.7E, 2165 m asl). The NO2 slant column is obtained using DOAS (Differential Optical Absorption Spectroscopy) methodology and the PBL_NO2_C is retrieved by comparing the simultaneous measurements. A typical plot provided is similar to that shown in Fig. 2 where:
---->Black squares = Mt. Cimone slant column measurements
---->Red squares = Mt. Cimone slant column measurements interpolated at Bologna measurement time
---->Continous black line = cloud cover
---->Grey area = NO2 tropospheric slant column (PBL_NO2_C)
---->Continous bold black line = hourly average of Grey area
---->Black squares = in situ measurements of NO2 at the ground (right scales in ppbv and 1010molec/cm3)
---->Dashed coloured lines = PBL_NO2_C simulated with the PROMSAR multiple scattering model for a NO2 vertical layer extension up to 1500 m (lower right corner) and different constant values (colour scales)
---->Grey arrows = wind intensity and direction
Meteo information and in situ ground measurements have been kindly provided by the ARPA Emilia Romagna.
Figure 2: NO2 PBL column measured with DOAS methodoloy at Bologna.
For each avaialbe satellite overpass we provide (if avaiable) also links to maps of measurements or simulations of lightning occurence, cloud cover, PV, wind and T fields.
Few case studies on the interpretation of high NO2 levels observed by the satellite sensors will be shown. The goal is to provide some general examples on how to use auxiliary information such as back trajectories, cloud cover, lightings events and ground based measurements to give an interpretation to the NO2 maps from space that is mainly to find an explanation to the observed NO2 tropospheric column field. In fact high NO2 tropospheric column can be due either to local production, or to trasport from other places or to lightings. The events described in the Case Studies section will show how is possible sometimes to identify such phenomena. To access more detailed case studies visit the web page of the POLPO-ESA project.