Norsk institutt for luftforskning
The input data used for this modeling initiative is from the operational version of the ECMWF model. The ECMWF data has 60 model levels and was retrieved fully mass-consistently at ECMWF. The gridded data has 1x1 degree resolution globally and 0.5x0.5 degree resolution from 90W-20E and 40S-20N.
All calculations have been done with the particle dispersion model FLEXPART. For emission input for carbon monoxide, nitrogen oxides and sulfur dioxide, the EDGAR version 3.2 emission inventory for the year 2000 (fast track) on a 1 x 1 degree grid is used outside North America. Over most of North America, the inventory of Frost and McKeen (2004) is used. This inventory has a resolution of 4 km and also includes a list of point sources. Previous experience with the 1995 inventory has shown that Asian emissions of CO are underestimated (probably by as much as a factor of 2 or more) in the EDGAR inventory, while American CO emissions may be overestimated.
Backward simulations are done from along the flight track. Whenever an aircraft changes its position by more than 0.2 degrees, a backward simulation is initiated. Also, whenever it changes its altitude by 50 m below 300 m, 150 m below 1000 m, 200 m below 3000 m, or 400 m above, a new backward simulation is initiated. Every simulation consists of 40.000 particles released in the volume of air sampled. The backward simulations are done with full turbulence and convection parameterizations. Their theory is described by Seibert and Frank (Source-receptor matrix calculation with a Lagrangian particle dispersion model in backward mode, Atmos. Chem. Phys. 4, 51-63, 2004), and an application to aircraft measurements was presented by Stohl et al. (A backward modeling study of intercontinental pollution transport using aircraft measurements, J. Geophys. Res., 108, 4370, doi:10.1029/2002JD002862, 2003). Output is produced every 24 hours (particle positions plus so-called "residence times" accumulated over the 24 hours, see below). The "residence times" are stored on a 3-d grid with three levels (0-100 m, 100-3000 m, and above). The horizontal resolution of the output grid is 1 x 1 degree globally, with a 0.5 x 0.5 degree resolution nest over Europe.
Plots are shown for three plotting regions: Global, European, and a polar stereographic projection centered at the North Pole. You can always toggle between projection regions. Products are organized station-wise on a monthly basis. Once you have selected one of the products (see below), you can enter at a particular day and time of the month and, from there, you can navigate backward and forward in time. You can also change the product displayed for the active time by a simple mouse-click, or you can go back to the overview page to enter at a different time, or you can go back to the main page.
This is perhaps the most complex product and uses a technique described by Stohl et al. (A replacement for simple back trajectory calculations in the interpretation of atmospheric trace substance measurements, Atmos. Environ., 36, 4635-4648, 2002) to display 5-dimensional data. Every 24 hours, particle positions are assigned to one of 5 groups using a clustering algorithm. At the position of every cluster a circle is drawn with the circle's radius scaled with the number of particles the cluster represents (i.e., the fraction of sampled air for which it is representative). The color of the circle indicates the altitude, and the number on top gives the time backward in days. The retroplume's centroid is also displayed by a trajectory, but as plumes get complex back in time, the centroid may not be very representative of the true plume position. It takes some time to get acquainted, but once you know how it can be used, this product tells you where the air sampled was at what time and at what altitude, all in one plot. Also shown are time series of the mean altitude of the retroplume (and the five clusters, red circles in the time series, size again indicating the relative fraction of sampled air it represents), the fraction of particles in the boundary layer, and the fraction of particles in the stratosphere (2 pvu polewards from 30 degree, thermal tropopause in the tropics).
Column residence time
This product shows the vertically integrated residence time of the particles. It is recommended to inspect this product first, because it always shows the entire retroplume and gives the quickest impression where the air did come from (but without altitude information). Strictly, this is not a residence time, but the response an emission release of unit source strength would have at the receptor (i.e., at the measurement point) assuming no chemical transformations, deposition, etc. This response function is proportional to the residence time of all particles over a unit area (hence the name I have chosen), but involves scaling with the specific volume of air. The unit shown is nanoseconds times meters divided by kilograms. The numbers superimposed on the shading are the days back in time for the retroplume centroid (see above). They give an approximate indication of where the plume was at what time (but note that the centroids become poorly representative for the plume if the plume shape is too complex. Numbers typically become unrepresentative when they leave the main stream of particles (i.e., a well confined streamer in the column residence time) or if there are multiple such streamers.
You may notice that individual particle trajectories become visible as "lines" of low values of residence times. This is due to the logarithmic scale used and typically occurs far backward in time when particle trajectories have already diverged strongly and the 40.000 particles used are not many enough to fully characterize the retroplume's complexity. Also note that low values of residence times often can be found appearantly "downwind" of the measurement location. This normally is due to particles having circled the globe.
Footprint residence time
Same as above, but averaged over the lowest 150 m instead of vertically integrated. As anthropogenic emissions are mostly located at the surface, this gives an indication where emissions were likely taken up. The unit shown is nanoseconds divided by kilograms.
CO, NO2, SO2, PCB-153 source contributions
This is the product between the "residence time" (or response function, or source-receptor-relationship; there are different names in the literature) and the anthropogenic emission flux (in kilograms per square meter and second) taken from the inventories. The result is an emission contribution in ppb per square meter. If the emission contribution is integrated over the earth's surface, a "tracer" mixing ratio at the sampling location is obtained. It is also reported on the plot and, furthermore, Asian, American and European contributions are listed separately. These mixing ratios are quantitatively comparable to the measurements under the assumption that the species is conserved (no chemistry, no deposition).
The tracers reported for the European domains is different from that shown in the global and polar stereographic domains. This occurs because a 2 degree residence time output is used for the latter domains, whereas for the former a 0.5x0.5 degree domain is used that is limited to Europe. If emissions within a 2 degree grid cell are inhomogeneous, substantial differences can occur between the two resolutions, even though the residence times are exactly the same when averaged over the coarse grid cell. Also, the European domain does not cover North America and Asia, such that these values are zero (or close to zero for Asia) in the European plots.
Emission tracer time series
These plots show time series of the above tracers constructed from the backward simulations for the entire month, displayed seperately for total anthropogenic, Asian, North American, and European pollution.
Back to main menu