Norsk institutt for luftforskning
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Norsk institutt for luftforskning |
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Two meteorological input data sets have been used here: 1x1 degree data on 26 pressure levels from the Global Forecast System (GFS) model of the National Center for Environmental Prediction (NCEP), and data from the ECMWF model. The ECMWF data has 91 model levels and was retrieved fully mass-consistently from the T799 spherical harmonics data at ECMWF. The gridded data has 1x1 degree resolution globally, but two 0.36x0.36 degree nests were used in the regions 108W-27W, 9N-54N, and 27W-54E, 36N-81N.
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 0.18 degrees in either longitude or latitude, a backward simulation is initiated. Also, whenever it changes its altitude by 8 hPa below 850 hPa, 12 hPa between 850 hPa and 700 hPa, or 15 hPa above, a new backward simulation is initiated. For surface stations, a constant time interval of 3 hours is chosen for model simulations.
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 emission sensitivities accumulated over the 24 hours, see below). The emission sensitivities 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 0.25 x 0.25 degree resolution nest over the area of most interest.
Plots are shown for three plotting regions: Global, regional, and local, with the regional and local plotting regions adjusted to the area of main interest. You can always toggle between plotting 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, you can toggle between ECMWF and GFS data displays, or you can go back to the overview page to enter at a different time, or you can go back to the main page.
Retroplume summary
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).
Emission sensitivity integrated over the entire atmospheric column
This product shows the vertically integrated emission sensitivity, which is proportional to the residence time of the particles over a unit area. 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). The emission sensitivity is based on the assumption that transport alone occurs; it does not account for any removal processes, such as wet or dry deposition.
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 of high values in the column emission sensitivity) or if there are multiple such streams.
You may notice that individual particle trajectories become visible as "lines" of low values of the emission sensitivity. 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 the emission sensitivity often can be found appearantly "downwind" of the measurement location. This normally is due to particles having circled the globe.
Footprint emission sensitivity
Same as above, but averaged over the lowest 100 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, and SO2 source contributions
This is the product between the emission sensitivity 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, contributions from different continents are listed separately. These mixing ratios are quantitatively comparable to the measurements under the assumption that the species is conserved (no chemistry, no deposition).
Importantly, source contributions are integrated either for the global output domain at 1 degree resolution, or over the nested regional domain at 0.25 degree resolution. Emissions from outside the domain are not accounted for in the latter. Also, the higher resolution of the nest is not accounted for in the global sum, such that occasionally contributions from the nested region can be higher than for the entire globe.
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.
Biomass burning CO contributions
Hot spot locations are obtained daily from measurements made with MODIS onboard the Aqua and Terra satellites and processed using the MOD14 (MYD14) algorithm described by Louis Giglio (MODIS Collection 4 Active Fire Product User's Guide).
The data are downloaded from a server at the University of Maryland in ASCII format and only those hot spot detections with a confidence of 75% or greater are used.
Fire hot spots are shown also in the column-integrated and footprint emission sensitivity maps.
They are compared to a landuse inventory with 1-km resolution and if they are on forested land, they are marked as red dots (overlaid over larger black dots), for all other landuses as black dots only.
The hot spots are shown on the footprint emission sensitivity map only in grid cells where the DAILY footprint emission sensitivity on the very day of the hot spot identification is above 0.005 ps/kg.
They are shown on the column-integrated emission sensitivity map only in grid cells where the DAILY column-integrated emission sensitivity on the very day of the hot spot identification is above 8 ns m /kg.
The emissions are estimated assuming an area burned of 180 ha/fire detection.
They depend on a parameterization based on biomass available to burning, fraction actually burned, and emission factors, all dependent on landuse.