.. _docs-trajstat-introduction:
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Introduction
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Statistical analysis of air mass back trajectories combined with long-term ambient air pollution
measurements are useful tools for source identification. Using these methods, the geographic
information system (GIS) based software, TrajStat, was developed to view, query, and cluster
the trajectories and compute the potential source contribution function (PSCF) and concentration
weighted trajectory (CWT) analyses when measurement data are included.
The HYSPLIT model is used to calculate trajectories, which are loaded into the system as an external
process. The trajectory files with three-dimensional endpoint data could be converted to the ESRI
“PolylineZ” shape file format. In this type of shape file the x, y and z properties of each point
are defined by its longitude, latitude and air pressure along the trajectory. The trajectories can
be shown in various spatial patterns. For instance, using only the level (x, y) or height (z)
coordinates, each trajectory can be shown as a two-dimensional figure. When combined height with
longitude and latitude values, the three-dimensional trajectories can be plotted. The long-term
measurement data could be assigned to their corresponding trajectories. A query function was
developed to identify the trajectories to which a user can distinguish the polluted trajectories
with high measurement concentration from a large number of trajectories and then the pollutant
pathway could be roughly estimated. Euclidean distance or angle distance (Sirois and Bottenheim,
1995) can be selected as the cluster model. A reasonable maximum cluster number can be decided
through visual inspection and comparison of the mean-trajectory maps. The mean pollutant
concentration for each cluster can be computed using the cluster statistics function. Pollutant
pathways could then be associated with the high concentration clusters. After calculating the PSCF
and CWT value, an arbitrary weight function (Polissar et al., 1999) is applied to reduce the
uncertainty of cells with few endpoints. Then the potential source regions with high PSCF or CWT
value could be identified.
Publication:
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- Wang, Y.Q., Zhang, X.Y. and Draxler, R., 2009. TrajStat: GIS-based software that uses various trajectory statistical analysis methods to identify potential sources from long-term air pollution measurement data. Environmental Modelling & Software, 24: 938-939
Author:
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Yaqiang Wang
email: yaqiang.wang@gmail.com
Chinese Academy of Meteorological Sciences, CMA
Source code
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https://github.com/meteoinfo/TrajStat