Background Traj. Analysis

General            -            Specific Features            -            Background on Traj. Analysis

Coupling concentration data and air mass history can be a powerful approach for the investigation of potentially advected pollution over large geographical scales. Most of available approach use a gridded map and trajectory residence time to evaluate potential zones responsible for high concentrations measured at a receptor site.

-          Potential Source Contribution Function (PSCF)

PSCF specifically investigate the probability of an air parcel to be responsible for measured concentrations at the receptor site above a user-defined criteria. This criteria is generally a percentile value (75th or 90th):

Where nij is the total count of trajectory endpoints in the ijth cell, and mij the count of trajectory endpoints in the ijth cell associated to concentrations above the defined threshold.

-          Concentration-Weighted Trajectory (CWT)

CWT (Hsu et al., 2003) takes the entire dataset into account (i.e. no threshold), and, instead of adding “1” in grid cells, the m matrix consists in summing actual concentration values, which is eventually divided by the residence time matrix (n).

-          Concentration Field (CF)

CF is very similar to CWT, but it uses log values of concentrations. It can be useful when concentrations values fall within a broad range.

-          Multi-site merging

-          Cluster analysis

Cluster analysis consists in trying to regroup trajectories into more global pathways. The calculation is an iterative process, based on the minimisation of the total spatial variance. More information is available here.