Based on the aerosol data available from 2000-2004,
source factors were generated from the PMF modeling. These PMF factor scores
then were used to weight backtrajectories generated with HYSPLIT (click on the chart to see
the full size figure).
The PMF weighted backtrajectories weight
the residence time over a given area during each backtrajectory by the PMF
factor score for the day when the backtrajectory started. These weights can
only be done for backtrajectories starting on days with IMPROVE data. The
weighting gives higher weights to regions the air passed over on days with
high impacts from a particular source (factor) and low weights to areas
associated with low impacts from that factor.
Difference maps: The PMF weighted
backtrajectory difference map is the PMF weighted residence time minus the
unweighted residence time. The difference then represents the enhancement
due to the PMF weighting. This has the effect of emphasizing geographical
areas contributing significantly to a PMF factor.
Ratio maps: The ratio maps give the ratio
of the PMF weighted residence time divided by the unweighted residence time.
So this is more of a multiplicative effect. Ratio maps can be misleading
when the residence time over an area is low.