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| Boundary Waters
Canoe Area Wilderness
Area |
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| The role of meteorology in the causes of haze |
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Figure 5 indicates that the air transport patterns are
quite different for 20% best and 20% worst days. During the 20% best days,
air usually comes from southeast of the site. While during the 20% worst
haze days, air most frequently comes from north. |
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Figure 5. Difference (left) and ratio (right)
of normalized residence time in 20% worst nitrate days and all days during
2000-2002 (Reddish means that more than 20% of the time when air passed
through the area, the site was in 20% worst nitrate days) |
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Figure 6 indicates that NO3 generally comes from local
and regional sources in Minnesota as shown in
Figure 7. The
emissions of NOx from the local and regional sources (need specific source
type information…), together with cold temperatures, which favors the
partition of nitric acid to the particle phase, and the frequently occurring
temperature inversion in the winter season are most likely to be the major
causes of haze at the Boundary Waters Canoe Area Wilderness Area in the
Winter.
During the summer, there is a higher frequency of southerly flow,
which passes through southern Minnesota, eastern Nebraska and Kansas, Iowa,
Missouri and western Illinois to the site, and result in high sulfate days.
Subsidence inversions associated with buildup and stagnation of synoptic
high pressure ridges are most likely to occur during the summer. They tend
to cover a large area and are regional in nature, and may persist for
periods of days, which helps to build up sulfate and results in worst haze
days. |
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| Figure 6. Difference (left) and ratio (right) of
normalized residence time in 20% worst sulfate days and all days in
2000-2002 (possible important source regions are shown up as blue in the
maps) |
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| For an in depth discussion on the role of meteorology
at Boundary Waters see the
Meteorological
Discussion section. HYSPLIT backtrajectories for Boundary Waters can
be viewed in the
backtrajectory gallery page. |
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