Causes of Haze Assessment (COHA)

As shown in Figure 1, northerly and northwesterly flows from Canadian interior are dominant in the winter. During the summer, flows from south and southeast become important.

Figure 1. Normalized residence in February (left) and August (right)

Figure 2 suggests that flows are usually from south to southeast of the site during the 20% worst sulfate and OC days, while flows are generally from north and west during the 20% worst nitrate days. Difference and ratio maps in Figure 3 and 4 indicate that North and South Dakota and central and the major sources regions of sulfate and OC. Although flows from northeast US are not frequent, it may bring high sulfate to the site when flows come from the northeast.

Figure 2. Normalized residence in 20% worst visibility (top left), 20% worst sulfate (top right), 20% worst OC (bottom left) and 20% worst nitrate (bottom right) days.

In figure 3 below the difference plot shows areas to the east are more likely transport paths than other directions during the worst 20% sulfate days. The ratio plot on the right shows southeast flows bring in conditions favorable to high sulfate. Considering the density of sulfur emissions to the far southeast and directly east in North Dakota, most of the 20% worst sulfate days can be attributed to these sources.

Figure 3. Difference (left) and ratio (right) of normalized residence time in 20% worst sulfate days and all days

In both maps in figure 4 air transported from areas to the southeast are likely during the 20% worst OC days. These southeasterly transport paths coincide with a high frequency of fires as detected by satellites. A map of fires in July of 2004 shows a high density in the central states.

Figure 4. Difference (left) and ratio (right) of normalized residence time in 20% worst OC days and all days

For an in depth discussion on the role of meteorology at Fort Peck see the Meteorological Discussion section.

Last updated 7 June 2005