1. The average PM2.5 mass concentration based on IMPROVE data available from September 2000 to December 2002 is 3.3 mg/m3
The highest occurrence of the 20% worst days happened in the winter time. Nitrate is the largest aerosol contributor to haze in the winter, with a contribution of ~50%. In the summer, OMC is the largest aerosol contributor to haze, and its contribution is about 50% in July and August.

2. Dominated westerly synoptic flows throughout the year, with frequent easterly flows in the fall and winter.

3. Figure 8. Regional nitrate emissions
Northern and Southeastern Idaho, Northern Utah, Northwestern Wyoming and Northeastern California are the possible important source regions of nitrate in the winter.

4. Meteorological Indicators
The Snake River Plain is a regional topographic depression, lower than surrounding mountainous terrain, and is subject to severe winter inversion conditions that can persist for days or weeks at a time. During these periods weather systems may move over the area without stirring the air below resulting in buildup of aerosols and other pollutants. The condition can persist until valley air is displaced by colder arctic air from the east that may in turn stagnate repeating the cycle.
Forest fire emissions are important in the Northwestern US during the summer, which may result in 20% worst days with organics as the biggest contributor to haze, especially in August. In the summer regional subsidence inversions during periods of high pressure and stagnation can result in aerosol buildup over periods of days. Subsidence inversion heights are typically at elevations of 2,000 to 3,000 m (6,000 to 10,000 ft). High regional aerosol concentrations may occur during summertime stagnation and subsidence inversion periods in conjunction with western wildland fires.

5. Causes of Haze in the Craters of the Moon Wilderness Area (CRMO1)
Regional nitrate and nitrate transported from Northern Utah, Northwestern Wyoming and Northeastern California in the winter, as well as organics emitted from forest fires in Idaho, Oregon, Washington and Northern UT in the summer are the major causes of haze at the Craters of the Moon Wilderness Area.

6. Preliminary Conceptual Model - Causes of Haze in the Joshua Tree Wilderness Area (JOSH1)
The Joshua Tree Wilderness located in the eastern extent of the Mohave Desert of southern California, with the eastern portions also within the Sonoran Desert Physiographic province. The IMPROVE site JOSH1 is located near the northwestern Wilderness Boundary at an elevation of 1,228 m (4,028 ft).

7. The average PM2.5 mass concentration during March 2000 to December 2002 is 5.6 mg/m3. The average total light extinction coefficient (Bext) is 42.0 Mm-1 (Visual Range ~ 93 Km; Deciview ~ 14.4).
The highest occurrence of the 20% worst days happened in August. Sulfate, CM and OMC each contributes ~ 20% to haze in August in the 20% worst days.
Nitrate is the largest aerosol contributor to haze except in the summer from June to August, during which sulfate is the largest aerosol contributor.

8. Main flow direction from the north
Main flow direction from the north. Eastern and southeastern flows become important in the summer.

9. South Coast Air Basin is the most important source region of nitrate
South Coast Air Basin is the most important source region of nitrate. Air transported from central US around northern Texas and Oklahoma resulted in high nitrate concentrations at JOSH1, although it did not happen very frequently.

10. August has the highest occurrence of 20% worst days, with sulfate, CM and OMC each contributing ~ 20% to haze in the 20% worst days. Figure 8 suggests that most air flows are from the coast of California and northern Mexico area in August, which may be the major source regions of sulfate and OMC. While windblown dust from the exposed desert surfaces is believed to be the major source of CM and fine soil.

11. Causes of Haze in the Joshua Tree Wilderness Area (JOSH1)
Nitrate from the California South Coast Air Basin to the west, and occasionally from the central US around northern Texas and Oklahoma are the major causes of haze at the Joshua Tree Wilderness Area in the cool season.
In the summer, sulfate and organics from the California coast area and northern Mexico, and dust from the surrounding desert terrain are all important contributors to haze in the 20% worst days.

12. Preliminary Conceptual Model - Causes of Haze in the Mount Rainer National Park (MORA1)
The Mount Rainer National Park IMPROVE site is located at an elevation of 427 m (1,401 ft) within the Nisqually River Valley, some 30 km (18.5 mi) west-southwest from the summit of Mount Rainer

13. The average PM2.5 mass concentration based on IMPROVE data available from 1997 to 2002 is 4.1 mg/m3. The average total light extinction coefficient (Bext) is 37.9 Mm-1 (Visual Range ~ 103 Km; Deciview ~ 13.3).
The major chemical components that contribute to haze at MORA1 are sulfate and OMC.
The highest occurrence of the 20% worst days happened in July and August. Sulfate is the largest aerosol contributor to haze, with a contribution of ~ 50%

14. Dominated northwesterly synoptic flows in the summer, with frequent south and southwesterly flows in other seasons.

15. Northwestern Washington is the major source region of sulfate.

16. Causes of Haze in the Mount Rainer National Park (MORA1)
Sulfate transported from the northwestern Washington is the major cause of haze at the Mount Rainer National Park.