FOPE1 (Fort Peck, MT, Lat. 48.5, Long. -105.3)
Based on the data available from 6/2002-12/2003, sulfate and nitrate in the cool season, and OMC and sulfate in the warm season are the major causes of haze at the Fort Peck.
In Fort Peck, the average PM2.5 mass concentration during 6/2002-12/2003 is 3.8 mg/m3, and the average total light extinction coefficient (Bext) is 34 Mm-1 (Visual Range ~ 115 Km; Deciview ~ 12). Sulfate is the largest contributor to haze, with an average contribution of 26%.
Figure 1. Average contributions of major aerosol chemical components to light extinction
Based on the data in 2003, the average light extinctions in the 20% best, middle 60% and 20% worst haze days are calculated and shown in Figure 2. The light extinction coefficient of 55 Mm-1 in the 20% worst days is about 1.7 and 2.8 times of the values in the middle 60% and 20% best days. Sulfate is the largest contributor to haze in the 20% worst days in 2003, with an average contribution of ~33%. Figure 3 suggests that, in 2003, the highest occurrence of the 20% worst days happened in February and August, in which ~50-60% of the sampling days are the 20% haziest days at Fort Peck. Sulfate and nitrate contribute in average ~37% and 32% to light extinction in the 20% worst days during February 2003. In August, OMC is the largest contributor to light extinction in 20% worst days, with a contribution of 32%. CM and sulfate each contributes ~20% in the 20% worst days of August.
Figure 2 Average contributions of major aerosol chemical components to light extinction in 20% best, middle 60% and 20% worst days
Figure 3 Percentage of sampling days that are 20% worst days in each month
Figure 4 Average contributions of major aerosol chemical components to light extinction during 20% worst days in each month