In Boundary Waters Canoe Area, the average PM2.5 mass concentration during the years of 1997 to 2002 is 4.8 µg/m3. The average total light extinction coefficient (Bext) is 40.4 Mm-1 (Visual Range ~ 96 Km; Deciview ~ 14.0). The average contributions of the major aerosol components to Boundary Waters Canoe Area haze are particulate sulfate 37.4%, nitrate 13.2%, organic matter (OMC) 14.9%, elemental carbon (light absorbing carbon, LAC) 4.5%, fine soil 0.9% and coarse mass (CM) 4.4%. During the 20% worst haze days, sulfate is the largest contributor to aerosol light extinction, with an average contribution of 47%. Nitrate also contributes about 25% in the 20% worst days.

Figure 3 suggests that the highest occurrence of the 20% worst days happened in January, July, August and November, in which ~30% of the sampling days are the 20% haziest days at Boundary Waters Canoe Area.

During the winter season, nitrate particle is the largest contributor to haze, and contributes ~45% to haze in January and December, respectively as shown in Figure 4. While, in the summer, sulfate dominates the light extinction and contributes more than 50% to haze during the 20% worst days. Organics from fire emissions may have a significant contribution to haze during the summer fire episodes.