Figure 2 Average contributions of major aerosol chemical components to light extinction in 20% best, middle 60% and 20% worst days (Based on data available in 1997-2002)

As Figure 2 indicates, the average aerosol light extinction coefficient (Bext – Rayleigh Scattering (10 Mm^-1)) during the 20% worst days is 49.8 Mm^-1, which is about 4.5 times of the value of 11.1 Mm^-1 during the 20% best days and 2.1 times of the value of 23.9 Mm^-1 during the middle 60% days. Sulfate and nitrate are two of the largest contributors to aerosol light extinction during the 20% worst days, and the contributions of sulfate and nitrate to aerosol light extinction are  ~ 37.2% and 32.5% in worst days.

Figure 3 suggests that the highest occurrence of the 20% worst days happened in January, in which ~ 40% of the sampling days are the 20% haziest days at Medicine Lake. As shown in Figure 4, during the winter, early spring and later fall, both nitrate and sulfate contribute ~ 30 - 45% to regional haze in the 20% worst days. While in the summer, later spring and early fall, sulfate, OMC, and CM are important contributors to haze.

Figure 3 Percentage of sampling days that are 20% worst days in each month (Based on data available in 1997-2002)

Figure 4 Average contributions of major aerosol chemical components to light extinction during 20% worst days in each month (Based on data available in 1997-2002)