1. Worst 20% Hazes Across the Country Based on IMPROVE Speciation Data by Marc Pitchford August 2001

2. What Causes Worst Day & When do the Occur Southeastern US –Sulfate dominates worst case days –Most worst case days are in the late summer, rare to have any during cold periods Northeastern US –Sulfates dominate with occasional high nitrate –Many worst days are in the summer, but every season has some worst days

5. What Causes Worst Day & When do the Occur Northern Tier –Sulfates contribute any time of year –Nitrates contribute primarily in the Winter –Organics are occasionally important in the Summer

8. What Causes Worst Day & When do the Occur Interior Western US –Sulfates, Organics, & Coarse Mass share the blame on most worst case days –Apparent fire emissions (very high organic & elemental carbon) are sporadically responsible for unusually high haze levels –Apparent dust storms (very high coarse mass & fine soil) are occasionally responsible for unusually high haze levels

13. What Causes Worst Day & When do the Occur California –Nitrate, sulfate, & organic species contribute most on worst days –Worst days are distributed throughout the year

16. What Causes Worst Day & When do the Occur Alaska –Sulfate is important especially in the cold seasons (arctic haze from Eurasia?) –Organic is important especially in the warm seasons (apparently from forest fires)