COMPARISONS OF PARTICULATE-EMISSIONS AND SMOKE IMPACTS FROM PRESETTLEMENT, FULL SUPPRESSION, AND PRESCRIBED NATURAL FIRE PERIOD IN THE SELWAY-BITTERROOT WILDERNESS

Total particulate matter (PM) emissions were estimated for recent fire s (1979-1990) and the presettlement period (prior to 1935) in the Selw ay-Bitterroot Wilderness (SBW) in Idaho and Montana. Recent period emi ssions were calculated by 10-day periods for surface fire and crown fi re based on estimates of percentage fuel consumption and emission fact ors applied to fuel models constructed for each of eight fire regime t ypes. Presettlement emissions were derived from estimates of historica l fire occurrence. The occurrence and duration of smoke episodes in a large adjoining valley were documented for a 30-year period (1960-1990 ). During the presettlement period, smoke emissions were 1.3 times gre ater than during the recent period. Dispersion modeling indicated that during presettlement, visibility averaged 25% less than during the re cent period. Valley smoke episodes ranged from 2 to 10 days in length and were 1.3 times more likely to occur during the presettlement perio d. Regression analysis indicated that a valley smoke event was likely above a 10-day PM threshold of 1,500 Mg. Characteristics of smoke epis odes were described for none, minor, moderate, and extreme smoke categ ories. Occurrence of smoke did not differ between periods of full fire suppression and prescribed natural fire management.