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 fires (1979-1990) and the presettlement period (prior to 1935) in the Selway-Bitterroot Wilderness (SBW) in Idaho and Montana. Recent period emissions were calculated by 10-day periods for surface fire and crown fire based on estimates of percentage fuel consumption and emission factors applied to fuel models constructed for each of eight fire regime types. Presettlement emissions were derived from estimates of historical 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 greater than during the recent period. Dispersion modeling indicated that during presettlement, visibility averaged 25% less than during the recent period. Valley smoke episodes ranged from 2 to 10 days in length and were 1.3 times more likely to occur during the presettlement period. Regression analysis indicated that a valley smoke event was likely above a 10-day PM threshold of 1,500 Mg. Characteristics of smoke episodes were described for none, minor, moderate, and extreme smoke categories. Occurrence of smoke did not differ between periods of full fire suppression and prescribed natural fire management.