Better estimates of particulate matter from fire

By Kevin Robertson, PhD, Fire Ecology Program Director

Federal and state air quality agencies, which indirectly regulate prescribed burning, model the amount of aerial emissions from wildland fires by multiplying estimates of the area burned, fuel consumed per unit area, and the amount of emission per unit fuel consumed, called the "emission factor". This estimate is particularly important for particulate matter smaller than 2.5 microns (PM2.5), which is a health risk regulated by the Environmental Protection Agency.            

At present, most models for predicting emissions use a fixed emission factor for PM2.5 for widely varying fuel conditions over large areas. However, there are many factors that can affect the emission factor, including how the fuel burns (flaming versus smoldering combustion), fuel type, fuel arrangement, fuel moisture, and weather conditions, which in part reflect season of year and time since the last burn. We measured burns under many different conditions to determine what factors have the strongest effect on PM2.5 emission factors.

Results showed that fuel moisture had a strong effect on PM2.5 emission factors for both flaming and smoldering combustion, with greater moisture resulting in a higher emission factor.  That means that the proportion of fuel composed of green vegetation, which has many times the water content of dead vegetation and leaf litter, has a strong effect, such that emission factors are much higher during growing season than dormant season burns (see figure). Also, burns at longer times since the last fire have lower emission factors because they are composed of a smaller fraction of live fuel. These results should help models more accurately predict PM2.5 emissions from wildland fire, which is important for avoiding overly conservative restriction of prescribed burning.  


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