Aerosol emissions from prescribed fires in the United States: A synthesis of laboratory and aircraft measurements

Abstract

Aerosol emissions from prescribed fires can affect air quality on regional scales. Accurate representation of these emissions in models requires information regarding the amount and composition of the emitted species. We measured a suite of submicron particulate matter species in young plumes emitted from prescribed fires (chaparral and montane ecosystems in California; coastal plain ecosystem in South Carolina) and from open burning of over 15 individual plant species in the laboratory. We report emission ratios and emission factors for refractory black carbon (rBC) and submicron nonrefractory aerosol and compare field and laboratory measurements to assess the representativeness of our laboratory-measured emissions. Laboratory measurements of organic aerosol (OA) emission factors for some fires were an order of magnitude higher than those derived from any of our aircraft observations; these are likely due to higher-fuel moisture contents, lower modified combustion efficiencies, and less dilution compared to field studies. Nonrefractory inorganic aerosol emissions depended more strongly on fuel type and fuel composition than on combustion conditions. Laboratory and field measurements for rBC were in good agreement when differences in modified combustion efficiency were considered; however, rBC emission factors measured both from aircraft and in the laboratory during the present study using the Single Particle Soot Photometer were generally higher than values previously reported in the literature, which have been based largely on filter measurements. Although natural variability may account for some of these differences, an increase in the BC emission factors incorporated within emission inventories may be required, pending additional field measurements for a wider variety of fires.

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“…The mass ratio of BC and K + apportioned to this factor was similar to that of a biomass burning plume, particularly the high K + proportion typical of herbaceous burning (Turn et al, 1997;Saarikoski et al, 2007;McMeeking et al, 2009;May et al, 2014). However, both BC and K + loading showed significant uncertainty.…”

“…This separation of BC-and K + -rich factors was persistent for all PMF solutions with four or more factors. Furthermore, the mass ratios of SO 2− 4 and NO − 3 to BC were much higher than would be expected for biomass burning with enrichment ratios above 10 (Turn et al, 1997; Hays et al, 2005;Saarikoski et al, 2007;McMeeking et al, 2009;May et al, 2014); the relative loading of SO 2− 4 compared to NH + 4 and NO − 3 was also higher than expected for biomass burning . This factor also showed an acidic signature with a neutralization ratio of 0.12.…”

“…However, both BC and K + loading showed significant uncertainty. The loadings of formate, acetate, Cl − , Br − , C 2 O 2− 4 , and NH + 4 appeared to be higher than expected for biomass burning emissions; the mass ratio of these analytes to BC were enriched by a factor of 3 to 75 relative to typical mass ratios of biomass burning emissions, based on a review of measured herbaceous and woody emissions (Turn et al, 1997;Andreae and Merlet, 2001;Hays et al, 2005;Saarikoski et al, 2007;McMeeking et al, 2009;May et al, 2014). The observed enrichment ratios of this factor above typical biomass burning plumes could be explained by atmospheric processing, for example, the cloud processing suggested by Legrand and de Angelis (1995).…”

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“…Corroborating these results, Liu et al [2014] showed that fire smoke particles are composed of approximately 5-10% (by mass) BC as compared to about 50-70% OC. May et al [2014] analyzed a large number of different smoke plumes in the laboratory and for prescribed fires in the United States, and found a range of BC and particulate organic matter (POM; i.e., the sum of OC and associated chemical elements) emission ratios depending on fuel type, fuel composition, and combustion conditions [see Table 4 in May et al, 2014]. However, POM typically dominates the sub-micron aerosol particle composition in most samples.…”

“…The choice of the BC/POM ratios must be considered rather arbitrary given the very wide range of possible BC/POM ratios observed during different type of fires [e.g., May et al, 2014]. An estimate of burnable organic material for Pakistan and India is not available.…”