Abstract
Abstract. An extensive program of experiments focused on biomass burning emissions began with a laboratory phase in which vegetative fuels commonly consumed in prescribed fires were collected in the southeastern and southwestern US and burned in a series of 71 fires at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The particulate matter (PM2.5) emissions were measured by gravimetric filter sampling with subsequent analysis for elemental carbon (EC), organic carbon (OC), and 38 elements. The trace gas emissions were measured by an open-path Fourier transform infrared (OP-FTIR) spectrometer, proton-transfer-reaction mass spectrometry (PTR-MS), proton-transfer ion-trap mass spectrometry (PIT-MS), negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS), and gas chromatography with MS detection (GC-MS). 204 trace gas species (mostly non-methane organic compounds (NMOC)) were identified and quantified with the above instruments. Many of the 182 species quantified by the GC-MS have rarely, if ever, been measured in smoke before. An additional 153 significant peaks in the unit mass resolution mass spectra were quantified, but either could not be identified or most of the signal at that molecular mass was unaccounted for by identifiable species. In a second, "field" phase of this program, airborne and ground-based measurements were made of the emissions from prescribed fires that were mostly located in the same land management units where the fuels for the lab fires were collected. A broad variety, but smaller number of species (21 trace gas species and PM2.5) was measured on 14 fires in chaparral and oak savanna in the southwestern US, as well as pine forest understory in the southeastern US and Sierra Nevada mountains of California. The field measurements of emission factors (EF) are useful both for modeling and to examine the representativeness of our lab fire EF. The lab EF/field EF ratio for the pine understory fuels was not statistically different from one, on average. However, our lab EF for "smoldering compounds" emitted from the semiarid shrubland fuels should likely be increased by a factor of ~2.7 to better represent field fires. Based on the lab/field comparison, we present emission factors for 357 pyrogenic species (including unidentified species) for 4 broad fuel types: pine understory, semiarid shrublands, coniferous canopy, and organic soil. To our knowledge this is the most comprehensive measurement of biomass burning emissions to date and it should enable improved representation of smoke composition in atmospheric models. The results support a recent estimate of global NMOC emissions from biomass burning that is much higher than widely used estimates and they provide important insights into the nature of smoke. 31–72% of the mass of gas-phase NMOC species was attributed to species that we could not identify. These unidentified species are not represented in most models, but some provision should be made for the fact that they will react in the atmosphere. In addition, the total mass of gas-phase NMOC divided by the mass of co-emitted PM2.5 averaged about three (range ~2.0–8.7). About 35–64% of the NMOC were likely semivolatile or of intermediate volatility. Thus, the gas-phase NMOC represent a large reservoir of potential precursors for secondary formation of ozone and organic aerosol. For the single lab fire in organic soil about 28% of the emitted carbon was present as gas-phase NMOC and ~72% of the mass of these NMOC was unidentified, highlighting the need to learn more about the emissions from smoldering organic soils. The mass ratio of total NMOC to "NOx as NO" ranged from 11 to 267, indicating that NOx-limited O3 production would be common in evolving biomass burning plumes. The fuel consumption per unit area was 7.0 ± 2.3 Mg ha−1 and 7.7 ± 3.7 Mg ha−1 for pine-understory and semiarid shrubland prescribed fires, respectively.
Highlights
Biomass burning is considered the main source of primary fine carbonaceous particles in the global atmosphere as well the second largest source of total trace gases (Crutzen and Andreae, 2000; Bond et al, 2004; Akagi et al, 2011)
On 11 November 2009, the Block A fire sampled from the air in the morning and the Block B fire sampled from the air in the afternoon were in “coastal sage scrub” and “maritime chaparral” fuel types, respectively
We present a detailed retrospective analysis of a series of studies that included measurements of biomass burning trace gas emissions with the most comprehensive selection of instrumentation to date as well as measurements of fine particle emissions, selected particle species, and biomass fuel consumption per unit area on prescribed fires
Summary
Biomass burning is considered the main source of primary fine carbonaceous particles in the global atmosphere as well the second largest source of total trace gases (Crutzen and Andreae, 2000; Bond et al, 2004; Akagi et al, 2011). Biomass burning is estimated to be the second largest global atmospheric source of gas-phase nonmethane organic compounds (NMOC) after biogenic emissions (∼ 1000 Tg yr−1, Guenther et al, 2006; Yokelson et al, 2008) contributing ∼ 400–700 Tg yr−1 (Akagi et al, 2011). The identified and unidentified NMOC emitted by biomass burning, especially the lower volatility species, are expected to be reactive and contribute to secondary formation of ozone (O3) or organic aerosol as observed and/or modeled in many plume aging studies Fire-adapted ecosystems depend on the regular occurrence of fire for survival (Keeley et al, 2009) In these ecosystems, land managers may implement prescribed burning as often as every ∼ 1–4 yr under conditions when fuel consumption can be limited and smoke dispersion can be at least partially controlled.
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