AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
How Well Do Laboratory Studies Represent Microphysical Properties of Soot Emitted from Wildfires?
RAJAN K. CHAKRABARTY, Nicholas Beres, Hans Moosmuller, Swarup China, Claudio Mazzoleni, Manvendra Dubey, Li Liu, Michael I Mishchenko, Desert Research Institute
Abstract Number: 425 Working Group: Biomass Burning Aerosol: From Emissions to Impacts
Abstract Soot emitted from wildfires has been implicated in rapid global warming, accelerated melting of glaciers, changing monsoon patterns, and degradation of human health and the environment. It contributes to greater than threefold uncertainty in current estimates of climate forcing. This large uncertainty is primarily attributable to poor understanding of the microphysical properties of wildfire-emitted soot and their parameterizations in models and satellite retrieval algorithms. In recent years, researchers have made thorough efforts to characterize these properties for soot emitted from laboratory-scale combustion of wildland fuels (for e.g., the Fire Laboratory at Missoula Experiments (FLAME)) as a function of various process parameters such as fuel type, temperature and scale of flaming phase, environmental conditions driving the combustion process, and interrelationships of these parameters. These studies have been conducted under the assumption that they may closely mimic natural wildfires in their soot formation mechanism and emitted particle properties. Consequently, the current view holds that in flaming wildfires, soot is formed via the cluster–dilute aggregation mechanism and is emitted as aggregates with fractal dimension Df ~1.8 and mobility diameter Dm <= 1µm.
Here, we provide evidence of significant presence of a hitherto unrecognized form of soot– superaggregates (SAs)–in the outflow from two wildfires in Karnataka (India) and California (USA). SAs are porous, low-density aggregates of cluster-dilute aggregates with characteristic Df ~2.6 and Dm > 1µm that form via the cluster-dense aggregation mechanism in the flaming phase of wildfires. Their greater than one micrometer Dm render SAs undetectable using conventional soot aerosol-sizing instruments, such as the Scanning Mobility Particle Analyzer (SMPS). Using numerically-exact superposition T-Matrix optical modeling, we estimate SAs to contribute, per unit optical depth, up to 35% less atmospheric warming than freshly-emitted (Df ~1.8) aggregates, about 25% more warming than aged aggregates (1.8 <= Df <= 3.0), and ~90% more warming than spherical particles currently used in climate models.