Upper Feather River Watershed

Soot superaggregates from flaming wildfires and thEIR direct radiative forcing

In a paper published this month in Nature Scientific Reports, a team of scientists led by Rajan Chakrabarty, Ph.Dand graduate student Nicholas Beres from Nevada's Desert Research Institute report the observation of apreviously unrecognized form of soot particle, identified by the authors as 'superaggregates,' from wildfire emissions. These newly identified particles were detected in smoke plumes from wildfires in Northern California, New Mexico, Mexico City, and India. Wildfires contribute significantly to global soot emissions, yet thEIR aerosol formation mechanisms and resulting particle properties are poorly understood and parameterized in climate models. The conventional view holds that soot is formed via the cluster-dilute aggregation mechanism in wildfires and emitted as aggregates with fractal dimension Df ≈1.8 mobility diameter Dm â‰_ 1 Î_m, and aerodynamic diameter Da â‰_ 300 nm. Here we report the ubiquitous presence of soot superaggregates (SAs) in the outflow from a major wildfire in India. SAs are porous, low-density aggregates of cluster-dilute aggregates with characteristic Df ≈ 2.6, Dm > 1 Î_m, and Da â‰_ 300 nm that form via the cluster-dense aggregation mechanism. We present additional observations of soot SAs in wildfire smoke-laden air masses over Northern California, New Mexico, and Mexico City. We estimate that SAs contribute, per unit optical depth, up to 35% less atmospheric warming than freshly-emitted (Df ≈ 1.8) aggregates, and ≈90% more warming than the volume-equivalent spherical soot particles simulated in climate models.

address the need for future research to better understand and characterize the detection and atmospheric processing of soot SAs for quantitatively estimating thEIR impact on climate and health.

2015-08-21

SCIENTIFIC REPORTS

Environmental hazards, Wildfire

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