title: Soot superaggregates from flaming wildfires and their direct radiative forcing.
abstract: 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 ≤ 1 μm, and aerodynamic diameter Da ≤ 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 μm, and Da ≤ 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. At 550 nm wavelength, [corrected] we estimate that SAs contribute, per unit optical depth, up to 35% less atmospheric warming than freshly-emitted (D(f) ≈ 1.8) [corrected] aggregates, and ≈90% more warming than the volume-equivalent spherical soot particles simulated in climate models.
authors: 1] Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA [2] Desert Research Institute, Nevada System of Higher Education, Reno, Nevada, USA.
authors: Rajan K
authors: RK
authors: Chakrabarty
authors: Desert Research Institute, Nevada System of Higher Education, Reno, Nevada, USA.
authors: Nicholas D
authors: ND
authors: Beres
authors: Desert Research Institute, Nevada System of Higher Education, Reno, Nevada, USA.
authors: Hans
authors: H
authors: Moosmüller
authors: Atmospheric Sciences Program, Michigan Technological University, Houghton, Michigan, USA.
authors: Swarup
authors: S
authors: China
authors: Atmospheric Sciences Program, Michigan Technological University, Houghton, Michigan, USA.
authors: Claudio
authors: C
authors: Mazzoleni
authors: Earth System Observations, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.
authors: Manvendra K
authors: MK
authors: Dubey
authors: NASA Goddard Institute for Space Studies, New York, NY 10025, USA.
authors: Li
authors: L
authors: Liu
authors: NASA Goddard Institute for Space Studies, New York, NY 10025, USA.
authors: Michael I
authors: MI
authors: Mishchenko