Soot superaggregates from flamingwildfires and their direct radiative forcingRajan K. Chakrabarty1,2, Nicholas D. Beres2, Hans Moosmuller2, Swarup China3, Claudio Mazzoleni3,Manvendra K. Dubey4, Li Liu5 & Michael I. Mishchenko5

1Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA,2Desert Research Institute, Nevada System of Higher Education, Reno, Nevada, USA, 3Atmospheric Sciences Program, MichiganTechnological University, Houghton, Michigan, USA, 4Earth System Observations, Los Alamos National Laboratory, Los Alamos,New Mexico, USA, 5NASA Goddard Institute for Space Studies, New York, NY 10025, USA.

Wildfires contribute significantly to global soot emissions, yet their aerosol formation mechanisms andresulting particle properties are poorly understood and parameterized in climate models. The conventionalview holds that soot is formed via the cluster-dilute aggregation mechanism in wildfires and emitted asaggregates with fractal dimension Df < 1.8 mobility diameter Dm # 1 mm, and aerodynamic diameter Da #300 nm. Here we report the ubiquitous presence of soot superaggregates (SAs) in the outflow from a majorwildfire in India. SAs are porous, low-density aggregates of cluster-dilute aggregates with characteristicDf < 2.6, Dm . 1 mm, and Da # 300 nm that form via the cluster-dense aggregation mechanism. We presentadditional observations of soot SAs in wildfire smoke-laden air masses over Northern California, NewMexico, and Mexico City. We estimate that SAs contribute, per unit optical depth, up to 35% lessatmospheric warming than freshly-emitted (Df < 1.8) aggregates, and

temperature). Exceptions have been the aircraft sampling studiesconducted during the 1990s in the over-fire regions of flaming forestfires in Brazil and southern Africa17,18. Observations of unusuallylarge soot aggregates were made from these fires, but the investiga-tors failed to distinguish the microphysical properties of these part-icles from conventional sub-micron soot aggregates. As a result,these unique observations have gone unnoticed, and there has beenno follow-up investigation conducted on the formation mechanismand frequency of occurrence of these unusually large soot aggregates,their microphysical properties, and their potential impact on radi-ative forcing and health.

Here, we investigate particles contained in the flaming-phaseplumes of the Nagarhole National Forest fire (NNFF)24 inKarnataka (India) and find the ubiquitous occurrence of superag-gregates (SAs), a hitherto unrecognized form of soot distinct fromconventional sub-micron aggregates. We report additional obser-vation of these SAs in wildfire smoke-laden air masses overSacramento (Northern California, USA), Los Alamos (NewMexico, USA), and the Mexico City metropolitan area (Mexico),respectively. Based on the unique morphological properties of SAs,we discuss their possible formation mechanism and their potentialimpact on human health. We also compute numerically-exact opticalproperties of these particles and compare them with those of sub-micron size soot particles. We make use of the optical properties tocalculate direct radiative forcing efficiencies of SAs at the top of theatmosphere and discuss their net warming or cooling of the atmo-sphere. Finally, we address the need for future research to betterunderstand and characterize the detection and atmospheric proces-sing of soot SAs for quantitatively estimating their impact on climateand health.

ResultsWe collected aerosol samples for scanning electron microscopy (SEM)analysis downwind of the NNFF over the Indian Ocean at the MaldivesClimate Observatory on Hanimaadhoo Island (MCOH) (6.78u N,73.18u E). The NNFF, which lasted for a week beginning February27, 2012, burned approximately 35 km2 of dry deciduous forest con-taining dry bamboo and teak trees. Dense smoke from intense flamingcombustion was reported, with the event turning into a firestormwithin a day24. The MCOH aerosol number concentration increasedfrom about 800 to 3000 cm23 during this period. The months ofNovember through May constitute the dry season in South Asia, whenlow-level flow brings a polluted air mass from Asia to the IndianOcean25. Ensemble back-trajectory analyses (Fig. 1a; Fig. S1 in supple-mentary information) coupled with satellite imagery and the CloudAerosol Lidar and Infrared Pathfinder Satellite (CALIPSO) measure-ments (Fig. 1 b and c) show a low-level polluted air massbetween 1to 3 km above sea leveltransported from the forest fire site flowingsouthwest over the Indian Ocean. Gas chromatography interfaced withmass spectrometry analyses of aerosol samples, revealed the presenceof levoglucosana molecular marker for biomass burning emis-sions26in trace amounts (about 0.09 ng/m3).

To investigate how commonly these SAs occur in different geo-graphical locations and atmospheric conditions, we sampled aerosolscontained in wildfire smoke-laden air masses over Sacramento dur-ing the CARES (Carbonaceous Aerosol and Radiative EffectsStudy)27 in June 2010 and over Mexico City as part of theMILAGRO (Megacity Initiative: Local And Global ResearchObservations)28,29 study during March-June 2006. Finally, in 2011at Los Alamos, we sampled the downwind plumes of the LasConchas wildfire19, the second largest wildfire in the states history.

Figure 1 | The 2012 Nagarhole forest wildfire smoke plume transport. (A) Average of NOAA HYSPLIT ensemble trajectories ending at the MaldivesClimate ObservatoryHanimaadhoo, Maldives (MCOH) on 01, 02, and 03 March 2012. (Image created using Adobe PhotoshopTM); (B) Visible imagery of

the Indian Peninsula from the MODIS sensor aboard the Terra satellite for 28 February 2012. (Image obtained from NASA Near Real-time (NRT) data

archive); (C) 532 nm backscatter return signal from the CALIOP Lidar aboard the CALIPSO satellite showing vertical distribution of aerosols (Image

obtained from NASA CALIPSO data archive). The color scale on the right indicates the strength of the LIDAR return signal: boundary layer clouds usually

show up as grey or white; cirrus clouds range from yellow to grey; and aerosols show up as green, yellow, and red (indicating low, medium, and high

loadings, respectively).

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SCIENTIFIC REPORTS | 4 : 5508 | DOI: 10.1038/srep05508 2

Morphological properties of soot superaggregates. We measuredthe structural and fractal properties of individual carbonaceousparticles collected from smoke plumes over the four locationsusing established image analysis routines5. The fractal properties ofindividual particles were quantified using the perimeter andensemble methods3032. Electron micrographs of typical SAsobserved at the four sampling sites are shown in Fig. 2 and 3. TheSAs consisted of aggregates of sub-micrometer size, cluster-dilutesoot aggregates with characteristic Df 5 1.9 6 0.2 (Fig. 2c). Themean maximum length scales of SAs were between 10 and 20 mm,and SAs had distinct Df 5 2.6 6 0.1.

Our analysis showed that a typical SA consisted of around 3000monomers, after accounting for apparent monomer overlap, which isparameterized by a power-law factor 1.0933. Observation of Df 5 1.9sub-micron aggregates within individual SAs confirms that the SAswere formed via percolation of these aggregates in the fires. Highmagnification images (Fig. 2d) of NNFF revealed minimal coating ofcondensed organic matter on the monomers. This suggested thatthese particles were formed under near unity net equivalence ratio34

and resisted atmospheric processing during long-range transport11.Alternatively, it could be that there was not enough condensableorganic matter available in the flaming fire plumes of NNFF (seetable S1 in supplementary information) to coat the SAs.

We analyzed 69 individual carbonaceous particles collected onSEM filters from the NNFF. Approximately 99% and 75% of theparticle mass and number, respectively, were soot SAs9,31,32,35, withthe remaining being aggregates (Fig. 4). We probed the elementalcomposition of the particles using energy dispersive X-ray spectro-scopy (EDX), finding carbon and oxygen to be their primary con-stituents. No tar balls22,36 or particles with inorganic inclusions wereobserved. We analyzed 580 particles from the CARES campaign andfound approximately 16% of the total aerosol particle number wasSAs. The remaining 57% and 27% of particles were externally sub-micron soot (occurring as bare and mixed with organic carbon) anddust particles, respectively. The SAs observed in samples fromMILAGRO and the Las Conchas fires were less than 1% in number.Aerosol types in these two locations were mostly organic carbon and

tar balls19,20,28,29,37, suggesting dominant emission from the smolder-ing phase of wildfires.

For the SAs and cluster-dilute aggregates observed at MCOH andduring CARES, we calculated their mobility diameters Dm14,38 andmass distributions (Fig. 4 and 5) based on their single-particle pro-jected area equivalent diameters14. The SAs had a range of Dmbetween 1 and 20 mm with a mean Dm < 3 mm. The monomernumber size distribution could be described by a mono-modal log-normal size distribution with a mean monomer diameter of 50 nmand a standard deviation of 5 nm. We observed a majority (95%) ofSAs in the third stage, Da , 0.3 mm, of the impactor used for aerosolcollection. Although characterized by very large geometric diameters(Dm), the low Da of SAs suggest that they are highly porous, have loweffective densities, and could get deposited in the innermost lungairways and alveoli via the process of diffusion deposition, similarto soot aggregates10,39. However, the extent of lung penetration of SAsdepends on particle Da40. Super-micron size porous carbon aggre-gates, similar to soot SAs in this study, have been synthesized in thelaboratory41, and were observed to have effective particle densities aslow as 2.5 mg/cm3.

DiscussionSuperaggregate formation mechanism. SAs are formed whencluster-dilute aggregates enter into a cluster-dense aggregationregime in flames35. This regime, defined by a small ratio of themean aggregate nearest-neighbor separation to aggregate size andby enhanced kinetics, results in the aggregates sticking together andpercolating to form a volume spanning SA with a universal Df