Spherical tarball particles form through rapid chemical and physical changes of organic matter in biomass-burning smoke

Abstract

Biomass burning (BB) emits enormous amounts of aerosol particles and gases into the atmosphere and thereby significantly influences regional air quality and global climate. A dominant particle type from BB is spherical organic aerosol particles commonly referred to as tarballs. Currently, tarballs can only be identified, using microscopy, from their uniquely spherical shapes following impaction onto a grid. Despite their abundance and potential significance for climate, many unanswered questions related to their formation, emission inventory, removal processes, and optical properties still remain. Here, we report analysis that supports tarball formation in which primary organic particles undergo chemical and physical processing within ∼3 h of emission. Transmission electron microscopy analysis reveals that the number fractions of tarballs and the ratios of N and O relative to K, the latter a conserved tracer, increase with particle age and that the more-spherical particles on the substrates had higher ratios of N and O relative to K. Scanning transmission X-ray spectrometry and electron energy loss spectrometry analyses show that these chemical changes are accompanied by the formation of organic compounds that contain nitrogen and carboxylic acid. The results imply that the chemical changes increase the particle sphericity on the substrates, which correlates with particle surface tension and viscosity, and contribute to tarball formation during aging in BB smoke. These findings will enable models to better partition tarball contributions to BB radiative forcing and, in so doing, better help constrain radiative forcing models of BB events.

Optical and Chemical Analysis of Absorption Enhancement by Mixed Carbonaceous Aerosols in the 2019 Woodbury, AZ, Fire Plume

“…OA with these characteristics has been hypothesized to be related to “tar balls,” spherical, carbonaceous particles observed by electron microscopy (Chakrabarty et al, 2010; Hand et al, 2005). Recently, tar balls have also been observed in ambient wildfire plumes (Adachi et al, 2019; Girotto et al, 2018). Optical attribution of red absorption to BrC is complicated because its mass is not quantified separately from BC by laser‐induced incandescence (Adler et al, 2019; Sedlacek et al, 2018) or by thermal‐optical experiments (Adler et al, 2019; Liu et al, 2015).…”

Detection of tar brown carbon with a single particle soot photometer (SP2)

“…While it is obvious that molecular differences will exist between minimally processed biomass-burning emissions and residual-fuel emissions, the material properties of tar brC produced from either fuel appear to be similar, according to studies which have comprehensively characterized tar brC from either residual-fuel combustion or biomass (Adler et al, 2019) combustion. Note that although Adler et al (2019) did not refer to their studied particles as tarballs, they characterized macromolecular, low-volatility, highly light-absorbing, spherical particles that were stable under an electron beam and therefore possessed all of the properties of tar brC without exception.…”

“…Examples of less carbonized tarballs include the hygroscopic wildfire tarballs described by Hand et al (2005) and the soluble laboratory tarballs described by Li et al (2019). Studies must therefore carefully characterize the LAC types discussed here, using a combination of techniques as necessary (Adler et al, 2019;, and keeping in mind the response of different techniques to tar brC of varying levels of carbonization. Since tar brC is of particular importance for its light-absorbing properties (Adachi et al, 2019), techniques which are biased towards more absorbing tar brC should be favoured over those which are biased towards less absorbing tar brC.…”

“…This inclusive, material-based definition avoids the unnecessary confusion of requiring several separate names for the tarball-like particles measured in unprocessed wildfire plumes (Pósfai et al, 2003(Pósfai et al, , 2004Semeniuk et al, 2006;Adachi and Buseck, 2011;China et al, 2013); laboratory biomass smoke (Vernooij et al, 2009;Chakrabarty et al, 2010;Adler et al, 2019); dry-distilled, heat-treated laboratory biomass (Tóth et al, 2014;Hoffer et al, 2016a;Li et al, 2019); fresh marine-engine exhaust Jiang et al, 2019); or atmospheric air masses of unmeasured or unreported photochemical age (Niemi et al, 2006;Hand et al, 2005;Tivanski et al, 2007;Alexander et al, 2008;Zhu et al, 2013). As all of these particles possess similar material properties, a single name, tar brC in general and tarballs for individual spheres, is most appropriate for them.…”

“…It is would therefore also be possible to characterize tar particles in real time by heating an aerosol sample to remove nonrefractory material (leaving only soot BC, tar brC, and potentially char BC) before using a combination of two different particle classifiers to produce an aerosol composed primarily of tar. This approach has been demonstrated by Adler et al (2019), although those authors did not refer to their particles as tar particles, as noted above.…”