Abstract
Abstract. Particles in smoke emitted from biomass combustion have a large impact on global climate and urban air quality. There is limited understanding of how particle optical properties – especially the contributions of black carbon (BC) and brown carbon (BrC) – evolve with photochemical aging of smoke. We analyze the evolution of the optical properties and chemical composition of particles produced from combustion of a wide variety of biomass fuels, largely from the western United States. The smoke is photochemically aged in a reaction chamber over atmospheric-equivalent timescales ranging from 0.25 to 8 d. Various aerosol optical properties (e.g., the single-scatter albedo, the wavelength dependence of absorption, and the BC mass absorption coefficient, MACBC) evolved with photochemical aging, with the specific evolution dependent on the initial particle properties and conditions. The impact of coatings on BC absorption (the so-called lensing effect) was small, even after photochemical aging. The initial evolution of the BrC absorptivity (MACBrC) varied between individual burns but decreased consistently at longer aging times; the wavelength dependence of the BrC absorption generally increased with aging. The observed changes to BrC properties result from a combination of secondary organic aerosol (SOA) production and heterogeneous oxidation of primary and secondary OA mass, with SOA production being the major driver of the changes. The SOA properties varied with time, reflecting both formation from precursors having a range of lifetimes with respect to OH and the evolving photochemical environment within the chamber. Although the absorptivity of BrC generally decreases with aging, the dilution-corrected absorption may actually increase from the production of SOA. These experimental results provide context for the interpretation of ambient observations of the evolution of particle optical properties in biomass-combustion-derived smoke plumes.
Highlights
Open and contained biomass combustion contributes substantial amounts of particulate matter to the atmosphere (Bond et al, 2004)
We suggest that the small Eabs,coat for photochemically aged biomass combustion particles results from three phenomena: (i) condensation occurring onto a polydisperse refractory black carbon (rBC) distribution, leading to some particles having very thick coatings and some quite thin, yielding a large average Rcoat-rBC yet small enhancement (Fierce et al, 2016, 2020); (ii) the coated biomassburning-derived rBC particles not having a core–shell morphology and reduced Eabs,coat (Helgestad, 2016; Liu et al, 2017); and (iii) weak absorption by brown carbon (BrC) at 781 nm by both internally and externally mixed BrC that becomes notable only when the total [organic aerosol (OA)] greatly exceeds [black carbon (BC)] (McClure et al, 2020)
While there remain some class-specific differences, this indicates that the secondary organic aerosol (SOA) that is forming has similar optical properties independent of the initial burn conditions
Summary
Open and contained biomass combustion contributes substantial amounts of particulate matter to the atmosphere (Bond et al, 2004). The chemical, optical, and physical properties of freshly emitted BB particles produced from burning of various biomass fuel types under various conditions are reasonably well studied (e.g., Lewis et al, 2008; McMeeking et al, 2009; Levin et al, 2010; Cheng et al, 2016; Fortner et al, 2018; McClure et al, 2020) Such measurements have established that some fraction of the emitted OA is light absorbing, with the absorptivity dependent upon the burn conditions (Saleh et al, 2014). The importance of BrC to light absorption tends to increase as wavelength decreases
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