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Wildfires emit large amounts of black carbon and light-absorbing organic carbon, known as brown carbon, into the atmosphere. These particles perturb Earth’s radiation budget through absorption of incoming shortwave radiation. It is generally thought that brown carbon loses its absorptivity after emission in the atmosphere due to sunlight-driven photochemical bleaching. Consequently, the atmospheric warming effect exerted by brown carbon remains highly variable and poorly represented in climate models compared with that of the relatively nonreactive black carbon. Given that wildfires are predicted to increase globally in the coming decades, it is increasingly important to quantify these radiative impacts. Here we present measurements of ensemble-scale and particle-scale shortwave absorption in smoke plumes from wildfires in the western United States. We find that a type of dark brown carbon contributes three-quarters of the short visible light absorption and half of the long visible light absorption. This strongly absorbing organic aerosol species is water insoluble, resists daytime photobleaching and increases in absorptivity with night-time atmospheric processing. Our findings suggest that parameterizations of brown carbon in climate models need to be revised to improve the estimation of smoke aerosol radiative forcing and associated warming.

 

Abstract. Emission of organic aerosol (OA) from wood combustion is not well constrained;understanding the governing factors of OA emissions would aid in explainingthe reported variability. Pyrolysis of the wood during combustion is theprocess that produces and releases OA precursors. We performed controlledpyrolysis experiments at representative combustion conditions. The conditionschanged were the temperature, wood length, wood moisture content, and woodtype. The mass loss of the wood, the particle concentrations, and light-gasconcentrations were measured continuously. The experiments were repeatable asshown by a single experiment, performed nine times, in which the real-timeparticle concentration varied by a maximum of 20 %. Highertemperatures increased the mass loss rate and the released concentration ofgases and particles. Large wood size had a lower yield of particles than thesmall size because of higher mass transfer resistance. Reactions outside thewood became important between 500 and 600 ∘C. Elevatedmoisture content reduced product formation because heat received was sharedbetween pyrolysis reactions and moisture evaporation. The thermophysicalproperties, especially the thermal diffusivity, of wood controlled thedifference in the mass loss rate and emission among seven wood types. Thiswork demonstrates that OA emission from wood pyrolysis is a deterministicprocess that depends on transport phenomena. 

Abstract. Biomass burning emits vapors and aerosols into the atmosphere thatcan rapidly evolve as smoke plumes travel downwind and dilute, affectingclimate- and health-relevant properties of the smoke. To date, theory hasbeen unable to explain observed variability in smoke evolution. Here, we useobservational data from the Biomass BurningObservation Project (BBOP) field campaign and show that initial smokeorganic aerosol mass concentrations can help predict changes in smokeaerosol aging markers, number concentration, and number mean diameterbetween 40–262 nm. Because initial field measurements of plumes aregenerally >10 min downwind, smaller plumes will have alreadyundergone substantial dilution relative to larger plumes and have lowerconcentrations of smoke species at these observations closest to the fire.The extent to which dilution has occurred prior to the first observation isnot a directly measurable quantity. We show that initial observed plumeconcentrations can serve as a rough indicator of the extent of dilutionprior to the first measurement, which impacts photochemistry, aerosolevaporation, and coagulation. Cores of plumes have higher concentrationsthan edges. By segregating the observed plumes into cores and edges, we findevidence that particle aging, evaporation, and coagulation occurred beforethe first measurement. We further find that on the plume edges, the organicaerosol is more oxygenated, while a marker for primary biomass burningaerosol emissions has decreased in relative abundance compared to the plumecores. Finally, we attempt to decouple the roles of the initialconcentrations and physical age since emission by performing multivariatelinear regression of various aerosol properties (composition, size) on thesetwo factors. 

Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC’s radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model–measurement differences. We show that accounting for these two effects—variability in per-particle composition and deviations from the core-shell approximation—reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC’s radiative effect on climate. 

Abstract. During the first phase of the Biomass Burn Operational Project (BBOP) fieldcampaign, conducted in the Pacific Northwest, the DOE G-1 aircraft was usedto follow the time evolution of wildfire smoke from near the point ofemission to locations 2–3.5 h downwind. In nine flights we maderepeated transects of wildfire plumes at varying downwind distances andcould thereby follow the plume's time evolution. On average there was littlechange in dilution-normalized aerosol mass concentration as a function ofdownwind distance. This consistency hides a dynamic system in which primaryaerosol particles are evaporating and secondary ones condensing. Organicaerosol is oxidized as a result. On all transects more than 90 % ofaerosol is organic. In freshly emitted smoke aerosol, NH4+ isapproximately equivalent to NO3. After 2 h of daytime aging, NH4+ increased and is approximately equivalent tothe sum of Cl, SO42, and NO3. Particle size increased with downwind distance,causing particles to be more efficient scatters. Averaged over nine flights,mass scattering efficiency (MSE) increased in ∼ 2 h by 56 % and doubled in one flight. Mechanisms for redistributing mass from small to large particles are discussed. Coagulation is effective at movingaerosol from the Aitken to accumulation modes but yields only a minor increase in MSE. As absorption remained nearly constant with age, the timeevolution of single scatter albedo was controlled by age-dependentscattering. Near-fire aerosol had a single scatter albedo (SSA) of 0.8–0.9. After 1 to 2 h of aging SSAs were typically 0.9 and greater. Assuming global-average surface and atmospheric conditions, the observedage dependence in SSA would change the direct radiative effect of a wildfire plume from near zero near the fire to a cooling effect downwind.