Search for: All records

Abstract. The evolution of organic aerosol (OA) and aerosol sizedistributions within smoke plumes is uncertain due to the variability inrates of coagulation and OA condensation/evaporation between different smokeplumes and at different locations within a single plume. We use aircraftdata from the FIREX-AQ campaign to evaluate differences in evolving aerosolsize distributions, OA, and oxygen to carbon ratios (O:C) between and withinsmoke plumes during the first several hours of aging as a function of smokeconcentration. The observations show that the median particle diameterincreases faster in smoke of a higher initial OA concentration (>1000 µg m−3), with diameter growth of over 100 nm in 8 h – despite generally having a net decrease in OA enhancementratios – than smoke of a lower initial OA concentration (<100 µg m−3), which had net increases in OA. Observations of OA and O:Csuggest that evaporation and/or secondary OA formation was greater in lessconcentrated smoke prior to the first measurement (5–57 min afteremission). We simulate the size changes due to coagulation and dilution andadjust for OA condensation/evaporation based on the observed changes in OA.We found that coagulation explains the majority of the diameter growth, withOA evaporation/condensation having a relatively minor impact. We found thatmixing between the core and edges of the plume generally occurred ontimescales of hours, slow enough to maintain differences in aging betweencore and edge but too fast to ignore the role of mixing for most of our cases. 

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.