Complexity in the Evolution, Composition, and Spectroscopy of Brown Carbon in Aircraft Measurements of Wildfire Plumes

Abstract

Biomass burning is a major source of light‐absorbing organic aerosol (brown carbon), but its composition, chemical evolution, and lifetime are not well known. We measured water‐soluble brown carbon absorption from 310 to 500 nm on the National Oceanic and Atmospheric Administration Twin Otter aircraft during flights downwind of western United States wildfires in summer 2019. The sampling strategy was near‐Lagrangian and the plume ages spanned 0–5 hr. Trends in brown carbon mass absorption coefficient with plume age varied between flights, and did not show an exponential decay over these short time scales. The measured absorption spectra were smoothly varying, without identifiable contributions from individual chromophores with structured absorption. Using aerosol tracer ions and reference absorption spectra, the calculated contribution of 4‐nitrocatechol to total absorption was <22 ± 9% and <11 ± 5%, although spectral fitting showed that it may be as low as <1.1% and <0.6% at 365 and 405 nm, respectively.

Beyond the Ångström Exponent: Probing Additional Information in Spectral Curvature and Variability of In Situ Aerosol Hyperspectral (0.3–0.7 μm) Optical Properties

“…Both observations (variable but increasing O:C and inconsistent trends in BrC) are consistent with a varying suite of chromophores for each plume where oxidation alters the chromophores differently across the set of plumes. Looking just at photolytic degradation of the filter samples (i.e., complete set of smoke aerosols from a given burn) from their laboratory study, Fleming et al (2020) found that the absorption changes varied by fuel type, fuel components burned, and combustion condition, consistent with the variable trend observed by Washenfelder et al (2022). Examining the photodegradation of individual chromophores, Fleming et al (2020) found that lignin-derived and distillation (specifically, flavonoids) products tended to have the longest lifetimes.…”

“…(2019) reveal that any given organic compound within the set that comprise BBOA will only contribute a fraction to the total mass, a very small fraction for most. Further, important aerosol chromophores appear to be present in trace amounts compared to non‐absorbing molecules that may dominate aerosol mass (e.g., Bones et al., 2010; Palm et al., 2020; Updyke et al., 2012; Washenfelder et al., 2022). In the same 2016 FIREX study, Fleming et al.…”

“…This complexity in chromophoric composition likely accounts for the inconsistent variability observed in 0.365 μm BrC evolution of aqueous extracts of BBOA in Lagrangian plume sampling from the Twin Otter during FIREX‐AQ (Washenfelder et al., 2022 ). To look at evolution of BrC absorption over time or to relate it to other observations of chemical, optical, or microphysical parameters it is convenient to select one wavelength from the BrC spectrum; 0.365 μm is the wavelength typically used for this purpose.…”

“…The results from Jen et al (2019) reveal that any given organic compound within the set that comprise BBOA will only contribute a fraction to the total mass, a very small fraction for most. Further, important aerosol chromophores appear to be present in trace amounts compared to non-absorbing molecules that may dominate aerosol mass (e.g., Bones et al, 2010;Palm et al, 2020;Updyke et al, 2012;Washenfelder et al, 2022). In the same 2016 FIREX study, Fleming et al (2020) performed a similarly comprehensive examination of the BrC components within the aerosols identifying 46 chromophores (MWt ≤400 amu with above-detection light absorption limited to λ ≤0.55 μm over a measured wavelength range of 0.3-0.7 μm) from a diverse set of biomass samples (from 7 coniferous [gymnosperm] and 3 flowering [angiosperm] species, including duff, litter, and canopy material).…”

“…To look at evolution of BrC absorption over time or to relate it to other observations of chemical, optical, or microphysical parameters it is convenient to select one wavelength from the BrC spectrum; 0.365 μm is the wavelength typically used for this purpose. Washenfelder et al (2022) determined plume age both in terms of chemical and physical age, and observed BBOA evolution over the first 5 hr after emission (Washenfelder et al, 2022). O:C ratios varied between the flights, but oxidation of BBOA consistently increased with plume age for all of them.…”