Characteristics and Evolution of Brown Carbon in Western United States Wildfires

Abstract

Abstract. Brown carbon (BrC) associated with aerosol particles in western United States wildfires was measured between Jul. and Aug. 2019 onboard the NASA DC-8 research aircraft during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) study. Two BrC measurement methods are investigated; highly spectrally-resolved light absorption in solvent (water and methanol) extracts of particles collected on filters and in-situ bulk aerosol particle light absorption measured at three wavelengths (405, 532, 664 nm) with a photo acoustic spectrometer (PAS). A light absorption closure analysis for wavelengths between 300 and 700 nm was performed. The combined light absorption of particle pure black carbon material, including enhancements due to internally mixed materials, plus soluble BrC and a Mie-predicted factor for conversion of soluble BrC to aerosol particle BrC, was compared to absorption spectra from a power law fit to the three PAS wavelengths. For the various parameters used, at a wavelength of roughly 400 nm they agreed, at lower wavelengths the individual component-predicted particle light absorption significantly exceeded the PAS and at higher wavelengths the PAS absorption was consistently higher, but more variable. Limitations with extrapolation of PAS data to wavelengths below 405 nm and missing BrC species of low solubility that more strongly absorb at higher wavelengths may account for the differences. Based on measurements closest to fires, the emission ratio of PAS measured BrC at 405 nm relative to carbon monoxide (CO) was on average 0.13 Mm−1 ppbv−1, emission ratios for soluble BrC are also provided. As the smoke moved away from the burning regions the evolution over time of BrC was observed to be highly complex; BrC enhancement, depletion, or constant levels with age were all observed in the first 8 hours after emission in different plumes. Within 8 hours following emissions, 4-nitrocatechol, a well characterized BrC chromophore commonly found in smoke particles, was largely depleted relative to the bulk BrC. In a descending plume where temperature increased by 15 K, 4-nitrocatechol dropped possibly due to temperature-driven evaporation, but bulk BrC remained largely unchanged. Evidence was found for reactions with ozone, or related species, as a pathway for secondary formation of BrC under both low and high oxides of nitrogen (NOx) conditions, while BrC was also observed to be bleached in regions of higher ozone and low NOx, consistent with complex behaviors of BrC observed in laboratory studies. Although the evolution of smoke in the first hours following emission is highly variable, a limited number of measurements of more aged smoke (15 to 30 hours) indicate a net loss of BrC. It is yet to be determined how the near-field BrC evolution in smoke affects the characteristics of smoke over longer time and spatial scales, where its environmental impacts are likely to be greater.

Examination of Brown Carbon Absorption from Wildfires in the Western U.S. During the WE-CAN Study

“…Overall, during WE-CAN ~45% (ranging from 31% to 65%) of the BrC absorption at Abs 405 was due to water-soluble species. This is similar to what was observed from off-line LWCC analysis of water and methanol extracts from filter samples collected during sampling of biomass burning plumes as part of the DC3 (Deep Convective Clouds and Chemistry), SEAC4RS (Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys), and FIREX aircraft campaigns [Forrister et al, 2015;Liu et al, 2015;Zeng et al, 2022].…”

“…Forrister et al [2015], who used plume transect averages of SEAC4RS data, reported a decrease in the total Abs 365/ΔCO from ~0.13 to 0.07 Mm -1 /ppbv in 5 h for smoke from the Rim Fire. Observations of smoke during FIREX, by contrast, indicated no clear trend with plume age [Zeng et al, 2022] in a dataset where the majority of plume ages were less than 10 h. These varying results also suggest that other factors that contribute to changes in BrC absorption over time may still need to be explored.…”

“…CC BY 4.0 License.and a ratio of 3.6 for Mie calculated total Abs 365 to measured water-soluble Abs 365 were observed. InZeng et al [2022], Mie Theory was used to calculate the factor to convert solution to particle light absorption (i.e., ratio of Mie calculated to measured water-soluble absorption) as a function of wavelength for the FIREX data. At 405 nm a factor of ~1.7 was determined, similar to what was determined from the WE-CAN data.…”

Chromophores and chemical composition of brown carbon characterized at an urban kerbside by excitation-emission spectroscopy and mass spectrometry

“…2c). Zeng et al (2022) found that the brown carbon in western United States wildfires was mainly moderately absorptive BrC. And…”

Examination of brown carbon absorption from wildfires in the western US during the WE-CAN study

“…In that work, a ratio of 2 for Mie-calculated water-soluble Abs 365 to measured water-soluble Abs 365 and a ratio of 3.6 for Mie-calculated total Abs 365 to measured water-soluble Abs 365 were observed. In Zeng et al (2022), Mie theory was used to calculate the factor to convert solution to particle light absorption (i.e., ratio of Mie-calculated to measured water-soluble ab-sorption) as a function of wavelength for the FIREX data. At 405 nm a factor of ∼ 1.7 was determined, similar to what was determined from the WE-CAN data.…”