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Characteristics and evolution of brown carbon in western United States wildfires
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2022
Source: Atmospheric Chemistry and Physics, 22(12), 8009-8036
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Journal Title:Atmospheric Chemistry and Physics
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Description:Abstract. Brown carbon (BrC) associated with aerosol particles in western UnitedStates wildfires was measured between July and August 2019 aboard the NASADC-8 research aircraft during the Fire Influence on Regional to GlobalEnvironments and Air Quality (FIREX-AQ) study. Two BrC measurement methodsare investigated, highly spectrally resolved light absorption in solvent(water and methanol) extracts of particles collected on filters and in situbulk aerosol particle light absorption measured at three wavelengths (405,532 and 664 nm) with a photoacoustic spectrometer (PAS). A light-absorptionclosure analysis for wavelengths between 300 and 700 nm was performed. Thecombined light absorption of particle pure black carbon material, includingenhancements due to internally mixed materials, plus soluble BrC and aMie-predicted factor for conversion of soluble BrC to aerosol particle BrC,was compared to absorption spectra from a power law fit to the three PASwavelengths. For the various parameters used, at a wavelength of roughly 400nm they agreed, at lower wavelengths the individual component-predictedparticle light absorption significantly exceeded the PAS and at higherwavelengths the PAS absorption was consistently higher but more variable.Limitations with extrapolation of PAS data to wavelengths below 405 nm andmissing BrC species of low solubility that more strongly absorb at higherwavelengths may account for the differences. Based on measurements closestto fires, the emission ratio of PAS-measured BrC at 405 nm relative tocarbon monoxide (CO) was on average 0.13 Mm−1 ppbv−1; emissionratios for soluble BrC are also provided. As the smoke moved away from theburning regions, the evolution over time of BrC was observed to be highlycomplex; BrC enhancement, depletion or constant levels with age were allobserved in the first 8 h after emission in different plumes. Within 8 h following emissions, 4-nitrocatechol, a well-characterized BrCchromophore commonly found in smoke particles, was largely depleted relativeto the bulk BrC. In a descending plume where temperature increased by 15 K,4-nitrocatechol dropped, possibly due to temperature-driven evaporation, butbulk BrC remained largely unchanged. Evidence was found for reactions withozone, or related species, as a pathway for secondary formation of BrC underboth low and high oxides of nitrogen (NOx) conditions, while BrC wasalso 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 ishighly variable, a limited number of measurements of more aged smoke (15 to30 h) indicate a net loss of BrC. It is yet to be determined how thenear-field BrC evolution in smoke affects the characteristics of smoke overlonger timescales and spatial scales, where its environmental impacts are likelyto be greater.
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Source:Atmospheric Chemistry and Physics, 22(12), 8009-8036
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ISSN:1680-7324
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Rights Information:CC BY
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Compliance:Submitted
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