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Brown carbon light absorptivity is associated with organic aerosol volatility and elemental carbon concentrations. 

Abstract 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. Organic aerosol (OA) emissions from biomass burning havebeen the subject of intense research in recent years, involving acombination of field campaigns and laboratory studies. These efforts haveaimed at improving our limited understanding of the diverse processes andpathways involved in the atmospheric processing and evolution of OAproperties, culminating in their accurate parameterizations in climate andchemical transport models. To bring closure between laboratory and fieldstudies, wildfire plumes in the western United States were sampled andcharacterized for their chemical and optical properties during theground-based segment of the 2019 Fire Influence on Regional to GlobalEnvironments and Air Quality (FIREX-AQ) field campaign. Using acustom-developed multiwavelength integrated photoacoustic-nephelometerspectrometer in conjunction with a suite of instruments, including anoxidation flow reactor equipped to generate hydroxyl (OH⚫) ornitrate (NO3⚫) radicals to mimic daytime or nighttimeoxidative aging processes, we investigated the effects of multipleequivalent hours of OH⚫ or NO3⚫ exposure onthe chemical composition and mass absorption cross-sections (MAC(λ)) at 488 and 561 nm of OA emitted from wildfires in Arizona and Oregon. Wefound that OH⚫ exposure induced a slight initial increase inabsorption corresponding to short timescales; however, at longer timescales, the wavelength-dependent MAC(λ) decreased by a factor of0.72 ± 0.08, consistent with previous laboratory studies and reportsof photobleaching. On the other hand, NO3⚫ exposure increasedMAC(λ) by a factor of up to 1.69 ± 0.38. We also noted somesensitivity of aerosol aging to different fire conditions between Arizonaand Oregon. The MAC(λ) enhancement following NO3⚫ exposure was found to correlate with an enhancement in CHO1N andCHOgt1N ion families measured by an Aerodyne aerosol mass spectrometer. 

Abstract Emission factors (EFs) are crucial in understanding the effects of wildfire emissions on air quality. We examined the variability of EFs of three wildfires (Nethker, Castle, and 204 Cow) during the 2019 Western US wildfire season using the Aerodyne Mobile Laboratory (AML) and compared them to previous studies. The AML sampling captured the high degree of variability present in wildfires, and we report results for a range of combustion conditions that is more extensive than previous field and laboratory studies. For instance, we captured emissions from freshly started flaming fuels and we report rare EF measurements at very high modified combustion efficiencies (MCEs); MCEs >0.9. Differences in emissions between AML‐observed wildfires were attributed to burning state/MCE rather than fuel type. A comparison of EFs versus MCE was made and linear fits were compared to previous observations to reveal important differences that incorporate these high MCEs. For some species, there remains an EF dependence on MCE at these high values, while others reach a minimum value and exhibit either no or a weak dependence above it. EF differences were found for many of the studied compounds when comparing ground‐based and airborne observations, with generally greater airborne EFs possibly due to photochemical oxidation. The largest differences were from monoterpenes and acetaldehyde. Comparisons were made between AML‐observed wildfires, aircraft observations, and the values in literature for EFs and emission ratios, with mixed agreement due to the high degree of variability caused by differences in MCE. Differences in MCE drove the diurnal EF differences.