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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. Recent studies have shown that organic aerosol (OA) could have a nontrivialrole in atmospheric light absorption at shorter visible wavelengths. Goodestimates of OA light absorption are therefore necessary to better estimateradiative forcing due to these aerosols in climate models. One of the commontechniques used to measure OA light absorption is the solvent extractiontechnique from filter samples which involves the use of a spectrophotometerto measure bulk absorbance by the solvent-soluble organic fraction ofparticulate matter. Measured solvent-phase absorbance is subsequentlyconverted to particle-phase absorption coefficient using scaling factors.The conventional view is to apply a correction factor of 2 to absorptioncoefficients obtained from solvent-extracted OA based on Mie calculations.The appropriate scaling factors are a function of biases due to incompleteextraction of organic carbon (OC) by solvents and size-dependent absorption properties of OA.The range for these biases along with their potential dependence on burnconditions is an unexplored area of research. Here, we performed a comprehensive laboratory study involving three solvents(water, methanol, and acetone) to investigate the bias in absorptioncoefficients obtained from solvent-extraction-based photometry techniques ascompared to in situ particle-phase absorption for freshly emitted OA frombiomass burning. We correlated the bias with OC∕TC (total carbon) mass ratio and singlescattering albedo (SSA) and observed that the conventionally used correctionfactor of 2 for water and methanol-extracted OA might not be extensible toall systems, and we suggest caution while using such correction factors toestimate particle-phase OA absorption coefficients. Furthermore, a linearcorrelation between SSA and the OC∕TC ratio was also established. Finally, fromthe spectroscopic data, we analyzed the differences in absorptionÅngström exponents (AÅE) obtained from solution- andparticulate-phase measurements. We noted that AÅE fromsolvent-phase measurements could deviate significantly from their OAcounterparts.