Diel Cycle Impacts On The Chemical And Light Absorption Properties Of Organic Carbon Aerosol From Wildfires In The Western United States

Organic aerosol (OA) emissions from biomass burning have been the subject of intense research in recent years, involving a combination of field campaigns and laboratory studies. These efforts have aimed at improving our limited understanding of the diverse processes and pathways involved in the atmospheric processing and evolution of OA properties, culminating in their accurate parameterizations in climate and chemical transport models. To bring closure between laboratory and field studies, wildfire plumes in the western United States were sampled and characterized for their chemical and optical properties during the ground-based segment of the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign. Using a custom-developed multiwavelength integrated photoacoustic-nephelometer spectrometer in conjunction with a suite of instruments, including an oxidation flow reactor equipped to generate hydroxyl (OH center dot) or nitrate (NO3 center dot) radicals to mimic daytime or nighttime oxidative aging processes, we investigated the effects of multiple equivalent hours of OH center dot or NO3 center dot exposure on the chemical composition and mass absorption cross-sections (MAC(lambda)) at 488 and 561 nm of OA emitted from wildfires in Arizona and Oregon. We found that OH center dot exposure induced a slight initial increase in absorption corresponding to short timescales; however, at longer timescales, the wavelengthdependent MAC(lambda) decreased by a factor of 0.72 +/- 0.08, consistent with previous laboratory studies and reports of photobleaching. On the other hand, NO3 center dot exposure increased MAC(lambda) by a factor of up to 1.69 +/- 0.38. We also noted some sensitivity of aerosol aging to different fire conditions be- tween Arizona and Oregon. The MAC(lambda) enhancement following NO3 center dot exposure was found to correlate with an enhancement in CHO1N and CHOgt1N ion families measured by an Aerodyne aerosol mass spectrometer.