Diel Cycle Impacts on the Chemical and Light Absorption Properties of Organic Carbon Aerosol from Wildfires in the Western United States

Abstract. 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 (MIPN) spectrometer in conjunction with a suite of instruments, including an oxidation flow reactor equipped to generate hydroxyl (OH∙) or nitrate (NO3∙) radicals to mimic daytime or nighttime oxidative aging processes, we investigated the effects of multiple equivalent days or nights of OH∙/NO3∙ exposure on the chemical composition and mass absorption cross-sections (MAC(λ)) at 488 and 561 nm of OA emitted from wildfires in Arizona and Oregon. We found that OH∙ exposure reduced the wavelength-dependent MAC(λ) by a factor of 0.72 ± 0.08, consistent with previous laboratory studies. On the other hand, NO3∙ exposure increased it by a factor of up to 1.69 ± 0.38. The MAC enhancement following NO3∙ exposure was correlated with an enhancement in CHO1N and CHOgt1N ion families measured with an aerosol mass spectrometer.