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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 The NOAA/NASA Fire Influence on Regional to Global Environments and Air Quality (FIREX‐AQ) experiment was a multi‐agency, inter‐disciplinary research effort to: (a) obtain detailed measurements of trace gas and aerosol emissions from wildfires and prescribed fires using aircraft, satellites and ground‐based instruments, (b) make extensive suborbital remote sensing measurements of fire dynamics, (c) assess local, regional, and global modeling of fires, and (d) strengthen connections to observables on the ground such as fuels and fuel consumption and satellite products such as burned area and fire radiative power. From Boise, ID western wildfires were studied with the NASA DC‐8 and two NOAA Twin Otter aircraft. The high‐altitude NASA ER‐2 was deployed from Palmdale, CA to observe some of these fires in conjunction with satellite overpasses and the other aircraft. Further research was conducted on three mobile laboratories and ground sites, and 17 different modeling forecast and analyses products for fire, fuels and air quality and climate implications. From Salina, KS the DC‐8 investigated 87 smaller fires in the Southeast with remote and in‐situ data collection. Sampling by all platforms was designed to measure emissions of trace gases and aerosols with multiple transects to capture the chemical transformation of these emissions and perform remote sensing observations of fire and smoke plumes under day and night conditions. The emissions were linked to fuels consumed and fire radiative power using orbital and suborbital remote sensing observations collected during overflights of the fires and smoke plumes and ground sampling of fuels.