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Abstract. Light absorbing organic carbon, or brown carbon (BrC), can be a significantcontributor to the visible light absorption budget. However, the sources ofBrC and the contributions of BrC to light absorption are not wellunderstood. Biomass burning is thought to be a major source of BrC.Therefore, as part of the WE-CAN (Western Wildfire Experiment for CloudChemistry, Aerosol Absorption and Nitrogen) study, BrC absorption data werecollected on board the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C-130 aircraft as it intercepted smoke fromwildfires in the western US in July–August 2018. BrC absorptionmeasurements were obtained in near real-time using two techniques. The firstcoupled a particle-into-liquid sampler (PILS) with a liquid waveguidecapillary cell and a total organic carbon analyzer for measurements ofwater-soluble BrC absorption and WSOC (water-soluble organic carbon). Thesecond employed a custom-built photoacoustic aerosol absorption spectrometer(PAS) to measure total absorption at 405 and 660 nm. The PAS BrC absorption at 405 nm (PAS total Abs 405 BrC) was calculated by assuming the absorption determined by the PAS at 660 nm was equivalent to the black carbon (BC) absorption and the BC aerosol absorption Ångström exponent was 1. Data from the PILS and PAS were combined to investigate the water-soluble vs. total BrC absorption at 405 nm in the various wildfire plumes sampled during WE-CAN. WSOC, PILS water-soluble Abs 405, and PAS total Abs 405 tracked each other in and out of the smoke plumes. BrC absorption was correlated with WSOC (R2 value for PAS =0.42 and PILS =0.60) and CO (carbon monoxide) (R2 value for PAS =0.76 and PILS =0.55) for all wildfires sampled. The PILS water-soluble Abs 405 was corrected for thenon-water-soluble fraction of the aerosol using the calculated UHSAS(ultra-high-sensitivity aerosol spectrometer) aerosol mass. The correctedPILS water-soluble Abs 405 showed good closure with the PAS total Abs 405BrC with a factor of ∼1.5 to 2 difference. This differencewas explained by particle vs. bulk solution absorption measured by the PASvs. PILS, respectively, and confirmed by Mie theory calculations. DuringWE-CAN, ∼ 45 % (ranging from 31 % to 65 %) of the BrCabsorption was observed to be due to water-soluble species. The ratio of BrC absorption to WSOC or ΔCO showed no clear dependence on firedynamics or the time since emission over 9 h.
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Shortwave absorption by wildfire smoke dominated by dark brown carbon
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.
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- Award ID(s):
- 1926817
- PAR ID:
- 10439314
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Geoscience
- Volume:
- 16
- Issue:
- 8
- ISSN:
- 1752-0894
- Page Range / eLocation ID:
- p. 683-688
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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