Examination of Brown Carbon Absorption from Wildfires in the Western U.S. During the WE-CAN Study

Abstract

Abstract. Light absorbing organic carbon, or brown carbon (BrC), can be a significant contributor to the visible light absorption budget. However, the sources of BrC and the contributions of BrC to light absorption are not well understood. Biomass burning is thought to be a major source of BrC. Therefore, as part of the WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen) Study BrC absorption data was collected aboard the NSF/NCAR C-130 aircraft as it intercepted smoke from wildfires in the Western U.S. in July–August 2018. BrC absorption measurements were obtained in near real-time using two techniques. The first coupled a Particle-into-Liquid Sampler (PILS) with a Liquid Waveguide Capillary Cell and a Total Organic Carbon analyzer for measurements of water-soluble BrC absorption and WSOC (water-soluble organic carbon). The second 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 the non-water-soluble fraction of the aerosol using the calculated UHSAS (Ultra-High-Sensitivity Aerosol Spectrometer) aerosol mass. The corrected PILS water-soluble Abs 405 showed good closure with the PAS total Abs 405 BrC with a factor of ~1.5 to 2 difference. This difference was explained by particle vs. bulk solution absorption measured by the PAS vs. PILS, respectively, and confirmed by Mie Theory calculations. During WE-CAN, ~45 % (ranging from 31 % to 65 %) of the BrC absorption was observed to be due to water-soluble species. The ratio of BrC absorption to WSOC or ΔCO showed no clear dependence on fire dynamics or the time since emission over 9 h.

Evolution of Brown Carbon Aerosols during Atmospheric Long-Range Transport in the South Asian Outflow and Himalayan Cryosphere

“…; Table S1 of the Supporting Information) were converted to aerosol particle absorption using the Mie theory while calculating SFE. Many recent studies ,− have experimentally validated the applicability of the Mie theory − for the same. Further, only the WS fraction of BrC was considered to convert from bulk to aerosol particle absorption for SFE calculation.…”

“…It is noted that the absorption properties retrieved from these studies were based on bulk aqueous solution absorbance measurements. Although solution absorption is not a perfect reflection of aerosol absorption, it still serves the purpose of an intercomparison between the source region and receptor sites. ,, A number of studies experimentally investigated the conversion of bulk solution absorption to aerosol particle absorption and have ascertained the applicability of the Mie theory − for the same. ,− We used the Mie theory to convert from bulk to aerosol optical properties for the radiative forcing calculations (details are given in section S3 of the Supporting Information). Moreover, only the water-soluble (WS) fraction of BrC was considered during conversion from bulk to aerosol particle absorption in the Mie theory calculations.…”

Connecting the Light Absorption of Atmospheric Organic Aerosols with Oxidation State and Polarity

“…The light absorption spectrum of water- and methanol-soluble extracts (WS and MS) were also measured by a UV–vis spectrophotometer (Analytik-jena, Specord 50 Plus, Germany) in the wavelength range of 200–900 nm with 0.5 nm resolution. The measured attenuation of solution can be converted to the ambient σ abs using , where A (λ) is the light absorbance of solution at wavelength λ measured by UV–vis, where absorbance at λ = 900 nm (A900) is subtracted to minimize the interference of baseline shift assuming there is no absorption of BrC at near-infrared …”

“…The light absorption spectrum of water-and methanol-soluble extracts (WS and MS) were also measured by a UV−vis spectrophotometer (Analytik-jena, Specord 50 Plus, Germany) in the wavelength range of 200−900 nm with 0.5 nm resolution. The measured attenuation of solution can be converted to the ambient σ abs using 67,68 V V l ( ) ( ( ) (900)) ln (10) abs abs abs…”