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Aerosol absorption of visible light has an important impact on global radiative forcing. Wildfires are one of the major sources of light-absorbing aerosol, but there remains significant uncertainty about the magnitude, wavelength dependence and bleaching of absorption from biomass burning aerosol. We collected and analyzed data from 21 Western United States wildfire smoke plumes during the 2018 WE-CAN airborne measurement campaign to determine the contribution of black carbon (BC), brown carbon (BrC), and lensing to the aerosol mass absorption cross-section (MAC). MACBC, MAC of organics (MACBrC+lensing), and the MAC of water-soluble BrC (MACws_BrC660) are calculated using Photoacoustic Absorption Spectrometer, Single Particle Soot Photometer and Particle-into-Liquid Sampler measurements. MACBC660 does not change significantly with physical age, organic aerosol (OA) concentration, oxygen to carbon ratio (O:C), gas-phase toluene:benzene ratio, modified combustion efficiency (MCE), altitude, or temperature, and has a relatively stable average value of 10.9 ± 2.1 m2 g-1. On average, 54 % of non-BC absorption (23 % total absorption) at 660 nm is from water-soluble BrC. MACws_BrC660 is 0.06 ± 0.04 m2 g-1 while MACBrC+lensing is 0.11 ± 0.06 m2 g-1 at 660 nm, increasing to 0.59 ± 0.19 m2 g-1 at 405 nm. MACBrC+lensing is constant with physical age and MCE, but increases slightly with increasing O:C or decreasing toluene:benzene, while total absorption (normalized to CO) slightly decreases with increasing O:C or decreasing toluene:benzene due to decreasing OA. No evidence of BrC bleaching is observed. Comparison to commonly used parameterizations, modeling studies, and the FIREX-AQ observations suggest model overestimation of absorption is likely due to incorrect BrC refractive indices. Quantification of significant brown carbon in the red wavelengths and the stability of MACBC, the observation of minimal bleaching, and the observation of changes in OA with O:C and toluene:benzene markers all serve as important constraints on aerosol absorption in regional and global climate models. [ABSTRACT FROM AUTHOR]

Aerosol absorption of visible light has an important impact on global radiative forcing. Wildfires are one of the major sources of light-absorbing aerosol, but there remains significant uncertainty about the magnitude, wavelength dependence and bleaching of absorption from biomass burning aerosol. We collected and analyzed data from 21 Western United States wildfire smoke plumes during the 2018 WE-CAN airborne measurement campaign to determine the contribution of black carbon (BC), brown carbon (BrC), and lensing to the aerosol mass absorption cross-section (MAC). MACBC, MAC of organics (MACBrC+lensing), and the MAC of water-soluble BrC (MACws_BrC660) are calculated using Photoacoustic Absorption Spectrometer, Single Particle Soot Photometer and Particle-into-Liquid Sampler measurements. MACBC660 does not change significantly with physical age, organic aerosol (OA) concentration, oxygen to carbon ratio (O:C), gas-phase toluene:benzene ratio, modified combustion efficiency (MCE), altitude, or temperature, and has a relatively stable average value of 10.9 ± 2.1 m² g-1. On average, 54% of non-BC absorption (23% total absorption) at 660 nm is from water-soluble BrC. MACws_BrC660 is 0.06 ± 0.04 m² g-1 while MACBrC+lensing is 0.11 ± 0.06 m² g-1 at 660 nm, increasing to 0.59 ± 0.19 m² g-1 at 405 nm. MACBrC+lensing is constant with physical age and MCE, but increases slightly with increasing O:C or decreasing toluene:benzene, while total absorption (normalized to CO) slightly decreases with increasing O:C or decreasing toluene:benzene due to decreasing OA. No evidence of BrC bleaching is observed. Comparison to commonly used parameterizations, modeling studies, and the FIREX-AQ observations suggest model overestimation of absorption is likely due to incorrect BrC refractive indices. Quantification of significant brown carbon in the red wavelengths and the stability of MACBC, the observation of minimal bleaching, and the observation of changes in OA with O:C and toluene:benzene markers all serve as important constraints on aerosol absorption in regional and global climate models. [ABSTRACT FROM AUTHOR]

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 were collected on board the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C-130 aircraft as it intercepted smoke from wildfires in the western US 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. [ABSTRACT FROM AUTHOR]

Other Test Method 33A (OTM 33A) is a near-source flux measurement method developed by the Environmental Protection Agency (EPA) primarily used to locate and estimate emission fluxes of methane from oil and gas (O&G) production facilities without requiring site access. A recent national estimate of methane emissions from O&G production included a large number of flux measurements of upstream O&G facilities made using OTM 33A and concluded the EPA National Emission Inventory underestimates this sector by a factor of ∼2.1. The study presented here investigates the accuracy of OTM 33A through a series of test releases performed at the Methane Emissions Technology Evaluation Center (METEC), a facility designed to allow quantified amounts of natural gas to be released from decommissioned O&G equipment to simulate emissions from real facilities (Fig.). This study includes test releases from single and multiple points, from equipment locations at different heights, and spanned methane release rates ranging from 0.16 to 2.15 kg h -1. Approximately 95 % of individual measurements (N=45) fell within ±70 % of the known release rate. A simple linear regression of OTM 33A versus known release rates at the METEC site gives an average slope of 0.96 with 95 % CI (0.66,1.28) , suggesting that an ensemble of OTM 33A measurements may have a small but statistically insignificant low bias. [ABSTRACT FROM AUTHOR]

Other Test Method 33A (OTM 33A) is a near-source flux measurement method developed by the Environmental Protection Agency (EPA) primarily used to locate and estimate emission fluxes of methane from oil and gas (O&G) production facilities without requiring site access. A recent nation-wide estimate of methane emissions from O&G production included a large number of flux measurements of upstream O&G facilities made using OTM 33A and concluded the EPA National Emission Inventory underestimates this sector by a factor of ~ 2.1 (Alvarez et al., 2018). The study presented here investigates the accuracy of OTM 33A through a series of test releases performed at the Methane Emissions Technology Evaluation Center (METEC), a facility designed to allow quantified amounts of natural gas to be released from decommissioned O&G equipment to simulate emissions from real facilities (Fig. 1). This study includes test releases from single and multiple points, from equipment locations at different heights, and spanned methane release rates ranging from 0.16 to 2.15 kg h-1. Approximately 95 % of individual measurements (N = 45) fell within ±70 % of the known release rate. A simple linear regression of OTM 33A versus known release rates at the METEC site gives an average slope of 0.96 with 95 % CI (0.66,1.28), suggesting that an ensemble of OTM 33A measurements may have a small but statistically insignificant low bias. [ABSTRACT FROM AUTHOR]

A wide range of globally significant biomass fuels were burned during the fourth Fire Lab at Missoula Experiment (FLAME-4). A multi-channel photoacoustic absorption spectrometer (PAS) measured dry absorption at 405, 532, and 660 nm and thermally denuded (250 °C) absorption at 405 and 660 nm. Absorption coefficients were broken into contributions from black carbon (BC), brown carbon (BrC), and lensing following three different methodologies, with one extreme being a method that assumes the thermal denuder effectively removes organics and the other extreme being a method based on the assumption that black carbon (BC) has an Ångström exponent of unity. The methodologies employed provide ranges of potential importance of BrC to absorption but, on average, there was a difference of a factor of 2 in the ratio of the fraction of absorption attributable to BrC estimated by the two methods. BrC absorption at shorter visible wavelengths is of equal or greater importance to that of BC, with maximum contributions of up to 92% of total aerosol absorption at 405 nm and up to 58% of total absorption at 532 nm. Lensing is estimated to contribute a maximum of 30% of total absorption, but typically contributes much less than this. Absorption enhancements and the estimated fraction of absorption from BrC show good correlation with the elemental-carbon-to-organic-carbon ratio (EC/OC) of emitted aerosols and weaker correlation with the modified combustion efficiency (MCE). Previous studies have shown that BrC grows darker (larger imaginary refractive index) as the ratio of black to organic aerosol (OA) mass increases. This study is consistent with those findings but also demonstrates that the fraction of total absorption attributable to BrC shows the opposite trend: increasing as the organic fraction of aerosol emissions increases and the EC/OC ratio decreases. [ABSTRACT FROM AUTHOR]

A wide range of globally significant biomass fuels were burned during the fourth Fire Lab at Missoula Experiment (FLAME-4). A multi-channel photoacoustic absorption spectrometer (PAS) measured dry absorption at 405, 532, and 660 nm and thermally denuded (250 �C) absorption at 405 and 660 nm. Absorption coefficients were broken into contributions from black carbon (BC), brown carbon (BrC) and lensing following three different methodologies, with one extreme being a method that assumes the thermal denuder effectively removes organics and the other extreme being a method based on the assumption that black carbon (BC) has an angstrom exponent of unity. The methodologies employed provide ranges of potential importance of BrC to absorption but, on average, there was a factor of 2 difference in the ratio of the fraction of absorption attributable to brown carbon estimated by the two methods. BrC absorption at shorter visible wavelengths is of equal or greater importance to that of BC with maximum contributions of up to 92 % of total aerosol absorption at 405 nm and up to 58 % of total absorption at 532 nm. Lensing is estimated to contribute a maximum of 30 % of total absorption, but typically contributes much less than this. Absorption enhancements and the estimated fraction of absorption from BrC show good correlation with the elemental to organic carbon ratio (EC/OC) of emitted aerosols and weaker correlation with the modified combustion efficiency (MCE). Previous studies have shown that brown carbon grows darker (larger imaginary refractive index) as the ratio of black to organic aerosol (OA) mass increases. This study is consistent with those findings but also demonstrates that the fraction of total absorption attributable to BrC shows the opposite trend: increasing as the organic fraction of aerosol emissions increases and the EC/OC ratio decreases. [ABSTRACT FROM AUTHOR]

Single-scattering albedo (SSA) and absorption Ångström exponent (AAE) are two critical parameters in determining the impact of absorbing aerosol on the Earth's radiative balance. Aerosol emitted by biomass burning represent a significant fraction of absorbing aerosol globally, but it remains difficult to accurately predict SSA and AAE for biomass burning aerosol. Black carbon (BC), brown carbon (BrC), and non-absorbing coatings all make substantial contributions to the absorption coefficient of biomass burning aerosol. SSA and AAE cannot be directly predicted based on fuel type because they depend strongly on burn conditions. It has been suggested that SSA can be effectively parameterized via the modified combustion efficiency (MCE) of a biomass burning event and that this would be useful because emission factors for CO and CO2, from which MCE can be calculated, are available for a large number of fuels. Here we demonstrate, with data from the FLAME-4 experiment, that for a wide variety of globally relevant biomass fuels, over a range of combustion conditions, parameterizations of SSA and AAE based on the elemental carbon (EC) to organic carbon (OC) mass ratio are quantitatively superior to parameterizations based on MCE.We show that the EC/OC ratio and the ratio of EC / (EC+OC) both have significantly better correlations with SSA than MCE. Furthermore, the relationship of EC / (EC+OC) with SSA is linear. These improved parameterizations are significant because, similar to MCE, emission factors for EC (or black carbon) and OC are available for a wide range of biomass fuels. Fitting SSA with MCE yields correlation coefficients (Pearson's r) of ~0.65 at the visible wavelengths of 405, 532, and 660 nm while fitting SSA with EC / OC or EC / (EC+OC) yields a Pearson's r of 0.94-0.97 at these same wavelengths. The strong correlation coefficient at 405 nm (r =0.97) suggests that parameterizations based on EC / OC or EC / (EC+OC) have good predictive capabilities even for fuels in which brown carbon absorption is significant. Notably, these parameterizations are effective for emissions from Indonesian peat, which have very little black carbon but significant brown carbon (SSA=0.990±0.001 at 532 and 660 nm, SSA=0.937±0.011 at 405 nm). Finally, we demonstrate that our parameterization based on EC / (EC+OC) accurately predicts SSA during the first few hours of plume aging with data from Yokelson et al. (2009) gathered during a biomass burning event in the Yucatán Peninsula of Mexico. [ABSTRACT FROM AUTHOR]