Light absorption by brown carbon over the South-East Atlantic Ocean

Abstract

Abstract. Biomass burning emissions often contain brown carbon (BrC), which represents a large family of light-absorbing organics that are chemically complex, thus making it difficult to estimate their absorption of incoming solar radiation, resulting in large uncertainties in the estimation of the global direct radiative effect of aerosols. Here we investigate the contribution of BrC to the total light absorption of biomass burning aerosols over the South-East Atlantic Ocean with different optical models, utilizing a suite of airborne measurements from the ORACLES 2018 campaign. An effective refractive index of black carbon (BC), meBC=1.95+ikeBC, that characterizes the absorptivity of all absorbing components at 660 nm wavelength was introduced to facilitate the attribution of absorption at shorter wavelengths, i.e. 470 nm. Most values of the imaginary part of the effective refractive index, keBC, were larger than those commonly used for BC from biomass burning emissions, suggesting contributions from absorbers besides BC at 660 nm. The TEM-EDX single-particle analysis further suggests that these long-wavelength absorbers might include iron oxides, as iron is found to be present only when large values of keBC are derived. Using this effective BC refractive index, we find that the contribution of BrC to the total absorption at 470 nm (RBrC,470) ranges from ∼8 %–22 %, with the organic aerosol mass absorption coefficient (MACOA,470) at this wavelength ranging from 0.30±0.27 to 0.68±0.08 m2 g−1. The core–shell model yielded much higher estimates of MACOA,470 and RBrC,470 than homogeneous mixing models, underscoring the importance of model treatment. Absorption attribution using the Bruggeman mixing Mie model suggests a minor BrC contribution of 4 % at 530 nm, while its removal would triple the BrC contribution to the total absorption at 470 nm obtained using the AAE (absorption Ångström exponent) attribution method. Thus, it is recommended that the application of any optical properties-based attribution method use absorption coefficients at the longest possible wavelength to minimize the influence of BrC and to account for potential contributions from other absorbing materials.

Shortwave absorption by wildfire smoke dominated by dark brown carbon

“…It is noteworthy that the distribution of BC among diverse particles of varied composition, as well as the assumed value for MAC BC and the fixed absorption enhancement factor, could give rise to deviation in the assumed mean AÅE value of 1 (refs. 49 , 51 ). The light absorption coefficient by the non-BC component at a particular wavelength was estimated as the difference between the b abs and the b abs,BC values.…”

Seasonal variations in fire conditions are important drivers in the trend of aerosol optical properties over the south-eastern Atlantic

“…Figure 2 shows the evaluated contribution from BrC on total absorption at 464 nm is generally around 1 %-2 %, suggesting BrC has a minimal influence on the aerosol absorption measured on ASI. Zhang et al (2022) found around ∼ 10 % of BrC at 470 nm near the African continent, while Taylor et al (2020) found ∼ 10 % BrC at a much lower wavelength (405 nm) in the free troposphere near ASI. In contrast to these measurements in the free troposphere, our observed BB plumes are generally considered to have experienced cloud processing, which could further reduce the BrC content (Che et al, 2022a).…”

“…AAE 529-648 is the absorption Ångström exponent calculated between 529 and 648 nm. This method is similar to the one used by Taylor et al (2020); however, it may bring uncertainties as the AAE of BC is not always 1 and the MAC BC at longer wavelengths might contain contributions from other absorbing materials, such as dust (Lack and Langridge, 2013;Zhang et al, 2022). Note that most of the BC observation for September and October 2017 is unreliable due to issues with the inlet system.…”

“…Although this can prove the presence of Fe-bearing aerosols on ASI, the limitations of the sample numbers make it difficult to estimate the proportion of Febearing aerosols and whether they contribute significantly to the observed E abs . Zhang et al (2022) noticed a small contribution of Fe-bearing aerosols in the absorption based on an optical closure study with filter samples from the ORACLES aircraft campaign (ObseRvations of Aerosols above CLouds and their intEractionS) near the African continent. However, even considering that a 10 % absorption contribution is from dust, the upper limit, according to their conclusions, is still not enough to explain the high value of E abs we observed on ASI.…”

Enhanced Light Absorption and Elevated Viscosity of Atmospheric Brown Carbon through Evaporation of Volatile Components

“…BrC exhibits a broad range of optical properties due to its diverse sources and dynamic transformations during atmospheric aging. − Recent studies have begun revealing the relationship between BrC’s optical properties and its chemical composition. − Accordingly, BrC is classified into four optically based groups: very weakly (VW), weakly (W), moderately (M), or strongly (S)absorbing BrC . The VW group portrays BrC formed in secondary organic aerosols, while the W and M groups represent BrC originating from primary BB emissions. − The S group delineates highly viscous particles that have undergone significant transformations, such as refractory carbonaceous spheres (tar balls). , Chemical analysis of BrC materials representing these groups has revealed distinct physicochemical properties, including unique wavelength absorption spectra, molecular weight, elemental and structural composition, oxidation state, volatility, aqueous solubility, and more. ,,,, These characteristics serve as metrics for comparing and apportioning BrC observed in real-world studies within the proposed optically based classification framework.…”