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 is chemically complex and therefore 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 by introducing an effective refractive index of black carbon (BC), meBC = neBC+ikeBC, that accounts for all possible absorbing components at 660 nm wavelength to facilitate the attribution of absorption at shorter wavelengths. Most values of the imaginary part of the refractive index, keBC, were larger than those commonly used for BC from biomass burning emissions, suggesting contributions from absorbers beyond 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 ~5–15 %, with the organic aerosol mass absorption coefficient (MACOA,470) at this wavelength ranging from 0.25 ± 0.34 m2 g−1 to 0.43 ± 0.12 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. Another key finding was that estimates of the BrC contribution at 470 nm from the commonly used AAE (absorption Ångström exponent) attribution method (< 5 %) are much lower than the BrC contribution estimates (RBrC,470) using our new methodology that accounts for contributions from both BrC and non-carbonaceous, long-wavelength absorbers, such as magnetite. Thus, it is recommended that application of any optical properties-based attribution method use absorption coefficients at the longest possible wavelength to minimize the influence of BrC at the long wavelength and to account for potential contributions from other absorbing materials.

An attribution of the low single-scattering albedo of biomass-burning aerosol over the southeast Atlantic

“…Following Carter et al (2021), we also evaluate the MAC relative to the BC+OA mass concentration at a λ of 470 nm. At λ=660 nm, the solar absorption will be dominated by the black carbon, whereas at λ=470 nm the MAC BC+OA will contain contributions from both OA-induced brown carbon as well as other wavelength-dependent absorbers (Zhang et al, 2022). -S8).…”

“…Taylor et al ( 2020) place an upper estimate of 11% on shortwave absorption by brown carbon (BrC) at 405 nm wavelength by the time the BBA plume reaches Ascension Island, with . Zhang et al (2022) indicate that other non-BrC materials absorb sunlight over the southeast Atlantic as a function of wavelength, so that BrC may contribute even less than < 10% of the total.…”

“…2020), but does suggest that perhaps some of the secondary OA is absorbing of sunlight. An important caveat remains that the 470 nm wavelength may already be too long to be responsive to additional absorption by OA-produced BrC (Zhang et al, 2022). Another assessment of brown carbon absorption can be done using the mass absorption coefficients at 470 nm relative to the sum of the BC and OA mass concentration (MAC 470,BC+OA ; following Carter et al, 2021) (Fig.…”

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

“…2 shows the evaluated contribution from BrC on total absorption at 464 nm is generally around 1-2%, suggesting BrC have a minimal influence on the 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 Ascension Island. In contrast to these measurements in the free 220 troposphere, our observed BB plumes are generally considered to have experienced cloud processing, which could further reduce the BrC content (Che et al, 2022a).…”

“…This method is similar to the one used by Taylor et al (2020), while it may bring uncertainties as the AAE of BC is not always , 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.…”