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

Abstract

Abstract. From June to October, southern Africa produces one-third of the global biomass burning (BB) emissions by widespread fires. BB aerosols are transported westward over the south-eastern Atlantic with the mid-tropospheric winds, resulting in significant radiative effects. Ascension Island (ASI) is located midway between Africa and South America. From June 2016 to October 2017, a 17-month in situ observation campaign on ASI found a low single-scattering albedo (SSA) as well as a high mass absorption cross-section of black carbon (MACBC), demonstrating the strong absorbing marine boundary layer in the south-eastern Atlantic. Here we investigate the monthly variations of critical optical properties of BB aerosols, i.e. SSA and MACBC, during the BB seasons and the driving factors behind these variations. Both SSA and MACBC increase from June to August and decrease in September and October. The average SSA during the BB seasons is 0.81 at 529 nm wavelength, with the highest mean ∼ 0.85 in October and the lowest ∼ 0.78 in August. The absorption enhancement (Eabs) derived from the MACBC shows similar trends with SSA, with the average during the whole of the BB seasons at ∼ 1.96 and ∼ 2.07 in 2016 and 2017, respectively. As the Eabs is higher than the ∼ 1.5 commonly adopted value by climate models, this result suggests the marine boundary layer in the south-eastern Atlantic is more absorbing than model simulations. We find the enhanced ratio of BC to CO (ΔBC/ΔCO, equal to BC/ΔCO as the BC background concentration is considered to be 0) is well correlated with SSA and MACBC, providing a simple way to estimate the aerosol optical characteristics in the south-eastern Atlantic. The exponential function we proposed can approximate SSA and MACBC with BC/ΔCO, and when BC/ΔCO is small it can capture the rapid growth of SSA as BC/ΔCO decreases. BC/ΔCO is influenced by combustion conditions and aerosol scavenging. From the analysis of the location of BB, the primary source fuel, the water content in the fuel, combined with the mean cloud cover and precipitation in the transport areas of the BB plume, we conclude that the increase in BC/ΔCO from June to August is likely to be caused by burning becoming more flaming. The reduction in the water content of fuels may be responsible for the change in the burning conditions from June to August. The decrease in BC/ΔCO in September and October may be caused by two factors, one being a lower proportion of flaming conditions, possibly associated with a decrease in mean surface wind speed in the burning area, and the other being an increase in precipitation in the BB transport pathway, leading to enhanced aerosol scavenging, which ultimately results in an increase in SSA and MACBC.

Burning conditions and transportation pathways determine biomass-burning aerosol properties in the Ascension Island marine boundary layer

“…These mass ratios can be used as an indicator of burning conditions at the source because rBC is chemically inert (Wang 2004;Cape et al, 2012;Lund et al, 2018) and the average lifetime of CO is approximately two months (Khalil et al, 1990), much longer than typical transport times. rBC:CO was used to classify fires as either inefficient or efficient, with inefficient fires having values of rBC:CO less than 0.01 and efficient fires having values of rBC:CO greater than 0.01, consistent with previous classifications (Vakkari et al, 2018;Che et al, 2022a). Surface RH fields provided by the NOAA National Center for Environmental Prediction (NCEP) reanalysis are independently used to assess the burning condition classification.…”

“…The BBA in the FT is highly absorbing of shortwave radiation (Pistone et al, 2019;Denjean et al, 2020a;Taylor et al, 2020;Wu et al, 2020;Dobracki et al, 2023) with August-September mean single scatting albedo (the ratio of the aerosol scattering coefficient to the sum of the aerosol scattering and absorption coefficients) at wavelength 530 nm (SSA530) values near 0.84 (Pistone et al, 2019;Wu et al, 2020. Measurements in the MBL at Ascension Island (-7.95˚N, -14.36˚E), a remote location in the tropical Atlantic, yield an even lower SSA530, with June-August monthly-mean values near 0.80 Che et al, 2022a). The lower SSA530 at Ascension Island in the MBL compared to that in the FT above the island has not been previously explained (Barrett et al, 2022;Sedlacek et al, 2022).…”

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

“…The absorption enhancement, E Abs , defined as the ratio of MAC BC to the value for uncoated BC reported by Bond and Bergstrom (2006), is 2.3 ± 0.1 for all three wavelengths. To the best of our knowledge, except for modelling or laboratory studies of thickly coated particles (Bond et al, 2006;Jacobson, 2012;Peng et al, 2016;Che et al, 2022a), such high values of E Abs , particularly at long visible wavelengths, are rarely reported in field measurements (Cui et al, 2016). Taylor et al (2020) presented relatively high MAC BC values of 20 ± 4, 15 ± 3, and 12 ± 2 m 2 g −1 at 405, 514, and 655 nm, respectively and an E Abs of 1.85 ± 0.45 for CLARIFY 2017 (Cloud-Aerosol-Radiation Interaction and Forcing 2017 measurement campaign; Haywood et al, 2021), which are generally comparable to or smaller than those for RF05_3.…”

Assessing Radiative Impacts of African Smoke Aerosols Over the Southeastern Atlantic Ocean

“…This influences not only the Earth's surface temperature but also the amount of energy needed to drive numerous atmospheric dynamic processes (Pan et al, 2020;Tian et al, 2017). Carbonaceous aerosols derived from biomass burning via natural and anthropogenic activities have extensive global source regions (Che et al, 2022). Numerous studies have examined the relationship between biomass burning aerosols and their ability to readily activate as cloud condensation nuclei (CCN) with respect to aerosol transport and regional/seasonal variability (Gallo et al, 2023;Logan et al, 2020;McCoy et al, 2021;Redemann et al, 2021;Zheng et al, 2022).…”

“…Several studies have been published regarding LASIC and focused primarily on continental and marine aerosols, cloud development, and biomass burning smoke plume physico‐chemical properties (Barrett et al., 2022; Che et al., 2022; Dedrick et al., 2022; Kuete et al., 2020; Marquardt Collow et al., 2020; Ryoo et al., 2021, 2022; Sedlacek et al., 2022; Zhang & Zuidema, 2019, 2021; Zuidema et al., 2016, 2018). This study seeks to relate the short‐term variability of biomass burning smoke plume radiative properties to their impacts on near‐surface and upper atmosphere radiative forcing and marine boundary layer cloud development.…”