Seasonal variations in fire conditions are important drivers to the trend of aerosol optical properties over the south-eastern Atlantic
- 1Department of Geophysics, Porter School, Tel Aviv University, Tel Aviv, 69978, Israel
- 2Bay Area Environmental Research Institute, NASA Ames Research Center, Moffett Field, CA, USA
- 3NASA Ames Research Center, Moffett Field, California, 94035, USA
- 4Universities Space Research Association, Columbia, Maryland, 21046, USA
- 5Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL, USA
- 6Brookhaven National Laboratory, Upton, NY, USA
- 7State Key Laboratory of Severe Weather (LASW) and Key Laboratory of Atmospheric Chemistry of CMA, Chinese Academy of Meteorological Sciences, Beijing 100081, China
- 8School of Meteorology, University of Oklahoma, Norman, OK, USA
Abstract. From June to October, southern Africa produces one-third of the global biomass burning (BB) emissions by widespread fires. BB aerosols are westward transported 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. A 17-month in-situ 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 BB seasons at ~ 1.96 and ~ 2.07 in 2016 and 2017. 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 (BC / ∆CO) is well correlated with SSA and MACBC, providing a simple way to estimate those aerosol optical characteristics in the south-eastern Atlantic. The exponential function we proposed can approximate SSA and MACBC with BC / ∆CO, and can better capture the growth of SSA as the decrease of BC / ∆CO, especially when BC / ∆CO is small. BC / ∆CO is influenced by combustion conditions and aerosol scavenging. From the analysis of the location of BB burning, the primary source fuel, the water content in the fuel, combined with the mean cloud cover and precipitation in the transport areas of BB plume, we conclude that the increase in BC / ∆CO 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 changing of the burning conditions during these periods. The decrease in BC / ∆CO 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.
Haochi Che et al.
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Haochi Che et al.
Haochi Che et al.
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