Vertical variability of the properties of highly aged biomass burning aerosol transported over the southeast Atlantic during CLARIFY-2017

Abstract

Abstract. Seasonal biomass burning (BB) from June to October in central and southern Africa leads to absorbing aerosols being transported over the South Atlantic Ocean every year and contributes significantly to the regional climate forcing. The vertical distribution of submicron aerosols and their properties were characterized over the remote southeast Atlantic, using airborne in situ measurements made during the CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017 (CLARIFY-2017) campaign. BB aerosols emitted from flaming-controlled fires were intensively observed in the region surrounding Ascension Island, in the marine boundary layer (MBL) and free troposphere (FT) up to 5 km. We show that the aerosols had undergone a significant ageing process during > 7 d transit from source, as indicated by the highly oxidized organic aerosol. The highly aged BB aerosols in the far-field CLARIFY region were also especially rich in black carbon (BC), with relatively low single-scattering albedos (SSAs), compared with those from other BB transported regions. The column-weighted dry SSAs during CLARIFY were observed to be 0.85, 0.84 and 0.83 at 405, 550 and 658 nm respectively. We also found significant vertical variation in the dry SSA, as a function of relative chemical composition and size. The lowest SSA in the column was generally in the low FT layer around 2000 m altitude (averages: 0.82, 0.81 and 0.79 at 405, 550 and 658 nm). This finding is important since it means that BB aerosols across the southeast Atlantic region are more absorbing than currently represented in climate models, implying that the radiative forcing from BB may be more strongly positive than previously thought. Furthermore, in the FT, average SSAs at 405, 550 and 658 nm increased to 0.87, 0.86 and 0.85 with altitude up to 5 km. This was associated with an enhanced inorganic nitrate mass fraction and aerosol size, likely resulting from increased partitioning of ammonium nitrate to the existing particles at higher altitude with lower temperature and higher relative humidity. After entrainment into the boundary layer (BL), aerosols were generally smaller in dry size than in the FT and had a larger fraction of scattering material with resultant higher average dry SSA, mostly due to marine emissions and aerosol removal by drizzle. In the BL, the SSA decreased from the surface to the BL top, with the highest SSA in the column observed near the surface. Our results provide unique observational constraints on aerosol parameterizations used in modelling regional radiation interactions over this important region. We recommend that future work should consider the impact of this vertical variability on climate models.

The CLoud–Aerosol–Radiation Interaction and Forcing: Year 2017 (CLARIFY-2017) measurement campaign

“…The size distributions for BBA from Peers et al (2019) and Wu et al (2021) are consistent with those determined from SAFARI-2000, although Haywood et al (2003) chose to describe the accumulation mode with two log-normal distributions rather than a single log-normal distribution. The corresponding refractive indices retrieved over the CLAR-IFY period (16 August-7 September 2017) derived from AERONET Version 2 algorithms for the Ascension Island site are 1.47-0.020i at a wavelength of 550 nm.…”

“…Thus, in general, the aerosol at lower altitudes can be significantly older compared to that located at elevated altitudes (see analysis of ORACLES data by Dobracki et al, 2021). There is clear evidence from both Wu et al (2021) and Dobracki et al (2021) that aerosol higher up in the residual CBL exhibits a higher SSA (i.e. it is less absorbing on a per particle basis) than that lower down.…”

“…An extensive set of measurements of stratocumulus clouds has been performed during VOCALS off the Pacific coast of South America (Wood et al, 2011), with one of the foci being aerosol-cloud interactions (e.g. Yang et al, 2011;Painemal and Zuidema, 2013). However, the aerosol composition, sources, and interaction with the clouds in the VOCALS region are very different to those over the SE Atlantic, which is dominated by relatively strongly absorbing biomass burning aerosol (e.g.…”

Observation of absorbing aerosols above clouds over the south-east Atlantic Ocean from the geostationary satellite SEVIRI – Part 2: Comparison with MODIS and aircraft measurements from the CLARIFY-2017 field campaign

“…The relationship between the top-of-cloud reflectance and the COT is convex, which means that the COT derived from the mean reflectance of a pixel is smaller than the COT calculated from the mean COTs within the pixel. Zeng et al (2012) have shown that subpixel inhomogeneities cause satellite sensors with a coarser spatial resolution, such as SEVIRI, to retrieve a smaller COT. Also, as the clouds becomes thicker, the visible and the near-infrared (NIR) measurements become less sensitive to the COT for both instruments.…”

Unexpected Biomass Burning Aerosol Absorption Enhancement Explained by Black Carbon Mixing State

“…A number of field campaigns—including Dynamics‐Aerosol‐Chemistry‐Clouds Interactions in West Africa (DACCIWA), Aerosols, Radiation and Clouds in southern Africa (AEROCLO‐sA), Layered Atlantic Smoke Interactions with Clouds (LASIC), ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), and Cloud and Aerosols Radiative Impact and Forcing (CLARIFY)—have recently been carried out over the eastern Atlantic Ocean, with the aim to better quantify the BBA radiative effects (Flamant, Knippertz, et al, 2018; Formenti et al, 2019; Zuidema et al, 2016, 2018). Both in situ and remote‐sensing observations acquired during these campaigns have all emphasized a strong shortwave (SW) absorption (low single scattering albedo, SSA ) in BBA transported from the coast of southern Africa to the far north over southern West Africa (SWA) (Chylek et al, 2019; Denjean et al, 2020; Pistone et al, 2019; Wu et al, 2020; Zuidema et al, 2018). This means that BBA over this climatically important region is more absorbing than is currently represented in climate models (Mallet et al, 2019; Peers et al, 2016; Stier et al, 2013).…”