Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke

Abstract

Black carbon (BC) aerosols influence the Earth’s atmosphere and climate, but their microphysical properties, spatiotemporal distribution, and long-range transport are not well constrained. This study presents airborne observations of the transatlantic transport of BC-rich African biomass burning (BB) smoke into the Amazon Basin using a Single Particle Soot Photometer (SP2) as well as several complementary techniques. We base our results on observations of aerosols and trace gases off the Brazilian coast onboard the HALO (High Altitude and LOng range) research aircraft during the ACRIDICON-CHUVA campaign in September 2014.{}/p{} {}p{}During flight AC19 over land and ocean at the northeastern coastline of the Amazon Basin, we observed a BC-rich layer at {}span class=“inline-formula”{}∼3.5{}/span{} km altitude with a vertical extension of {}span class=“inline-formula”{}∼0.3{}/span{} km. Backward trajectories suggest that fires in African grasslands, savannas, and shrublands were the main source of this pollution layer and that the observed BB smoke had undergone more than 10 d of atmospheric transport and aging over the South Atlantic before reaching the Amazon Basin. The aged smoke is characterized by a dominant accumulation mode, centered at about 130 nm, with a particle concentration of {}span class=“inline-formula”{}{}math xmlns=“http://www.w3.org/1998/Math/MathML” id=“M3” display=“inline” overflow=“scroll” dspmath=“mathml”{}{}mrow{}{}msub{}{}mi{}N{}/mi{}{}mi mathvariant=“normal”{}acc{}/mi{}{}/msub{}{}mo{}={}/mo{}{}mn mathvariant=“normal”{}850{}/mn{}{}mo{}±{}/mo{}{}mn mathvariant=“normal”{}330{}/mn{}{}/mrow{}{}/math{}{}span{}{}svg:svg xmlns:svg=“http://www.w3.org/2000/svg” width=“83pt” height=“12pt” class=“svg-formula” dspmath=“mathimg” md5hash=“e52a253c59b68f2a3e88c5951f4d18c9”{}{}svg:image xmlns:xlink=“http://www.w3.org/1999/xlink” xlink:href=“acp-20-4757-2020-ie00001.svg” width=“83pt” height=“12pt” src=“acp-20-4757-2020-ie00001.png”/{}{}/svg:svg{}{}/span{}{}/span{} cm{}span class=“inline-formula”{}\(^{\textrm{−3}}\){}/span{}. The rBC particles account for {}span class=“inline-formula”{}∼15{}/span{} % of the submicrometer aerosol mass and {}span class=“inline-formula”{}∼40{}/span{} % of the total aerosol number concentration. This corresponds to a mass concentration range from 0.5 to 2 {}span class=“inline-formula”{}µ{}/span{}g m{}span class=“inline-formula”{}\(^{\textrm{−3}}\){}/span{} (1st to 99th percentiles) and a number concentration range from 90 to 530 cm{}span class=“inline-formula”{}\(^{\textrm{−3}}\){}/span{}. Along with rBC, high {}span class=“inline-formula”{}\(_{\textrm{CO}}\){}/span{} ({}span class=“inline-formula”{}150±30{}/span{} ppb) and {}span class=“inline-formula”{}{}math xmlns=“http://www.w3.org/1998/Math/MathML” id=“M12” display=“inline” overflow=“scroll” dspmath=“mathml”{}{}mrow{}{}msub{}{}mi{}c{}/mi{}{}mrow class=“chem”{}{}msub{}{}mi mathvariant=“normal”{}O{}/mi{}{}mn mathvariant=“normal”{}3{}/mn{}{}/msub{}{}/mrow{}{}/msub{}{}/mrow{}{}/math{}{}span{}{}svg:svg xmlns:svg=“http://www.w3.org/2000/svg” width=“18pt” height=“12pt” class=“svg-formula” dspmath=“mathimg” md5hash=“f3967245e31a334b782549bf7ec73a28”{}{}svg:image xmlns:xlink=“http://www.w3.org/1999/xlink” xlink:href=“acp-20-4757-2020-ie00002.svg” width=“18pt” height=“12pt” src=“acp-20-4757-2020-ie00002.png”/{}{}/svg:svg{}{}/span{}{}/span{} ({}span class=“inline-formula”{}56±9{}/span{} ppb) mixing ratios support the biomass burning origin and pronounced photochemical aging of this layer. Upon reaching the Amazon Basin, it started to broaden and to subside, due to convective mixing and entrainment of the BB aerosol into the boundary layer. Satellite observations show that the transatlantic transport of pollution layers is a frequently occurring process, seasonally peaking in August/September.{}/p{} {}p{}By analyzing the aircraft observations together with the long-term data from the Amazon Tall Tower Observatory (ATTO), we found that the transatlantic transport of African BB smoke layers has a strong impact on the northern and central Amazonian aerosol population during the BB-influenced season (July to December). In fact, the early BB season (July to September) in this part of the Amazon appears to be dominated by African smoke, whereas the later BB season (October to December) appears to be dominated by South American fires. This dichotomy is reflected in pronounced changes in aerosol optical properties such as the single scattering albedo (increasing from 0.85 in August to 0.90 in November) and the BC-to-CO enhancement ratio (decreasing from 11 to 6 ng m{}span class=“inline-formula”{}\(^{\textrm{−3}}\){}/span{} ppb{}span class=“inline-formula”{}\(^{\textrm{−1}}\){}/span{}). Our results suggest that, despite the high fraction of BC particles, the African BB aerosol acts as efficient cloud condensation nuclei (CCN), with potentially important implications for aerosol–cloud interactions and the hydrological cycle in the Amazon.

Citation

@article{Holanda2020,
    title = {Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke},
    volume = {20},
    copyright = {All rights reserved},
    issn = {1680-7316},
    url = {https://acp.copernicus.org/articles/20/4757/2020/},
    doi = {https://doi.org/10.5194/acp-20-4757-2020},
    language = {English},
    number = {8},
    urldate = {2021-04-27},
    journal = {Atmospheric Chemistry and Physics},
    author = {Holanda, Bruna A. and Pöhlker, Mira L. and Walter, David and Saturno, Jorge and Sörgel, Matthias and Ditas, Jeannine and Ditas, Florian and Schulz, Christiane and Franco, Marco Aurélio and Wang, Qiaoqiao and Donth, Tobias and Artaxo, Paulo and Barbosa, Henrique M. J. and Borrmann, Stephan and Braga, Ramon and Brito, Joel and Cheng, Yafang and Dollner, Maximilian and Kaiser, Johannes W. and Klimach, Thomas and Knote, Christoph and Krüger, Ovid O. and Fütterer, Daniel and Lavrič, Jošt V. and Ma, Nan and Machado, Luiz A. T. and Ming, Jing and Morais, Fernando G. and Paulsen, Hauke and Sauer, Daniel and Schlager, Hans and Schneider, Johannes and Su, Hang and Weinzierl, Bernadett and Walser, Adrian and Wendisch, Manfred and Ziereis, Helmut and Zöger, Martin and Pöschl, Ulrich and Andreae, Meinrat O. and Pöhlker, Christopher},
    month = apr,
    year = {2020},
    pages = {4757--4785},
}