Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke
Authors: Holanda Bruna A. , Pöhlker Mira L. , Walter David , Saturno Jorge , Sörgel Matthias , Ditas Jeannine , Ditas Florian , Schulz Christiane , Franco Marco Aurélio , Wang Qiaoqiao , Donth Tobias , Artaxo Paulo , Barbosa Henrique M. J. , Borrmann Stephan , Braga Ramon , Brito Joel , Cheng Yafang , Dollner Maximilian , Kaiser Johannes W. , Klimach Thomas , Knote Christoph , Krüger Ovid O. , Fütterer Daniel , Lavrič Jošt V. , Ma Nan , Machado Luiz A. T. , Ming Jing , Morais Fernando G. , Paulsen Hauke , Sauer Daniel , Schlager Hans , Schneider Johannes , Su Hang , Weinzierl Bernadett , Walser Adrian , Wendisch Manfred , Ziereis Helmut , Zöger Martin , Pöschl Ulrich , Andreae Meinrat O. , Pöhlker Christopher 
Published: April 2020

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. During flight AC19 over land and ocean at the northeastern coastline of the
Amazon Basin, we observed a BC-rich layer at ∼3.5 km altitude
with a vertical extension of ∼0.3 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 Nacc=850±330 cm−3. The
rBC particles account for ∼15 % of the submicrometer
aerosol mass and ∼40 % of the total aerosol number
concentration. This corresponds to a mass concentration range from 0.5 to
2 µg m−3 (1st to 99th percentiles) and a number
concentration range from 90 to 530 cm−3. Along with rBC, high
cCO (150±30 ppb) and cO3 (56±9 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. 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−3 ppb−1). 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.