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

Abstract

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.

Impact of biomass burning aerosols on radiation, clouds, and precipitation over the Amazon: relative importance of aerosol–cloud and aerosol–radiation interactions

“…Wet deposition of aerosols includes in-and below-cloud removal through being collected by rain, graupel, and snow (Chapman et al, 2009) and through being scavenged by precipitation washout (Easter et al, 2004), respectively. Other major schemes utilized, e.g., the RRTMG longwave and shortwave radiation scheme (Mlawer et al, 1997;Pincus et al, 2003), the Yonsei University (YSU) boundary layer scheme (Hong, 2010), the Rapid Update Cycle (RUC) land surface scheme (Smirnova et al, 1997(Smirnova et al, , 2000, the Grell-Devenyi cumulus parameterization (Grell and Devenyi, 2002), and the Fast-J photolysis rate scheme (Wild et al, 2000), are described in Table 1.…”

Concentrations and biosphere–atmosphere fluxes of inorganic trace gases and associated ionic aerosol counterparts over the Amazon rainforest

“…It is possible that a biogenic source may have contributed to SO 2 measured during the relatively pristine conditions. For example, SO 2 could derive from the oxidation by OH of dimethyl sulfide emitted from the rainforest (Jardine et al, 2015).…”

Long-term INP measurements from four stations across the globe

“…Back trajectories differ significantly between the wet season (February to May: northeast) and the dry season (August to November: southeast). There is also a distinct seasonality in pollution markers measured at the site (Saturno et al, 2018;Holanda et al, 2020). During the dry season biomass burning heavily influences the site, resulting in an order of magnitude increase in aerosol number concentration.…”