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 are influencing the Earth's atmosphere and climate, but their microphysical 35 properties, spatiotemporal distribution and long-range transport are not well constrained. This study analyzes the transatlantic transport of BC-rich African biomass burning (BB) pollution into the Amazon Basin, based on airborne observations of aerosol particles and trace gases in and off the Brazilian coast during the ACRIDICON-CHUVA campaign in September 2014, combining in-situ measurements on the research aircraft HALO with satellite remote-sensing and numerical model results. 40 During flight AC19 over land and ocean at the Brazilian coastline in the northeast of the Amazon Basin, we observed a BC-rich atmospheric layer at ~3.5 km altitude with a vertical extension of ~0.3 km.Backward trajectory analyses 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 days of atmospheric transport and aging. The BC mass concentrations in the layer ranged from 0.5 to 2 μg m -3 , and 45 the BC particle number fraction of ~40 % was about 8 times higher than observed in a fresh Amazonian BB plume, representing the highest value ever observed in the region. Upon entering the Amazon Basin, the layer 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. 50By analyzing the aircraft observations within the broader context of 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 north-central Amazonian aerosol population during the BB-influenced season (July to November). Specifically, the early BB season in this part of the Amazon appears to be dominated by African smoke, whereas the later BB season appears to be dominated by South American fires. 55This dichotomy is reflected in pronounced changes of 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 7.4 to 4.4 ng m -3 ppb -1 ). Our results suggest that, despite the high amount of BC particles, the African BB aerosol act as efficient cloud condensation nuclei (CCN) with potentially important implications for aerosol-cloud interactions and the hydrological cycle in the Amazon Basin. 60 The role of the southern African easterly jet in modifying the southeast Atlantic aerosol and cloud environments, Q.

Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

“…Wildfires occur annually in many tropical regions across the globe (e.g., the Amazon, central Africa, Borneo) and often cover vast areas with optically thick layers of aerosol that can persist for weeks to months. These plumes can transport the carbonaceous biomass burning aerosol (BBA) high into the atmosphere and far from the source (Holanda et al., 2020), demonstrating the potential for widespread non‐local impacts. BBA influences the environment via both ARI and ACI.…”

Overview: On the transport and transformation of pollutants in the outflow of major population centres – observational data from the EMeRGe European intensive operational period in summer 2017

“…In the PBL, measured BC mass concentrations were as high as 1 µg m -3 . The stratification of pollution plumes above the PBL is a typical feature for BB emissions (Holanda et al, 2020). Mixing ratios of CH 4 comparable to those in urban plumes were measured in this BB event during E-EU-07 (not shown).…”

Isolating large-scale smoke impacts on cloud and precipitation processes over the Amazon with convection permitting resolution

“…Wildfires occur annually in many tropical regions across the globe (e.g., the Amazon, central Africa, Borneo) and often cover vast areas with optically thick layers of aerosol that can persist for weeks to months. These plumes can transport the carbonaceous biomass burning aerosol (BBA) high into the atmosphere and far from the source (Holanda et al, 2020), demonstrating the potential for widespread non-local impacts. BBA influences the environment via both ARI and ACI.…”