Strong impact of wildfires on the abundance and aging of black carbon in the lowermost stratosphere

Wildfires inject large amounts of black carbon (BC) particles into the atmosphere, which can reach the lowermost stratosphere (LMS) and cause strong radiative forcing. During a 14-month period of observations on board a passenger aircraft flying between Europe and North America, we found frequent and widespread biomass burning (BB) plumes, influencing 16 of 160 flight hours in the LMS. The average BC mass concentrations in these plumes (∼140 ng·m$^{-3}$, standard temperature and pressure) were over 20 times higher than the background concentration (∼6 ng·m$^{-3}$) with more than 100-fold enhanced peak values (up to ∼720 ng·m$^{-3}$). In the LMS, nearly all BC particles were covered with a thick coating. The average mass equivalent diameter of the BC particle cores was ∼120 nm with a mean coating thickness of ∼150 nm in the BB plume and ∼90 nm with a coating of ∼125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of ∼1 nm·h$^{-1}$ due to the BC particle coatings. The observed high concentrations and thick coatings of BC particles demonstrate that wildfires can induce strong local heating in the LMS and may have a significant influence on the regional radiative forcing of climate

On the small-scale dynamics of cloud edges

Clouds are one of the major uncertainties in climate change predictions caused by their complex structure and dynamics. Numerous cloud processes are acting from cloud-scale down to mm-scale and interplay with each other as well as with atmospheric processes. This complexity on the one hand and the high spatial resolution required to analyse the small scale processes on the other hand cause difficulties in cloud research. One important and until now insufficiently understood process in cloud microphysics is the entrainment process. It defines the turbulent transport of sub-saturated environmental air into the cloud region. Subsequent mixing leads to the evaporation of cloud droplets resulting in negatively buoyant air at cloud edge. One distinguishes between two types of entrainment processes: cloud top and lateral entrainment. While the first type is mostly detected at the top of stratiform clouds, lateral entrainment plays an important role for the dynamics of cumulus clouds. Within in this thesis, highly-resolved measurements with a resolution down to the centimeter scale performed with the helicopter-borne measurement payload ACTOS (Airborne Cloud Turbulence Observation System) are used to study both types of entrainment processes. Shear-induced cloud top entrainment leads to a turbulent inversion layer (TIL) atop of a stratocumulus layer consisting of clear air. The TIL seems to be coupled with the underlying cloud layer due to the turbulence intensity. With increasing thickness of the TIL the turbulence inside is damped monotonically leading to a maximum layer thickness and inhibiting direct mixing between cloud top and free troposphere. At the edges of shallow trade wind cumuli, shear-induced lateral entrainment generates a subsiding shell. Its evolution is analysed based on detailed measurements in continuously developing shallow cumuli. With the cloud evolution, the subsiding shell grows at the expense of the cloud core region and an increasing downdraft velocity is observed within this region. These observations are confirmed with the simulation of an idealised subsiding shell. The results present unique observations at the edges of clouds and are an appreciable progress in cloud research which decisively influence future research

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

Black carbon (BC) aerosols are influencing the Earth’s atmosphere and climate, but their microphysical 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. 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 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. By 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. This 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