Light absorption by carbonaceous aerosols over the Amazon rain forest

Assessing the role of atmospheric aerosol particles in the Earth system prior to the man-made perturbations of the industrial era has been a key research topic in the last decades. The climate response to direct and indirect radiative effects by aerosol particles is still not well understood. Consequently, studying the current anthropogenic perturbations to the natural aerosol cycling has become an important task. The Amazon rain forest is one of the few continental locations where the atmosphere can be near-pristine to pristine during certain periods. Man-made biomass burning (BB) due to agricultural expansion is considered the largest source of pollution to the Amazonian atmosphere. Biomass burning emits large amounts of light absorbing aerosol particles like black carbon (BC). The BB contributions to the Amazonian aerosol burden are not only regional but also transatlantic, with African savannah and woodland emissions being seasonally significant. A comprehensive aerosol instrumentation setup has been used at the Amazon Tall Tower Observatory (ATTO) in central Amazonia to continuously measure the aerosol's physical and chemical properties. The optical measurements comprise light scattering and absorption measurements. Additionally, microscopy, chemical and particle sizing techniques have been used to characterize and measure the aerosol properties on a long-term basis. The combination of systematic field measurements at ATTO and dedicated modeling studies as presented here provides a robust long-term data set of aerosol optical properties. Specifically, the contribution of light-absorbing organic aerosol particles — so-called 'brown carbon' (BrC) — is compared to BC in the context of BB-dominated regimes. The occurrence of El Niño Southern Oscillation (ENSO) in its positive phase in 2015 caused drought in the Amazon Basin. Consequently, more frequent fire events occurred in the forest and its peripheries. The increased BB emissions affected the aerosol optical properties and near-by fire emissions allowed the observation of enhanced BrC contribution to light absorption. Long-range transport of African air masses to the Amazon rain forest was also studied by analyzing a particular volcanic emission event in the Congo. The transatlantic transport of the volcanogenic sulfur plume was traced by transport models and observed by satellite, ground-based and airborne measurements. This episode is considered an exemplary case of transatlantic aerosol transport that helps to understand the African contribution to the Amazonian aerosol population. This dissertation provides new substantial insights into the role and relevance of atmospheric aerosols in the Amazon region. Different absorption measurement techniques are evaluated and inter-compared. The results represented here will serve as a basis for follow-up studies on particle mixing state as well as the transport and life cycle of light-absorbing aerosol particles in central Amazonia. In addition, the results contribute to a better understanding of the influence of ENSO on the Amazonian atmosphere, offering prospects to warmer and drier scenarios. Finally, this work may help to constrain the role of absorbing aerosols in the Earth’s climate system