# Large-Scale Modeling of Absorbing Aerosols and Their Semi-Direct Effects
## 1 Introduction  
Atmospheric aerosol particles impact the atmospheric radiation budget in various ways. The direct radiative effect by aerosol particles is the change in the Earth's radiation budget by scattering and absorption of incoming solar radiation. Coarse aerosol particles such as mineral dust can also absorb and re-emit outgoing longwave radiation. Aerosol radiative forcing is understood as the radiative effect of anthropogenic aerosols considering present-day distributions in comparison to pre-industrial conditions, when human impact on the aerosol load was considered negligible. In contrast, the aerosol radiative effect is due to aerosol species from both anthropogenic and natural sources. [PERSON] et al. [1] pointed out that the both natural and anthropogenic aerosol species are relevant to fully understand interactions of aerosol and climate. Effective radiative forcing of aerosols consists of both the instantaneous forcing by aerosols and the rapid adjustment of the atmosphere to forcing including the impacts of heating rate changes from aerosol absorption on boundary layer stability or cloud cover. Such adjustment is also named the \"semi-direct\" effect, which is the radiative effect of changes in cloud cover due to aerosol-induced atmospheric heating rate changes. In contrast, indirect aerosol forcing is the effect of aerosol particles acting as cloud condensation or ice nuclei on cloud optical properties or lifetimes, which are, thus, indirectly impacting on the radiation balance. The 5 th Assessment Report by the Intergovernmental Panel for Climate Change [2] specifies the global average direct radiative forcing by all anthropogenic aerosol types (including secondary sulfate, soot and organic particles from anthropogenic sources) as \(-\)0.35 Wm\({}^{-2}\) (estimates ranging from \(-\)0.85 Wm\({}^{-2}\) to +0.15 Wm\({}^{-2}\)) and the indirect radiative forcing as \(-\)0.45 Wm\({}^{-2}\) (estimates ranging from \(-\)1.2 Wm\({}^{-2}\) to 0.0 Wm\({}^{-2}\)). In addition to uncertainties in the global distribution of the differentaerosol species [3], a main uncertainty in estimates of direct aerosol forcing is the uncertain contribution of absorbing soot or black carbon (BC) particles to the aerosol load, which are strongly absorbing in the solar spectral range and contribute a positive atmospheric direct radiative forcing. In contrast, secondarily formed aerosol particles (ammonium sulfate, ammonium nitrate, SOA), have a reflective effect. In the industrialized regions of North America, Europe, and Asia, BC is primarily emitted by the combustion of fossil fuels or biomass [4]. Sources are mainly industrial emissions, traffic, house fires, and vegetation fires, including burning of crop residues on arable land [5]. While anthropogenic soot aerosol from fossil fuel burning usually remains within the boundary layer, BC from vegetation fires can be released into the atmosphere at emission several kilometers high and, thus, like mineral dust aerosol, is subject to the long-distance transport in the free troposphere [6; 7]. Estimates of BC direct radiative forcing are as high as +0.7 Wm\({}^{-2}\) in the global annual average for BC from anthropogenic sources, and the direct radiative effect can be a high as +0.9 Wm\({}^{-2}\) for all BC aerosol from both natural and anthropogenic sources [4]. Another partly absorbing aerosol is mineral dust, which originates from mostly natural sources, although part of the dust emissions may stem from anthropogenically influenced surfaces [8]. Mineral dust produced by strong wind events is a main component of the aerosol mixture in desert regions. Dust emission occurs mainly in the dust belt from the Sahara on the Arabian Peninsula and the desert areas in Central Asia and China. Other source regions are located in Australia, Patagonia and North America [9]. Mineral dust particles are slightly absorbing in the solar part of the spectrum depending on their mineral composition, in particular their content of the iron oxides hematite and goethite.