Downward surface solar radiation (SSR) trends for the first decade of the 2000s were computed using a radiative transfer model and satellite and reanalysis input data and were validated against measurements from the reference global station networks Global Energy Balance Archive (GEBA) and Baseline Surface Radiation Network (BSRN). Under all-sky conditions, in spite of a somewhat patchy structure of global dimming and brightening (GDB), an overall dimming was found that is weaker in the Northern than in the Southern Hemisphere (−2.2 and −3.1 W m⁻², respectively, over the 2001–2009 period). Dimming is observed over both land and ocean in the two hemispheres, but it is more remarkable over land areas of the Southern Hemisphere. The post-2000 dimming is found to have been primarily caused by clouds, and secondarily by aerosols, with total cloud cover contributing −1.4 W m⁻² and aerosol optical thickness −0.7 W m⁻² to the global average dimming of −2.65 W m⁻². The evaluation of the model-computed GDB against BSRN and GEBA measurements indicates a good agreement, with the same trends for 65% and 64% of the examined stations, respectively. The obtained model results are in line with other studies for specific world regions and confirm the occurrence of an overall solar dimming over the globe during the first decade of 21st century. This post-2000 dimming has succeeded the global brightening observed in the 1990s and points to possible impacts on the ongoing global warming and climate change.

The Arctic is influenced by long-range transport of aerosols, for example, sulphate, black carbon, and dust from mid-latitude emissions, especially in winter and spring, leading to the formation of Arctic Haze with enhanced aerosol concentrations. However, more recently, local sources of aerosols, such as wood-burning or resource extraction, are highlighted as already being important, but many uncertainties about sources and aerosol processes still remain. For example, the formation of secondary aerosols, such as sulphate, in winter despite very low temperatures and the absence of sunlight. In this study, which contributes to the international PACES-ALPACA initiative, the Weather Research Forecasting (WRF) and WRF-Chem models are used to investigate wintertime pollution over Alaska with a focus on regions influenced by local pollution, such as Fairbanks and by Arctic Haze, such as Utqiagvik (formerly known as Barrow). Fairbanks is the most polluted city in the United States during wintertime due to high emissions and the occurrence of strong surface temperature inversions. As a first step, background aerosols originating from remote sources were evaluated in large- scale quasi-hemispheric WRF-Chem runs using ECLIPSE anthropogenic emissions. The model performs quite well over Alaska at background sites (e.g. Denali Park) compared to observations from the US Environmental Protection Agency (EPA). Discrepancies in modelled aerosols due to formation mechanisms and aerosol acidity are being investigated. Secondly, in order to better simulate Arctic aerosols and local pollution episodes, different schemes in WRF were tested over Alaska with a particular focus on improving simulations of the Arctic boundary layer structure and, in particular, wintertime temperature inversions which trap pollution at the ground. In order to simulate these extreme/cold meteorological conditions, different schemes linked to boundary layer physics, surface layer dynamics and the land surface have been tested and evaluated against Integrated Global Radiosonde Archive (IGRA2) and Integrated Surface Database (ISD). The model captures the cold meteorological conditions over Alaska, for example, capturing strong temperature inversions over Utqiagvik and Fairbanks in winter 2012.Thirdly, WRF-Chem is used to simulate background and local Arctic air pollution, using the improved WRF setup for meteorology over Alaska for winter 2013-2014. The model is being run with Hemispheric Transport of Air Pollution version 2 (HTAP v2) and other high-resolution emission inventories and evaluated against available aerosol data (PM2.5, black carbon, sulphate) over Alaska including data on aerosol chemical properties. The model is used to examine aerosol composition in locally produced and remote aerosols and to identify the origins contributing to aerosol distributions. The sensitivity of modelled aerosols to, for example, meteorological factors, such as humidity, is examined.