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Substantial changes in anthropogenic aerosols and precursor gas emissions have occurred over recent decades due to the implementation of air pollution control legislation and economic growth. The response of atmospheric aerosols to these changes and the impact on climate are poorly constrained, particularly in studies using detailed aerosol chemistry-climate models. Here we compare the HadGEM3-UKCA (Hadley Centre Global Environment Model-United Kingdom Chemistry and Aerosols) coupled chemistry-climate model for the period 1960-2009 against extensive ground-based observations of sulfate aerosol mass (1978-2009), total suspended particle matter (SPM, 1978-1998), PM10 (1997-2009), aerosol optical depth (AOD, 2000-2009), aerosol size distributions (2008-2009) and surface solar radiation (SSR, 1960-2009) over Europe. The model underestimates observed sulfate aerosol mass (normalised mean bias factor (NMBF) =-0.4), SPM (NMBF=-0.9), PM10 (NMBF =-0.2), aerosol number concentrations (N30 NMBF =-0.85; N50 NMBF =-0.65; and N100 NMBF =-0.96) and AOD (NMBF =-0.01) but slightly overpredicts SSR (NMBF =-0.02). Trends in aerosol over the observational period are well simulated by the model, with observed (simulated) changes in sulfate of -68% (-78 %), SPM of -42% (-20 %), PM10 of -9% (-8 %) and AOD of -11% (-14 %). Discrepancies in the magnitude of simulated aerosol mass do not affect the ability of the model to reproduce the observed SSR trends. The positive change in observed European SSR (5 %) during 1990-2009 ("brightening") is better reproduced by the model when aerosol radiative effects (ARE) are included (3 %), compared to simulations where ARE are excluded (0.2 %). The simulated topof- the-atmosphere aerosol radiative forcing over Europe under all-sky conditions increased by >3.0Wm-2 during the period 1970-2009 in response to changes in anthropogenic emissions and aerosol concentrations. [ABSTRACT FROM AUTHOR]

Black carbon-containing aerosol particles play an important role in the direct and indirect radiative forcing of climate. However, the magnitude and sign of the net radiative effect is strongly dependent on the physical properties of the black carbon (BC) component of the particles, such as mass concentration, number size distribution and mixing state. Here we use a global aerosol model combined with aircraft measurements of BC particle number and size from the Single Particle Soot Photometer (SP2) to assess the realism with which these physical properties are predicted by global models. The comparison reveals a substantial mismatch between the measured and modelled BC size distribution over the size range of the SP2 instrument (90-400 nm BC diameter). The model predicts BC particle number concentrations a factor ~3.5-5.7 higher than measured and a mode diameter that is ~40-65 nm smaller than observed. More than ~90% of the model particles with dry diameters ≳260 nm contain BC, while the observations suggest only 14% on average. These model-observation biases in the BC properties are considerably greater than for the overall particle distribution, suggesting that the discrepancy is associated with model assumptions about the size and mixing state of the emitted carbonaceous particles. We expect the discrepancy in BC size distribution to be common among most global aerosol models, with implications for model estimates of absorption optical depth and direct radiative forcing. [ABSTRACT FROM AUTHOR]