The impact of changing anthropogenic emissions on European atmospheric aerosols over the second half of the 20th Century

Across Europe anthropogenic emissions have changed substantially over recent decades, due in part to the implementation of air pollution control legislation. It is important to understand the effect that such changes in emissions have on atmospheric aerosols and their impact on air quality and climate. The composition climate model HadGEM3-UKCA and long term observations of aerosols are used to simulate and evaluate the change in aerosols over the period 1960 to 2009. Simulated sulphate mass, total aerosol mass and aerosol number concentrations from HadGEM3-UKCA were underestimated but temporal trends were in better agreement when compared to observations. The inclusion of aerosols was found to be essential when reproducing the observed brightening trend in surface solar radiation across Europe since 1990. The all-sky aerosol radiative forcing (relative to a 1980-2000 mean) increased by >3 W m-2 during the period 1970-2009 due to reductions in aerosol concentrations. The sensitivity of sulphate aerosol formation and aerosol radiative effects to an increase in cloud water pH were investigated over the period 1970 to 2009. Sulphate aerosol mass increased over regions with large SO2 emissions but decreased at other locations. A positive response in the all-sky aerosol radiative effect was simulated due to reductions in aerosol number concentrations. Over Europe the aerosol radiative forcing between 1970 and 2009 varied from +1.5 to +7 W m-2 depending on whether an increase or decrease in cloud water pH was assumed. This shows the additional uncertainty that could exist in aerosol radiative effects if temporal changes in such parameters are not considered. Two emission scenarios have been used to assess the effect of European air quality legislation and technology improvements since the 1970s on particulate air quality, human health and climate. European annual mean PM2.5 concentrations reduced by 35% and are calculated to have prevented 80,000 premature deaths annually across the European Union, a perceived financial benefit to society of US$232 billion. A positive response in the all-sky aerosol radiative effect was simulated, which has increased European annual mean surface temperatures by 0.45 °C. The implementation of legislation and technology improvements has improved air quality and human health across Europe but also had an unintended impact on climate.