Thomas Holst

Atmospheric aerosol particles influence the climate system directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei(1-4). Apart from black carbon aerosol, aerosols cause a negative radiative forcing at the top of the atmosphere and substantially mitigate the warming caused by greenhouse gases(1). In the future, tightening of controls on anthropogenic aerosol and precursor vapour emissions to achieve higher air quality may weaken this beneficial effect(5-)7. Natural aerosols, too, might affect future warming(2,3,8,9). Here we analyse long-term observations of concentrations and compositions of aerosol particles and their biogenic precursor vapours in continental mid-and high-latitude environments. We use measurements of particle number size distribution together with boundary layer heights derived from reanalysis data to show that the boundary layer burden of cloud condensation nuclei increases exponentially with temperature. Our results confirm a negative feedback mechanism between the continental biosphere, aerosols and climate: aerosol cooling effects are strengthened by rising biogenic organic vapour emissions in response to warming, which in turn enhance condensation on particles and their growth to the size of cloud condensation nuclei. This natural growth mechanism produces roughly 50% of particles at the size of cloud condensation nuclei across Europe. We conclude that biosphere-atmosphere interactions are crucial for aerosol climate effects and can significantly influence the effects of anthropogenic aerosol emission controls, both on climate and air quality.