An Implicit Air Quality Bias Due to the State of Pristine Aerosol

Air pollution is largely attributed to anthropogenic aerosols, with the role of natural aerosols, including sea salt, dust, and other terrestrial emissions considered to be less important. However, natural aerosols have strong geographic gradients and this suggests that spatially invariant air quality guidelines may handicap regions close to natural sources. We use climate models to construct a view of pre-industrial "pristine" air quality, including fine particulate matter with diameters less than 2.5 mu m (PM2.5). Under pristine conditions, PM2.5 levels over regions in geographic proximity to dust sources, including parts of Africa and Asia, exceed World Health Organization air quality guidelines. We estimate that this pristine air pollution, which is unassociated with human activities, impacts up to about one billion people globally. The results show that natural aerosols, with strong geographic gradients, can lead to poor air quality over regions close to sources, and that in many areas no amount of anthropogenic emission reductions will result in clean air.