The Impact of Changes in Cloud Water pH on Aerosol Radiative Forcing

Oxidation of sulfur dioxide (SO₂) in cloud water by reaction with ozone is an important sulfate aerosol formation mechanism and strongly dependent on the acidity of cloud water. Decadal reductions in Northern Hemisphere sulfur emissions have contributed to higher cloud water pH, thereby altering sulfate formation rates. Here we use a global composition-climate model to show that changes in cloud water pH over the 1970–2009 period strongly affects the aerosol particle size distribution, cloud condensation nuclei concentrations, and the magnitude of aerosol radiative forcing. The simulated all-sky aerosol radiative forcing (1970–2009) over the North Atlantic is +1.2 W m‾² if pH remains constant at 5.0, as in many climate models. However, the forcing increases to +5.2 W m‾² if pH is assumed to increase by 1.0 unit over this period. Global composition climate models need to account for variations in cloud water pH to improve the representation of sulfate aerosol formation and aerosol radiative effects.