Background Conditions Influence the Estimated Cloud Radiative Effects of Anthropogenic Aerosol Emissions from Different Source Regions

Using the Community Earth System Model, with the Community Atmosphere Model version 5.3, we investigate the cloud radiative effects of anthropogenic aerosols emitted from different source regions and global shipping. We also analyze aerosol burdens, cloud condensation nuclei concentration, liquid water path, and ice water path. Due to transboundary transport and sublinearity in the response of clouds to aerosols, the cloud radiative effects of emissions from a given source region are influenced by emissions from other source regions. For example, the shortwave cloud radiative effect of shipping is W/m(2) when other anthropogenic emissions sources are present (the present-day background assumption) compared with W/m(2) when other anthropogenic emissions sources are absent (the natural background assumption). In general, the cloud radiative effects are weaker if present-day background conditions are assumed compared with if natural background conditions are assumed. Assumptions about background conditions should be carefully considered when investigating the climate impacts of aerosol emissions from a given source region. Plain Language Summary Particulate air pollutants interact with clouds, influencing Earth's radiation budget. We use a global climate model to investigate the cloud radiative effects of particulate emissions from different source regions and global shipping. Due to nonlinearity, the radiative effects of emissions from one region are influenced by emissions from other regionsthis should be taken into account when investigating the climate impacts of particulate pollutants.