The relationship between forcing and emission is investigated for black carbon (BC) and primary organic carbon (OC) emitted from North America and Asia. Direct and indirect radiative forcing (DRF and IRF) of BC and OC are simulated with CAM5.1. Two diagnostics are introduced to aid policy‐relevant discussions: linearity and emission‐normalized forcing. DRF is linearly related to emission for both BC and OC from two regions, and the linear relationship is similar, within 15%. IRF is linear in emissions when emissions are lower and regions are far from sources (North American BC and OC). Indirect radiative forcing is sublinear for strong sources and near‐source regions (Asian OC). Emission‐normalized IRF in North America is two to four times higher than that in Asia. The difference among regions and species is primarily caused by particle density as high density of BC results in fewer emitted particles and by the processes for accumulation mode particles to become cloud condensation nuclei and then to activate into cloud droplet. Lower emission‐normalized IRF in more polluted regions means that reductions of OC in these regions would be relatively climate‐neutral rather than causing significant warming via IRF reduction. An optimal aggregation area (30° × 30°) is identified for analysis of the forcing‐to‐emission relationship. For IRF, only 15–40% of the Earth's surface is significantly affected by an emission region, but forcing in these regions comprises most of the global impact. Emission‐normalized forcing can be used to estimate forcing changes due to emission reductions, as long as causes of nonlinearity are identified and considered.
Journal of Geophysical Research: Atmospheres – Wiley
Published: Jan 16, 2018
Keywords: ; ;
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