Source-Normalized Radiative Effects of Spatially Non-Uniform CO2 During the Active-Emission Phase: Contrasting Emissions from China and India

During rapidly changing emission conditions, including climate overshoot pathways, the conventional globally well-mixed representation of CO2 may not fully resolve its short-term spatial heterogeneity and associated radiative effects. This study aims to diagnose how source-specific three-dimensional transport and the meteorological and radiative environments sampled by emitted CO2 are associated with differences in source-normalized radiative effect (RE) during the active emission phase, using China and India as two contrasting source-region cases. Fossil-fuel CO2 concentration increments from China and India in 2010 were isolated using IAP-AACM with online source tagging, driven by WRF meteorology and CarbonTracker flux and initial fields. Top-of-atmosphere longwave radiative perturbations relative to the realistic baseline atmosphere were then calculated using libRadtran to quantify the RE, while CO2-mass-weighted meteorological fields were analyzed to characterize the environments sampled along the transport pathways. In the 2010 case, the global annual mean RE was 3.46 mW·m−2·GtC−1 for Indian emissions and 3.15 mW·m−2·GtC−1 for Chinese emissions, corresponding to a 9.8% difference. The maximum local annual mean RE reached 10.0 mW·m−2·GtC−1 for India and 5.5 mW·m−2·GtC−1 for China, indicating a substantially stronger source-dependent spatial contrast than suggested by the global mean alone. CO2-mass-weighted diagnostics showed that the Indian source sampled higher surface skin temperatures (Tsk), lower atmospheric temperatures (Tk), and lower cloud water paths (CWP) than the Chinese source, conditions that are physically consistent with its larger RE. In contrast, the higher precipitable water vapor (PW) sampled along the Indian transport pathways would tend to enhance spectral masking and partially offset this contrast. These results indicate that, before global mixing is approached, the realized radiative effect per unit CO2 emission reflects the combined influence of source features, three-dimensional atmospheric transport, and the meteorological and radiative environments sampled by the transported CO2.