Lifetimes, direct and indirect radiative forcing, and global warming potentials of ethane (C2H6), propane (C3H8), and butane (C4H10)

The atmospheric abundance of the non-methane volatile organic compounds (NMVOCs) ethane, propane, and butane increased during the industrial era. In addition to weak absorption and emission of longwave radiation, these gases influence the atmospheric radiative balance indirectly, mainly as precursors for ozone (O-3), and through reaction with the hydroxyl radical (OH), which leads to less OH and thereby longer atmospheric lifetime of methane (CH4). In this study, we have calculated lifetimes, direct and indirect radiative forcing (RF), and global warming potentials (GWPs) for the three compounds, using a self-consistent methodology. Results show net RF per unit emission of 1.0, 0.9, and 0.6 mWm(-2) (Tgyear(-1))(-1) for ethane, propane, and butane, respectively. For all compounds, the direct effect is considerably smaller than the indirect effects (6% or less of the total). The indirect O-3 and CH4 effects are approximately of the same magnitude. Net GWPs for a 100-year time horizon are 10 for ethane and propane, and 7 for butane, whereof the direct GWPs are <1 for all compounds. The net GWPs are generally higher than previous estimates, mainly because our calculations include emissions for a full year rather than one season. For the compounds studied here, 100-year GWP values do not differ substantially between each compound, considering the large uncertainties involved, and this may indicate that using values representative for a lump of NMVOCs may be sufficient. However, the climate effects may differ more between NMVOC compounds other than alkanes, such as alkenes and aromatics.