Fossil fuel combustion releases carbon dioxide into the atmosphere along with co-pollutants such as sulfur dioxide, nitrogen oxides, and others. These emissions result in environmental externalities primarily in terms of climate and air quality. Here we quantify the cost of co-pollutant emissions per ton of CO ₂ emissions from US electric power generation. We measure the co-pollutant cost of carbon (CPCC) as the total value of statistical life associated with US-based premature mortalities attributable to co-pollutant emissions, per mass of CO ₂. We find an average CPCC of ∼$45 per metric ton (mt) of CO ₂ for the year 2011 (in 2017 USD). This is ∼20% higher than the central Social Cost of Carbon (SCC) measure of climate damages that was used by the Obama administration in its regulatory impact analysis for the Clean Power Plan (CPP), and >8 times higher than the SCC used by the Trump administration in its analysis for the Plan's repeal. At the state-level, the CPCC ranged from ∼$7/mt CO ₂ for Arizona to ∼$96/mt CO ₂ for New Jersey. We calculate the CPCC trends from 2002 to 2017 and find a 71% decrease at the national level, contributing to total savings of ∼$1 trillion in averted mortality from power plant emissions over this period. By decomposing the aggregate and fuel-specific co-pollutant intensities into simultaneous (CO ₂-driven) and autonomous components, we conclude that the CPCC trends originated mainly from targeted efforts to reduce co-pollutant emissions, e.g. through fuel switching (from coal to natural gas) and autonomous changes in co-pollutant emissions. The results suggest that the overall benefit to society from policies to curtail carbon emissions may be enhanced by focusing on pollution sources where the associated air-quality co-benefits are greatest. At the same time, continued efforts to reduce co-pollutant intensities, if technologically feasible, could help to mitigate the air-quality damages of the CPP's repeal and replacement.

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Aviation emissions have been found to cause5%of global anthropogenic radiative forcing and ∼16 000 premature deaths annually due to impaired air quality. When aiming to reduce these impacts, decision makers often face trade-offs between different emission species or impacts in different times and locations. To inform rational decision-making, this study computes aviation’s marginal climate and air quality impacts per tonne of species emitted and accounts for the altitude, location, and chemical composition of emissions. Climate impacts are calculated using a reduced-order climate model, and air quality-related health impacts are quantified using marginal atmospheric sensitivities to emissions from the adjoint of the global chemistry-transport model GEOS-Chem in combination with concentration response functions and the value of statistical life. The results indicate that 90% of the global impacts per unit of fuel burn are attributable to cruise emissions, and that 64% of all damages are the result of air quality impacts. Furthermore, nitrogen oxides (NOx), carbon dioxide (CO2), and contrails are collectively responsible for 97% of the total impact. Applying our result metrics to an example, we find that a 20%NOx stringency scenario for new aircraft would reduce the net atmospheric impacts by 700mUSD during the first year of operation, even if theNOx emission reductions cause a small increase inCO2 emissions of 2%. In such a way, the damage metrics can be used to rapidly evaluate the atmospheric impacts of market growth as well as emissions trade-offs of aviation-related policies or technology improvements. @en