Beyond SOX reductions from shipping: Assessing the impact of NOX and carbonaceous-particle controls on human health and climate

Abstract:

Historically, cargo ships have been powered by low-grade fossil fuels, which emit particles and particle-precursor vapors that impact human health and climate. We used a global chemical-transport model with online aerosol microphysics (GEOS-Chem-TOMAS) to estimate the aerosol health and climate impacts of four emission-control policies: (1) 85% reduction in sulfur oxide (SO_X) emissions (Sulf); (2) 85% reduction in SO_X and black carbon (BC) emissions (Sulf-BC); (3) 85% reduction in SO_X, BC, and organic aerosol (OA) emissions (Sulf-BC-OA); and (4) 85% reduction in SO_X, BC, OA, and nitrogen oxide (NO_X) emissions (Sulf-BC-OA-NO_X). The SO_X reductions reflect the 0.5% fuel-sulfur cap implemented by the International Maritime Organization (IMO) on January 1st, 2020. The other reductions represent feasible best-case emissions controls. We estimate that these policies could reduce fine particulate matter (PM_2.5)-attributable mortalities by 16,400 (Sulf) to 42,800 (Sulf-BC-OA-NO_X) mortalities per year. These changes represent 0.4% and 1%, respectively, of annual PM_2.5-attributable mortalities from all PM_2.5 sources. Comparing simulations, we estimate that adding the NO_X cap has the greatest health benefit. In contrast to the health benefits, all scenarios lead to a simulated climate warming tendency. The combined aerosol direct radiative effect (DRE) and cloud-albedo indirect effects (AIE) are 32 mW m^-2 (Sulf) and 69 mW m^-2 (Sulf-BC-OA-NO_X). These changes are about 4% (Sulf) to 8% (Sulf-BC-OA-NO_X) of the total anthropogenic aerosol radiative forcing. Each additional emission cap increases the aerosol radiative forcing by 7.5 to 32 mW m^-2. The emission control policies examined here yield larger relative changes in the aerosol radiative forcing (4-8%) than in health effects (0.4-1%), because most shipping emissions are distant from populated regions.