Potential impact of a US climate policy and air quality regulations on future air quality and climate change.

We have investigated how future air quality and climate change are influenced by the US air quality regulations that existed or were proposed in 2013 and a hypothetical climate mitigation policy that aims to reduce 2050 CO₂ emissions to be 50% below 2005 emissions. Using the NASA GISS ModelE2 general circulation model, we look at the impacts for year 2030 and 2055. The US energy-sector emissions are from the GLIMPSE project (GEOS-Chem LIDORT Integrated with MARKAL (MARKet ALlocation) for the Purpose of Scenario Exploration), and other US emissions data sets and the rest of the world emissions data sets are based on the RCP4.5 scenario. The US air quality regulations are projected to have a strong beneficial impact on US air quality and public health in year 2030 and 2055 but result in positive radiative forcing. Under this scenario, no more emission constraints are added after 2020, and the impacts on air quality and climate change are similar between year 2030 and 2055. Surface particulate matter with a diameter smaller than 2.5 μm (PM_2.5) is reduced by ∼2 μgm^-3 on average over the USA, and surface ozone by ∼8 ppbv. The improved air quality prevents about 91 400 premature deaths in the USA, mainly due to the PM_2.5 reduction (∼74 200 lives saved). The air quality regulations reduce the light-reflecting aerosols (i.e., sulfate and organic matter) more than the lightabsorbing species (i.e., black carbon and ozone), leading to a strong positive radiative forcing (RF) over the USA by both aerosols' direct and indirect forcing: the total RF is ∼0.04 W m^-2 over the globe, and ∼0.8 W m^-2 over the USA. Under the hypothetical climate policy, a future CO₂ emissions cut is achieved in part by relying less on coal, and thus SO₂ emissions are noticeably reduced. This provides air quality co-benefits, but it could lead to potential climate disbenefits over the USA. In 2055, the US mean total RF is +0.22 W m^-2 due to positive aerosol direct and indirect forcing, while the global mean total RF is -0.06Wm₂ due to the dominant negative CO₂ RF (instantaneous RF). To achieve a regional-scale climate benefit via a climate policy, it is critical (1) to have multinational efforts to reduce greenhouse gas (GHG) emissions and (2) to simultaneously target emission reduction of light-absorbing species (e.g., BC and O₃) on top of long-lived species. The latter is very desirable as the resulting climate benefit occurs faster and provides cobenefits to air quality and public health. [ABSTRACT FROM AUTHOR]