Climate policies targeting CO2 emissions from fossil fuels can simultaneously reduce emissions of air pollutants and their precursors, thus mitigating air pollution and associated health impacts. Previous work has examined co-benefits of climate policy from reducing PM2.5 in rapidly-developing countries such as China, but have not examined co-benefits from ozone and its transboundary impact for both PM2.5 and ozone. Here, we compare the air quality and health co-benefits of China’s climate policy on both PM2.5 and ozone in China to their co-benefits in three downwind and populous countries (South Korea, Japan and the United States) using a coupled modeling framework. In a policy scenario consistent with China’s pledge to peak CO2 emissions in approximately 2030, avoided premature deaths from ozone reductions are 54 300 (95% confidence interval: 37 100–71 000) in China in 2030, nearly 60% of those from PM2.5. Total avoided premature deaths in South Korea, Japan, and the US are 1200 (900–1600), 3500 (2800–4300), and 1900 (1400–2500), respectively. Total avoided deaths in South Korea and Japan are dominated by reductions in PM2.5-related mortality, but ozone plays a more important role in the US. Similar to co-benefits for PM2.5 in China, co-benefits of China’s policy for ozone and for both pollutants in those downwind countries also rise with increasing policy stringency.

We find that summertime air quality in the eastern U.S. displays strong dependence on North Atlantic sea surface temperatures, resulting from large-scale ocean-atmosphere interactions. Using observations, reanalysis data sets, and climate model simulations, we further identify a multidecadal variability in surface air quality driven by the Atlantic Multidecadal Oscillation (AMO). In one-half cycle (\textasciitilde35 years) of the AMO from cold to warm phase, summertime maximum daily 8 h ozone concentrations increase by 1–4 ppbv and PM2.5 concentrations increase by 0.3–1.0 μg mÀ3 over much of the east. These air quality changes are related to warmer, drier, and more stagnant weather in the AMO warm phase, together with anomalous circulation patterns at the surface and aloft. If the AMO shifts to the cold phase in future years, it could partly offset the climate penalty on U.S. air quality brought by global warming, an effect which should be considered in long-term air quality planning.

A number of remotely sensed land cover datasets with spatial resolutions 1 km have recently become available or are in the process of being mapped. The application of these higher resolution and more up-to-date land cover datasets in chemical transport models (CTMs) is expected to improve the simulation of dry deposition and biogenic emissions of non-methane volatile organic compounds (NMVOCs), which affect ozone and other secondary air pollutants. In the present study, we updated the land cover dataset in the nested-grid GEOS-Chem CTM with the 1 km resolution GLC2000 land cover map and examined the resulting changes in the simulation of surface ozone and sulfate over China in July 2007. Through affecting the dry deposition velocities of ozone and its precursors, using GLC2000 in the dry deposition module can decrease the simulated surface ozone by 3% (up to 6 ppb) over China. Simulated surface sulfate shows an increase of 3% in northwestern China and a decrease of 1% in northern China. Applying GLC2000 in the biogenic emissions of the NMVOC module can lead to a 0.5–4.5 ppb increase in simulated surface ozone over East China, mainly driven by the larger coverage of broadleaf trees in East China in the GLC2000 dataset. Our study quantifies the large sensitivity to land cover datasets with different spatial resolutions and time periods of simulated secondary air pollutants over China, supporting ongoing research efforts to produce high resolution and dynamically updated land cover datasets over China, as well as for the globe.

Previous studies have indicated that the regression slope between the interhemispheric difference (IHD) of CO2 mixing ratios and fossil fuel (FF) CO2 emissions was rather constant at about 0.5 ppm/Pg C yr(1 during 1957Á2003. In this study, we found that the average regression slopes between the IHD of CO2 mixing ratios and IHD of FF emissions for 16 sites in the Northern Hemisphere (NH) decreased from 0.6990.12 ppm/Pg C yr(1 during 1982Á1991 to 0.3790.06 ppm/Pg C yr(1 during 1996Á2008 (IHD of CO2 defined as the differences between each site and the South Pole, SPO). The largest difference was found in summer and autumn. The change in the spatial distribution of FF emissions driven by fast increasing Asian emissions may explain the slope change at three sites located north of 608N but not at the other sites. A 30-yr SF6 simulation with time-varying meteorology and constant emissions suggests no significant difference in the decadal average and seasonal variation of interhemispheric exchange time (tex) between the two periods. Based on the hemispheric net carbon fluxes derived from a two-box model, we attributed 75% of the regression slope decrease at NH sites south of 608N to the acceleration of net carbon sink increase in the NH and 25% to the weakening of net carbon sink increase in the SH during 1996Á2008. The growth rate of net carbon sink in the NH has increased by a factor of about three from 0.02890.023 [mean92s] Pg C yr(2 during 1982Á1991 to 0.09390.033 Pg C yr(2 during 1996Á2008, exceeding the percentage increase in the growth rate of IHD of FF emissions between the two periods (45%). The growth rate of net carbon sink in the SH has reduced 62% from 0.05890.018 Pg C yr(2 during 1982Á1991 to 0.02290.012 Pg C yr(2 during 1996Á2008.