Implications of RCP emissions on future PM2.5 air quality and direct radiative forcing over China

Severe PM2.5 air pollution in China and the First Grand National Standard (FGNS), implemented in 2016 (annual PM2.5 concentration target of less than 35 mu g m(-3)), necessitate urgent reduction strategies. This study applied the nested-grid version of the Goddard Earth Observing System (GEOS) chemical transport model (GEOS-Chem) to quantify 2000-2050 changes in PM2.5 air quality and related direct radiative forcing (DRF) in China, based on future emission changes under the representative concentration pathway (RCP) scenarios of RCP2.6, RCP4.5, RCP6.0, and RCP8.5. In the near term (2000-2030), a projected maximum increase in PM2.5 concentrations of 10-15 mu g m(-3) is found over east China under RCP6.0 and RCP8.5 and less than 5 mu g m(-3) under RCP2.6 and RCP4.5. In the long term (2000-2050), PM2.5 pollution clearly improves, and the largest decrease in PM2.5 concentrations of 15-30 mu g m(-3) is over east China under all RCPs except RCP6.0. Focusing particularly on highly polluted regions, we find that Beijing-Tianjin-Hebei (BTH) wintertime PM2.5 concentrations meeting the FGNS occur after 2040 under RCP2.6, RCP4.5, and RCP8.5, and summertime PM2.5 concentrations reach this goal by 2030 under RCP2.6 and RCP4.5. In Sichuan Basin (SCB), wintertime PM2.5 concentrations below the FGNS occur only in 2050 under RCP2.6 and RCP4.5, although future summertime PM2.5 will be well controlled. The difficulty in controlling future PM2.5 concentrations relates to unmitigated high levels of nitrate, although NOx and SO2 emissions show substantial reductions during 2020-2040. The changes in aerosol concentrations lead to positive aerosol DRF over east China (20 degrees-45 degrees N, 100 degrees-125 degrees E) by 1.22, 1.88, and 0.66 W m(-2) in 2050 relative to 2000 under RCP2.6, RCP4.5, and RCP8.5, respectively. When considering both health and climate effects of PM2.5 over China, for example, PM2.5 concentrations averaged over east China under RCP4.5 (RCP2.6) decrease by 54% (43%) in 2050 relative to 2000, but at the cost of warming with DRF of 1.88 (1.22) W m(-2). Our results indicate that it will be possible to mitigate future PM2.5 pollution in China, but it will likely take two decades for polluted regions such as BTH and SCB to meet the FGNS, based on all RCP scenarios. At the same time, the consequent warming effects from reduced aerosols are also significant and inevitable.