Regional differences in Chinese SO<sub>2</sub> emission control efficiency and policy implications

Abstract

Abstract. SO2 emission control has been one of the most important air pollution policies in China since 2000. In this study, we assess regional differences in SO2 emission control efficiencies in China through the modeling analysis of four scenarios of SO2 emissions, all of which aim to reduce the national total SO2 emissions by 8% or 2.3 Tg below the 2010 emissions level, the target set by the current twelfth Five-Year Plan (FYP; 2011–2015), but differ in spatial implementation. The GEOS-Chem chemical transport model is used to evaluate the efficiency of each scenario on the basis of four impact metrics: surface SO2 and sulfate concentrations, population-weighted sulfate concentration (PWC), and sulfur export flux from China to the western Pacific. The efficiency of SO2 control (β) is defined as the relative change of each impact metric to a 1% reduction in SO2 emissions from the 2010 baseline. The S1 scenario, which adopts a spatially uniform reduction in SO2 emissions in China, gives a β of 0.99, 0.71, 0.83, and 0.67 for SO2 and sulfate concentrations, PWC, and export flux, respectively. By comparison, the S2 scenario, which implements all the SO2 emissions reduction over North China (NC), is found most effective in reducing national mean surface SO2 and sulfate concentrations and sulfur export fluxes, with β being 1.0, 0.76, and 0.95 respectively. The S3 scenario of implementing all the SO2 emission reduction over South China (SC) has the highest β in reducing PWC (β = 0.98) because SC has the highest correlation between population density and sulfate concentration. Reducing SO2 emissions over Southwest China (SWC) is found to be least efficient on the national scale, albeit with large benefits within the region. The difference in β by scenario is attributable to the regional difference in SO2 oxidation pathways and the source–receptor relationship. Among the three regions examined here, NC shows the largest proportion of sulfate formation through gas-phase oxidation, which is more sensitive to SO2 emissions change than aqueous oxidation. In addition, NC makes the largest contribution to inter-regional transport of sulfur within China and to the transport fluxes to the western Pacific. The policy implication of this is that China needs to carefully design a regionally specific implementation plan of realizing its SO2 emissions reduction target in order to maximize the resulting air quality benefits, not only for China but for the downwind regions, with emphasis on reducing emissions from NC, where SO2 emissions have decreased at a slower rate than national total emissions in the previous FYP period.