Regional differences in Chinese SO₂ emission control efficiency and policy implications

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.