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.
Highlights
SO2 is the precursor of ambient sulfate, which is a major component of particulate matter (PM) with a dynamic diameter less than 2.5 μm (PM2.5) and it makes up about 20–35 % of total PM2.5 mass (Pathak et al, 2009)
In this study we extend the previous model evaluation by using four additional data sets over China: (1) aerosol optical depths (AOD) retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) for January, July, and annual mean of 2010; (2) SO2 total columns retrieved by the Ozone Monitoring Instrument (OMI) satellite instrument; (3) sulfate concentrations observed at three surface sites in China: the Miyun site (40◦29 N, 116◦47 N) in North China (NC), the Jinsha site (29◦38 N, 114◦12 N) in South China (SC) (Zhang F. et al, 2014), and the Chengdu site (30◦39 N, 104◦2 N) in Southwest China (SWC) (Tao et al, 2014); and (4) monthly wet deposition fluxes at five sites from January 2009 to December 2010, which are from the Acid Deposition Monitoring Network in East Asia (EANET, http: //www.eanet.asia/)
To better understand the regional differences in the efficiency factors presented in Sect. 3 mechanistically, we investigate the regional differences in the conversion of SO2 to sulfate and inter-regional transport of the major sulfur compounds (SO2 and sulfate) on the basis of Goddard Earth Observation System (GEOS)-Chem model outputs
Summary
SO2 is the precursor of ambient sulfate, which is a major component of particulate matter (PM) with a dynamic diameter less than 2.5 μm (PM2.5) and it makes up about 20–35 % of total PM2.5 mass (Pathak et al, 2009). Zhang et al.: Regional differences in Chinese SO2 emission control emission reduction target was set in both the tenth Five-Year Plan (FYP; 2000–2005) and the eleventh FYP (2006–2010). Given the global impact of changing Chinese emissions, it is important to understand the response of pollution outflow to different emission control strategies in China. Since sulfate aerosols exhibit regionally specific formation and transport characteristics (Wang et al, 2013), the response of a given impact metric to the same amount of SO2 emission reduction is expected to differ by region. 4 analyzes sulfate formation and sulfur transport by region to understand the mechanisms behind the regional difference of SO2 emission control efficiency, followed by sensitivity tests of our results.
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