Interannual variation in the fine-mode MODIS aerosol optical depth and its relationship to the changes in sulfur dioxide emissions in China between 2000 and 2010

Abstract. Anthropogenic SO₂ emissions increased alongside economic development in China at a rate of 12.7% yr⁻¹ from 2000 to 2005. However, under new Chinese government policy, SO₂ emissions declined by 3.9% yr⁻¹ between 2005 and 2009. Between 2000 and 2010, we found that the variability in the fine-mode (submicron) aerosol optical depth (AOD) over the oceans adjacent to East Asia increased by 3–8% yr⁻¹ to a peak around 2005–2006 and subsequently decreased by 2–7% yr⁻¹, based on observations by the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA's Terra satellite and simulations by a chemical transport model. This trend is consistent with ground-based observations of aerosol particles at a mountainous background observation site in central Japan. These fluctuations in SO₂ emission intensity and fine-mode AOD are thought to reflect the widespread installation of fuel-gas desulfurization (FGD) devices in power plants in China, because aerosol sulfate is a major determinant of the fine-mode AOD in East Asia. Using a chemical transport model, we confirmed that the contribution of particulate sulfate to the fine-mode AOD is more than 70% of the annual mean and that the abovementioned fluctuation in fine-mode AOD is caused mainly by changes in SO₂ emission rather than by other factors such as varying meteorological conditions in East Asia. A strong correlation was also found between satellite-retrieved SO₂ vertical column density and bottom-up SO₂ emissions, both of which were also consistent with observed fine-mode AOD trends. We propose a simplified approach for evaluating changes in SO₂ emissions in China, combining the use of modeled sensitivity coefficients that describe the variation of fine-mode AOD with changes in SO₂ emissions and satellite retrieval. Satellite measurements of fine-mode AOD above the Sea of Japan marked a 4.1% yr⁻¹ decline between 2007 and 2010, which corresponded to the 9% yr⁻¹ decline in SO₂ emissions from China during the same period.