Up / Down trend in the MODIS Aerosol Optical Depth and its relationship to the Sulfur Dioxide Emission Changes in China during 2000 and 2010

Introduction Conclusions References


Conclusions References
Tables Figures

Introduction
Atmospheric aerosols play an important role in the global energy budget and in modifying cloud properties, precipitation efficiency, and the characteristics of the atmospheric circulation.It is essential to investigate their distribution, microphysical properties, longterm variability, and impact on climate.However, the short lifetime of aerosols in the atmosphere makes it difficult to study their chemical and physical properties and their spatial and temporal distributions with limited surface network observations.In a short-term episodic observation of East Asian aerosol characteristics and components, Carmichael et al. (1996) showed annual mean non-sea-salt (nss) sulfate (nss-SO 2− 4 ) and nitrate mass concentrations of 6.9 µg m −3 and 1.2 µg m −3 , respectively, from ground-based tape-air sampler measurements at Jeju Island (33.17 • N, 126.10 • E), Korea, between March 1992and May 1993. Osada et al. (2007) also found that nss-SO 2− 4 was a major aerosol component (mean concentration was 2.4 µg m −3 , >86 % by anion aerosol weight base) within the free tropospheric aerosol at Mt. Norikura (36.06 • N, 137.33 • E, 2770 m above sea level) in central Japan between May and October in 2001 and 2002.Similar results were obtained in observations made during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field campaign in spring 2001 (Huebert et al., 2003).For example, Matsumoto et al. (2003) reported that the mean concentrations of nss-SO 2− 4 , NO − 3 , and NH + 4 in aerosols were 2.48, 0.64, and 0.72 µg m −3 , respectively, at Rishiri Island (45.07 • N, 141.12 • E), Japan from March to May 2001.These aerosol measurements indicated that anthropogenic sulfate aerosols are a dominant contributor to the aerosol component in East Asia.
Recently, continuous observation data from space, retrieved by various satellites, have become available and can be used to study variations in atmospheric pollution.One of an important sensor is the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA's Terra and Aqua satellites.MODIS can observe the column characteristics of atmospheric aerosols and the accumulated aerosol products over decades.Among the MODIS products, the aerosol optical depth (AOD) represents the Introduction

Conclusions References
Tables Figures

Back Close
Full attenuation of sunlight by aerosols and serves as an important measure of the aerosol column concentration.As sulfate aerosols are mainly chemically oxidized from sulfur dioxide (SO 2 ) and are generally a fine-mode aerosol.From the observed results in the downwind region over East Asia, sulfate contributes largely to the aerosol component, accordingly, have a dominant role to fine-mode AOD.It is expected that the trends in fine-mode AOD are important when examining SO 2 emission variation in this region.
Because of the lack of long-period observations of aerosol which cover the vast East Asia, satellite retrieved data provide a valuable information.
The temporal behavior of historical sulfur emissions is consistent with the sulfate AOD over North America and Europe, where sulfur emissions increased continuously until the early 1980s, after which they declined because of mandated reductions in air pollution (Streets et al., 2006).East Asia produces huge amounts of SO 2 emissions from burning fossil fuels.Most SO 2 emissions in East Asia originate from coal combustion (68 %), with this figure reaching 85 % in China (Ohara et al., 2007).In the past three decades, anthropogenic SO 2 emissions in East Asia have increased dramatically in parallel with the region's rapid economic growth.Especially from 2000 to 2005, anthropogenic SO 2 emissions from China increased at a rate of 12.7 % yr −1 from 28 to 51 Mt yr −1 (Ohara et al., 2007).In this situation, the AOD also increased over East Asia from 1980 to 2006, as shown by a global model and observed surface solar radiation (Streets et al., 2009).
Recently, however, substantial declines in SO 2 emissions in China have been reported, with a 3.9 % yr −1 reduction in the total SO 2 emissions in the period [2005][2006][2007][2008][2009] (Ministry of Environmental Protection in China, 2009), after reaching a maximum in 2006.A major reason for this decline could be the widespread installation of fuel-gas desulfurization (FGD) devices in power plants to control air pollution (Lu et al., 2010;Li et al., 2010).Zhao et al. (2008) estimated that the capacity of units with FGD in mainland China would reach 477 GW (∼70 % of total capacity) in 2010, from 45 GW (13 % of total capacity) in 2005, with the requirement for fitting FGD devices to all units constructed after 2004.We can infer that recent installations of FGD devices have Introduction

Conclusions References
Tables Figures

Back Close
Full played a significant role in reducing SO 2 emissions.As noted in the MEP report, FGD installation in Chinese coal-fired power plants is ongoing, and the associated reduction in SO 2 emissions is expected to correlate with changes in fine-mode AOD over East Asia.
The decline in SO 2 emissions has resulted in increased visibility (Wang et al., 2009) and is believed to be in line with satellite observations of the total AOD (Ruckstuhl et al., 2008).As the major anthropogenic aerosols that cool the climate by reflecting solar radiation and by indirect effects on the reflectivity and lifetime of clouds (Haywood and Boucher., 2000), sulfate aerosol was considered a positive factor in slowing the rate of climate warming.If the Asian emissions of sulfur dioxide decline significantly, the warming process may be faster than we expect.Therefore, it is important to investigate whether sulfate aerosol over East Asia will be reduced with the change in emissions by analyzing the trend in AOD.To understand the recent trend in AOD and its relationship to the changes in anthropogenic emissions in East Asia, this study analyzed the trend in AOD between 2000 and 2010 using emission inventory data with a bottom-up approach, space-based observations, and the results of a chemical transport model simulation.
2 Observation data and model description

Satellite measurements and ground observation
As mentioned in the previous section, short-term observation data on aerosol concentrations are available; however, long-term aerosol observations are quite limited.In this study, to investigate long-term trends in AOD, we used MODIS/Terra data from satellite observations.The MODIS algorithm for determining aerosol characteristics performs well within an expected error when compared to co-located ground-based observation data obtained from the Aerosol Robotic Network (AERONET) direct Sun/sky radiometers over both land (Chu et al., 1996) and ocean (Remer et al., 2002).Introduction

Conclusions References
Tables Figures

Back Close
Full The Level 3 MODIS/Terra daily products (MOD08 D3) for a 1 • ×1 • equal-angle global grid were obtained from http://ladsweb.nascom.nasa.gov/.Collections 5 and 5.1 contain the entire time series of data from March 2000 to the present.To avoid the uncertainties and contribution of coarse particles, especially mineral dust particles, which are dominant in spring over East Asia, we used the aerosol optical depth at 550 nm together with the fine-mode fraction to derive the fine-mode AOD (AOD f hereafter) over the ocean (Kaufman et al., 2005).Remer et al. (2008) noted that the ocean product contains inherently more information content than does the land product, which is sensitive to assumptions made about spectral surface reflectance.They also indicated that the size parameters of the ocean algorithm are more reliable than those for land.
Considering these factors, we used only ocean AOD f data from MODIS.Generally, submicron particles originate from combustion and chemical reactions, and AOD f is a suitable indicator for examining the trends in AOD attributed to anthropogenic activity, despite the estimated MODIS error over ocean, with values of ±0.03 and ±0.05 due to the uncertainty in the ocean state and aerosol properties, respectively (Kaufman et al., 2005).Details of the MODIS products and their validation can be found in the study of Remer et al. (2008) and references therein.
The satellite retrieved the SO 2 vertical column density (VCD) from the Global Ozone Monitoring Experiment (GOME) and the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) was used to examine the recent trend in SO 2 emissions.The GOME on board ERS-2 and the SCIAMACHY on board ENVISAT are passive remote-sensing spectrometers that observe backscattered, reflected, transmitted, or emitted radiation from the atmosphere and Earth's surface and monitor the most prominent species, permitting studies of the status of the Earth's atmosphere (see Gottwald and Bovensmann, 2011).Lee et al. (2009) examined the ability of satellite retrievals of SO 2 to discern information about anthropogenic SO 2 emissions by conducting a sensitivity simulation with GEOS-Chem and found that the retrieval of the vertical column of sulfur dioxide from SCIAMACHY was sufficient to reflect the Chinese emissions trend.Introduction

Conclusions References
Tables Figures
For comparison with the satellite AOD f , the monthly averaged volume concentrations were calculated for months with >50 % coverage of the daily nighttime data for the submicrometer (0.3-1.0 µm) size range (Osada et al., 2009).

Chemical transport model
To support the analysis of AOD trends, we used AOD from the Community Multiscale Air Quality (CMAQ; version 4.4) (Byun and Schere, 2006).The CMAQ model simulation results have been validated and reported by Uno et al. (2007) and Itahashi et al. (2010).Therefore, here we only describe aspects relevant to this study.The horizontal model domain covers the whole of East Asia, comprising 78×68 grids with a resolution of 80 km on a rotated polar stereographic map projection centered at 25 • N, 115 • E. The vertical resolution includes 14 layers extending from the earth's surface to 23 km with stretching grid layers.The anthropogenic emissions data for 2000-2005 are from the Regional Emission Inventory in Asia (REAS) (Ohara et al., 2007).REAS was constructed based on energy data, emission factors, and other socioeconomic information, giving the ten chemical species as an Asian emission inventory at 0.5 Environment (IMPROVE) program, for the USA.Thus, we must assume that aerosol characteristics in East Asia are the same as those in the USA.This assumption has been reported to be reliable and well validated (e.g., Song et al., 2008).We conducted two sets of numerical experiments.First, we performed a 5-yr simulation for 2000-2005 using emission data sets and meteorological fields for each year (designated "E yy M yy ").Second, we conducted a simulation for the same period using fixed meteorology for 2000 and the emission fields for each year ("E yy M 00 ").The purpose of E yy M 00 was to evaluate AOD sensitivity to interannual variation in meteorology by comparing these results with those of E yy M yy .

Results and discussion
Figure 1 shows anthropogenic SO 2 emissions over East Asia from the REAS emission inventory for 2005, region numbers, and location names.Huge amounts of SO 2 are emitted from East Asia, especially from central eastern China (CEC).The spatial distributions of the averaged AOD determined from CMAQ and MODIS/Terra are shown in Fig. 2a and b.The AOD f had a high value (>0.5) from the Yellow Sea to the East China Sea.A ridge of high AOD f extended from the Yellow Sea to Korea, the Sea of Japan, and north of Japan and then decreased toward the southeast, clearly reflecting the pollutant transport patterns from China via the Asian monsoon.In East Asia, sulfate aerosol is a major contributor to AOD f , contributing more than 70 % near the Asian continent and exceeding 80 % around Japan (Fig. 2b).Streets et al. (2009) reported that the average contribution of sulfur to estimated anthropogenic AOD exceeded 80 % in East Asia between 1980 and 2006, based on the GOCART global model.On the other hand, Park et al. (2011) pointed out that the importance of nitrate contribution, especially in winter (∼23 %, loccaly ∼53 %), though, the high nitrate regions are restricted to the mainland China compared with that of sulfate from the modeled spatial distribution.Moreover, they also showed that the contribution of organic aerosols and elemental carbon are relatively small.In this work, we focused on the AODf over oceans of the Introduction

Conclusions References
Tables Figures

Back Close
Full downwind region in East Asia, and the modeled dominant contribution of sulfate is well consistent with observation data in Korea and Japan (Carmichael et al., 1996;Osada et al., 2007).Comparing the other modeling studies and observations, the simulated sulfate fraction to AOD in China might be overpredicted, but we believe that this might not cause big issue as long as we are discussing the relative trends.
The differences in the 3-year averaged AOD f between 2000-2002 (the early 2000s) and 2004-2006 (the mid-2000s) and that between 2004-2006 and 2008-2010 (the late 2000s) are shown in Fig. 2c and d, respectively.There was a significant, dramatic change in AOD f between 2000 and 2010 over East Asia, with a turning point around 2005-2006, when the AOD f peaked.From the early 2000s to the mid-2000s, the AOD f increased over most of East Asia, whereas from the mid-2000s to the late 2000s, it decreased, except in the region around Shanghai and over northern Japan, where submicron particles originating from a wildfire in Siberia likely affected the AOD f .
The temporal variation in the monthly mean AOD f was examined over the numbered rectangles shown in Fig. 1 and is presented in Fig. 3.The regression coefficient of MODIS/Terra and CMAQ and the linear approximation to the annual mean AOD f are shown for each region.Note that the data for 2003 were excluded from the trend analysis for all regions because of the low insolation in eastern Asia (e.g., Lu et al., 2010) and the anomalous wildfire in Siberia.The significant increase in AOD f over East Asia of 4-8 % yr served with the LPC (assuming 1 g cm −3 density) corresponded well to the AOD f = 0.1 retrieved by MODIS/Terra.Between 2000 and 2010, the AOD f showed a dramatic increase and subsequent decrease over all of the marked regions, and the decline in sulfate aerosol is thought to have been the predominant contributor to this trend.Similar background stratospheric aerosol changes in Hawaii and Colorado, USA, were found by Hofmann et al. (2009) with possible connections with Chinese SO 2 emissions.Finally, we examined the relationship between Chinese SO 2 emissions and AOD f over the downwind region, focusing on the Sea of Japan, which is located under the main transport path of continental aerosols, and investigated whether the emission intensity changed based on satellite AOD f observations.Figure 4a shows the temporal variation in the SO 2 emissions from China, the SO 2 VCD above CEC (data from Gottwald and Bovensmann (2011), Fig. 10.11), and the AOD f over the Sea of Japan (MODIS/Terra and CMAQ).It clearly shows that SO 2 VCD is representative of the SO 2 emission changes, and there is a good correlation with the SO 2 emission inventories of REAS and (Lu et al., 2010), with correlation coefficients of 0.97 and 0.91, respectively (Fig. 4b), and both are significant with 99 % confidence level.The SO 2 VCD above CEC increased until 2007 and then decreased.The decreasing trend in SO 2 after 2007 in China may have been due to the rapid expansion of FGD coverage, as mentioned above.The OMI SO 2 data over Inner Mongolia show a similar trend (Li et al., 2010).By contrast, the trend in SO 2 estimated from the emission inventory and satellite-retrieved SO 2 VCD was consistent with the trend in AOD f over the Sea of Japan between 2000 and 2010, also demonstrating that aerosol sulfate is the key component of the AOD f in East Asia.During this period, SO 2 emissions from Korea did not increase significantly, and ranged from 0.4-0.5 Mt yr −1 , decreasing slightly.Consequently, Korean emissions did not make a dominant contribution to the observed AOD f variation.Sensitivity studies comparing E yy M 00 and E yy M yy indicated that meteorological variability with the same emission intensity (i.e., patterns of transportation and mixing) could influence changes in AOD f .The fluctuations of AOD f in these two sensitivity simulations (E yy M yy and E yy M 00 ) range from 0.005 to 0.023, and Introduction

Conclusions References
Tables Figures

Back Close
Full are correspond to 3.3-10.1 % of the annual mean AOD f over the Sea of Japan.This results indicated that the variability in emissions is clearly the dominant factor in our study region.We can see the difference in the peak year of SO 2 VCD (peak in 2007) and AOD f (peak in 2005-2006) from Fig. 4a.The AOD f are examined above the Sea of Japan, which is located in the downwind region of China, the meteorological condition could be a one of the possible reason in this difference.However, variability in emissions is clearly the dominant factor in our study region.Figure 4b is a scatterplot of AOD f and SO 2 VCD with the SO 2 emissions in Fig. 4a.There is a good linear relationship between the SO 2 emissions from China and the AOD f over the Sea of Japan, which affords a new measure for estimating the Chinese emissions variation via satellite measurements and historical emissions databases.Similarly, Lamsal et al. (2011) proposed a simplified emissions-update method for NO x based on satellite observations.Their basic equation is; where E is emissions, Ω is the vertical column density, ∆Ω is the change in the vertical column density with the change in emissions ∆E (here, we calculated this using a 15 % emission perturbation), and β represents the local sensitivity of the change in the column density to the change in emissions.As there was a positive correlation among the AOD f , SO 2 emissions, and SO 2 VCD, this equation could be extended to the SO 2 VCD and AOD f variation as where γ is a local sensitivity coefficient.First, we applied the results of a chemicaltransport model to determine the sensitivity coefficients (β and γ).Then, the emissionchange ratio was calculated using satellite observations.The spatial distributions of ∆Ω and ∆ AOD are shown in Fig. 5.The sensitivity coefficients β and γ reflect the Introduction

Conclusions References
Tables Figures

Back Close
Full 4 fraction within the total AOD.Some errors will also arise depending on the chemical mechanisms and perturbation ratio chosen for the model.However, SO 2 chemistry has a more linear response than does NO x chemistry, and thus the error from chemical non-linearity should be minor.Note that the original method by Lamsal et al. (2011) was applied to a grid-by-grid estimation of emission inventories by satellite observations, while our estimates are over a much larger scale and changes in emission are averaged using a regional averaging method Hereafter, we try to estimate the SO 2 emissions from China based on the REAS emission inventory using values from 2005 as the base emission levels.As a demon-  2010) reported reductions of 2.9 % yr −1 , and 2.0 % yr −1 using our method.
The retrieval of SO 2 VCD depends strongly on the surface conditions (e.g., reflection) and a state-of-the-art retrieval model based on several assumptions (e.g., air mass factors), whereas the AOD measurement over the ocean is relatively reliable for wide application.As long as the major AOD component is sulfate and the long-range transport pathway does not change interannually, our simplified AOD approach gives a reasonable inverse estimate of SO 2 emissions.Introduction

Conclusions References
Tables Figures

Back Close
Full including over the land surface, is needed to understand and quantify the complex and rapidly changing air quality in East Asia.
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Such bottom-up emission estimate methods often suffer from time lags of several years; at the time of this study, only data up to the year 2005 were available.The aerosol calculation AERO3 module was employed, and the aerosol concentration at the nearest MODIS/Terra flyover time (10:30 local time) was used to calculate the AOD at 550 nm based on the formula proposed by Malm et al. (1994).The aerosol types used included sulfate, nitrate, black carbon, and organic carbon.This formula was originally calculated on the basis of the Interagency Monitoring of Projected Visual 21978 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | −1 between 2001 and 2005 was caused mainly by a continuous increase in emissions in China.However, as shown in Fig. 2c-d and 3, the AOD f decreased from 2006 to 2010, at a rate of 4-7 % yr −1 .AOD f are affected by the changes of both meteorology and emissions of its precursors, to exclude the effect of temporal changes within a year to year, we applied the 3-year moving average for the significance test of these trends.Based on this, we confirmed that the increasing trend in 2001-2005 and subsequently decreasing trend in 2006-2010 in all regions were respectively significant with the 95 % and 99 % confidence level.A very clear trend was also captured by the ground-based observations at Mt. Tateyama, a mountain site in central Japan (closed gray circle in region 3 of Fig. 1).At this site, the aerosol concentration of 1 µg m −3 ob-Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | feedback of emissions to chemical mechanisms, the horizontal distribution of transport/removal efficiency, and the SO 2− interannual variability in the aerosol optical depth (AOD) over East Asia and its relationship to the change in Chinese sulfur dioxide (SO 2 ) emissions between 2000 and 2010.The fine-mode AOD (AOD f ) from MODIS/Terra measurements and the results from the CMAQ modeling system were investigated.AOD f over the oceans adjacent to East Asia was found to increase from 2001 to 2005 and then decreased until 2010 at a rate of 4-7 % yr −1 .This trend is consistent with groundbased observations of the number-size distribution of aerosol particles at a mountainous background observation site in central Japan.One of the reasons for these fluctuations in SO 2 emission intensity and AOD is the widespread installation of fuel-gas desulfurization (FGD) devices in power plants in China because sulfate aerosol is a major determinant of the AOD in East Asia.Only 13 % of coal-fired power plants were equipped with FGDs in 2005, while this ratio exceeded 70 % in 2010.Variability in meteorological conditions such as the Asian monsoon could influence variations in AOD; however, our sensitivity analysis with the chemical transport model showed that the observed trends in AOD f were dominated by variability in emissions.Regression analysis showed that the observed AOD f over the Sea of Japan decreased by 4.1 % yr −1 between 2007 and 2010, which is equivalent to a reduction in SO 2 emissions over China of approximately 9 % yr −1 , according to our approach using AOD data based on MODIS/Terra measurements.Our results indicate that the usefulness of integrated approach of satellite measurement and modeling study in the analysis of Asian air quality and emissions, under the limitation of long-range ground-based observation data.As reported from space-based NO 2 observation, anthropogenic NO x emission in East Asia, especially in mainland China, has been increasing during the 2000s, contrary to the trends in SO 2 .Nitrate aerosol formation from NO x emission after photochemical reactions could be another factor in AOD variation.Nitrate aerosol usually contributes to coarse mode aerosols and could be less sensitive to AOD f .Further study of in situ and satellite observations of aerosol components and fine-mode AOD, Discussion Paper | Discussion Paper | Discussion Paper | stration, the SO 2 emissions on 2009 and 2010, which are expected to score the decline trends, are estimated via this simplified inversion method.Based on the β and SO 2 VCD over CEC in 2009, the SO 2 emissions from China in 2009 were estimated at 42.5 Mt yr −1 , and this level was equivalent to that in 2004.Based on γ and AOD f over the Sea of Japan, the retrieved emissions from China for 2009 were 40.6 Mt yr 2 emissions from China decreased by an estimated ∼9 % yr −1 , with the peak of 49.0 Mt yr −1 reduced to 32.3 Mt yr −1 .By comparison, between 2006 and 2008, which was the first phase of the widespread installation of FGD systems, Lu et al. (