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Contributions of open crop straw burning emissions to PM2.5 concentrations in China

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2016 Environ. Res. Lett. 11 014014

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Environ. Res. Lett. 11 (2016) 014014 doi:10.1088/1748-9326/11/1/014014

LETTER

Contributions of open crop straw burning emissions to PM2.5

concentrations in China

LiboZhang1, Yongqiang Liu2 and LuHao1

1 International Center for Ecology,Meteorology and Environment, College of AppliedMeteorology, NanjingUniversity of InformationScience andTechnology, Nanjing, People’s Republic of China

2 Center for Forest Disturbance Science, USDAForest Service, Athens, GA,USA

E-mail: yliu@fs.fed.us

Keywords: straw burning, air pollution,monthly and annual emissions, PM2.5 inventories inChina cities

Supplementarymaterial for this article is available online

AbstractPM2.5 inventories have been developed inmajor Chinese cities to quantify the contributions fromvarious sources based on annual emissions. This approach, however, could substantially under-estimate the contribution fromopen straw burning during the harvest or other active burning periods.This study examines this issue by estimatingmonthly and annual straw-burning PM2.5 emissions inChina and comparingwith themwith the corresponding emissions fromother anthropogenicsources. Annually burned strawPM2.5 emissions during 1997∼2013 for 31China provinces werecalculated based on crop and related burning information for 12months based on satellite detectionof agricultural burning. Annual emissions fromother anthropogenic sources were collected from theliterature and allocated tomonthly values using air pollution indexmeasurements. The resultsindicate that the annual PM2.5 emissions fromopen straw burning inChinawere 1.036m tons. Themonthly PM2.5 emission ratios of straw burning to other anthropogenic sources during June, theharvest period formany regions, were several times larger than the annual ratios at national, regional,and province levels, suggesting that, in contrast to annual emissions that were used in the PM2.5

inventories inChinese cities to assess the contributions fromother sources,monthly emissions shouldbe used to assess the contributions from straw burning during the harvest or other active burningperiods. The larger contributions from straw burning shown in this study also suggest that substantialreduction of openfield straw burningwould dramatically improve air quality inmanyChinese regionsduring the harvest or other active burning periods.

1. Introduction

China is among the major agricultural nations in theworld. Crop straw resources in China, ranking the firstin the world, account for 17.29% of the globalproduction (Bi et al 2010). Corn,wheat, and rice strawsare the major crop straw resources in China. Farmersneed to remove a large amount of crop straws in ashort period of time after the harvest in order to plantcrops for the next growing season. In comparison withother approaches such as cutting, burning is the mosteffective and is less expensive to remove straws.

However, straw burning has many adverse envir-onmental and ecological impacts (Shi et al 2014).

Straw burning wastes valuable biological resourcesand, more importantly, often causes serious air pollu-tion. Straw burning releases a large amount of pollu-tants such as PM2.5, SO2, CO, NH3, VOC, and NOX

(Koppmann et al 2005, Li et al 2008). Further, strawburning is one of the primary contributors to haze andsmog formation during the harvest periods in China(Zha et al 2013).

Air pollution is a serious environmental problemin China. Regional haze and smog events have drama-tically increased in recent years, due to the rapid devel-opment of the economy since the late 1970s. Thissituation has worsened in recent decades due to dra-matic urban expansion and an in increase in

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automobile use in China (Huang et al 2013). Forexample, there were 124 recorded haze and smog daysin Beijing in 2012, 25 of which occurred in January (13more than the monthly average over previous years),and 18 in June (three times the monthly average overthe previous 10 years) (http://beijing.tianqi.com/

news/19519.html).Reversing these declining air quality trends and

eventually solving the air pollution problem is one ofthe fundamental goals of China today. Many researchprojects have been initiated to assess the intensity of airpollution and to identify the major contributors.China startedmonitoring PM10, SO2, andNO2 as earlyas 2000 in more than 40 major cities to formulate theair pollution index (API). In 2012, China added PM2.5,the most important contributor to smog and haze andone of the major air pollution indicators used by othercountries, to the monitoring list together with O3 andCO to formulate a new air pollution index (i.e., the airquality index (AQI)).

A recent major effort has been the development ofPM2.5 inventories and the identification of majoremission sources based on the PM2.5 data togetherwith various analysis tools. This task was accom-plished for nine cities in the three most industrializedcities in Beijing-Tianjin-Hebei, the Yangtze RiverDelta, and the Pearl River Delta in 2014 and 26 morecities will be added to the list in 2015. The inventoriesare essential for air pollutant control, prediction, andprevention.

The PM2.5 particles come from both local andtransported sources. In Beijing, for example, theyaccounted for 64%∼72% and 28%∼36%, respec-tively. The local sources were mainly automobiles(31.1%), coal consumption (22.4%), industry(18.1%), construction dust (14.3%), and othersections (14.1%). The percentage of transported sour-ces were not specified, but smoke particles from crop-straw burning were expected to be one of the majorcomponents for Beijing (Cheng et al 2013, Wanget al 2014).

Straw-burning emissions in China have been esti-mated for 2000∼2003 (Cao et al 2008), 2006 (Wangand Zhang 2008), and 2007 (Lu et al 2011), based oncrop yields and burning properties (Ortiz de Zarateet al 2005, Li et al 2007, Zhang et al 2008, Wang andZhang 2008, Bi et al 2009, Zuo et al 2015). Accordingto Guan et al. (2014) and Lu et al (2011), annual bio-mass burning contributed about 26% of total particu-late matter (PM) emissions, half of which were fromopen-field straw burning. These results provide usefulinformation to assess how important straw-burningemissions are as a transported source to the PM2.5

inventories inChina cities.However, straw burning is essentially a seasonal

activity and it peaks with crop harvests. The harvestperiods are mostly in June—July and around Octoberin southern China and late May—early June, and lateSeptember—early October in northern China (Wang

and Zhang 2008). This strong seasonal dependencesuggests that the PM2.5 emission ratios of straw burn-ing to other anthropogenic sources during the harvestperiods are much larger than the annual ratios. Thus,the current approach to developing PM2.5 inventoriesin China cities using annual values underestimates thecontributions of straw-burning emissions during theharvest periods. For the PM2.5 inventory in Beijing, thecontributions from the transported sources weremorethan 50% during severe air pollution periods,15%∼20% more than the annual values. This is acommon feature of biomass burning.

The aim of this study was to examine the aboveissue by estimating and analyzing the monthly PM2.5

emissions from open straw burning during1997∼2013 in China and comparing the emissionratios of straw burning to other anthropogenic sour-ces. The results are expected to provide useful infor-mation for improving our understanding of the rolesof straw-burning emissions in air pollution formationand for better assessing the contributions of trans-ported sources in the PM2.5 inventories in Chinesecities.

2.Methods

Themethods used in this study are briefly described asfollows. An illustration of the calculation procedure(figure S1) with a detailed description of emissionestimation is provided in the supplementalmaterial.

2.1. EmissionsAnnual emissions of open straw burning wereobtained using (Lu et al 2011)

M P N R F EF , 1im

m m r i ( )å= ´ ´ ´ ´

whereMi is emission of ith straw-burning pollutant,mcrop species, P annual crop yield, N yield-to-strawratio, Rr portion of open straw burning in region r, Fcombustion efficiency, and EFi emission factor of thepollutant.

To estimate emissions from open straw burning,the annual crop yields of 31 China provinces during1997∼2013 (Chinese Statistical Yearbook), theyield-to-straw rates from Bi et al (2009) and Zuo et al(2015), the straw-burning proportions from Wangand Zhang (2008), and the average combustion effi-ciency from Zhang et al (2008) were used. The emis-sion factors of the straw burning were provided inmany studies (e.g. Cao et al 2008, Li et al 2007, Zhanget al 2015, Zhu et al 2005). This study used the resultsfrom Ni et al (2005) and Li et al (2007) with the PM2.5

emission factors of 8.5, 9.5, 12.0, and 9.5 g kg−1 forrice, wheat, corn, and other crops, respectively. Anaverage value of 9.65 g kg−1 was also used without dis-tinguishing crop types. In addition, PM10 emissionswere calculated using an emission factor of 11.7 g kg−1

based on Li et al (2007), Lu et al (2011), and Zhu et al

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Environ. Res. Lett. 11 (2016) 014014

(2012). The results for 2006 were also used for com-parison to examine interannual variability.

Annual PM2.5 emissions from other anthro-pogenic sources (mainly traffic, energy, industry, andconstruction) were estimated for 2001 (Zhang et al2007), 2007 (Cao et al 2011), and multiple years (XieandHan 2014). The 2006 PM2.5 emissions fromZhanget al (2009)were used, which were part of an inventoryof air pollutant emissions of INTEX-B in 2006 from allmajor anthropogenic sources except biomass burning,with PM2.5 emission of 13.3 m tons and PM10 of18.2 m tons (1 m ton=1Tg=1012 g).

2.2.MODISmonthly agricultural burningemissionsThe Moderate Resolution Imaging Spectroradiometer(MODIS) is the most frequently used satellite remotesensing (RS) sensor to detect straw burning in China(Li et al 2009). The monthly global fire emission datafor agricultural burning from GFED4.0 s (van derWerf et al 2010, Giglio et al 2013) (http://www.globalfiredata.org)were used to allocate the calculatedannual straw-burning emissions to monthly values.The data combineMODIS information on fire activityand vegetation productivity to estimate griddedmonthly burned area and fire emissions. The RSdetection has a spatial resolution of 0.25 degrees and isavailable from 1997 and onwards. Note that GFED4.0sincludes an algorithm to detect small fires that ishelpful to improve the accuracy of the estimation ofagricultural straw burning. However, straw burning inChina is scattered in residential areas with small burnsizes. In addition, the MODIS detections during theday are available around 10 am and 1 pm local time.The flaming stage of straw burning may be last shorterthan the interval between the two detection times.Thus, it is likely that the GFED data could have missedmany small burns inChina.Our comparison indicatedthat the detected emissions were much smaller thanthose obtained using crop yield data (figure S2).Nonetheless, little impact on the detection of strawburning season is expected (He et al 2007), Liet al 2009). Thus, we estimated straw-burning emis-sions based on crop data while using the GFED dataformonthly allocation.

2.3. Air pollution indexMonthly means of API in 42 key cities were obtainedbased on daily values (http://www.mep.gov.cn) andaveraged over 2001∼2012. They were used toallocate the monthly variations of PM emissions fromother anthropogenic sources. The API values aredivided into five grades of 0∼50, 51∼100,101∼200, 201∼300, and 301∼500 with increas-ing severity of air pollution.

3. Results

3.1. Annual PM2.5 emissionsThe annual straw yield averaged during 1997∼2013and calculated using the crop yield data in China was626.41 m tons, or province average of about 20 m tons.Straw yields varied dramatically across regions andprovinces (figure 1(a)). Straw yields were relativelysmall in South andNorthwest China and large in otherregions (see the horizontal coordinate of figure 1 foreach of the seven China geographic regions and 31provinces included in each region; the geographiclocations of the provinces are shown in figure 2). Tenprovinces had yields much more than the provinceaverage, including Henan (60 m tons) of CentralChina, Shandong (52 m tons) of North China, andHeilongjiang (45 m tons) of Northeast China. Theregional distributions of burned straw in open fields(figure 1(a)) were similar, but the values became largefor South China and small for Southwest and North-east China. The largest burned amounts at provincelevel occurred in Jiangsu and Anhui (more than 10 mtons) of East China, followed by Hunan and Henan(about 9 m tons) of Central China, and Shandong (8 mtons) of North China. Heilongjiang of NortheastChina only burned less than 5 m tons per year despitethe large straw yield. Apparently the more economic-ally developed regions such as East China had largerburning rates than the less developed regions such asNortheast China.

The annual PM2.5 emission fromopen straw burn-ing in China was 1.036 m tons, calculated using onesingle emission factor for all kinds of crops, which wasvery close to the value of 1.044 m tons calculated usingthree different emission factors for cereal crops andone emission factor for other crops. The emissionvalue is comparable to the estimate by Cao et al (2008),but about half of the estimate by Wang and Zhang(2008), who usedmuch larger yield-to-straw rate, pro-portion of burning, burning efficiency, and emissionfactors. Consistent with the amounts of burned strawshown in figure 1(a), larger PM emissions from strawburning occurred in East, Central, and North China(figures 1(b)) and 2(a)). The PM2.5 emissions fromJiangsu and Anhui in East China reached 107.04 and99.64 k tons, respectively (1 k ton=1 Gg=109 g). Incomparison, the annual PM10 emission was 1.256 mtons nationally, and 129.77 and 120.80 k tons, respec-tively, for Jiangsu and Anhui. This spatial distributionis consistent with other studies (e.g., Lu et al 2011).

The PM2.5 emissions from other anthropogenicsources occurred mainly in North and East China, aswell as in Sichuan of Southwest China andGuangdongof South China (figure 1(b)). These regions have highpopulation density and more advanced industry andeconomy than other regions. In contrast, PM2.5 emis-sions in the less industrialized areas of Xizang, Qin-ghai, Ningxia of Southwest and Northwest China, andthe tourist-oriented Hainan of South China were very

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Environ. Res. Lett. 11 (2016) 014014

low. In addition, emissions were low in the fourmuni-cipalities of Beijing, Tianjin, Shanghai, and Chongq-ing. The largest PM2.5 emission of about 1200 k tonsoccurred in Shandong of North China (figure 2(b)),followed by about 800∼1000 k tons in Hebei,Jiangsu, Henan, and Sichuan. Different from PM2.5

emissions from straw burning, all regions exceptNorthwest China had one or more provinces withconsiderably large emissions from anthropogenicsources. Note that the proportion rate for Hunan is32.9% (district 6), which is nearly two times largerthan the rate of 10.7% for Hubei (district 1) (table S2).Thus, despite having very similar straw yields, theamount of burned straw is about two times larger inHunan than in Hubei. Also note that the emissions inBeijing, Shanghai, and Tianjin are relatively small.There are two reasons: (1) The emissions for each areais the total amount rather than the amount per unit.All three cities are extensively urbanized with littlefarming land.

3.2.Monthly PM2.5 emissionsThemaximummonthly variations of PM2.5 emissionsfrom open straw burning of 110, 76, and 45 k tonsoccurred in June in East, North, and Central China

(figure 3(a)). At the province level in these regions,large PM2.5 emissions in June were 58.51 k tons(Jiangsu), 48.10 k tons (Anhui), 41.79 k tons (Shan-dong), 40.98 k tons (Henan), and 21.29 k tons (Hebei).In other regions, the maximum emissions occurred inspring and/or fall months, with much smaller valuesof about 25 k tons. The large emissions in June andduring the fallmonths were a result of the summer andfall harvest seasons, respectively. Some areas did notburn straw immediately after the fall harvest, butwaited until the next spring planting season. Thiscaused large emissions in some springmonths.

The monthly API looked different than themonthly straw-burning emissions. Only one patternwas found for all regions (figure S3), with the worst airpollution (the highest API) found in winter, the best insummer, and a second peak in spring. Heating usingcoal is one of the major air pollution sources in China,mostly in the winter. In addition, the atmosphere ismore stable and rainfall is the lowest in winter but theopposite is true in summer. Dust storms are a majorenvironmental hazard, mainly in northern China dur-ing spring. These natural and human activities explainthe seasonal API pattern. The main seasonal cycle isconsistent with Zhang et al (2009) who estimated the

Figure 1.Annual properties of crop straw at province level. (a) Straw yields (solid bars) and open burned straw (shadow lines) during1997-2013. (b)PM2.5 emissions from straw burning (solid bars) during 1997-2013, and PM2.5 emissions fromother anthropogenicsources (shadow lines) for 2006.

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monthly variations based on the monthly distribu-tions of energy consumption of residents according totemperature and other energy and industrial emis-sions according to power generation, cement produc-tion, and gross domestic product.

Consistent with the seasonal API variations, largeremissions from other anthropogenic sources in Chinaoccurred in the winter months of January and Decem-ber with PM2.5 of about 1.303 and 1.325 m tons,respectively. In the spring months ofMarch and April,PM2.5 emissions were about 1.207 and 1.181 m tons,respectively. At the regional level, the largest monthlyvariations occurred in North China, followed by EastChina, and relatively small variations were seen inNorthwest, South, andNortheast China (figure 3(b)).

3.3. PM2.5 emission ratiosThe annual PM2.5 emission ratio of open crop strawburning averaged over 1997∼2013 to other anthro-pogenic sources for the year of 2006 was 7.8% at thenational level (table 1). The monthly PM2.5 emissionratioswere 26% in June, 11% inMay, 9.7% inOctober,

and 8% in March and April. The June ratio was about3.4 times the annual ratio.

At the regional level, the annual ratios were about10% for Central, East, and South China, and as low asabout 5% for Southwest and North China. The Juneratios were about 56% in East China and 30% inNorthand Central China. The October ratios were about29% in Northwest China and 22% in Northeast China(figure 4 and table 1). The monthly ratios were about3∼5 times the annual ratios.

At the province level, the annual emission ratioswere the largest at nearly 35% in Xizang of SouthwestChina, over 20% in Hunan of Central China, over15% in Guangxi of South China, Anhui of East China,and Xinjiang of Northwest China, and below 10% in21 out of the 31 provinces (figure S4; also see figure S5(a) for the geographic distribution). The June ratios(figures S4 and S5(b); table 1) were about 107% inAnhui and 81% in Jiangsu of East China, 63% inHenan of Central China, 47% in Shaanxi of NorthwestChina, and 45% in Shandong of North China, whichwere about 6∼7 times the corresponding annualratios. The other months with relatively large ratioswere about 33% in Guangxi of South China and 41%

Figure 2. Spatial distributions of annual PM2.5 emissions (k tons) from straw burning during 1997-2013 (a) and other anthropogenicsources for 2006 (b).

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Environ. Res. Lett. 11 (2016) 014014

in Hunan of Central China in March (figure S5(c)),97% in Xinjiang of Northwest China in October(figure S5(d)), 88% in Xizang of Southwest China inFebruary, and 41% in Jilin of Northeast China inApril.

According to the analysis described in the supple-mentary material, the provinces with extremely largemonthly ratios were Jiangsu and Anhui of East China,

Shandong of North China, and Hunan and Henan ofCentral China; the provinces with largemonthly ratioswere Guangxi of South China, Hebei of North China,Jilin and Heilongjiang of Northeast China, andShaanxi andXinjiang ofNorthwest China (table S3).

Straw-burning emissions had some interannualvariability (figure S6). To examine the possible impactof the variability on the emission ratios, the PM2.5

emission ratios using open crop straw burning for theyear 2006 only instead of the average over1997∼2013 were calculated. The results are the sameas those described above, with slightly larger magni-tude (figure S7 and table S4).

4.Discussion

Thefindings from this study, the PM2.5 emission ratiosof straw burning compared to other anthropogenicsources, may be valuable for the development of thePM2.5 inventories in China cities. First, it provides newevidence for the important contribution of straw-burning emissions to air pollution in China, suggest-ing that the PM2.5 inventories should include strawburning as an emission source. Previous studies haveprovided this evidence based on burning informationof single or several years, while this study analyzed thelong-term burning information of 17 years and thus ismoremeaningful from a statistical point of view.

Secondly, this study further indicates that the con-tributions of straw-burning emissions during harvestor other active burning periods are much larger thanthe contributions from previous assessments, whichwere based on annual emissions of straw burning andother anthropogenic sources. This suggests that the

Figure 3.Monthly distributions of PM2.5 emissions for Chinese regions fromopen straw burning during 1997-2013 (a) and otheranthropogenic sources for 2006 (b).

Table 1.Comparisons betweenmonthly PM2.5 emission ratios (%)of straw burning during 1997-2013 to other anthropogenic sourcesfor 2006 during harvest or other active burning period (measured bythe largestmonthly value) and annual ratios (%) at national,regional, and province levels.

Level Area

Monthly

ratio

Annual

ratio

Monthly

ratio/

annual ratio

Nation 26.3 7.8 3.4

South 14.9 9.7 1.5

Southwest 7.9 4.0 2.0

Central 32.8 11.4 2.9

Region East 56.3 10.4 5.4

North 29.5 5.1 5.8

Northeast 22.4 8.1 2.8

Northwest 29.1 8.5 3.4

Guangxi 32.6 17.9 1.8

Xizang 87.6 34.5 2.5

Hunan 41.0 21.4 1.9

Henan 63.0 10.6 5.9

Jiangsu 81.0 12.1 6.7

Province Anhui 107.2 17.4 6.1

Shandong 45.2 6.3 7.2

Jilin 41.3 11.0 3.8

Shaanxi 47.0 6.0 7.8

Xinjiang 96.8 17.4 5.6

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Environ. Res. Lett. 11 (2016) 014014

PM2.5 inventories should use monthly rather thanannual straw-burning emissions while assessing itscontribution as a transported source. The importanceof more detailed wildland fire information than theannual data from theUS EPAnational emission inven-tories for improving air quality simulations is indi-cated (e.g., Battye and Battye 2002). This studyprovides evidence for this by showing the large differ-ence between monthly and annual contributions ofstraw-burning PM2.5 emissions to other anthro-pogenic emissions inChina.

Thirdly, this study identifies the specific time of ayear (June, October, March, etc) for a region or

province when the monthly contribution from straw-burning emissions was the largest. Thus, the PM2.5

inventories for a specific city could focus on thecorresponding time of year to determine the straw-burning contribution. Dynamic models such asCMAQ (Byun and Schere 2006) and HYSPLIT (Drax-ler and Hess 1998, Zhu et al 2010) are needed to simu-late transport of smoke particles from the ruralburning sites to urban area where the impact of smokeon the air quality conditions are assessed (Liuet al 2009, Su et al 2012). Simulations of such modelsrequires a large amount of computational resources,while much less is needed for simulations if a specific

Figure 4.Monthly PM2.5 emission ratios of straw burning during 1997-2013 to other anthropogenic sources for 2006 for all regionsandChina (%).

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Environ. Res. Lett. 11 (2016) 014014

harvest or other active burning period is used insteadof an entire year.

The June peak in the major straw-burning regionsof China is consistent with the finding from a globalMODIS detection over 2001∼2003 for this region(Korontzi et al 2006). The peak times, however, aredifferent in other regions, as shown in that study. Thepeaks occur during April to May and August for theglobal average. North America has a similar two-peakpattern during March to June and September toNovember. One peak pattern is dominant in othercontinents during February and March in South andSoutheast Asia, April and May in Central and North-east Asia and Central America, July and August in Eur-ope, and November and December in North Africa. Inthe southern hemisphere, the peak occurs duringMarch and April in Australia and NewZealand, July toSeptember in South Africa, and August to October inSouth America. The amount of PM2.5 emissions fromagricultural burning in the continental United Statesdetected using MODIS from 2003∼2007(McCarty 2011) is much smaller than in China, prob-ably because of the extensive use of powerful mechan-ical chopping. The largest emissions occur mainly inseveral regions of the Southeast, the Great Plains, andthe PacificNorthwest.

There are alternatives to burning for removingcrop straw residues in China. Straw can be choppedand directly decomposed in soil or composted outsidefields as nutrients and organic materials. However,farmers in China usually use simple tools that can’tchop straw fine enough to be well decomposed andabsorbed by soil before the next crop season. Usingpower generation has been explored but has largelyfailed due to the low efficiency and high costs of strawcutting and transportation. Crop straws were onceused as domestic fuel for some farmers. However, thisis no longer a practice for themajority of famers due toextensive usage of electric power and natural gas inrural areas in recent years. The Chinese governmenthas banned crop straw burning in most regions in aneffort to reduce air pollution but has compensatedfarmers for the economic burdens of using alternativesto burning. However, this policy is unlikely to be along-term solution.

5. Conclusions

It can be concluded from the findings of this study thatthe contributions of straw-burning PM2.5 emissionsduring harvest or other active burning periods to airpollutions in China are substantially larger thanprevious assessments based on annual emissions ofstraw burning and other anthropogenic sources in themajor burning provinces. China recently identifiedmain urban air pollutant sources from transportation,energy, industrial, and construction dust based onannual emissions. For sources such as crop straw

burning, monthly emissions should be used to esti-mate their contributions to air pollutants. Otherwise,the contributions will be significantly underestimatedfor certain periods of a year.

The larger contribution of straw burning to airpollution in China for harvest or other active burningperiods suggests that a substantial reduction or com-plete abandonment of open-field straw burning woulddramatically improve air quality in many Chinaregions during these periods.

Acknowledgments

The authors would like to thank two reviewers fortheir constructive and insightful comments and sug-gestions, which contributed to the improvement of themanuscript in both science and writing. This studywas supported by the International Center for Ecology,Meteorology and Environment, Nanjing University ofInformation Science and Technology, Nanjing, Chinaand USDA Forest Service. The funds were fromUSDA-NIFA-2013-35100-20516 and NSFC-71373130. The satellite remote sensed fire data wereobtained from the global fire emissions dataset(GFED). The air pollution index (API) data wereobtained from the Ministry of Environmental Protec-tion of China.

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