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Quantification of crop residue burning in the field and its influence on ambient air quality in Suqian, China
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Atmospheric Environment 42 (2008) 1961–1969 www.elsevier.com/locate/atmosenv
Quantification of crop residue burning in the field and its influence on ambient air quality in Suqian, China Shijian Yanga,b, Hongping Hea,, Shangling Luc, Dong Chena,b, Jianxi Zhua a
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China c Environmental Monitoring Central Station of Suqian City, Suqian 223800, PR China Received 5 June 2007; received in revised form 13 November 2007; accepted 3 December 2007
Abstract In China, many pollutants are released because of crop residue burning in the field, resulting in serious pollution of ambient air. Suqian with 4523 km2 of total area under cultivation was selected as a case to be studied, where wheat–rice double cropping system is widely adopted. Based on the data of crop output from 2001 to 2005, the annual average amount of crop residue generated was estimated as 3.04 106 t. About 82% of wheat straw and 37% of rice straw were burned in the field, so the proportion of crop residue burned in the field was about 43%. In combination with emission factors proposed by some literatures, the total amounts, the amounts in summer harvest and in autumn harvest of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2, emitted from crop residue burning in the field, were estimated. The total amounts of them were 11,051, 7572, 525, 3280, 1707, 3544, 905, 4331, 20,606, 120,747, and 1,988,376 t, respectively, and about 78% of them were emitted in summer harvest. During the summer harvest from June 4 to 13 in 2006, influenced by crop residue burning in the field, the daily average concentrations of PM10, NO2, and SO2 were 0.266, 0.051, and 0.063 mg m3, respectively. And the daily average concentration of PM10 kept exceeding 0.250 mg m3, the Third Standard Level of National Ambient Air Quality (China). Based on hourly concentration changes of PM10 and meteorological condition, crop residue burning in the field was characterized. According to the field survey, it is regarded that combine harvester acts as an important role in crop residue burning in the field. r 2007 Elsevier Ltd. All rights reserved. Keywords: Crop residue; Field burning; Pollutant emissions; PM10; Combine harvester
1. Introduction With rapid economic growth, China has faced severe air pollution problem for decades, especially in urban areas with particulate matter as the Corresponding author. Tel.: +86 2085290257;
fax: +86 2085290708. E-mail addresses: [email protected] (S. Yang), [email protected] (H. He). 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.12.007
principal pollutant in most cases. So aerosol emission from China has been of particular concern. China has a large rural population whose major energy source was biofuel (crop residue, fuel wood). However, rapid economic development has increased rural access to commercial energy, and the use of biofuel is decreasing. As a result, because crop residue is not in such high demand as fuel, it increasingly is being burned in the field; this is reflected in the mass media’s frequent claim that air
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pollution results from crop residue burning in the field (Yan et al., 2006). This has resulted in some serious environmental consequences, including temporary closings of airports and highways due to heavy smog. Furthermore, active trace gases emitted from crop residue burning such as CO and nitrogen oxide (NOx) are precursors of tropospheric ozone (O3) and can reduce the tropospheric concentration of the extremely reactive hydroxyl radical (OH) (Logan et al., 1981; Mauzerall et al., 1998). The government of China has enacted a series of regulations and laws to prohibit crop residue burning in the field. Peasants are encouraged to return crop residue to agricultural soil as organic fertilizer. However, this operation increases labor, cost and has some side effects on crop growth; thus, it is not adopted by most peasants, and a large proportion of crop residue is still burned in the field (Yan et al., 2006). In the survey of Gao et al. (2002), only 6.6% of the crop residue was burned in the field on average, and 36.6% of the crop residue was returned to the soil directly. But the estimates of most other scholars on the proportion of crop residue directly returned to the field were between 15% and 20% either in the whole country (MOA/DOE Project Expert Team, 1998; Yang, 1994) or in an individual province (Li, 2003; Song, 1995). Trace gas emissions from the burning of crop residue in China were estimated by Zhuang et al. (1996), without distinguishing between domestic burning and field burning, and only using a single fraction of the burning of crop residue for the whole country. Streets et al. (2003b) prepared a more thorough estimate of trace gas and aerosol emissions from biomass burning in China, including biofuel combustion, crop residue burning in the field, and forest and grassland fires. But in these researches, the proportion of crop residue burned in the field may be understated, so some controversial results appear in their data. At the same time the influence of crop residue burning in field on ambient air quality during burning is seldom studied. Now in China crop residue burning in the field is the most important source of biomass burning and it influences ambient air quality seriously. Therefore, crop residue burning in the field deserves special investigation. In this research, not only the pollutant emissions from crop residue burning in the field were estimated with up-to-date data, but also its influence on ambient air quality during burning was analyzed with environment monitoring and meteorological data.
2. Methods To estimate the pollutant emissions from crop residue burning in the field and its influence on ambient air quality, field survey and environmental monitoring were carried out in Suqian, a typical agricultural region in the northwest of Jiangsu province, China. Its total area is 8555 km2 (the proportion of water area is 22.4%), and its total area under cultivation is 4523 km2. In Suqian, wheat–rice double cropping system is widely adopted. 2.1. Pollutant emissions from crop residue burning in the field Pollutant emissions from crop residue burning in the field annually can be calculated using the product of the emission factor of a particular pollutant by the total amount of crop residue burned in the field, according to the expression used by Delmas et al. (1995): E ¼ EF M,
(1)
where E is the amount of a particular pollutant emitted annually (g year1); EF is the amount of a particular pollutant emitted per unit of crop residue burned in the field (g kg1); and M is the amount of crop residue burned in the field annually (kg year1). The EF values of TSP, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 were proposed by Streets et al. (2003a), and the EF value of PM10 was declared by US EPA (Cao et al., 2005). These values were once used in China by Cao et al. (2005). In the case of crop residue burning in the field, many authors, such as Rodriguez (1994), Gately (1975), and Donald and Hamblin (1976), showed that the amount of crop residue generated could be calculated by means of a constant ratio of crop residue to the crop produced. This ratio is identified as g and its product by the output of crop (gG) is used to estimate total crop residue generated (de Zarate et al., 2005). The output of crop is easy to know with accuracy since it bears economic value, and g values of all kinds of crop were proposed by CAREI (Cao et al., 2005). So the total amount of crop residue burned in the field can be expressed as X M¼ ji gi G i ; (2) where Gi is the output of a particular crop (kg year1); gi the ratio of the particular crop
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residue to crop produced; and ji the proportion of the particular crop residue burned in the field. It is possible to estimate the pollutant emissions from crop residue burning in the field in Suqian, according to values of G published annually by Suqian statistical bureau. However, j parameters have to be investigated. 2.2. Estimate of j After field survey, it is found that in Suqian the proportions of different crop residue burned in the field are varied. In Suqian, crop residues of legume, corn, potato, cotton, and oil plant are seldom directly burned in the field, they are always disposed as fuel, feed, or directly decomposed in the field. So the crop residues burned in the field are mainly wheat straw in summer harvest and rice straw in autumn harvest. The information about proportion of crop residue burned in the field provided by some peasants may be unreliable, because they may be afraid to be punished due to crop residue burning in the field. Based on the characteristics of agricultural production and peasants’ habits in Suqian, a project was designed to investigate the proportion of crop residue burned in the field. During field survey, it is found that if crop is harvested by combine harvester, the proportion of crop residue burned in the field is much more than harvested by man. So this project includes two parts: one is that crop is harvested by man and the other is that crop is harvested by combine harvester. The proportion of crop harvested by combine harvester can be provided by Suqian agricultural bureau, and it is about 90% in these years. For crop harvested by man, the proportion of crop residue burned in the field is small and difficult to be investigated, so it is estimated by some agricultural technicians as about 10% based on their experience. For crop harvested by combine harvester, the proportion of crop residue burned in the field was investigated with field survey. Firstly, the proportion of the field with crop residue burning was investigated. If crop residue is burned in the field, the color of the field will be obviously black with ash. The proportion of the field with crop residue burning may be estimated as a using this obvious characteristic. Field with crop residue burning consists of two parts: one is the field where only stubble is burned and the left crop residue is called back, the other is the field where no crop residue is called back and all of the crop residues are burned. Secondly, the
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proportion of the families who call back crop residue was investigated as b. It is regarded that crop residue is called back by the families if there are some proper haystacks around their houses. The proportion of families who call back crop residue may approximately equal to the proportion of the field where crop residue is called back. The field where crop residue is called back consists of three parts: one is the field where crop is harvested by man and this proportion is 10%; another is the field where no crop residue is burned (most of crop residue is called back, but stubble is not burned) and this proportion can be expressed as 1–a; and the other is the field where only stubble is burned and the left crop residue is called back. There should be some superposition among these three parts. Because the proportion of crop harvested by man is small, its contributions to the proportion of the field with crop residue burning, the proportion of the field with no crop residue (including stubble) burning, and the proportion of the field with only stubble burning can be neglected. So for crop harvested by combine harvester, the proportion of the field where only stubble is burned and the left crop residue is called back can be expressed as b0.1(1a), and the proportion of crop residue burned in the field can be estimated approximately with the following empirical formula: j ¼ a ½b 0:1 ð1 aÞ ð1 0:4Þ,
(3)
where j is the proportion of crop residue burned in the field; a is the proportion of the field with crop residue burning; b is the proportion of the families who call back crop residue; 0.1 is the proportion of crop harvested by man; and 0.4 is the ratio of the stubble to total crop residue estimated approximately by some agricultural technicians. The proportion of the field with crop residue burning was provided by the inspecting groups of Suqian government whose duty was to prohibit crop residue burning in the field, and the survey scope was more than half of Suqian. Some of us took part in the inspecting groups to ensure the veracity of survey result. After harvest, about 10 villages including more than 500 families which are all far from cities were randomly sampled to investigate the proportion of the families who call back crop residue. 2.3. Ambient air quality monitoring during burning There are two sets of ambient air quality auto-monitoring systems made by API (Advanced
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Pollution Instruments Inc., San Diego, CA 92121) at different monitoring sites (named Suqian College and Monitoring Station) in Suqian. At the two sites, PM10, NO2, SO2 are monitored. PM10 is determined by MET ONE BAM-1020 once per hour and NO2, SO2 are determined online by API MODEL 100A and API MODEL 200A, respectively. There is about 8 km between the two sites. Potassium in precipitation was determined by using flame atomic absorption (FAAS) using a Varian, SpectrAA-50B spectrometer after filtrating. The detection limit and precision of FAAS method were 0.005 mg L1 and 3.0%, respectively. The meteorological data were provided by Suqian meteorological bureau.
3. Results and discussion 3.1. Pollutant emissions from crop residue burning in the field The total amount of crop residue generated from 2001 to 2005 in Suqian can be calculated according to the values of G provided by Suqian statistical bureau (Table 1). Through field survey, it was found that if wheat is harvested by combine harvester (in summer harvest), the values of a and b were about 100% and 25%, respectively; if rice was harvested by combine harvester (in autumn harvest), the values of a and b were about 70% and 90%, respectively. So if crop was harvested by combine harvester, the j values of
wheat and rice were about 90% and 40%, respectively. It can be determined that the j value of wheat straw was about 82%, and the j value of rice straw was about 37%. Afterward the amount of crop residue burned in the field can be calculated using Eq (2) (Table 2). Using the emission factors, emissions of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 generated from crop residue burning in the field can be obtained by means of Eq (1) (Table 3). Some results can be provided in these tables, and the results are summarized in the following way: (1) The annual amount of crop residue in Suqian generated from the crop output varied between 2.06 and 3.48 106 t; therefore, an average amount of 3.04 106 t (s ¼ 5.59 105 t) was obtained. (2) The annual amount of crop residue burned in the field ranged between 9.66 105 t and 1.53 106 t, the average value of which was 1.31 106 t (s ¼ 2.33 105 t). So the proportion of crop residue burned in the field in Suqian was about 43%. (3) The annual pollutant emissions of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 from crop residue burning in the field were 11,051, 7572, 525, 3280, 1707, 3544, 905, 4331, 20,606, 120,747, and 1,988,376 t, respectively, and 78% of them were emitted in summer harvest.
Table 1 Amount of crop residue generated in Suqian from 2001 to 2005 (t) Summer harvest
Autumn harvest
gi (unitless)
Wheat 1.366
Rice 0.623
Corn 2
Legume 1.5
Potato 0.5
Oil plant 2
Cotton 3
2001
Gi gi Gi
918,018 1,254,012
6188 9292
1,292,344 805,130
282,675 565,350
73,142 109,713
234,413 117,206
174,937 349,874
24,980 74,940
3,285,507
2002
Gi gi Gi
836,198 1,142,246
4695 7042
1,243,259 774,550
322,102 644,204
74,826 112,239
248,796 124,398
182,960 365,920
15,600 46,800
3,217,399
2003
Gi gi Gi
718,863 972,404
7176 10,764
732,542 456,374
104,131 208,262
45562 68,343
165,110 82,555
119,884 239,768
7612 22,836
2,061,306
2004
Gi gi Gi
1,047,669 1,431,116
5565 8348
1,499,186 933,993
289,705 579,410
63,989 95,984
150,094 75,047
155,132 310,264
14,849 44,547
3,478,709
2005
Gi gi Gi
1,075,705 1,469,413
4910 7365
1,399,685 872,004
225,209 450,418
45,640 68,460
114,761 57,380
96,261 192,522
7244 21,732
3,139,294
Legume 1.5
Total amount of crop residue
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Table 2 Amount of crop residue burned in the field in Suqian from 2001 to 2005 (t) Amount of crop residue burned in the field
2001
Summer harvest Autumn harvest
1,028,290 297,898
Total
1,326,188
2002
2003
2004
2005
Average
936,642 286,584
797,371 168,858
1,173,515 345,577
1,204,919 322,641
1,028,147 284,312
1,223,226
966,229
1,519,092
1,527,560
1,312,459
Table 3 Average amount of pollutant emissions from crop residue burning in the field annually in Suqian in 2001–2005 (t)
E
TSP
PM10
SO2
NOx
NH3
CH4
EC
OC
VOC
CO
CO2
EF (g kg1) Summer harvest Autumn harvest
8.42 8657 2394
5.77 5932 1640
0.4 411 114
2.5 2570 710
1.3 1337 370
2.7 2776 768
0.69 709 196
3.3 3393 938
15.7 16,142 4464
92 94,590 26,157
1515 1,557,643 430,733
Total
11,051
7572
525
3280
1707
3544
905
4331
20,606
120,747
1,988,376
3.2. Influence of crop residue burning in field on ambient air quality Because in these years the pollutant emissions in summer harvest were much more than in autumn harvest, the summer harvest in 2006 was sampled as a typical case to describe the influence of crop residue burning in the field on ambient air quality during burning. This summer harvest was from June 4 to 13 in Suqian, and lasted about 10 days. If the pollutant emissions in 2006 were equal to the average of 2001–2005, the intensity of particle (TSP) emitting to ambient air was about 36 t h1 in these days, which was more than the total emissions of 100 larger coal-fired power stations. In these days, ambient air quality was much worse. With the investigation of industrial pollution sources, it was found that no industrial accidents took place. The concentration of potassium (K) in precipitation at wee hours on June 14 was analyzed, and it was 8.42 mg L1, much more than the data in May when no crop residue was burned in the field. Potassium (K) is the tracer element of biomass burning (Duan et al., 2004), so it can be determined that this ambient air pollution was caused by crop residue burning in the field. The daily average concentration changes of ambient air pollutants (PM10, SO2, and NO2) from June 1 to 16 in Suqian are shown in Fig. 1. The daily average concentrations of PM10, NO2, and SO2 from June 4 to 13 were 0.266, 0.051, and
0.063 mg m3, respectively. The daily average concentration of PM10 kept exceeding 0.250 mg m3, the Third Standard Level of National Ambient Air Quality (China). And the daily average concentration of SO2 kept exceeding 0.050 mg m3, the First Standard Level of National Ambient Air Quality. Based on the obvious correlation between the quantity of crop residue burned in the field and PM10 concentration in ambient air, crop residue burning in the field was characterized with the meteorological condition and hourly concentration of PM10. As shown in Fig. 2, from morning on June 4, ambient air quality in Suqian began to deteriorate. But little wheat in Suqian was reaped. With further investigation, it was found that wheat was being harvested massively in Xuyi and Chuzhou to the southeast of Suqian, and crop residue burning in the field happened prevalently after dark on June 3. With southeast wind from 4:00 to 10:00 in Suqian, ambient air quality was seriously affected by the particles from the southeast areas. Because the wind speeded up and the source of particles declined, ambient air quality became better after 12:00. But when southeast wind turned back and crop residue was burned in the field again after dark in the southeast areas, ambient air quality turned worse again from 22:00. Following last night, ambient air quality continued to be worse on June 5. From 7:00 to 20:00, the main wind direction was southwest and the
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0.4
PM10
SO2
0.25
NO2
0.2
PM10 (mg m-3)
0.3
0.15 0.2 0.1 0.1
0.05
0
SO2 and NO2 (mg m-3)
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0 6-1
6-3
6-5
6-7
6-9
6-11
6-13
6-15
Fig. 1. Daily average concentration changes of PM10, SO2, and NO2 in Suqian from June 1 to 16.
0.1 0 0:00 4:00 8:00 12:00 16:00 20:00 June 5
0 0:00 4:00 8:00 12:00 16:00 20:00 June 4
0.6 0.4 0.2
1
0.6
0.8
0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 8
PM10 (mg m-3)
1 0.8 0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 9
0.4 0.2
0.8
0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 10
0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 11
1
PM 10 (mg m-3)
0.8
PM10 (mg m-3)
0.6
0 0:00 4:00 8:00 12:00 16:00 20:00 June 7
0 0:00 4:00 8:00 12:00 16:00 20:00 June 6
0.8
PM10 (mg m-3)
PM10 (mg m-3)
0.2
PM10 (mg m-3)
0.2
0.3
PM10 (mg m-3)
0.4
PM10 (mg m-3)
0.6
0.8
0.8
0.4 PM10 (mg m-3)
PM10 (mg m-3)
0.8
0.6 0.4 0.2
0.8 0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 13
0 0:00 4:00 8:00 12:00 16:00 20:00 June 12 Monitoring Station
Suqian College
Fig. 2. Hourly concentration changes of PM10 in Suqian from June 4 to 13.
source of particles declined in daytime, so the concentration of PM10 declined. From 0:00 to 9:00 on June 6, with southeast wind, ambient air quality deteriorated influenced by crop residue burning in the southeast areas. After 10:00, because of the source of particle declining and wind speeding up, ambient air quality improved. Ambient air quality deteriorated after 18:00 because wheat began being harvested in Suqian and some crop residue began being burned in the field. From 0:00 to 7:00 on June 7, as wind turned southeast, ambient air quality was very terrible which was attributed to crop residue burning both in Suqian and southeast areas. After 7:00, ambient air quality improved because fire extinguished and
wind speed increased. After 19:00, ambient air quality deteriorated again, which was attributed to crop residue burning in the field in Suqian. From 0:00 to 5:00 on June 8, the wind direction was southeast and south, so the air pollution was also attributed to crop residue burning in the field both in Suqian and other areas. When wind speeded up, the concentration of PM10 declined after 13:00. After 19:00, ambient air quality became bad again. Before 11:00 on June 9, though main wind direction was southwest and wind speed increased, the pollution was very serious because crop residue was burned in the field in Suqian and around Suqian last night on a large scale. At noon, ambient air quality improved since wind speed was up to
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9 m s1. After 19:00, crop residue was burned in Suqian on a larger scale, so PM10 concentration was much larger than before. Before 2:00 on June 10, ambient air quality was seriously influenced by crop residue burning in Suqian. In this morning, Suqian government held a meeting against crop residue burning in the field, and strengthened some groups to prohibit this activity. Before 22:00, the phenomenon of crop residue burning in the field did not appear and ambient air quality was better. But after the inspection groups went back, crop residue was still burned in the field. So crop residue burning in the field at this night was not concentrative and last a long time. There was some crop residue burned in the morning of June 11, so the pollution was terrible from wee hours to 9:00. Since today was Sunday a holiday for government, crop residue began being burned at about 19:00. From June 9, because the quantity of crop residue burned in Suqian increased, the influence of crop residue burning in other areas on ambient air quality of Suqian could be neglected. On June 12, the harvest in Suqian came into the key time, and crop residue burning in the field also happened prevalently; ambient air quality was serious and the measure of Suqian government to prohibit crop residue burning in the field had no effect. On June 13, weather forecast declared that it would rain tomorrow, so the left crop must be harvested today. To avoid influencing summer plant, many peasants burned all left crop residue in the field. At last, this air pollution caused by crop residue burning in the field was over after the rainfall and gale. From above analysis, some characteristics of crop residue burning in the field in Suqian can be described: (1) The ambient air quality of Suqian is not only influenced by crop residue burning in Suqian, but also it may be seriously influenced by crop residue burning in the vicinages. It may be presumed that from June 5 to 9 the air pollution from last night to the next wee hours was attributed to crop residue burning in the field in Suqian, and the air pollution in the next morning was attributed to crop residue burning in the field in other areas. (2) During daytime, because the peasants are busy in harvesting and crop residue burning nearby
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will influence harvesting, crop residue is seldom burned. Crop residue burning in the field is almost concentrated on the several hours after dark, so ambient air quality is better in daytime than at night. But there were some peak values of pollutant in daytime probably because with bad meteorological conditions, pollutant was transferred to Suqian from other places by wind. (3) The ambient air quality during burning is very correlative with the meteorological condition. High wind speed can promote the transference of pollutants, so with high wind, the ambient air quality is better. Compared with high wind speed, precipitation is more effective to eliminate the influence of crop residue burning in the field on ambient air quality. (4) The effect of Suqian government’s measure to prohibit crop residue burning in the field is poor. In Suqian, the forced measure cannot reduce crop residue burning in the field, although lots of human and non-human sources are consumed. Its poor effect only postpones the burning time. It indicates that only government forced measure is inefficient to prohibit crop residue burning in the field. 3.3. Some reasons why crop residue burning in the field in Suqian In recent years, some scholars (Cao et al., 2005; Yan et al., 2006) regarded that with the economic developing, crop residue was not one kind of necessary fuel, collecting them was very troublesome, the places to store them were short, and there were few ways to use them were the main reasons why crop residue was burned in the field. With firsthand information from talking with some peasants, officers, and agricultural technicians, it was found that there were some other reasons why crop residue was burned in the field. The use of combine harvester is the main reason why crop residue is burned in the field. Before 1990 s of the 20th century, most of wheat and rice were harvested by man in Suqian. Crop, including crop residue, was transported to settled place, then was threshed. At that time, crop residue was a byproduct which did not need any extra labor and cost. To improve the benefit, the length of stubble was about 10 cm. But after using combine harvester, byproduct which does not need any extra labor and cost cannot be realized. Though the length of stubble is requested to be low to 20 cm by
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government, in practice the length of stubble is about 30–40 cm to avoid the damage of combine harvester and save harvest cost. It causes that after harvest almost half of the crop residue is stubble standing in the field and the other half is lying in the field. So the benefit of collecting crop residue is only half of before, but the cost is much more than before. If there is some crop residue in the field, it will emerge on the ground after plowing, which will influence transplanting rice seedlings and the growth of seedlings. So the peasants cannot but burn them in the field. The busyness of peasants in June is an important reason why crop residue burning in the field in summer harvest is more serious than it in autumn harvest in Suqian. June is the busiest month for peasants in Suqian. In this month, they are busy with harvesting cole, wheat, sowing beans, transplanting rice seedlings, and sericiculture, so the farming season for them is obviously pressing. Many peasants are so tired in this month that they are unwilling to spend the valued time on callback crop residue. But there is enough time between autumn harvest and autumn sowing, so lots of crop residue was called back. 4. Conclusions From the results obtained in this study, a few conclusions can be drawn: (1) In Suqian, the amount of crop residue burned in the field annually ranged between 9.66 105 t and 1.53 106 t, the average value was 1.31 106 t (s ¼ 2.33 105 t) during the analyzed period and the proportion of crop residue burned in the field was about 43%. The annual pollutant emissions of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 from crop residue burning in the field were 11,051, 7572, 525, 3280, 1707, 3544, 905, 4331, 20,606, 120,747, and 1,988,376 t, respectively, and 78% of them were emitted in summer harvest. (2) Influenced by crop residue burning in the field, ambient air quality during burning is terrible. From June 4 to 13 in 2006, the daily average concentration of PM10, NO2, and SO2 was 0.266, 0.051 and 0.063 mg m3, respectively. The daily average concentration of PM10 kept exceeding 0.250 mg m3, the Third Standard Level of National Ambient Air Quality.
(3) During burning, the ambient air quality at night is more serious than in daytime. The ambient air quality in Suqian is influenced by crop residue burning not only in Suqian, but also in the vicinages. The effect of forced measure adopted by Suqian government to prohibit crop residue burning in the field is poor. (4) The use of combine harvester is the main reason why crop residue is burned in the field in Suqian. And the busyness of peasants in June and the press of farming season are important reasons why crop residue burning in the field in summer harvest is much more serious than it in autumn harvest.
References Cao, L.G., Zhang, X.Y., Wang, D.L., Zheng, F.C., 2005. Inventory of atomspheric pollutants discharged from biomass in China continent. China Environmental Sciences 25 (4), 389–393 (in Chinese). de Zarate, I.O., Ezcurra, A., Lacaux, J.P., Van Dinh, P., de Argandona, J.D., 2005. Pollution by cereal waste burning in Spain. Atmospheric Research 73 (1–2), 161–170. Delmas, R., Lacaux, J.P., Menaut, J.C., Abbadie, L., Le Roux, X., Helaa, G., Lobert, J., 1995. Nitrogen compound emission from biomass burning in tropical African Savanna FOS/ DECAFE 1991 experiment. Journal of Atmospheric Chemistry 22, 175–193. Donald, C.M., Hamblin, J., 1976. The biological yield and harvest index of cereals as agronomic and plant breeding criteria. Advances in Agronomy 28, 361–405. Duan, F.K., Liu, X.D., Yu, T., Cachier, H., 2004. Identification and estimate of biomass burning contribution to the urban aerosol organic carbon concentrations in Beijing. Atmospheric Environment 38 (9), 1275–1282. Gao, X., Ma, C., Zhang, F., Wang, Y., 2002. Analysis of the current status of utilization of crop straw in China. Journal of Huazhong Agricultural University 21, 242–247 (in Chinese). Gately, T.F., 1975. Barely production, effects of fertilizer nitrogen and number of years in tillage on the yield of barely straw. Journal of Agricultural Research 14, 237–245. Li, A.X., 2003. Status and development measures of organic fertilizer utilization in Hunan Province. Hunan Agricultural Sciences 4, 8–10 (in Chinese). Logan, J., Prather, M.J., Wofsy, S.C., McElory, M.B., 1981. Tropospheric chemistry: a global perspective. Journal of Geophysical Research 86, 7210–7254. Mauzerall, D.L., Logan, J.A., Jacob, D.J., Anderson, B.E., Blake, D.R., Bradshaw, J.D., Heikes, B., Sachse, G.W., Singh, H., Talbot, B., 1998. Photochemistry in biomass burning plumes and implications for tropospheric ozone over the tropical South Atlantic. Journal of Geophysical Research—Atmospheres 103, 8401–8423. MOA/DOE Project Expert Team, 1998. Assessment of Biomass Resource Availability in China. China Environmental Science Press, Beijing, China, 270pp.
ARTICLE IN PRESS S. Yang et al. / Atmospheric Environment 42 (2008) 1961–1969 Rodriguez, J.C., 1994. Greenhouse gas emissions from burning of agricultural crop wastes. Informe para la OCDE 6, 34–47. Song, J.B., 1995. Survey and analysis of crop residue resources and quality in Zhejiang Province. Soil and Fertilizer 2, 23–24 (in Chinese). Streets, D.G., Bond, T.C., Carmichael, G.R., Fernandes, S.D., Fu, Q., He, D., Klimont, Z., Nelson, S.M., Tsai, N.Y., Wang, M.Q., Woo, J.H., Yarber, K.F., 2003a. An inventory of gaseous and primary aerosol emissions in Asia in the year 2000. Journal of Geophysical Research—Atmospheres 108, 8809. Streets, D.G., Yarber, K.F., Woo, J.H., Carmichael, G.R., 2003b. Biomass burning in Asia: Annual and seasonal
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estimates and atmospheric emissions. Global Biogeochemical Cycles 17 (4), 1099. Yan, X.Y., Ohara, T., Akimoto, H., 2006. Bottom-up estimate of biomass burning in mainland China. Atmospheric Environment 40 (27), 5262–5273. Yang, S.J., 1994. Emphasizing the use of crop residue as energy source. Rural Energy 58, 18–19 (in Chinese). Zhuang, Y.H., Cao, M.Q., Wang, X.K., Yao, H., 1996. Spatial distribution of trace-gas emissions from burning crop residue in China. In: Levine, J.S. (Ed.), Global Biomass Burning Atmospheric, Climatic and Biospheric Implications. MIT Press, Cambridge, MA, pp. 764–770.
Atmospheric Environment 42 (2008) 1961–1969 www.elsevier.com/locate/atmosenv
Quantification of crop residue burning in the field and its influence on ambient air quality in Suqian, China Shijian Yanga,b, Hongping Hea,, Shangling Luc, Dong Chena,b, Jianxi Zhua a
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China c Environmental Monitoring Central Station of Suqian City, Suqian 223800, PR China Received 5 June 2007; received in revised form 13 November 2007; accepted 3 December 2007
Abstract In China, many pollutants are released because of crop residue burning in the field, resulting in serious pollution of ambient air. Suqian with 4523 km2 of total area under cultivation was selected as a case to be studied, where wheat–rice double cropping system is widely adopted. Based on the data of crop output from 2001 to 2005, the annual average amount of crop residue generated was estimated as 3.04 106 t. About 82% of wheat straw and 37% of rice straw were burned in the field, so the proportion of crop residue burned in the field was about 43%. In combination with emission factors proposed by some literatures, the total amounts, the amounts in summer harvest and in autumn harvest of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2, emitted from crop residue burning in the field, were estimated. The total amounts of them were 11,051, 7572, 525, 3280, 1707, 3544, 905, 4331, 20,606, 120,747, and 1,988,376 t, respectively, and about 78% of them were emitted in summer harvest. During the summer harvest from June 4 to 13 in 2006, influenced by crop residue burning in the field, the daily average concentrations of PM10, NO2, and SO2 were 0.266, 0.051, and 0.063 mg m3, respectively. And the daily average concentration of PM10 kept exceeding 0.250 mg m3, the Third Standard Level of National Ambient Air Quality (China). Based on hourly concentration changes of PM10 and meteorological condition, crop residue burning in the field was characterized. According to the field survey, it is regarded that combine harvester acts as an important role in crop residue burning in the field. r 2007 Elsevier Ltd. All rights reserved. Keywords: Crop residue; Field burning; Pollutant emissions; PM10; Combine harvester
1. Introduction With rapid economic growth, China has faced severe air pollution problem for decades, especially in urban areas with particulate matter as the Corresponding author. Tel.: +86 2085290257;
fax: +86 2085290708. E-mail addresses: [email protected] (S. Yang), [email protected] (H. He). 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.12.007
principal pollutant in most cases. So aerosol emission from China has been of particular concern. China has a large rural population whose major energy source was biofuel (crop residue, fuel wood). However, rapid economic development has increased rural access to commercial energy, and the use of biofuel is decreasing. As a result, because crop residue is not in such high demand as fuel, it increasingly is being burned in the field; this is reflected in the mass media’s frequent claim that air
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pollution results from crop residue burning in the field (Yan et al., 2006). This has resulted in some serious environmental consequences, including temporary closings of airports and highways due to heavy smog. Furthermore, active trace gases emitted from crop residue burning such as CO and nitrogen oxide (NOx) are precursors of tropospheric ozone (O3) and can reduce the tropospheric concentration of the extremely reactive hydroxyl radical (OH) (Logan et al., 1981; Mauzerall et al., 1998). The government of China has enacted a series of regulations and laws to prohibit crop residue burning in the field. Peasants are encouraged to return crop residue to agricultural soil as organic fertilizer. However, this operation increases labor, cost and has some side effects on crop growth; thus, it is not adopted by most peasants, and a large proportion of crop residue is still burned in the field (Yan et al., 2006). In the survey of Gao et al. (2002), only 6.6% of the crop residue was burned in the field on average, and 36.6% of the crop residue was returned to the soil directly. But the estimates of most other scholars on the proportion of crop residue directly returned to the field were between 15% and 20% either in the whole country (MOA/DOE Project Expert Team, 1998; Yang, 1994) or in an individual province (Li, 2003; Song, 1995). Trace gas emissions from the burning of crop residue in China were estimated by Zhuang et al. (1996), without distinguishing between domestic burning and field burning, and only using a single fraction of the burning of crop residue for the whole country. Streets et al. (2003b) prepared a more thorough estimate of trace gas and aerosol emissions from biomass burning in China, including biofuel combustion, crop residue burning in the field, and forest and grassland fires. But in these researches, the proportion of crop residue burned in the field may be understated, so some controversial results appear in their data. At the same time the influence of crop residue burning in field on ambient air quality during burning is seldom studied. Now in China crop residue burning in the field is the most important source of biomass burning and it influences ambient air quality seriously. Therefore, crop residue burning in the field deserves special investigation. In this research, not only the pollutant emissions from crop residue burning in the field were estimated with up-to-date data, but also its influence on ambient air quality during burning was analyzed with environment monitoring and meteorological data.
2. Methods To estimate the pollutant emissions from crop residue burning in the field and its influence on ambient air quality, field survey and environmental monitoring were carried out in Suqian, a typical agricultural region in the northwest of Jiangsu province, China. Its total area is 8555 km2 (the proportion of water area is 22.4%), and its total area under cultivation is 4523 km2. In Suqian, wheat–rice double cropping system is widely adopted. 2.1. Pollutant emissions from crop residue burning in the field Pollutant emissions from crop residue burning in the field annually can be calculated using the product of the emission factor of a particular pollutant by the total amount of crop residue burned in the field, according to the expression used by Delmas et al. (1995): E ¼ EF M,
(1)
where E is the amount of a particular pollutant emitted annually (g year1); EF is the amount of a particular pollutant emitted per unit of crop residue burned in the field (g kg1); and M is the amount of crop residue burned in the field annually (kg year1). The EF values of TSP, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 were proposed by Streets et al. (2003a), and the EF value of PM10 was declared by US EPA (Cao et al., 2005). These values were once used in China by Cao et al. (2005). In the case of crop residue burning in the field, many authors, such as Rodriguez (1994), Gately (1975), and Donald and Hamblin (1976), showed that the amount of crop residue generated could be calculated by means of a constant ratio of crop residue to the crop produced. This ratio is identified as g and its product by the output of crop (gG) is used to estimate total crop residue generated (de Zarate et al., 2005). The output of crop is easy to know with accuracy since it bears economic value, and g values of all kinds of crop were proposed by CAREI (Cao et al., 2005). So the total amount of crop residue burned in the field can be expressed as X M¼ ji gi G i ; (2) where Gi is the output of a particular crop (kg year1); gi the ratio of the particular crop
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residue to crop produced; and ji the proportion of the particular crop residue burned in the field. It is possible to estimate the pollutant emissions from crop residue burning in the field in Suqian, according to values of G published annually by Suqian statistical bureau. However, j parameters have to be investigated. 2.2. Estimate of j After field survey, it is found that in Suqian the proportions of different crop residue burned in the field are varied. In Suqian, crop residues of legume, corn, potato, cotton, and oil plant are seldom directly burned in the field, they are always disposed as fuel, feed, or directly decomposed in the field. So the crop residues burned in the field are mainly wheat straw in summer harvest and rice straw in autumn harvest. The information about proportion of crop residue burned in the field provided by some peasants may be unreliable, because they may be afraid to be punished due to crop residue burning in the field. Based on the characteristics of agricultural production and peasants’ habits in Suqian, a project was designed to investigate the proportion of crop residue burned in the field. During field survey, it is found that if crop is harvested by combine harvester, the proportion of crop residue burned in the field is much more than harvested by man. So this project includes two parts: one is that crop is harvested by man and the other is that crop is harvested by combine harvester. The proportion of crop harvested by combine harvester can be provided by Suqian agricultural bureau, and it is about 90% in these years. For crop harvested by man, the proportion of crop residue burned in the field is small and difficult to be investigated, so it is estimated by some agricultural technicians as about 10% based on their experience. For crop harvested by combine harvester, the proportion of crop residue burned in the field was investigated with field survey. Firstly, the proportion of the field with crop residue burning was investigated. If crop residue is burned in the field, the color of the field will be obviously black with ash. The proportion of the field with crop residue burning may be estimated as a using this obvious characteristic. Field with crop residue burning consists of two parts: one is the field where only stubble is burned and the left crop residue is called back, the other is the field where no crop residue is called back and all of the crop residues are burned. Secondly, the
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proportion of the families who call back crop residue was investigated as b. It is regarded that crop residue is called back by the families if there are some proper haystacks around their houses. The proportion of families who call back crop residue may approximately equal to the proportion of the field where crop residue is called back. The field where crop residue is called back consists of three parts: one is the field where crop is harvested by man and this proportion is 10%; another is the field where no crop residue is burned (most of crop residue is called back, but stubble is not burned) and this proportion can be expressed as 1–a; and the other is the field where only stubble is burned and the left crop residue is called back. There should be some superposition among these three parts. Because the proportion of crop harvested by man is small, its contributions to the proportion of the field with crop residue burning, the proportion of the field with no crop residue (including stubble) burning, and the proportion of the field with only stubble burning can be neglected. So for crop harvested by combine harvester, the proportion of the field where only stubble is burned and the left crop residue is called back can be expressed as b0.1(1a), and the proportion of crop residue burned in the field can be estimated approximately with the following empirical formula: j ¼ a ½b 0:1 ð1 aÞ ð1 0:4Þ,
(3)
where j is the proportion of crop residue burned in the field; a is the proportion of the field with crop residue burning; b is the proportion of the families who call back crop residue; 0.1 is the proportion of crop harvested by man; and 0.4 is the ratio of the stubble to total crop residue estimated approximately by some agricultural technicians. The proportion of the field with crop residue burning was provided by the inspecting groups of Suqian government whose duty was to prohibit crop residue burning in the field, and the survey scope was more than half of Suqian. Some of us took part in the inspecting groups to ensure the veracity of survey result. After harvest, about 10 villages including more than 500 families which are all far from cities were randomly sampled to investigate the proportion of the families who call back crop residue. 2.3. Ambient air quality monitoring during burning There are two sets of ambient air quality auto-monitoring systems made by API (Advanced
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Pollution Instruments Inc., San Diego, CA 92121) at different monitoring sites (named Suqian College and Monitoring Station) in Suqian. At the two sites, PM10, NO2, SO2 are monitored. PM10 is determined by MET ONE BAM-1020 once per hour and NO2, SO2 are determined online by API MODEL 100A and API MODEL 200A, respectively. There is about 8 km between the two sites. Potassium in precipitation was determined by using flame atomic absorption (FAAS) using a Varian, SpectrAA-50B spectrometer after filtrating. The detection limit and precision of FAAS method were 0.005 mg L1 and 3.0%, respectively. The meteorological data were provided by Suqian meteorological bureau.
3. Results and discussion 3.1. Pollutant emissions from crop residue burning in the field The total amount of crop residue generated from 2001 to 2005 in Suqian can be calculated according to the values of G provided by Suqian statistical bureau (Table 1). Through field survey, it was found that if wheat is harvested by combine harvester (in summer harvest), the values of a and b were about 100% and 25%, respectively; if rice was harvested by combine harvester (in autumn harvest), the values of a and b were about 70% and 90%, respectively. So if crop was harvested by combine harvester, the j values of
wheat and rice were about 90% and 40%, respectively. It can be determined that the j value of wheat straw was about 82%, and the j value of rice straw was about 37%. Afterward the amount of crop residue burned in the field can be calculated using Eq (2) (Table 2). Using the emission factors, emissions of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 generated from crop residue burning in the field can be obtained by means of Eq (1) (Table 3). Some results can be provided in these tables, and the results are summarized in the following way: (1) The annual amount of crop residue in Suqian generated from the crop output varied between 2.06 and 3.48 106 t; therefore, an average amount of 3.04 106 t (s ¼ 5.59 105 t) was obtained. (2) The annual amount of crop residue burned in the field ranged between 9.66 105 t and 1.53 106 t, the average value of which was 1.31 106 t (s ¼ 2.33 105 t). So the proportion of crop residue burned in the field in Suqian was about 43%. (3) The annual pollutant emissions of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 from crop residue burning in the field were 11,051, 7572, 525, 3280, 1707, 3544, 905, 4331, 20,606, 120,747, and 1,988,376 t, respectively, and 78% of them were emitted in summer harvest.
Table 1 Amount of crop residue generated in Suqian from 2001 to 2005 (t) Summer harvest
Autumn harvest
gi (unitless)
Wheat 1.366
Rice 0.623
Corn 2
Legume 1.5
Potato 0.5
Oil plant 2
Cotton 3
2001
Gi gi Gi
918,018 1,254,012
6188 9292
1,292,344 805,130
282,675 565,350
73,142 109,713
234,413 117,206
174,937 349,874
24,980 74,940
3,285,507
2002
Gi gi Gi
836,198 1,142,246
4695 7042
1,243,259 774,550
322,102 644,204
74,826 112,239
248,796 124,398
182,960 365,920
15,600 46,800
3,217,399
2003
Gi gi Gi
718,863 972,404
7176 10,764
732,542 456,374
104,131 208,262
45562 68,343
165,110 82,555
119,884 239,768
7612 22,836
2,061,306
2004
Gi gi Gi
1,047,669 1,431,116
5565 8348
1,499,186 933,993
289,705 579,410
63,989 95,984
150,094 75,047
155,132 310,264
14,849 44,547
3,478,709
2005
Gi gi Gi
1,075,705 1,469,413
4910 7365
1,399,685 872,004
225,209 450,418
45,640 68,460
114,761 57,380
96,261 192,522
7244 21,732
3,139,294
Legume 1.5
Total amount of crop residue
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Table 2 Amount of crop residue burned in the field in Suqian from 2001 to 2005 (t) Amount of crop residue burned in the field
2001
Summer harvest Autumn harvest
1,028,290 297,898
Total
1,326,188
2002
2003
2004
2005
Average
936,642 286,584
797,371 168,858
1,173,515 345,577
1,204,919 322,641
1,028,147 284,312
1,223,226
966,229
1,519,092
1,527,560
1,312,459
Table 3 Average amount of pollutant emissions from crop residue burning in the field annually in Suqian in 2001–2005 (t)
E
TSP
PM10
SO2
NOx
NH3
CH4
EC
OC
VOC
CO
CO2
EF (g kg1) Summer harvest Autumn harvest
8.42 8657 2394
5.77 5932 1640
0.4 411 114
2.5 2570 710
1.3 1337 370
2.7 2776 768
0.69 709 196
3.3 3393 938
15.7 16,142 4464
92 94,590 26,157
1515 1,557,643 430,733
Total
11,051
7572
525
3280
1707
3544
905
4331
20,606
120,747
1,988,376
3.2. Influence of crop residue burning in field on ambient air quality Because in these years the pollutant emissions in summer harvest were much more than in autumn harvest, the summer harvest in 2006 was sampled as a typical case to describe the influence of crop residue burning in the field on ambient air quality during burning. This summer harvest was from June 4 to 13 in Suqian, and lasted about 10 days. If the pollutant emissions in 2006 were equal to the average of 2001–2005, the intensity of particle (TSP) emitting to ambient air was about 36 t h1 in these days, which was more than the total emissions of 100 larger coal-fired power stations. In these days, ambient air quality was much worse. With the investigation of industrial pollution sources, it was found that no industrial accidents took place. The concentration of potassium (K) in precipitation at wee hours on June 14 was analyzed, and it was 8.42 mg L1, much more than the data in May when no crop residue was burned in the field. Potassium (K) is the tracer element of biomass burning (Duan et al., 2004), so it can be determined that this ambient air pollution was caused by crop residue burning in the field. The daily average concentration changes of ambient air pollutants (PM10, SO2, and NO2) from June 1 to 16 in Suqian are shown in Fig. 1. The daily average concentrations of PM10, NO2, and SO2 from June 4 to 13 were 0.266, 0.051, and
0.063 mg m3, respectively. The daily average concentration of PM10 kept exceeding 0.250 mg m3, the Third Standard Level of National Ambient Air Quality (China). And the daily average concentration of SO2 kept exceeding 0.050 mg m3, the First Standard Level of National Ambient Air Quality. Based on the obvious correlation between the quantity of crop residue burned in the field and PM10 concentration in ambient air, crop residue burning in the field was characterized with the meteorological condition and hourly concentration of PM10. As shown in Fig. 2, from morning on June 4, ambient air quality in Suqian began to deteriorate. But little wheat in Suqian was reaped. With further investigation, it was found that wheat was being harvested massively in Xuyi and Chuzhou to the southeast of Suqian, and crop residue burning in the field happened prevalently after dark on June 3. With southeast wind from 4:00 to 10:00 in Suqian, ambient air quality was seriously affected by the particles from the southeast areas. Because the wind speeded up and the source of particles declined, ambient air quality became better after 12:00. But when southeast wind turned back and crop residue was burned in the field again after dark in the southeast areas, ambient air quality turned worse again from 22:00. Following last night, ambient air quality continued to be worse on June 5. From 7:00 to 20:00, the main wind direction was southwest and the
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0.4
PM10
SO2
0.25
NO2
0.2
PM10 (mg m-3)
0.3
0.15 0.2 0.1 0.1
0.05
0
SO2 and NO2 (mg m-3)
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0 6-1
6-3
6-5
6-7
6-9
6-11
6-13
6-15
Fig. 1. Daily average concentration changes of PM10, SO2, and NO2 in Suqian from June 1 to 16.
0.1 0 0:00 4:00 8:00 12:00 16:00 20:00 June 5
0 0:00 4:00 8:00 12:00 16:00 20:00 June 4
0.6 0.4 0.2
1
0.6
0.8
0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 8
PM10 (mg m-3)
1 0.8 0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 9
0.4 0.2
0.8
0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 10
0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 11
1
PM 10 (mg m-3)
0.8
PM10 (mg m-3)
0.6
0 0:00 4:00 8:00 12:00 16:00 20:00 June 7
0 0:00 4:00 8:00 12:00 16:00 20:00 June 6
0.8
PM10 (mg m-3)
PM10 (mg m-3)
0.2
PM10 (mg m-3)
0.2
0.3
PM10 (mg m-3)
0.4
PM10 (mg m-3)
0.6
0.8
0.8
0.4 PM10 (mg m-3)
PM10 (mg m-3)
0.8
0.6 0.4 0.2
0.8 0.6 0.4 0.2 0 0:00 4:00 8:00 12:00 16:00 20:00 June 13
0 0:00 4:00 8:00 12:00 16:00 20:00 June 12 Monitoring Station
Suqian College
Fig. 2. Hourly concentration changes of PM10 in Suqian from June 4 to 13.
source of particles declined in daytime, so the concentration of PM10 declined. From 0:00 to 9:00 on June 6, with southeast wind, ambient air quality deteriorated influenced by crop residue burning in the southeast areas. After 10:00, because of the source of particle declining and wind speeding up, ambient air quality improved. Ambient air quality deteriorated after 18:00 because wheat began being harvested in Suqian and some crop residue began being burned in the field. From 0:00 to 7:00 on June 7, as wind turned southeast, ambient air quality was very terrible which was attributed to crop residue burning both in Suqian and southeast areas. After 7:00, ambient air quality improved because fire extinguished and
wind speed increased. After 19:00, ambient air quality deteriorated again, which was attributed to crop residue burning in the field in Suqian. From 0:00 to 5:00 on June 8, the wind direction was southeast and south, so the air pollution was also attributed to crop residue burning in the field both in Suqian and other areas. When wind speeded up, the concentration of PM10 declined after 13:00. After 19:00, ambient air quality became bad again. Before 11:00 on June 9, though main wind direction was southwest and wind speed increased, the pollution was very serious because crop residue was burned in the field in Suqian and around Suqian last night on a large scale. At noon, ambient air quality improved since wind speed was up to
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9 m s1. After 19:00, crop residue was burned in Suqian on a larger scale, so PM10 concentration was much larger than before. Before 2:00 on June 10, ambient air quality was seriously influenced by crop residue burning in Suqian. In this morning, Suqian government held a meeting against crop residue burning in the field, and strengthened some groups to prohibit this activity. Before 22:00, the phenomenon of crop residue burning in the field did not appear and ambient air quality was better. But after the inspection groups went back, crop residue was still burned in the field. So crop residue burning in the field at this night was not concentrative and last a long time. There was some crop residue burned in the morning of June 11, so the pollution was terrible from wee hours to 9:00. Since today was Sunday a holiday for government, crop residue began being burned at about 19:00. From June 9, because the quantity of crop residue burned in Suqian increased, the influence of crop residue burning in other areas on ambient air quality of Suqian could be neglected. On June 12, the harvest in Suqian came into the key time, and crop residue burning in the field also happened prevalently; ambient air quality was serious and the measure of Suqian government to prohibit crop residue burning in the field had no effect. On June 13, weather forecast declared that it would rain tomorrow, so the left crop must be harvested today. To avoid influencing summer plant, many peasants burned all left crop residue in the field. At last, this air pollution caused by crop residue burning in the field was over after the rainfall and gale. From above analysis, some characteristics of crop residue burning in the field in Suqian can be described: (1) The ambient air quality of Suqian is not only influenced by crop residue burning in Suqian, but also it may be seriously influenced by crop residue burning in the vicinages. It may be presumed that from June 5 to 9 the air pollution from last night to the next wee hours was attributed to crop residue burning in the field in Suqian, and the air pollution in the next morning was attributed to crop residue burning in the field in other areas. (2) During daytime, because the peasants are busy in harvesting and crop residue burning nearby
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will influence harvesting, crop residue is seldom burned. Crop residue burning in the field is almost concentrated on the several hours after dark, so ambient air quality is better in daytime than at night. But there were some peak values of pollutant in daytime probably because with bad meteorological conditions, pollutant was transferred to Suqian from other places by wind. (3) The ambient air quality during burning is very correlative with the meteorological condition. High wind speed can promote the transference of pollutants, so with high wind, the ambient air quality is better. Compared with high wind speed, precipitation is more effective to eliminate the influence of crop residue burning in the field on ambient air quality. (4) The effect of Suqian government’s measure to prohibit crop residue burning in the field is poor. In Suqian, the forced measure cannot reduce crop residue burning in the field, although lots of human and non-human sources are consumed. Its poor effect only postpones the burning time. It indicates that only government forced measure is inefficient to prohibit crop residue burning in the field. 3.3. Some reasons why crop residue burning in the field in Suqian In recent years, some scholars (Cao et al., 2005; Yan et al., 2006) regarded that with the economic developing, crop residue was not one kind of necessary fuel, collecting them was very troublesome, the places to store them were short, and there were few ways to use them were the main reasons why crop residue was burned in the field. With firsthand information from talking with some peasants, officers, and agricultural technicians, it was found that there were some other reasons why crop residue was burned in the field. The use of combine harvester is the main reason why crop residue is burned in the field. Before 1990 s of the 20th century, most of wheat and rice were harvested by man in Suqian. Crop, including crop residue, was transported to settled place, then was threshed. At that time, crop residue was a byproduct which did not need any extra labor and cost. To improve the benefit, the length of stubble was about 10 cm. But after using combine harvester, byproduct which does not need any extra labor and cost cannot be realized. Though the length of stubble is requested to be low to 20 cm by
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government, in practice the length of stubble is about 30–40 cm to avoid the damage of combine harvester and save harvest cost. It causes that after harvest almost half of the crop residue is stubble standing in the field and the other half is lying in the field. So the benefit of collecting crop residue is only half of before, but the cost is much more than before. If there is some crop residue in the field, it will emerge on the ground after plowing, which will influence transplanting rice seedlings and the growth of seedlings. So the peasants cannot but burn them in the field. The busyness of peasants in June is an important reason why crop residue burning in the field in summer harvest is more serious than it in autumn harvest in Suqian. June is the busiest month for peasants in Suqian. In this month, they are busy with harvesting cole, wheat, sowing beans, transplanting rice seedlings, and sericiculture, so the farming season for them is obviously pressing. Many peasants are so tired in this month that they are unwilling to spend the valued time on callback crop residue. But there is enough time between autumn harvest and autumn sowing, so lots of crop residue was called back. 4. Conclusions From the results obtained in this study, a few conclusions can be drawn: (1) In Suqian, the amount of crop residue burned in the field annually ranged between 9.66 105 t and 1.53 106 t, the average value was 1.31 106 t (s ¼ 2.33 105 t) during the analyzed period and the proportion of crop residue burned in the field was about 43%. The annual pollutant emissions of TSP, PM10, SO2, NOx, NH3, CH4, EC, OC, VOC, CO, and CO2 from crop residue burning in the field were 11,051, 7572, 525, 3280, 1707, 3544, 905, 4331, 20,606, 120,747, and 1,988,376 t, respectively, and 78% of them were emitted in summer harvest. (2) Influenced by crop residue burning in the field, ambient air quality during burning is terrible. From June 4 to 13 in 2006, the daily average concentration of PM10, NO2, and SO2 was 0.266, 0.051 and 0.063 mg m3, respectively. The daily average concentration of PM10 kept exceeding 0.250 mg m3, the Third Standard Level of National Ambient Air Quality.
(3) During burning, the ambient air quality at night is more serious than in daytime. The ambient air quality in Suqian is influenced by crop residue burning not only in Suqian, but also in the vicinages. The effect of forced measure adopted by Suqian government to prohibit crop residue burning in the field is poor. (4) The use of combine harvester is the main reason why crop residue is burned in the field in Suqian. And the busyness of peasants in June and the press of farming season are important reasons why crop residue burning in the field in summer harvest is much more serious than it in autumn harvest.
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