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Abstract [Objectives] This study was conducted to explore the dissipation dynamics of chlorothalonil （2，4，5，6??tetrachloroisophthalonitrile） in scallion. [Methods] The level of residue and the dissipation of chlorothalonil and its main metabolite 4??hydroxy??2，5，6??trichloroisophthalonitrile in scallion in Beijing and Jiangsu experimental bases were determined via acetonitrile extraction and ultra??performance liquid chromatography coupled with ultraviolet detection （UPLC??UV）. [Results] The dissipation of chlorothalonil in scallion followed the first??order kinetic model and the half??life of chlorothalonil in scallion was 3.5-4.1 d. The final residue of chlorothalonil in scallion decreased over time after the last application. After chlorothalonil was applied at 1 050 or 1 575 g a.i./hm2 3 or 4 times， the maximum residue level of chlorothalonil was 1.48， 1.21， 0.72 and 0.45 mg/kg 5， 7， 10 and 14 d after the last application， while that of its 4??hydroxy metabolite was 0.25， 0.19， 0.13 and 0.09 mg/kg， respectively. [Conclusions] The results may provide experimental evidence for evaluating the safety of the use of chlorothalonil in scallion.
Key words Chlorothalonil； Scallion； Residue； Dissipation rate
Chlorothalonil （2，4，5，6??tetrachloroisophthalonitrile）， developed by Diamond Alkali Company， USA in 1963， is an organic compound mainly used as a broad spectrum fungicide， with other uses as a wood protectant[1]. Its structural formula is shown in Fig. 1.
As one of the most commonly used fungicides[2]， chlorothalonil is widely applied in various vegetables and fruits such as cucumber[3]， grape[4]， and yam in China[5]. In Japan， chlorothalonil is mainly used to control fungal diseases in vegetables and fruits[6]. In many tropical countries and regions， chlorothalonil is one of the most important fungicides used to control fungal diseases in bananas[7]. In addition， chlorothalonil can also be used as a paint additive[7]. However， chlorothalonil has been classified as a dangerous pesticide used in greenhouses because of its carcinogenic， mutagenic and teratogenic effects， and severe irritation to skin and eyes[8]. Studies have shown that chlorothalonil can lead to rat renal carcinogenesis. So， it is a chemical that may cause genetic mutations in the cells of amphibians， aquatic organisms and plants[9].
Chlorothalonil produces many metabolites in soil and water. Among them， 4??hydroxy??2，5，6??trichloroisophthalonitrile is one of its important metabolites， because it is more toxic， with higher soil??adsorbing capacity and longer half??life than chlorothalonil. In addition， it is highly soluble in water. Therefore， 4??hydroxy??2，5，6??trichloroisophthalonitrile will have a more serious impact to the environment and human health than chlorothalonil[10-12]. In this paper， the level of chlorothalonil and 4??hydroxy??2，5，6??trichloroisophthalonitrile residues in scallion in Beijing and Jiangsu experimental bases were determined by acetonitrile extraction and ultra??performance liquid chromatography with ultraviolet detection （UPLC??UV）， to investigate the dissipation pattern of chlorothalonil under different solar illumination and climatic conditions at different latitudes.
Materials and Methods
Materials
Drugs Chlorothalonil （purity 98.4%） and 4??hydroxy??2，5，6??trichloroisophthalonitrile （purity 98.5%） were both purchased from Dr. Ehrenstorfer GmbH， Germany.
Reagents Acetonitrile and n??hexane were of chromatographic grade， purchased from TIDEA， USA. Sodium chloride， anhydrous sodium sulfate and acetone were of analytical grade， purchased from Sinopharm Group Co.， Ltd.
Instruments and equipment Ultra Performance Liquid Chromatograph was a product of Waters Corporation. SHA??C thermostatic oscillator was purchased from Jintan Jieruier Electric Co.， Ltd. RV10 rotary evaporator was purchased from IKA Works Guangzhou. SC??3610 low speed centrifuge was purchased from Anhui USTC Zonkia Scientific Instruments Co.， Ltd. Florisil SPE column （1 000 mg/6 ml） and amino （NH2） column （500 mg/6 ml） were purchased from Tianjin Bonna??Agela Technologies Co.， Ltd.
Field trials
Dissipation of chlorothalonil in open field Scallion were grown in a plot of 50 m2， and 75% chlorothalonil wettable powder （WP） was evenly sprayed at 1 575 g a.i./hm2 only once to scallion plants at middle growth stage， and the plant samples were collected 2 h， 1， 3， 5， 7， 10， 14， 21 d later to determine the level of chlorothalonil residue.
Determination of final chlorothalonil residue Three replicates were set in this test， and each plot was 15 m2. 75% chlorothalonil WP was sprayed at 1 050 g a.i./hm2 to the scallion plants four times， once every 7 d. Samples were collected 5， 7， 10 and 14 d after the last spraying to determine the residue of chlorothalonil and its 4??hydroxy metabolite in aboveground part of scallion plants.
Pretreatment of samples
Extraction To extract residual chlorothalonil， 10.0 g of plant sample was accurately weighed， put into a 50 ml centrifuge tube， added with 20 ml of acetonitrile （containing 1% acetic acid）， mixed thoroughly， oscillated for 30 min at constant temperature. Then， the mixture was centrifuged at 4 000 r/min for 5 min， and the supernatant was transferred to a new 50 ml centrifuge tube， and added with 6 g of sodium chloride. The extraction was repeated once with 20 ml of acetonitrile （containing 1% acetic acid）. The extracts were combined， mixed by shaking vigorously for 5 min， allowed to stand for 30 min， and centrifuged at 4 000 r/min for 5 min. Subsequently， 20 ml of the supernatant was collected， passed through a funnel containing anhydrous sodium sulfate into a 150 ml pear??shaped flask， concentrated at 40 ?? to near dryness. Finally， the residue was dissolved in 3 ml of n??hexane. To extract residual 4??hydroxy chlorothalonil， 10.0 g of plant sample was accurately weighed， put into a 50 ml centrifuge tube， added with 20 ml of acetonitrile （containing 1% acetic acid）， mixed thoroughly， oscillated for 30 min at constant temperature. After 5 g of NaCl was added the mixture was oscillated for another 5 min and centrifuged at 4 000 r/min for 5 min. Then， 10 ml of the supernatant was transferred to a 100 ml pear??shaped flask， concentrated at 40 ?? to near dryness. Finally， the residue was dissolved in 3 ml of HPLC??grade acetonitrile.
Purification The chlorothalonil solution extracted above was purified by the steps as follows. The Florisil column was pre??washed with 5 ml of n??hexane： acetone （v?? v = 9?? 1） mixture and then 5 ml of n??hexane. After that， 3 ml of the chlorothalonil solution extracted above was loaded to the Florisil column， which was then eluted twice with 10 ml of n??hexane ：acetone （v?? v=9?? 1） mixture， and the eluent was collected into a 50 ml pear??shaped flask， concentrated to near dryness at 40 ??， dissolved in acetonitrile：water （v?? v =50?? 50） mixture to a total volume of 5ml， and passed through a 0.22 ??m filter membrane before UPLC analysis.
The 4??hydroxy chlorothalonil solution extracted above was purified by the steps as follows. The NH-2 column was pre??washed with 5 ml of HPLC??grade acetonitrile. After that， 3 ml of the chlorothalonil solution extracted above was loaded to the NH-2 column， which was then eluted twice with 10 ml of acetonitrile， and the eluent was collected into a 50 ml pear??shaped flask， concentrated to near dryness at 40 ??， dissolved in acetonitrile： water （v?? v=70?? 30） mixture to a total volume of 5ml， and passed through a 0.22 ??m filter membrane before UPLC analysis.
UPLC
UPLC analysis of chlorothalonil was performed under the conditions as follows. The mobile phase was prepared by mixing formic acid and methanol at a ratio of 52?? 48 （v?? v）. All determinations were performed at ambient temperature of 40 ?? using an ACQUITY UPLC BEH Shield RP 18 column （1.7 ??m， 2.1 mm??100 mm）. The column effluent was monitored at 232 nm. The injection volume was 10 ??l with a flow rate of 0.2 ml/min. The retention time was about 15.6 min.
UPLC analysis of 4??hydroxy chlorothalonil was performed under the conditions as follows. The mobile phase was prepared by mixing formic acid solution and acetonitrile at a ratio of 30?? 70 （v?? v）. All determinations were performed at ambient temperature of 40 ?? using an ACQUITY UPLC BEH Shield RP 18 column （1.7 ??m， 2.1 mm??100 mm）. The column effluent was monitored at 248 nm. The injection volume was 10 ??l with a flow rate of 0.2 ml/min. The retention time was about 15.8 min. Results and Analysis
Standard curves
In brief， 0.05， 0.1， 0.2， 0.5， 1.0， 2.0， 5.0 mg/L of chlorothalonil and 0.02， 0.05， 0.1， 0.2， 0.5， 1.0， 2.0 mg/L 4??hydroxy chlorothalonil were detected by UPLC as described above. The standard curves were made by plotting concentration on the x??axis and peak area on the y??axis （Table 1）. Fig. 2 and Fig. 3 show the chromatograms of chlorothalonil and its 4??hydroxy metabolite.
The injection concentration in the range of 0.05-5 mg/kg for chlorothalonil and in the range of 0.02-2 mg/kg for 4??hydroxy chlorothalonil was linearly correlated with peak area. The standard curve equation was y=836 400x+16 482 for chlorothalonil and y=502 075x+4 404.3 for 4??hydroxy chlorothalonil， and the correlation coefficient R2 was 0.999 9 for both equations. Under the UPLC conditions in this study， the lower limit of detection was 0.05 mg/kg for chlorothalonil and 0.02 mg/kg for 4??hydroxy chlorothalonil.
Standard addition analysis
Standard addition analysis showed that the recovery rate for chlorothalonil and 4??hydroxy chlorothalonil was 106.44%-77.90% and 106.99%-110.92%， respectively， when they were added at 0.05-2.00 mg/kg and 0.02-2.00 mg/kg， and the coefficient of variation was 4.07%-0.64% and 3.74%-1.92%， respectively. The results indicated that the UPLC system is a sensitive and precise method for the determination of residual chlorothalonil and 4??hydroxy chlorothalonil in scallion.
Dissipation dynamics of chlorothalonil in scallion
As can be seen from Fig. 4， the original concentration of chlorothalonil in Beijing and Jiangsu experimental bases was 3.71 and 10.94 mg/kg， respectively， after 75% chlorothalonil WP was applied at 1 575 g a.i./hm2. Chlorothalonil dissipated at a faster rate one to three days after application， and then the dissipation rate decreased. The dissipation dynamics of chlorothalonil in scallion in both experimental bases fitted the first??order kinetic models. The dissipation dynamics of chlorothalonil in Beijing experimental base fitted the model y=1.904e-0.198 2t， R2=0.844 5， and the half??life was 3.5 d. The dissipation dynamics of chlorothalonil in Jiangsu experimental base fitted the model y=2.030 2e-0.168 9t， R2=0.695 3， and the half??life was 4.1 d.
Final residues of chlorothalonil and 4??hydroxy chlorothalonil in scallion
75% chlorothalonil WP was applied to scallion plants at 1 050 or 1 575 g a.i./hm2 for three or four times in the field trials in both Beijing and Jiangsu， and the level of chlorothalonil （Table 3） and 4??hydroxy chlorothalonil （Table 4） residues was determined 5， 7， 10 and 14 d after the last application. The residues of chlorothalonil and its 4??hydroxy metabolite decreased over time after the last application， and also with the decrease in the dose and the number of times of application. The level of chlorothalonil residue was 0.34-1.48， 0.24-1.21， 0.14-0.72 and 0.08-0.45 mg/kg at 5， 7， 10 and 14 d after the last application， while that of 4??hydroxy chlorothalonil was 0.06-0.25， 0.05-0.19， 0.02-0.13 and 0.02-0.09 mg/kg.
Conclusions
Chlorothalonil dissipated at a faster rate one to three days after application in scallion plants. The dissipation dynamics within 21 d after application fitted the first??order kinetic model， and the half??life was 3.5-4.1 d.
The residues of both chlorothalonil and its 4??hydroxy metabolite decreased over time after the last application. When chlorothalonil was applied at 1 050 or 1 575g a.i./hm2 for three or four times， the maximum level of chlorothalonil residue in two experimental bases was 1.48， 1.21， 0.72 and 0.45 mg/kg 5， 7， 10 and 14 d after the last application， while that of 4??hydroxy chlorothalonil residue was 0.25， 0.19， 0.13 and 0.09 mg/kg， respectively.References
[1] LANG M， LI P， CAI ZC. The Degradation of chlorothalonil in soil and its environmental implications[J]. Chinese Agricultural Science Bulletin， 2012， 28（15）： 211-215.
[2] WANG ZW， LI FL， HE AF， et al. The distribution and degradation of chlorothalonil and chlorpyrifos in tomatoes in greenhouse[J]. Journal of Agro??Environment Science， 2011， 30（6）： 1076-1081.
[3] ZHAO B. Field efficacy of 43% cyazofamid??chlorothalonil suspension in controlling downy mildew in cucumber[J]. Modernizing Agriculture， 2018， 6： 7-8.
[4] ZHAO B. Field efficacy of 43% cyazofamid??chlorothalonil suspension in controlling downy mildew in grape[J]. Modernizing Agriculture， 2018， 7： 2-3.
[5] SHU R， JIAO J， YAO TT， et al. Control efficacy of common fungicides against yam stripe disease[J]. Biological Disaster Science， 2015， 38（1）： 46-48.
[6] BAIER??ANDERSON C， ANDERSON RS. Suppressionn of superoxide production by chlorothalonil in striped bass （Morone saxatilus） macrophages： The role of cellular sulfhydryls and oxidative stress[J]. Aquatic Toxicology， 2000， （50）： 85-96.
[7] CHAVES A， SHEA D， COPE WG. Environmental fate of chlorothalonil in a Costa Rican banana plantation[J]. Chemosphere， 2007， 69（7）： 1166-1174.
[8]XING H. The harm of pesticide residues on human health[J]. Modern Agriculture， 2012， （01）： 42.
[9] TANG MD， YI Y， CHEN YL. Mutagenic effect of chlorothalonil[J]. Journal of Environment and Health， 1989， 6（5）： 37-57.
[10] LYU P， ZHANG J， SHI TZ， et al. Procyanidolic oligomers enhance photodetradation of chlorothalonil in water via reductive dichlorination[J]. Applied Catalysis B： Environmtal， 2017， 15（217）： 137-143.
[11] KWON JW， ARMBRUST KL. Degradation of chlorothalonil in irradiated water/sediment systems[J]. J Agric Food Chem， 2006， 54（10）： 3651-3657.
[12] GAMBLE DS， LINDSAY E， BRUCCOLERI AG. et al. Chlorothalonil and it??s 4??hydroxy derivative in simple quartz sand soils： A comparison of sorption processes[J]. Environ Sci Technol， 2001， 35（11）： 2375-2380.
Key words Chlorothalonil； Scallion； Residue； Dissipation rate
Chlorothalonil （2，4，5，6??tetrachloroisophthalonitrile）， developed by Diamond Alkali Company， USA in 1963， is an organic compound mainly used as a broad spectrum fungicide， with other uses as a wood protectant[1]. Its structural formula is shown in Fig. 1.
As one of the most commonly used fungicides[2]， chlorothalonil is widely applied in various vegetables and fruits such as cucumber[3]， grape[4]， and yam in China[5]. In Japan， chlorothalonil is mainly used to control fungal diseases in vegetables and fruits[6]. In many tropical countries and regions， chlorothalonil is one of the most important fungicides used to control fungal diseases in bananas[7]. In addition， chlorothalonil can also be used as a paint additive[7]. However， chlorothalonil has been classified as a dangerous pesticide used in greenhouses because of its carcinogenic， mutagenic and teratogenic effects， and severe irritation to skin and eyes[8]. Studies have shown that chlorothalonil can lead to rat renal carcinogenesis. So， it is a chemical that may cause genetic mutations in the cells of amphibians， aquatic organisms and plants[9].
Chlorothalonil produces many metabolites in soil and water. Among them， 4??hydroxy??2，5，6??trichloroisophthalonitrile is one of its important metabolites， because it is more toxic， with higher soil??adsorbing capacity and longer half??life than chlorothalonil. In addition， it is highly soluble in water. Therefore， 4??hydroxy??2，5，6??trichloroisophthalonitrile will have a more serious impact to the environment and human health than chlorothalonil[10-12]. In this paper， the level of chlorothalonil and 4??hydroxy??2，5，6??trichloroisophthalonitrile residues in scallion in Beijing and Jiangsu experimental bases were determined by acetonitrile extraction and ultra??performance liquid chromatography with ultraviolet detection （UPLC??UV）， to investigate the dissipation pattern of chlorothalonil under different solar illumination and climatic conditions at different latitudes.
Materials and Methods
Materials
Drugs Chlorothalonil （purity 98.4%） and 4??hydroxy??2，5，6??trichloroisophthalonitrile （purity 98.5%） were both purchased from Dr. Ehrenstorfer GmbH， Germany.
Reagents Acetonitrile and n??hexane were of chromatographic grade， purchased from TIDEA， USA. Sodium chloride， anhydrous sodium sulfate and acetone were of analytical grade， purchased from Sinopharm Group Co.， Ltd.
Instruments and equipment Ultra Performance Liquid Chromatograph was a product of Waters Corporation. SHA??C thermostatic oscillator was purchased from Jintan Jieruier Electric Co.， Ltd. RV10 rotary evaporator was purchased from IKA Works Guangzhou. SC??3610 low speed centrifuge was purchased from Anhui USTC Zonkia Scientific Instruments Co.， Ltd. Florisil SPE column （1 000 mg/6 ml） and amino （NH2） column （500 mg/6 ml） were purchased from Tianjin Bonna??Agela Technologies Co.， Ltd.
Field trials
Dissipation of chlorothalonil in open field Scallion were grown in a plot of 50 m2， and 75% chlorothalonil wettable powder （WP） was evenly sprayed at 1 575 g a.i./hm2 only once to scallion plants at middle growth stage， and the plant samples were collected 2 h， 1， 3， 5， 7， 10， 14， 21 d later to determine the level of chlorothalonil residue.
Determination of final chlorothalonil residue Three replicates were set in this test， and each plot was 15 m2. 75% chlorothalonil WP was sprayed at 1 050 g a.i./hm2 to the scallion plants four times， once every 7 d. Samples were collected 5， 7， 10 and 14 d after the last spraying to determine the residue of chlorothalonil and its 4??hydroxy metabolite in aboveground part of scallion plants.
Pretreatment of samples
Extraction To extract residual chlorothalonil， 10.0 g of plant sample was accurately weighed， put into a 50 ml centrifuge tube， added with 20 ml of acetonitrile （containing 1% acetic acid）， mixed thoroughly， oscillated for 30 min at constant temperature. Then， the mixture was centrifuged at 4 000 r/min for 5 min， and the supernatant was transferred to a new 50 ml centrifuge tube， and added with 6 g of sodium chloride. The extraction was repeated once with 20 ml of acetonitrile （containing 1% acetic acid）. The extracts were combined， mixed by shaking vigorously for 5 min， allowed to stand for 30 min， and centrifuged at 4 000 r/min for 5 min. Subsequently， 20 ml of the supernatant was collected， passed through a funnel containing anhydrous sodium sulfate into a 150 ml pear??shaped flask， concentrated at 40 ?? to near dryness. Finally， the residue was dissolved in 3 ml of n??hexane. To extract residual 4??hydroxy chlorothalonil， 10.0 g of plant sample was accurately weighed， put into a 50 ml centrifuge tube， added with 20 ml of acetonitrile （containing 1% acetic acid）， mixed thoroughly， oscillated for 30 min at constant temperature. After 5 g of NaCl was added the mixture was oscillated for another 5 min and centrifuged at 4 000 r/min for 5 min. Then， 10 ml of the supernatant was transferred to a 100 ml pear??shaped flask， concentrated at 40 ?? to near dryness. Finally， the residue was dissolved in 3 ml of HPLC??grade acetonitrile.
Purification The chlorothalonil solution extracted above was purified by the steps as follows. The Florisil column was pre??washed with 5 ml of n??hexane： acetone （v?? v = 9?? 1） mixture and then 5 ml of n??hexane. After that， 3 ml of the chlorothalonil solution extracted above was loaded to the Florisil column， which was then eluted twice with 10 ml of n??hexane ：acetone （v?? v=9?? 1） mixture， and the eluent was collected into a 50 ml pear??shaped flask， concentrated to near dryness at 40 ??， dissolved in acetonitrile：water （v?? v =50?? 50） mixture to a total volume of 5ml， and passed through a 0.22 ??m filter membrane before UPLC analysis.
The 4??hydroxy chlorothalonil solution extracted above was purified by the steps as follows. The NH-2 column was pre??washed with 5 ml of HPLC??grade acetonitrile. After that， 3 ml of the chlorothalonil solution extracted above was loaded to the NH-2 column， which was then eluted twice with 10 ml of acetonitrile， and the eluent was collected into a 50 ml pear??shaped flask， concentrated to near dryness at 40 ??， dissolved in acetonitrile： water （v?? v=70?? 30） mixture to a total volume of 5ml， and passed through a 0.22 ??m filter membrane before UPLC analysis.
UPLC
UPLC analysis of chlorothalonil was performed under the conditions as follows. The mobile phase was prepared by mixing formic acid and methanol at a ratio of 52?? 48 （v?? v）. All determinations were performed at ambient temperature of 40 ?? using an ACQUITY UPLC BEH Shield RP 18 column （1.7 ??m， 2.1 mm??100 mm）. The column effluent was monitored at 232 nm. The injection volume was 10 ??l with a flow rate of 0.2 ml/min. The retention time was about 15.6 min.
UPLC analysis of 4??hydroxy chlorothalonil was performed under the conditions as follows. The mobile phase was prepared by mixing formic acid solution and acetonitrile at a ratio of 30?? 70 （v?? v）. All determinations were performed at ambient temperature of 40 ?? using an ACQUITY UPLC BEH Shield RP 18 column （1.7 ??m， 2.1 mm??100 mm）. The column effluent was monitored at 248 nm. The injection volume was 10 ??l with a flow rate of 0.2 ml/min. The retention time was about 15.8 min. Results and Analysis
Standard curves
In brief， 0.05， 0.1， 0.2， 0.5， 1.0， 2.0， 5.0 mg/L of chlorothalonil and 0.02， 0.05， 0.1， 0.2， 0.5， 1.0， 2.0 mg/L 4??hydroxy chlorothalonil were detected by UPLC as described above. The standard curves were made by plotting concentration on the x??axis and peak area on the y??axis （Table 1）. Fig. 2 and Fig. 3 show the chromatograms of chlorothalonil and its 4??hydroxy metabolite.
The injection concentration in the range of 0.05-5 mg/kg for chlorothalonil and in the range of 0.02-2 mg/kg for 4??hydroxy chlorothalonil was linearly correlated with peak area. The standard curve equation was y=836 400x+16 482 for chlorothalonil and y=502 075x+4 404.3 for 4??hydroxy chlorothalonil， and the correlation coefficient R2 was 0.999 9 for both equations. Under the UPLC conditions in this study， the lower limit of detection was 0.05 mg/kg for chlorothalonil and 0.02 mg/kg for 4??hydroxy chlorothalonil.
Standard addition analysis
Standard addition analysis showed that the recovery rate for chlorothalonil and 4??hydroxy chlorothalonil was 106.44%-77.90% and 106.99%-110.92%， respectively， when they were added at 0.05-2.00 mg/kg and 0.02-2.00 mg/kg， and the coefficient of variation was 4.07%-0.64% and 3.74%-1.92%， respectively. The results indicated that the UPLC system is a sensitive and precise method for the determination of residual chlorothalonil and 4??hydroxy chlorothalonil in scallion.
Dissipation dynamics of chlorothalonil in scallion
As can be seen from Fig. 4， the original concentration of chlorothalonil in Beijing and Jiangsu experimental bases was 3.71 and 10.94 mg/kg， respectively， after 75% chlorothalonil WP was applied at 1 575 g a.i./hm2. Chlorothalonil dissipated at a faster rate one to three days after application， and then the dissipation rate decreased. The dissipation dynamics of chlorothalonil in scallion in both experimental bases fitted the first??order kinetic models. The dissipation dynamics of chlorothalonil in Beijing experimental base fitted the model y=1.904e-0.198 2t， R2=0.844 5， and the half??life was 3.5 d. The dissipation dynamics of chlorothalonil in Jiangsu experimental base fitted the model y=2.030 2e-0.168 9t， R2=0.695 3， and the half??life was 4.1 d.
Final residues of chlorothalonil and 4??hydroxy chlorothalonil in scallion
75% chlorothalonil WP was applied to scallion plants at 1 050 or 1 575 g a.i./hm2 for three or four times in the field trials in both Beijing and Jiangsu， and the level of chlorothalonil （Table 3） and 4??hydroxy chlorothalonil （Table 4） residues was determined 5， 7， 10 and 14 d after the last application. The residues of chlorothalonil and its 4??hydroxy metabolite decreased over time after the last application， and also with the decrease in the dose and the number of times of application. The level of chlorothalonil residue was 0.34-1.48， 0.24-1.21， 0.14-0.72 and 0.08-0.45 mg/kg at 5， 7， 10 and 14 d after the last application， while that of 4??hydroxy chlorothalonil was 0.06-0.25， 0.05-0.19， 0.02-0.13 and 0.02-0.09 mg/kg.
Conclusions
Chlorothalonil dissipated at a faster rate one to three days after application in scallion plants. The dissipation dynamics within 21 d after application fitted the first??order kinetic model， and the half??life was 3.5-4.1 d.
The residues of both chlorothalonil and its 4??hydroxy metabolite decreased over time after the last application. When chlorothalonil was applied at 1 050 or 1 575g a.i./hm2 for three or four times， the maximum level of chlorothalonil residue in two experimental bases was 1.48， 1.21， 0.72 and 0.45 mg/kg 5， 7， 10 and 14 d after the last application， while that of 4??hydroxy chlorothalonil residue was 0.25， 0.19， 0.13 and 0.09 mg/kg， respectively.References
[1] LANG M， LI P， CAI ZC. The Degradation of chlorothalonil in soil and its environmental implications[J]. Chinese Agricultural Science Bulletin， 2012， 28（15）： 211-215.
[2] WANG ZW， LI FL， HE AF， et al. The distribution and degradation of chlorothalonil and chlorpyrifos in tomatoes in greenhouse[J]. Journal of Agro??Environment Science， 2011， 30（6）： 1076-1081.
[3] ZHAO B. Field efficacy of 43% cyazofamid??chlorothalonil suspension in controlling downy mildew in cucumber[J]. Modernizing Agriculture， 2018， 6： 7-8.
[4] ZHAO B. Field efficacy of 43% cyazofamid??chlorothalonil suspension in controlling downy mildew in grape[J]. Modernizing Agriculture， 2018， 7： 2-3.
[5] SHU R， JIAO J， YAO TT， et al. Control efficacy of common fungicides against yam stripe disease[J]. Biological Disaster Science， 2015， 38（1）： 46-48.
[6] BAIER??ANDERSON C， ANDERSON RS. Suppressionn of superoxide production by chlorothalonil in striped bass （Morone saxatilus） macrophages： The role of cellular sulfhydryls and oxidative stress[J]. Aquatic Toxicology， 2000， （50）： 85-96.
[7] CHAVES A， SHEA D， COPE WG. Environmental fate of chlorothalonil in a Costa Rican banana plantation[J]. Chemosphere， 2007， 69（7）： 1166-1174.
[8]XING H. The harm of pesticide residues on human health[J]. Modern Agriculture， 2012， （01）： 42.
[9] TANG MD， YI Y， CHEN YL. Mutagenic effect of chlorothalonil[J]. Journal of Environment and Health， 1989， 6（5）： 37-57.
[10] LYU P， ZHANG J， SHI TZ， et al. Procyanidolic oligomers enhance photodetradation of chlorothalonil in water via reductive dichlorination[J]. Applied Catalysis B： Environmtal， 2017， 15（217）： 137-143.
[11] KWON JW， ARMBRUST KL. Degradation of chlorothalonil in irradiated water/sediment systems[J]. J Agric Food Chem， 2006， 54（10）： 3651-3657.
[12] GAMBLE DS， LINDSAY E， BRUCCOLERI AG. et al. Chlorothalonil and it??s 4??hydroxy derivative in simple quartz sand soils： A comparison of sorption processes[J]. Environ Sci Technol， 2001， 35（11）： 2375-2380.