Studies on the Atmospheric Degradation of Chlorpyrifos-Methyl

Feb 2, 2011 - ABSTRACT: The gas-phase atmospheric degradation of chlorpyrifos-methyl (a widely used organophosphate insecticide in Southern...
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Studies on the Atmospheric Degradation of Chlorpyrifos-Methyl Amalia Mu~noz,†,* Teresa Vera,† Howard Sidebottom,‡ Abdelwahid Mellouki,§ Esther Borras,† Milagros Rodenas,† Eva Clemente,† and Monica Vazquez† †

Instituto Universitario CEAM-UMH, C/Charles R. Darwin, 14 Parque Tecnologico, 46980 Paterna, Valencia, Spain School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland § CNRS-ICARE, 1C Avenue de la Recherche Scientifique, 45071 Orleans Cedex 2, France ‡

ABSTRACT: The gas-phase atmospheric degradation of chlorpyrifos-methyl (a widely used organophosphate insecticide in Southern European regions) has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. Photolysis under sunlight conditions and reaction with ozone were shown to be unimportant. The rate constant for reaction of chlorpyrifos-methyl with OH radicals was measured using a conventional relative rate method with cyclohexane and n-octane employed as reference compounds with k = (4.1 ( 0.4)  10-11 cm3 molecule-1 s-1 at 300 ( 5 K and atmospheric pressure. The available evidence indicates that tropospheric degradation of chlorpyrifos-methyl is mainly controlled by reaction with OH radicals and that the tropospheric lifetime is estimated to be around 3.5 h. Significant aerosol formation was observed following the reaction of chlorpyrifos-methyl with OH radicals, and the main carbon-containing products detected in the gas phase were chlorpyrifos-methyl oxone and 3,5,6-trichloro-2-pyridinol.

’ INTRODUCTION Pesticides are extensively used in agriculture, horticulture, and a variety of household applications. The wide use of pesticides is of some concern since they may have a significant environmental impact. Their worldwide intensive use has led to ubiquitous contamination of soil, water, and air in exposed as well as remote areas. Once a pesticide is applied in the field, it can be partitioned into the soil, water, and atmosphere. Pesticides can be emitted into the atmosphere through dispersion during spraying or postapplication volatilization from ground or leaf surfaces, and the amount emitted is a function of their physical properties and their manner of application. In the atmosphere pesticides are distributed between the gas, particle, and aqueous phases depending on their physicochemical properties and environmental conditions.1 Chlorpyrifos-methyl (O,O-dimethyl-O-(3,5,6-trichloro-2-pyridyl)phosphorothioate) and the analogous compound chlorpyrifos (O,O-diethyl-O-(3,5,6-trichloro-2-pyridyl)phosphorothioate) are organophosphorous insecticides and among the most widely employed insecticides for agricultural crop protection with an annual usage of about 6 million kilograms in the United States. They are particularly important in the fruit-growing regions of Southern Europe. A number of studies have indicated that emissions of these insecticides into the atmosphere can be significant. Rice et al.2 investigated the volatilization of a series of pesticides from soil and found that following application of chlorpyrifos to soil approximately 10% of the compound had evaporated within 20 days. Concentrations of chlorpyrifos of up to 97.8 ng m-3 (∼2  108 molecules cm-3) have been reported in ambient air in both gas and particle phases (ref 3 and references therein).

The major pathways for the tropospheric degradation of volatile and semivolatile organic compounds involve photolysis and reactions with ozone and hydroxyl and nitrate radicals.4 Hence, it is expected that the gas-phase atmospheric oxidation of chlorpyrifos (CHL) and chlorpyrifos-methyl (CHLM) will be initiated by one or more of these processes. The photolysis of CHL has been studied in the liquid phase in methanol, hexane, and water and on soil and leaf surfaces,5-9 and the results suggest that photooxidation results mainly in desulfuration or dehalogenation. Hebert et al. investigated the photolysis10 and OH radical-initiated oxidation11 of CHL in the gas phase at elevated temperatures (60-80 °C) using a solar simulator as the radiation source. The atmospheric lifetime with respect to photolysis and to reaction with OH radicals was found to be between 1.4 and 2.2 h and 2 h respectively. In these studies, no information on the possible products of degradation was reported. It appears that no previous work on the photolysis or reactions of OH, NO3, and O3 with CHLM have been reported. From the available data on the degradation of organophosphorothioate- and phosphonothioate-based pesticides5,12 it is likely that the atmospheric oxidation of CHL and CHLM will lead to formation of the corresponding oxones (C5HNCl3O)(C2H5O)2PO and (C5HNCl3O)(CH3O)2PO, respectively, and 3,5,6-trichloro-2-pyridinol. CHL and CHLM have similar molecular structures and hence would be expected to show the same reactivity trends. Since the vapor pressure of CHML (molecular weight 322.55) is somewhat higher than that of CHL (molecular weight 350.59), it is more convenient to investigate CHLM. The present series of experiments was carried out in order to determine the major reaction pathways for the degradation of CHLM in the troposphere. The studies were carried out at the outdoor European Received: October 22, 2010 Accepted: January 7, 2011 Revised: December 23, 2010 Published: February 02, 2011

r 2011 American Chemical Society

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dx.doi.org/10.1021/es103572j | Environ. Sci. Technol. 2011, 45, 1880–1886

Environmental Science & Technology Photoreactor (EUPHORE). This system allows the reactions to be performed under realistic atmospheric conditions, particularly with respect to the intensity and wavelengths of solar radiation. The results provide information on the atmospheric lifetime of CHLM. The main primary products of the OH radical-initiated oxidation of CHLM were also determined and mechanisms for their formation proposed. The results are discussed in terms of the impact on the environment of emissions of CHLM into the atmosphere.

’ EXPERIMENTAL SECTION The experiments were performed under sunlight conditions at the outdoor European Photoreactor in Valencia, Spain (longitude = -0.5°, latitude = 39.5° N). The EUPHORE facility has been described in detail in the literature (ref 13 and references there in), and only information directly related to the present work is provided in the following section. EUPHORE consists of two identical half-spherical FEP (fluorineethene-propene) foil chambers with volumes of approximately 200 m3. A retractable steel housing surrounds the chamber and is used to control the time of exposure to sunlight. The chamber is filled to atmospheric pressure with purified dry air and the temperature and humidity inside the chamber are measured continuously using PT-100 thermocouples and a dew-point mirror system (Walz TS-2), respectively. The actinic flux was measured using a calibrated filter radiometer specific to the photolysis frequency of NO2. A Fourier transform infrared (FTIR) spectrometer coupled to a White-type mirror system (optical path length 553.5 m) was used to monitor reactants and products. Cyclohexane was also continuously analyzed during the experiments by a gas chromatograph (GC-FISONS 8130) equipped with a DB-624 column and a photoionization (PID) detector operated under isothermal conditions (120 °C). Ozone (Monitor Laboratories 9810), NOx (ECO-Physics CLD770 with PLC 760 photolytic converter), and SO2 (Thermo Scientific 43i) concentrations were measured using specific analyzers. The aerosol phase was characterized with a scanning mobility particle sizer (SMPS), TSI model 3080. The system consists of a differential mobility analyzer (DMA), model 3081, and a condensation particle counter (CPC), model 3022A. The instrument was operated at a 5 min scan rate and measured size distributions in the 10-1000 nm diameter range. Sheath and aerosol sampling flows were 2 and 0.30 L min-1, respectively. The products formed in the reactions of OH radicals with CHLM were identified using gas chromatography-mass spectrometry (Thermo Electron Corp.). For off-line gas-phase analysis, the air mixture was pumped through C18 cartridges at 1 L min-1 for 30 min, while for off-line analysis of the particle phase, the air mixture was pumped through a quartz filter device at 81 L min-1 for 60 min. The filter was extracted using 5 mL of an equimolar mixture of dichloromethane and acetonitrile. The extraction was performed by twice sonicating the sample for 15 min, and chemical analysis was carried out on the aliquots. The subsequent treatment and analysis of both cartridge and filter samples were the same. All analyses were conducted on a 5MS column of 30 m  0.25 mm i.d.  0.25 μm film thickness. The GC was programmed at 60 °C for 1 min, then ramped at a rate of 10 °C min-1 to 250 °C, then at a rate of 5 °C min-1 to 280 °C, and held at 280 °C for 10 min. The GC-MS system for analysis employed electron ionization at 70 eV.

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Relative rate studies on the reaction of OH radicals with CHLM and the products of the OH radical-initiated oxidation were carried out. The sunlight photolysis of CHLM and dark reaction with O3 were also investigated in the chamber. Measured amounts of 1,3,5-trimethylbenzene (TMB; 99% Fluka), n-octane (OCT; 99%, alkene free Fluka), and cyclohexane (CYCL; 99.7% Scharlau) were introduced into the chamber via a stream of purified air. Due to its relatively low vapor pressure, chlorpyrifos-methyl (CHLM; 99% Riedel de H€aen) was gently heated to accelerate its introduction into the chamber. In order to prevent condensation, the injection line into the chamber was also heated. The concentration of CHLM in the chamber was checked using FTIR spectroscopy to ensure that all the CHLM had been transferred. Ozone was generated by passing oxygen through an ozone generator and transferred directly into the chamber. Hydrogen peroxide (30% Scharlau) was used as the OH radical source for experiments performed in the absence of NOx and introduced into the chamber by a sprayer. Hydroxyl radicals were generated from photolysis of HONO for experiments carried out in the presence of NOx. Nitrous acid was synthesized by addition of a 1.5% solution of NaNO2 (Fluka) to a 30% solution of H2SO4 (Scharlau) and flushed directly into the chamber by a stream of purified air. The dilution rate was determined by adding about 20 ppbv (1 ppbv = 2.46  1010 molecules cm-3 at 298 K and 760 Torr) of the unreactive tracer gas SF6 to the reaction mixtures at the start of the experiments and monitored throughout the reactions using FTIR spectroscopy.

’ RESULTS AND DISCUSSION Photolysis of Chlorpyrifos-Methyl under Sunlight Conditions. Three experiments on the photolysis of CHLM were

carried out under springtime conditions, where the photolysis rate of NO2 was ∼8  10-3 s-1. The initial mixing ratios of CHLM were in the range 75-91 ppbv, and the photolyses were performed over approximately 3 h in the middle of the day. To prevent any reaction with Cl atoms or OH radicals, which may be generated by photolysis of CHLM or from photolysis of a surface source, respectively, an excess of TMB (∼300 ppbv) was added to the system in one of the experiments as a Cl and OH scavenger. The loss of CHLM in this experiment was identical to that observed in the other two runs, indicating that removal of CHLM by radicals was negligible. Under the experimental conditions employed, CHLM may be lost by photolysis, dilution, or adsorption at the chamber walls. CHLMþhν f products JðCHLMÞ

ð1Þ

CHLM f dilution kdil

ð2Þ

CHLM f wall loss kwall

ð3Þ

Thus, the measured rate constant for loss of CHLM is given by lnð½CHLMo =½CHLMt Þ ¼ ðJmeas Þt

ðIÞ

where Jmeas = J(CHLM) þ kdil þ kwall and [CHLM]o and [CHLM]t represent the concentrations of CHLM at the start of the photolysis and at time t, respectively. The leak rate constant during the experiment was derived from the loss of SF6 SF6 f dilution kdil 1881

ð4Þ

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Figure 1. Plot of ln([CHLM]o/[CHLM]t) versus time for the photolysis experiment in which the initial mixing ratio of CHLM was 91 ppbv. The CHLM concentrations were corrected for the leak rate derived from the loss of SF6. The slope of the plot before and after opening the chamber corresponds to the first-order loss rate of CHLM.

The temporal concentration-time profile for SF6 is given by lnð½SF6 o =½SF6 t Þ ¼ kdil t

ðIIÞ

where [SF6]o and [SF6]t are the concentrations of SF6 initially and at time t, respectively. The leak rate constants, kdil, determined during a particular photolysis experiment were constant and found to lie in the range (0.8-1.8)  10-5 s-1. The extent of loss of CHLM to the chamber wall was determined by starting the analytical sampling around 1 h before opening the chamber to sunlight and hence the onset of photolysis. Around 15-25% of the CHLM was lost at the wall during the dark period and corrected for dilution, providing an estimate for kwall = (6.3 ( 1.8)  10-5 s-1, where the quoted errors correspond to 2σ obtained from the leastsquares fit analysis of the data. Plots of ln([CHLM]o/[CHLM]t) versus time for the photolysis experiments, where the data have been corrected for dilution, showed quite clearly that the decay rate of CHLM when it was exposed to sunlight was indistinguishable from that measured in the dark. Concentration-time data from the experiment in which the initial mixing ratio of CHLM was 91 ppbv are shown plotted in the form of eq I in Figure 1. Hence, the rate of photolysis of CHLM under the springtime sunlight in which this study was performed was small compared to its loss due to wall adsorption. From these observations only an upper value for the photolysis rate constant J(CHLM) < 2  10-5 s-1 at 300 ( 5 K could be derived. It is of interest to compare the results of the present work with those previously reported on the photolysis of CHL by Hebert et al.10 These authors investigated the photolysis of CHL over the temperature range 60-80 °C using a solar simulator as the radiation source and determined a photolysis lifetime of around 10 h under their experimental conditions. In the experiments only 20% of the reaction vessel was illuminated by the collimated light beam, and the spectral distribution of the solar simulator used was slightly different than that of natural sunlight. The photolysis data was corrected to take account of these effects, and Hebert et al.10 estimated a considerably shorter photolysis lifetime of chlorpyrifos under atmospheric conditions of approximately 1.4-2.2 h by assuming that the effective quantum yield for photodissociation of chlorpyrifos in solar radiation is essentially

the same as that determined in the simulated solar photolysis conditions. The upper limit of the photolysis rate constant determined in this work gives a photolysis lifetime for chlorpyrisfosmethyl of over15 h, which is considerably longer than the value of about 2 h estimated by Hebert et al.10 for chlorpyrifos. Since CHL and CHLM have very similar molecular structures, it is unlikely that the reported differences in the photolysis lifetimes are due to structural differences in the molecules. It is suggested that the corrections to the experimental data obtained by Hebert et al.10 using a solar simulator are probably incorrect due to errors in the estimated effective quantum yield data. It is also possible that the quantum yield of dissociation is higher at the elevated temperatures employed by these workers, although this seems unlikely. Aschmann and Atkinson14 found no evidence for photolysis of the phosphonothioate (C2H5O)2P(S)CH3 and phosphorothioate (C2H5O)3PS at wavelengths > 300 nm in irradiated CH3ONO-NO-R3Pd S-air mixtures used to determine rate constants for reaction of OH radicals with these compounds. Reaction of Chlorpyrifos-Methyl with Ozone. A mixture of O3 (81 ppbv) and CHLM (108 ppbv) was left in the dark in the chamber for around 3 h at 295 ( 3 K. The loss of O3, after correction for leakage from the chamber, was 8 days was calculated, assuming that the 24-h average atmospheric concentration of O3 is 7  1011 molecules cm-3.21 The rate constant of 4.1  10-11 cm3 molecule-1 s-1 obtained in this work for reaction of OH with CHLM gives an atmospheric lifetime with respect to reaction with OH of approximately 3.5 h for an average 12-h daytime concentration of OH radicals of 2  106 molecules cm-3.22 The reaction of nitrate radicals with CHLM was not investigated in 1885

dx.doi.org/10.1021/es103572j |Environ. Sci. Technol. 2011, 45, 1880–1886

Environmental Science & Technology this work; however, a value of the rate constant for reaction of NO3 with (C2H5O)3PS has been reported, kNO3((C2H5O)3PS) = 1  10-15 cm3 molecule-1 s-1.14 Assuming a similar rate constant for reaction of NO3 radicals with CHLM, an estimate of the lifetime of CHLM with respect to reaction with NO3 of over 20 days is calculated for an average nighttime concentration of NO3 radicals of 5  108 molecules cm-3.23 The results of the present work indicate that in the gas phase, CHLM will have a relatively short lifetime due mainly to reaction with OH radicals. The mechanistic investigation of the reactions of OH radicals with CHLM showed that the main carbon-containing products were chlorpyrifos-methyl oxone and 3,5,6-trichloro-2-pyridinol, and a significant yield of sulfur dioxide was also detected in the system. Since the rate constants for reaction of OH radicals with alkyl phosphates and chlorinated aromatic compounds24 are significantly lower than for reaction with CHLM, then both the main carbon-containing products would be expected to have considerably longer lifetimes than CHLM. Hence, they will undergo long-range transport and their oxidation products merit investigation. In addition, the OH radical-initiated degradation of CHLM was found to generate organic aerosols in relatively high yields; however, the mechanism for their formation is not clear.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The research leading to these results received funding from the Spanish Ministry of Science and Innovation (project CGL2007-65223/CLI and CGL2010-18474), the CONSOLIDER-INGENIO 2010 Programme (GRACCIE, CSD2007-00067), and the European Community’s Seventh Framework Program under the grant agreement no. 228335 (Eurochamp2). This project is cofunded by Fundacion Bancaja (Spain). The CEAM Foundation is supported by Generalitat Valenciana (GVA). Prof. Sidebottom acknowledges the Feedbacks-Prometeo Programm (Prometeo 2009/006) for supporting his stay at CEAM. Eva Clemente acknowledges GVA for a Geronimo Forteza grant (FPA/2010/053). ’ REFERENCES (1) Tsal, W.; Cohen, Y. Dynamic partitioning of semivolatile organics in gas/particle/rain phases during rain scavenging. Environ. Sci. Technol. 1991, 25, 2012–2023. (2) Rice, C. P.; Nochetto, C. B.; Zara, P. Volatilization of trifluralin, atrazine, metolachlor, chlorpyrifos, alpha-endosulfan, and beta endosulfan from freshly tilled soil. J. Agric. Food Chem. 2002, 50, 4009– 4017. (3) Yusa, V.; Coscolla, C.; Mellouki, A.; Pastor, A.; de la Guradia, M. Pesticides in ambient air: sampling and analytical methods. J. Chromatogr. A 2009, 2972–2983. (4) Atkinson, R.; Arey, J. Atmospheric degradation of volatile organic compounds. Chem. Rev. 2003, 103, 4605–4638. (5) Barcelo, D.; Durand, G.; De Bertrand, N. Photodegradation of the organophosphorus pesticides chlorpyrifos, fenamiphos and vamidothion in water. Toxicol. Environ. Chem. 1993, 38, 183–199. (6) Bavcon Kralja, M.; Franko, M.; Trebse, P. Photodegradation of organophosphorus insecticides - Investigations of products and their toxicity using gas chromatography- mass spectrometry and AchE-thermal lens spectrometric bioassay. Chemosphere 2007, 67, 99–107.

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(7) Burkhard, N.; Guth, J. A. Photolysis of organophosphorus insecticides on soil surfaces. Pestic. Sci. 1979, 10, 313–319. (8) Chukwudebe, A.; March, R. B.; Othman, M.; Fukuto, T. R. Formation of trialkyl phosphorothioate esters from organophosphorus insecticides after exposure to either ultraviolet light or sunlight. J. Agric. Food Chem. 1989, 37, 539–545. (9) Floesser-Mueller, H.; Schwack, W. Photochemistry of organophosphorus insecticides. Rev. Environ. Contam. Toxicol. 2001, 172, 129– 228. (10) Hebert, V. R.; Hoonhout, C.; Miller, G. C. Use of stable tracer studies to evaluate pesticide photolysis at elevated temperatures. J. Agric. Food Chem. 2000, 48, 1916–1921. (11) Hebert, V. R.; Hoonhout, C.; Miller, G. C. Reactivity of certain organophosphorus insecticides toward hydroxyl radicals at elevated air temperatures. J. Agric. Food Chem. 2000, 48, 1922–1928. (12) Atkinson, R.; Guicherit, R.; Hites, R A.; Palm, W. U.; Seiber, J. N.; de Voogt, P. Transformations of pesticides in the atmosphere: A state of the art. Water, Air Soil Pollut. 1999, 115, 219–243. (13) Feigenbrugel, V; Le Person, A.; Le Calve, S.; Mellouki, A.; Mu~ noz, A.; Wirtz, K. Atmospheric fate of dichlorvos. Environ. Sci. Technol. 2006, 40, 850–8570. (14) Aschmann, S. M.; Atkinson, R. Kinetic and product study of the gas-phase reactions of OH radicals, NO3 radicals, and O3 with (C2H5O)2P(S)CH3 and (C2H5O)3PS. J. Phys. Chem. A 2006, 110, 13029–13035. (15) Atkinson, R. Kinetics of the gas-phase reactions of OH radicals with alkanes and cycloalkanes. Atmos. Chem. Phys. 2003, 3, 2233–2307. (16) Aschmann, S. M.; Atkinson, R. Rate constants for the gas-phase reactions of alkanes with Cl atoms at 296 ( 2 K. Int. J. Chem. Kinet. 1995, 27, 613–622. (17) Aschmann, S. M.; Long, W. D.; Atkinson, R. Temperaturedependent rate constants for the gas-phase reactions of OH radicals with 1,3,5-trimethyl benzene, triethyl phosphate, and a series of alkylphosphonates. J. Phys. Chem. A 2006, 110, 7393–7400. (18) Goodman, M. A; Aschmann, S. M.; Atkinson, R.; Winer, M. Kinetics of the atmospherically important gas-phase reactions of a series of trimethyl phosphorothioates. Arch. Environ. Contam. Toxicol. 1988, 17, 281–288. (19) Kwok, E. S. C.; Atkinson, R. Estimation of hydroxyl radical rate constants for gas-phase organic compounds using a structure-reactivity relationship: an update. Atmos. Environ. 1995, 29, 1685–1695. (20) Tuazon, E. C.; Aschmann, S. M.; Atkinson, R. Products of the gas-phase reactions of OH radicals with (C2H5O)2P(S)CH3 and (C2H5O)3PS. J. Phys. Chem. A 2007, 111, 916–924. (21) Logan, J. A. Tropospheric ozone: seasonal behavior, trends, and anthropogenic influence. J. Geophys. Res. 1985, 90, 10463–10482. (22) Prinn, R. G.; Huang, J.; Weiss, R. F.; Cunnold, D. M.; Fraser, P. J.; Simmonds, P. G.; McCulloch, A.; Harth, C.; Salameh, P.; O’Doherty, S.; Wang, R .H. J.; Porter, L.; Miller, B. R. Evidence for substantial variations of atmospheric hydroxyl radicals in the past two decades. Science 2001, 292, 1882–1888. (23) Atkinson, R. Kinetics and mechanisms of the gas-phase reactions of the NO3 radical with organic compounds. J. Phys. Chem. Ref. Data 1991, 20, 459–507. (24) Atkinson, R. Kinetics and mechanisms of the gas-phase reactions of the hydroxyl radical with organic compounds. J. Phys. Chem. Ref. Data, Monogr. 1 1989.

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