Atmospheric Fate of Dichlorvos: Photolysis and OH-Initiated Oxidation

The fraction of dichlorvos contained in the atmospheric cloud droplets can be calculated from its Henry's law constant H from the following expression...
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Environ. Sci. Technol. 2006, 40, 850-857

Atmospheric Fate of Dichlorvos: Photolysis and OH-Initiated Oxidation Studies V. FEIGENBRUGEL,† A. LE PERSON,‡ S . L E C A L V EÄ , * , † A . M E L L O U K I , * , ‡ A. MUN ˜ OZ,§ AND K. WIRTZ§ Centre de Ge´ochimie de la Surface, CNRS and Universite´ Louis Pasteur, 1 rue Blessig, F-67084 Strasbourg, Cedex France, Laboratoire de Combustion et Syste`mes Re´actifs, CNRS, 1C, Avenue de la recherche scientifique, 45071 Orle´ans Cedex 2, France, and Fundacion CEAM, Parque Tecnologico, E-46980 Paterna, Valencia, Spain

The OH-initiated oxidation of dichlorvos (a widely used insecticide) has been investigated under atmospheric conditions at the large outdoor European photoreactor (EUPHORE) in Valencia, Spain. The rate constant of OH reaction with dichlorvos, k, was measured by using a conventional relative rate technique where 1,3,5trimethylbenzene (TMB) and cyclohexane were taken as references. With the use of the rate constants of 5.67 × 10-11 and of 6.97 × 10-12 cm3 molecule-1 s-1 for the reactions OH + TMB and OH + cyclohexane, respectively, the resulting value of the OH reaction rate constant with dichlorvos was derived to be k ) (2.6 ( 0.3) × 10-11 cm3 molecule-1 s-1. The tropospheric lifetime of dichlorvos with respect to reaction with OH radical has been estimated to be around 11 h. The major carbon-containing products observed for the OH reaction with dichlorvos in air under sunlight condition were phosgene and carbon monoxide. The formation of a very stable toxic primary product such as phosgene associated with the relatively short lifetime of dichlorvos may make the use of this pesticide even more toxic for humans when released into the atmosphere.

Introduction The amount of pesticides routinely applied to agricultural commodities has dramatically increased in recent years which has led to serious concerns about the increasing risks to human health (1, 2). Organophosphate (OP) insecticides such as dichlorvos are commonly used as household, garden, and farm insecticides. The annual production of dichlorvos was as high as 4.2 million pounds (lbs) in the late 1970s and fell to 992 000 lbs by 1989. More recent estimates are not available but are likely to be lower due to the many recent cancellations of its use (3). Dichlorvos (2,2-dichlorovinyl dimethyl phosphate, DDVP) is an antihelmintic agent with widespread use. It has a high * Address correspondence to either author. Phone: +33-(0)3-9024-03-68 (S.L.C.). Fax: +33-(0)3-90-24-04-02 (S.L.C.); +33-(0)2-3869-60-04 (A.M.). E-mail: [email protected] (S.L.C.); [email protected] (A.M.). † Centre de Ge ´ ochimie de la Surface, CNRS and Universite´ Louis Pasteur. ‡ Laboratoire de Combustion et Syste ` mes Re´actifs, CNRS. § Fundacion CEAM. 850

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acute toxicity, showing a calculated LC50 of 15 mg m-3 in rats (4). It is classified by the WHO as a “highly hazardous” agent (5). Due to its relative high vapor pressure (7.0 Pa at 20 °C) (6), DDVP vaporizes quickly, and exposure by inhalation after its use in nonventilated or poorly ventilated areas has been reported to cause important cases of human poisoning (7). The atmosphere is considered as the major pathway for the dissemination of semivolatile organic compounds such as pesticides (8). The main input mechanisms of pesticides into the atmosphere are drift during spraying operations, volatilization from ground or leaf surfaces, and wind erosion (9). In the atmosphere, pesticides are distributed between the gas, particle, and aqueous phases. The partitioning in different phases depends on their physicochemical properties (such as equilibrium vapor pressures and Henry’s law constants) as well as on the environmental conditions (temperature and wind direction) (10). Gas-phase concentration of dichlorvos were found to be in the range of 0.2-6.6 ng m-3 in 1996 in Canada (11), while its levels in rainfall varied in the range of 2-107 ng L-1 in Canada (11) and reached up to 330 ng L-1 in Japan (12). After their entry into the atmosphere, their main removal routes are wet and dry deposition (8) and chemical reactions (13, 14). In the gas phase, pesticides are degraded by solar light photolysis and chemical reaction with OH and NO3 radicals and ozone. Reaction with OH radicals appears to be the major loss process for a large number of pesticides (15, 16). Therefore, kinetic and mechanistic information on the gas-phase reaction of OH with pesticides are needed, to assess their atmospheric fate, hence, their possible impact on air quality and humans. In this work, we report the first study on the gas-phase atmospheric chemistry of dichlorvos (C4H7Cl2O4P):

The reaction rate constant of OH with dichlorvos has been measured using a relative kinetic method, and the major products arising from this reaction have been determined under atmospheric conditions at the outdoor European photoreactor (EUPHORE). The data obtained enabled us to discuss the atmospheric fate of dichlorvos.

Experimental Section The OH-initiated oxidation of dichlorvos under natural sunlight was investigated at the European photoreactor (EUPHORE) during June 2004. The European photoreactor consists of two large outdoor simulation chambers integrated into the Centro de Estudios Ambientales del Mediterraneo (CEAM) in Valencia, Spain. Technical information concerning the installation has been previously reported in the literature (17-21), and hence, only details directly related to the present experiments are given here. The experiments were performed in a hemispherical reactor made of FEP (fluorinated ethylene propylene) foil with a volume of approximately 204 m3. The FEP foil is highly transparent to sunlight, with a transmission greater than 75% over the wavelength range of 290-550 nm. A retractable steel housing surrounds the chamber and is used to control the 10.1021/es051178u CCC: $33.50

 2006 American Chemical Society Published on Web 01/05/2006

time of exposure to sunlight. The floor of the reactor consists of aluminum panels covered with FEP foil and has a specially designed cooling system to compensate for heating of the chamber caused by solar radiation. A number of ports situated on the floor of the chamber are available for the introduction and sampling of reaction mixtures. The chamber is filled to atmospheric pressure with purified dry air generated from ambient air using an air purification system. Homogeneous gas mixtures are obtained by the use of two mixing fans located inside the chamber. Between the experiments the chamber is flushed with clean air. The reaction chamber is equipped with a variety of instruments for chemical and physical analysis and sensing. Temperature and humidity inside the chamber were measured continuously using PT-100 thermocouples and a dewpoint mirror system (Walz TS-2), respectively. The actinic flux was measured by using a calibrated filter radiometer specific to the photolysis frequency of NO2. A White mirror system installed inside the chamber and aligned with an optical path length of 553.5 m was used for in situ measurements by Fourier transform infrared (FTIR) spectroscopy. Infrared spectra were derived from the coaddition of 150-270 scans, collected over a 3-5 min period, and recorded using a resolution of 1 cm-1. O3, CO, and NOx were analyzed using specific analyzers, Monitor Labs 9810, Thermo Environment 48C, Monitor Labs 9841A and ECOPhysics CLD770 AL ppt with a PLC 760 photolytic converter, respectively. Additional chemical detection was provided by a gas chromatograph (Fisons 8000) equipped with flame ionization and photoionization detectors (FID and PID). The chromatograph was operated using a 30 m DB-624 fused silica capillary column (J&W Scientific, 0.32 mm i.d., 1.8 µm film). The reaction rate constant of OH with dichlorvos was measured using both 1,3,5-trimethyl benzene (TMB) and cyclohexane as references. Known amounts of dichlorvos (98%, Sipcam Phyteurop), TMB (99%, Fluka), and cyclohexane (99.7%, Scharlau) were introduced into the chamber via a stream of purified air. Due to their relatively low vapor pressures, dichlorvos and TMB were gently heated to accelerate their introduction into the chamber. NO was added to the mixture to initiate the OH radical formation under sunlight conditions. For each experiment, the gas mixture was analyzed for at least 30 min before exposing it to sunlight to check for any dark effect (wall loss of the compounds and/or reaction of dichlorvos with the reference compounds or NO). The temperature inside the chamber varied slightly during experiments but was always within the range of 293303 K. Chemical analysis was performed throughout the reaction. The reactants and products were quantified using calibrated reference infrared spectra and gas chromatographic sensitivity factors obtained by introducing known amounts of pure materials into the chamber. Complex infrared spectra were analyzed by successively subtracting the absorption features of the compounds using the calibrated spectra. To compensate for sampling losses, thermal expansion of the gas mixture, and leakage through the FEP foil, the chamber was refilled continuously with a clean air stream of 1 L s-1, using a thermal mass flow controller. The overall dilution rate was determined by adding at the start of the experiments about 20 ppbv (1 ppbv ) 2.46 × 1010 molecules cm-3 at 760 torr and 298 K) of the unreactive tracer gas SF6 to the chamber and measuring its loss by FTIR spectroscopy during the course of the experiments. The derived correction factors were applied to determine the amounts of reactants consumed and products formed. Phosgene (CCl2O) was calibrated using a 20% v/v solution in toluene (Fluka). The exact proportion of toluene present was determined by gas chromatography and FTIR spectroscopy, and the balance was attributed to phosgene.

FIGURE 1. Plot of ln([dichlorvos]0/[dichlorvos]t) vs time for the photolysis experiment. The dichlorvos concentrations are corrected from the leak rate derived from the loss of SF6. The first-order loss rate of dichlorvos, before and after opening the chamber, corresponds to the slope (see text).

Results and Discussion First-Order Loss Rate of Dichlorvos under Dark and Sunlight Conditions. A single experiment on the photolysis of dichlorvos was carried out at the EUPHORE simulation chamber in Valencia, Spain (longitude ) -0.5°, latitude ) 39.5°) in June. The run was conducted in the presence of 193.6 ppbv of dichlorvos and 24.5 ppbv of SF6. To prevent any reaction between Cl atoms which may be released from the photolysis of dichlorvos, an excess of cyclohexane of 11 ppmv was added to the gas mixture as Cl atoms scavenger (21). The leak rate constant during the experiment derived from the loss of SF6 was found to be (7.5 ( 0.4) × 10-6 s-1 where the quoted errors correspond to 2σ obtained from the leastsquares fit analysis. The concentrations of dichlorvos measured by FTIR spectroscopy were then corrected from the loss rate of SF6. First, dichlorvos was maintained in the dark for 90 min in order to quantify its possible adsorption rate on the Teflon walls of the chamber. Then, the chamber was opened for photolysis during approximately 4 h. No more than 30 ppbv of dichlorvos were lost during the 5 h and 30 min duration of the experiment. The plot of ln([dichlorvos]0/ [dichlorvos]t) versus time, shown in Figure 1 where the data have been corrected for dilution, highlights that the loss rate of dichlorvos when it was exposed to sunlight was similar to that obtained under dark conditions. This clearly indicates that the photolysis rate of dichlorvos under our sunlight conditions was small compared to its loss due to the wall adsorption. The wall loss rate of dichlorvos, before and after opening the chamber, corresponds to the slope of the linear plot shown in Figure 1 and was found to be (9.3 ( 0.3) × 10-6 s-1 where the quoted errors correspond to 2σ obtained from the least-squares fit analysis. From these observations, only an upper value of photolysis rate constant for dichlorvos could be derived: Jdichlorvos < 5 × 10-6 s-1. If dichlorvos was photolyzed at this rate, a decay with two different slopes, i.e., before and after opening the chamber, should be easily observed. OH Rate Constant for the OH Reaction with Dichlorvos. Two experiments were performed using both TMB and cyclohexane as references for OH reaction in order to VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Experimental Conditions and Results for the Relative Kinetic Study of OH Reaction with Dichlorvos at 298 ( 5 K and Atmospheric Pressure date

June 2, 2004

June 4, 2004

Experimental Conditions [dichlorvos]0 (ppbv)a [cyclohexane]0 (ppbv)a [TMB]0 (ppbv)a,b O3 range (ppbv) NO range (ppbv)

191.5 173.3 136.3 0.4-247.9 51.7-0

kdichlorvos/kTMB kdichlorvos/kcyclohexane kcyclohexane/kTMB kdichlorvose

0.49 ( 0.04c,d 3.80 ( 0.35c,d 0.126 ( 0.012d (2.8 ( 0.4) × 10-11

179.7 157.3 176.9 0.5-250.4 80.3-0.4

Results

0.48 ( 0.03c,d 3.22 ( 0.27c,d 0.148 ( 0.014d (2.7 ( 0.3) × 10-11

a Initial concentrations when the chamber is open. b TMB is 1,3,5trimethylbenzene. c Data obtained with corrections from wall loss of dichlorvos. d The errors correspond to 2σ + 5%. e Rate constant of OH reaction with dichlorvos calculated from a value of 5.67 × 10-11 cm3 molecule-1 s-1 for OH reaction with TMB (in units of cm3 molecule-1 s-1).

highlight any interference from the reaction of Cl atoms with dichlorvos since TMB and cyclohexane have very different reactivities toward Cl and OH. Cl atoms may be generated following the reaction of OH with dichlorvos. The rate constant of Cl reaction with TMB (2.42 × 10-10 cm3 molecule-1 s-1) (22) is of the order of that of Cl reaction with cyclohexane (3.08 × 10-10 cm3 molecule-1 s-1) (23), while the OH reaction with TMB is approximately 7.2 times faster than that with cyclohexane. Consequently, if Cl atoms are released through reaction of OH with dichlorvos, the decay of cyclohexane shoud be accelerated during the experiment. A summary of the initial experimental conditions and results is provided in Table 1. The initial concentration of each organic species was in the range of 130-190 ppbv. Both experiments were conducted with low initial NO levels (5080 ppbv) for approximately 3 h during the middle of the day. The loss of dichlorvos was monitored using FTIR spectroscopy and the trace gas analyzer while TMB and cyclohexane were quantified by using both FTIR and GC/PID. The rate constant for the OH reaction with dichlorvos was determined by the conventional relative rate method. OH radical reacts with dichlorvos and reference as follows: k

OH + dichlorvos 98 products kR

OH + reference 98 products

(1) (2)

where k and kR are the rate constants of OH reaction with dichlorvos and reference, respectively. Assuming that the reaction with OH is the only significant loss process for both dichlorvos and reference, it can be shown that

[dichlorvos]0 [reference]0 ln ) k/kR ln [dichlorvos]t [reference]t

(3)

where the subscripts 0 and t indicate concentrations at the beginning of the experiment and at time t, respectively. After corrections of the concentrations due to the leak (for all compounds) and due to the wall loss at the rate of 9.3 × 10-6 s-1 for dichlorvos (see above), plots of experimental data for ln([dichlorvos]0/[dichlorvos]t) versus either ln([TMB]0/[TMB]t) or ln([cyclohexane]0/[cyclohexane]t) and for ln([cyclohexane]0/[cyclohexane]t) versus ln([TMB]0/ [TMB]t) exhibit straight lines (see Figures 2-4) and permit us to determine the values of k/kR, reported in Table 1. 852

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FIGURE 2. Plot of ln([dichlorvos]0/[dichlorvos]t) vs ln([TMB]0/[TMB]t) for the experiments performed June 2 and 4, 2004 using FTIR. According to eq 3, the slope of the straight line is kdichlorvos/kTMB.

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FIGURE 3. Plot of ln([dichlorvos]0/[dichlorvos]t) vs ln([cyclohexane]0/ [cyclohexane]t) for the experiments performed June 2 and 4, 2004 using FTIR. According to eq 3, the slope of the straight line is kdichlorvos/kcyclohexane.

The rate constants of OH reactions with TMB and cyclohexane are well documented in the literature (24). We used the rate constant values of kcyclohexane ) 6.97 × 10-12 cm3 molecule-1 s-1 for the reaction OH + cyclohexane (24) and kTMB ) 5.67 × 10-11 cm3 molecule-1 s-1 for the reaction OH + TMB (24). Under our experimental conditions, we have obtained kcyclohexane/kTMB ) 0.126-0.148, which is in good agreement with that recommended by the literature (kcyclohexane/kTMB ) 0.123). This indicates that no complication occurred during the experiment and that all the data processing was correctly made. Thus, knowing the rate constants of OH reactions with TMB (24) and cyclohexane,

TABLE 2. Experimental Conditions and Results for the Mechanism Study of OH Reaction with Dichlorvos at 298 ( 5 K and Atmospheric Pressure date

June 1, 2004

Experimental Conditions [dichlorvos]0 (ppbv)a 191.5 O3 range (ppbv) 9.3-107.4 NO range (ppbv) 57.1-17.8 OH source HONO molar yield of CCl2O molar yield of CO

June 3, 2004 184.2 0.5-138.2 75.2-5.6 NOb

Results 0.66 ( 0.05c,d 0.69e

0.47 ( 0.03c,d 0.57e

0.20 ( 0.05c,d 0.73 ( 0.05c,f 0.78e

0.43 ( 0.06c,d 1.49 ( 0.11c,f 1.08e

a Initial concentrations when the chamber is open. b OH radicals production may involve photolysis of small amounts of HONO formed on the wall after the addition of NO (see text). c The quoted errors correspond to 2σ obtained from the least-squares analysis and the estimated systematic error of 5%. d Yield value obtained at the beginning of the experiment. e Yield value obtained at the end of the experiment. f Yield value obtained in the second part of the experiment.

FIGURE 4. Plot of ln([cyclohexane]0/[cyclohexane]t) vs ln([TMB]0/ [TMB]t) for the experiments performed June 2 and 4, 2004 using FTIR. According to eq 3, the slope of the straight line is kcyclohexane/ kTMB.

the rate constant of OH reaction with dichlorvos can be calculated according to eq 3. The rate constants ratios kdichlorvos/kTMB found in both experiments are in excellent agreement (see Table 1), while the ratio kdichlorvos/kcyclohexane obtained in the second run is 16% lower than that from the first one. From the mean values of 0.49 ( 0.04 and of 3.51 ( 0.35 for kdichlorvos/kTMB and kdichlorvos/ kcyclohexane, respectively, and the values of kTMB ) 5.67 × 10-11 and of kcyclohexane ) 6.97 × 10-12 (in units of cm3 molecule-1 s-1), the derived values of kdichlorvos are the following (in units of cm3 molecule-1 s-1): (2.78 ( 0.23) × 10-11 and (2.45 ( 0.24) × 10-11 using, respectively, TMB and cyclohexane as references. The quoted uncertainties correspond to 2σ obtained from the least-squares analysis and the estimated systematic error of 5%. Our two derived determinations of kdichlorvos are in good agreement. Then, our recommended value for the rate constant of the OH reaction with dichlorvos is kdichlorvos ) (2.6 ( 0.3) × 10-11 cm3 molecule-1 s-1. To our knowledge, this is the first experimental value to be reported for the rate constant of OH reaction with dichlorvos. This value can be compared to that estimated by using structure activity relationships (SAR method) (25). At 298 K, the rate constant of OH reaction with dichlorvos is estimated to be 9.2 × 10-12 cm3 molecule-1 s-1, which is 2.7 times lower than our determination. Dichlorvos is therefore typically an organic compound for which the SAR method does not permit an estimation within 50% uncertainty. This is probably due to the lack of experimental rate constants values for organic compounds containing a phosphorous atom. To evaluate the importance of each reaction pathway we have undertaken an experimental mechanistic study of the OH-initiated oxidation. Degradation Products of the Reaction of OH with Dichlorvos. Two experiments were conducted to investigate the OH-initiated oxidation of dichlorvos. In these runs, dichlorvos was introduced into the chamber along with either HONO or NO and SF6 (used as tracer), and the system was left for approximately 1 h for stabilization in the dark. The OH radicals formation was due to the photolysis of HONO which started when the chamber housing was opened. It has

to be mentioned that the formation of OH radicals in the run where NO was used instead of HONO is not completely clarified but most likely involves photolysis of small amounts of HONO formed on the wall after the addition of NO. The initial experimental conditions along with the obtained results are summarized in Table 2. The major carbon-containing products observed for the OH reaction with dichlorvos in air were phosgene and carbon monoxide. The complex infrared spectra were analyzed by successively subtracting the absorption features as shown in Figure 5. An example of the concentration-time profiles of the reactants (NO and dichlorvos) and products are plotted in Figure 6 (experiment performed June 3, 2004). Figures 7 and 8, where we have plotted the formation of CO and Cl2C(O) versus the consumption of dichlorvos, indicate that both are primary and secondary reaction products. During the run where HONO was used as the OH source (June 1, 2004), the consumption of dichlorvos was extremely rapid (90% consumed during the first 15 min corresponding to only three infrared spectra collected). We think that the yield values derived from this specific experiment may correspond to both primary and secondary productions. Consequently, the initial product yields have been determined from the run performed with addition of NO to initiate the OH radical production (June 3 2004) where the yields of primary products have been derived from the first part of the run (Figure 7) and found to be equal to (47 ( 3)% and (43 ( 6)% for Cl2C(O) and CO, respectively. The observed products can, at least partly, be explained by a mechanism proceeding by both H atom abstraction from the methoxy (CH3O-) group and OH addition to the carbon-carbon double bond. We first present the mechanism of H atom abstraction from CH3O- that can occur:

Cl2CdCH(OP(O)(OCH3)2) + OH + (O2) f Cl2CdCH(OP(O)(OCH3)OCH2OO•) (4) Cl2CdCH(OP(O)(OCH3)OCH2OO•) + NO f Cl2CdCH(OP(O)(OCH3)OCH2O•) + NO2 (5) The alkoxy radical formed may react with O2 leading to an aldehyde

Cl2CdCH(OP(O)(OCH3)OCH2O•) + O2 f Cl2CdCH(OP(O)(OCH3)OCHO) + HO2 (6) VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. (A) IR spectrum corresponding to the mixture in chamber B during the product experiment (June 3, 2004) recorded at 12:00 (p.m.); (B) IR spectrum of dichlorvos at 12:00; (C) IR spectrum of SF6 at 12:00; (D) IR spectrum of phosgene at 12:00; (E) residual IR spectrum at 12:00 obtained after successive subtractions of dichlorvos, SF6, and phosgene. or go through a rearrangement process

The OH addition channel may lead to Cl2C(O) through two possible pathways:

Cl2CdCH(OP(O)(OCH3)OCH2O•) f Cl2CdCH(OP(O)(OCH3)OH) + HC•O (7) This last process is supported by the recent work of Aschmann et al. which suggests a rearrangement of the alkoxy radical formed after H atom abstraction (26) from the degradation of similar compounds ((C2H5O)3PO). The observed CO in our experiment could be formed through the following reaction involving HCO radical produced in reaction 7:

HC•O + O2 f CO + HO2

(8)

Assuming that all CO observed is due to this mechanism, this would suggest that H atom abstraction from the CH3O groups accounts for 43 ( 6% of the overall reaction. 854

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Cl2CdCH(OP(O)(OCH3)2) + OH + (O2) f Cl2C(OH)-C(OO•)H(OP(O)(OCH3)2) (9a) f Cl2(OO•)C-C(OH)H(OP(O)(OCH3)2) (9b) In the presence of NO, the peroxy radical obtained by reaction 9a can then react as follows:

Cl2C(OH)-C(OO•)H(OP(O)(OCH3)2) + NO f Cl2C(OH)-C(O•)H(OP(O)(OCH3)2) + NO2 (10) Cl2C(OH)-C(O•)H(OP(O)(OCH3)2) f Cl2C•(OH) + HC(O)OP(O)(OCH3)2 (11) Cl2C•(OH) + O2 f Cl2C(O) + HO2 (12)

FIGURE 6. Concentration-time profiles of the reactant and products during the OH reaction with dichlorvos at EUPHORE (June 3, 2004). The initial concentrations were the following: [dichlorvos]0 ) 184.2 ppbv and [NO]0 ) 75.2 ppbv. CO has been quantified by FTIR between 2027 and 2241 cm-1.

FIGURE 8. Yield plots for products detected during the dichlorvos oxidation initiated by OH radical (June 1, 2004).

unidentified coproducts. Very weak bands around 17001800 cm-1 characteristic of carbonyl compounds have been also observed in the residual spectrum. However, in absence of reference spectra for HC(O)-OP(O)(OCH3)2, Cl2CdCHOP(O)(OCH3)(OCHO), the coproducts cannot be identified and quantified. Once formed they may rapidly react with OH radicals. Their OH-initiated oxidations can explain the secondary formation of both Cl2C(O) and CO, as observed in Figure 7. The observed yield of Cl2C(O) highlights that OH addition to the CdC bond accounts for 47 ( 3% of the overall reaction. Experimental primary yields of CO and Cl2CO show therefore that both pathways, i.e., OH addition to the CdC bond and H atom abstraction from the CH3O groups, contributes to 90 ( 6% of the overall reaction. This then implies that the rate constants of OH addition to the CdC bond and of H atom abstraction from the CH3O groups are (1.2 ( 0.1) × 10-11 and (1.1 ( 0.2) × 10-11 cm3 molecule-1 s-1, respectively.

FIGURE 7. Yield plots for products detected during the dichlorvos oxidation initiated by OH radical (June 3, 2004).

The second peroxy radical obtained by reaction 9b can also react with NO:

Cl2(OO•)C-C(OH)H(OP(O)(OCH3)2) + NO f Cl2(O•)C-C(OH)H(OP(O)(OCH3)2) (13) Cl2(O•)C-C(OH)H(OP(O)(OCH3)2) f Cl2C(O) + HC•(OH)(OP(O)(OCH3)2) (14) HC•(OH)(OP(O)(OCH3)2) + O2 f HO2 + HC(O)OP(O)(OCH3)2 (15) If Cl2C(O) and CO are the two main carbon-containing products, a major portion of the carbon is still in the

These two values can be compared to with those of OH with either other phosphorus-containing compounds or alkenes, reported in the literature. Tuazon et al. (27) and Martin et al. (28) have measured the rate constants of the OH reactions with trimethyl phosphonate (CH3O)3P(O) and with dimethyl phosphonate (CH3O)2P(O)H, respectively, and found them to be k ) (7.7 ( 0.47) × 10-12 and k ) (5.08 ( 0.53) × 10-12 cm3 molecule-1 s-1. These data enabled these authors to derive the rate constant for H atom abstraction from CH3O- as being 2.5 × 10-12 cm3 molecule-1 s-1. This value is about 2 times lower than that derived in this work, found to be of the order of 5.5 × 10-12 cm3 molecule-1 s-1. This suggests that the presence of the RO- group (R ) Cl2Cd CH- in dichlorvos) activates H atom abstraction from CH3Ogroups. The rate constant ((1.2 ( 0.1) × 10-11) corresponding to the addition of OH to the >CdC< contained in dichlorvos is of the same order of magnitude as that reported for the reaction of OH with CH2dCCl2 (1.1 × 10-11 cm3 molecule-1 s-1) (29). Then, the O-P(O)-(OCH3)2 group would not enhance the reactivity of the double bond toward OH radicals. However, we should mention here that is only speculation based on the observed reactivity. VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Atmospheric Implications The fraction of dichlorvos contained in the atmospheric cloud droplets can be calculated from its Henry’s law constant H from the following expression (30):

fdichlorvos,aq )

[dichlorvos]aq ) [dichlorvos]g + [dichlorvos]aq HRLwcT (16) 1 + HRLwcT

where [dichlorvos]aq and [dichlorvos]g are the concentrations of dichlorvos in both aqueous and gas phases, respectively, R the ideal gas constant (in L atm mol-1 K-1), and Lwc the dimensionless liquid water content of the cloud (typically 4.2 × 10-7). At 283 K which is the average temperature of tropospheric clouds and from the value of H ) 3.0 × 104 M atm-1 (31), the calculated aqueous fraction of dichlorvos reaches up to 11%. This implies that dichlorvos will be mainly oxidized in the atmospheric gas phase. The kinetic parameters in the gas phase obtained in this work have then been used to estimate the tropospheric lifetime of dichlorvos. The upper limit of the photolysis rate constant, J, of dichlorvos measured under atmospheric conditions (longitude ) -0.5°, latitude ) 39.5°) was less than 5 × 10-6 s-1 which enabled us to estimate the lifetime associated to photolysis (τphotolysis ) 1/J) to be higher than 56 h. The rate constant value determined in the present study has been used to estimate the tropospheric lifetimes of dichlorvos with respect to reaction with OH radicals (τOH ) 1/k[OH]). Assuming an average OH concentration of 1 × 106 molecule cm-3, we estimate that this tropospheric lifetime is of the order of 11 h. The other loss processes of dichlorvos in the atmosphere have not been counted for because of the lack of kinetic data. However, in the gas phase, the reactions of pesticide with either NO3 radicals or ozone are considered to be slow processes. Our mechanistic study of the OH reaction with dichlorvos has shown that carbon monoxide and phosgene are the two main carbon-containing products. Phosgene is extremely toxic by acute (short-term) inhalation exposure and was used as a chemical warfare agent in the past. Severe respiratory effects, including pulmonary edema, pulmonary emphysema, and death have been reported in humans. However, its LC50 in rats was calculated to be 1400 mg m-3 (4) which is 2 orders of magnitude higher than that of dichlorvos. Phosgene appears to be then less toxic than dichlorvos but much more stable since its tropospheric removal is predominantly via wet deposition with a lifetime of around 70 days (32). In regards to the high toxicity of dichlorvos and its relative short tropospheric lifetime to form a very stable toxic primary product such as phosgene, the use of dichlorvos for treatment might appear to be relatively dangerous for humans.

Acknowledgments Financial support was provided by the French Ministry of Environment through the PRIMEQUAL 2 and the Pesticides programs, CNRS through the PNCA program, the regions of Alsace and Centre, and the French Agency for Environment and Energy Management (ADEME). Fundacio´n CEAM is supported by the Generalitat Valenciana and Fundacio´n BANCAIXA.

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Received for review June 21, 2005. Revised manuscript received November 10, 2005. Accepted November 15, 2005. ES051178U

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