Particulate and gas-phase products from the atmospheric degradation of chlorpyrifos and chlorpyrifos-oxon

Atmospheric Environment 123 (2015) 112e120

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Particulate and gas-phase products from the atmospheric degradation of chlorpyrifos and chlorpyrifos-oxon
s, Milagros Ro
denas, Mo
nica Va
zquez, Teresa Vera, Amalia Mun ~ oz* Esther Borra
neo (Fundacio
n CEAM), Spain Centro de Estudios Ambientales del Mediterra

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Atmospheric OH-reaction and photolysis of organothiophosphorus.
Reaction profiles and yields of chlorpyrifos and chlorpyrifos-oxon determined.
Phosphorothioate derivatives were the main degradation products observed.
The degradation routes were formulated based on the structurally defined products.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 April 2015 Received in revised form 15 October 2015 Accepted 17 October 2015 Available online 26 October 2015

The phosphorothioate structure is highly present in several pesticides. However, there is a lack of information about its degradation process in air and the secondary pollutants formed. Herein, the atmospheric reactions of chlorpyrifos, one of the most world-used insecticide, and its main degradation product e chlorpyrifos-oxon e are described. The photo-oxidation under the presence of NOx was studied in a large outdoor simulation chamber for both chlorpyrifos and chlorpyrifos-oxon, observing a rapid degradation (Half lifetime < 3.5 h for both compounds). Also, the photolysis reactions of both were studied. The formation of particulate matter (aerosol mass yield ranged 6e59%) and gaseous products were monitored. The chemical composition of minor products was studied, identifying 15 multioxygenated derivatives. The most abundant products were ring-retaining molecules such as 3,5,6trichloropyridin-2-ol and ethyl 3,5,6-trichloropyridin-2-yl hydrogen phosphate. An atmospheric degradation mechanism has been amplified based on an oxidation started with OH-nucleophilic attack to P]S bond. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Chlorpyrifos Chlorpyrifos-oxon Insecticide SOA Photo-oxidation Multi-oxygenated products

1. Introduction Organophosphorus pesticides are extensively world-used in agriculture, horticulture, and a variety of household applications. They include herbicides, acaricides, and insecticides and are


neo * Corresponding author. Centro de Estudios Ambientales del Mediterra
n CEAM), C/Charles R. Darwin, 14, 46980 Paterna, Valencia, Spain. (Fundacio ~ oz). E-mail address: [email protected] (A. Mun http://dx.doi.org/10.1016/j.atmosenv.2015.10.049 1352-2310/© 2015 Elsevier Ltd. All rights reserved.

chemical or biological products used to prevent diseases, kill, repel, control plagues or interrupt the growth of plants. Once pesticides are applied in the field, they can be partitioned into the soil, water, and atmosphere with a significant environmental impact. Pesticides can be emitted into the atmosphere from ground or leaf surfaces, and the amount emitted is a function of their physical properties and their manner of application (Van den Berg et al., 1999; Unsworth et al., 1999). Moreover, they have a distribution into gas and condensed phases more or less displaced depending


s et al. / Atmospheric Environment 123 (2015) 112e120 E. Borra

on their physicochemical properties and environmental conditions
s et al., 2015). (Espallardo et al., 2012; Borra Chlorpyrifos (o,o-diethyl o-(3,5,6-trichloropyridin-2-yl) phosphorothioate) is considered as one of the most widely used insecticides in the world. It is applied on an important range of crops, particularly in the fruit-growing crops as well as and in numerous non-agricultural situations (Balinova et al., 2007). It is highly used in south-west Europe countries (France, Italy and Spain) and in USA. For example, this pesticide was detected in the gas phase in 39% of cases and in particulate phase in 18% of cases during
s 2008e2009 at different rural and agricultural Spanish areas (Borra et al., 2011). However, despite recent progress on the degradation of chlorpyrifos in atmosphere, soil, water, and heterogeneous re~ oz et al., 2014a; El Masri et al., 2014; Mackay et al., actions (Mun 2014), little is known about its degradation products in air beyond its transformation in the oxon compound. ~ oz et al., 2014a), after chlorpyrifos is As recently published (Mun emitted to the atmosphere, it is subjected to oxidation processes as happens to other semi-volatile organic compounds (SVOCs) (Atkinson et al., 1983). A set of new products, called residues or secondary pollutants, is formed. These gaseous and above all condensed products have a different atmospheric residence time, and sometimes exhibit worse toxicity than the original molecule (Hamilton et al., 2005; Mackay et al., 2014). However, the real atmospheric behavior is difficult to evaluate due to their low concentrations. In order to get a comprehensive overview of their atmospheric fate, the use of highly-equipped atmospheric simulation chambers solves some of these limitations (Finlayson-Pitts and Pitts, 2000). These facilities have allowed the examination of pollutant degradations under near-realistic atmospheric conditions ~ oz et al., 2012, 2014b). In fact, a previous (Le Person et al., 2007; Mun study performed at EUPHORE, one of the most high-volume atmospheric simulator chambers, demonstrated the general kinetics ~ oz et al., of the chlorpyrifos and its oxon towards OH-radicals (Mun 2014a). The present series of experiments performed in the EUPHORE simulators were carried out in order to improve the understanding of the degradation of chlorpyrifos, and its main degradation product e chlorpyrifos-oxon e in the troposphere. This research was focused on the contribution to particulate matter production and the identification of secondary organic aerosol (SOA) generated, being the novelty, the measurement and the qualitative analysis of SOA from photolysis and OH-degradation as well as a deeper investigation of the mechanism thanks to the detection of new degradation products. For that, measurements from a wide range of specific instruments and techniques such as mainly GCeMS, and also FTIR and several monitors were carried out to determine new products, both particulate and gas phase, providing information about the chemical composition. Some compounds formed were herein firstly detected for photodegradation of chlorpyrifos and chlorpyrifos-oxon. Also, a deeper analysis of results has been performed for the elucidation of the degradation pathway and the evaluation of the atmospheric impact of chlorpyrifos and one of its main degradation products. 2. Experimental section 2.1. Photoreactor and on eline instruments The experiments were carried out in the high volume outdoor smog chambers EUPHORE (European PHOtoREactor) (Valencia, Spain). These simulator photoreactors were designed to work under near-realistic atmospheric conditions minimizing losses and wall-interactions effects. These chambers consist of two half spherical fluoropolymeric bags, each one of 200 m3 with integrated

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measuring systems for monitoring pressure, humidity, tempera
s and ture, precursor species, and reaction products (Borra ~ oz et al., 2011a). Pressure, humidity Tortajada-Genaro, 2012a; Mun and temperature were measured using a pressure sensor (Air-DBVOC, Sirsa, Madrid, Spain) and a dew point hydrometer (TS-2, Walz, Effeltrich, Germany). An Eco Physics AG (AL-ppt-77312, Duernten, Switzerland), an API NOx monitor (API200AU, Teledyne API, San Diego, USA) and a NOx analyzer (ML9841A, Teledyne Monitor, Englewood, USA) were used for measuring NO, NO2 and NOx. White-type multi-reflection mirror system (path length of 553.5 m) coupled to a Fourier Transform Infrared spectrometer equipped with a MCT detector (NICOLET Magna 550, Thermo Scientific, USA) was used for recording concentrations of chlorpyrifos (760e1225 cm1), chlorpyrifos-oxone (760e1225 cm1), nitrous acid (762e956 cm1), SO2 (1050e1200 cm1), ozone (990e1150 cm1), formic acid (990e1150 cm1), nitric acid (762e956 cm1), hydrochloric acid (2710e2900 cm1), methyl glyoxal (2710e2900 cm1), pyrimidol (760e1225 cm1), SF6 (762e956 cm1) and formaldehyde (2710e2900 cm1). IR region bands analysis are included in parentheses. Concentration profiles
denas, 2008) that, were calculated by using specific software (Ro compared to the classic minimum least square method, improves the analysis of the complex gas mixtures when unknown compounds are present in the sample absorbing in the same region as the compounds of interest and interfering with them. Its effectiveness depends on the structure of their absorption bands. FTIR calibration procedure consisted on several introductions of each compound, using standards, into the EUPHORE simulation chamber at known concentrations. Moreover, concentration values for chlorpyrifos and chlorpyrifos-oxon were also experimentally determined and validated by off-line GCeMS plus derivatization protocol using C18 cartridges to sample them (see Section 2.2). Aerosol mass concentration was measured with two on-line instruments. One was a scanning mobility particle sizer (SMPS), model 3080 (TSI, Shoreview, USA). This system measured size distributions in the 11e789 nm diameter range in real time with a 5 min scan rate, and provided aerosol concentrations assuming spherical shapes and multi-charge correction for the condensed organic material. Sheath and aerosol sampling flows were 3 L min1 and 0.30 L min1, respectively. The other automated instrument was a tapered element oscillating monitor (TEOM) (model 1400a, Ruppercht and Patashnick, Albany, USA) with a 1 min scan rate, PM1 sampling head, and a sampling flow of 3 L min1. 2.2. Derivatization sample treatment and off-line analysis For fingerprint analysis, particles were collected at maximum aerosol formation, under a flow rate of 23 L min1 for 1 h, on quartz fiber filters that had been pre-baked at 500  C for 12 h. Gaseous products were sampled with C18 cartridges during reaction, under a flow rate of 1 L min1 for 0.5 h. The analysis of multi-oxygenated compounds was carried out after derivatization by gas chromatographyemass spectrometry (GCeMS) (Borr
as and TortajadaGenaro, 2012b). Briefly, 47 mm quartz fiber filters (Whatman, Brentford, England) were sonicated with 5 mL of CH2Cl2/CH3CN (1:1). C18 cartridges (Waters, Barcelona, Spain) were eluted with 2 mL of CH2Cl2/CH3CN (1:1). Then, a derivatization with o(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine hydrochloride (PFBHA) plus diluted N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA) e supplied by Sigma Aldrich (Barcelona, Spain) e was carried out. After that, the following reagents were added: 0.5 mL of CH3CN, 150 mL of PFBHA solution of 1000 mg L1 in CH3CN and 50 mL of a 4-fluorobenzaldehyde solution (15 mg L1) as surrogate of carbonyl derivatization. This mixture was left in darkness at


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room temperature for 24 h, after it was dried by a N2 air stream. It was then dissolved in 145 mL of MSTFA solution (1:150 in hexane), adding 5 mL of a chlorosuccinic acid solution (5 mg L1) as surrogate of hydroxyl derivatization and heated at 90  C for 1 h. Finally, it was injected in the gas chromatographemass spectrometer TRACE-DSQ II GCeMS (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a RTX-5MS column of 30 m  0.25 mm I.D  0.25 mm film thickness. The chromatograph was programmed at 60  C for 1 min, then ramped at a rate of 10  C min1 to 250  C, 5  C min1 to 280  C and held at 280  C for 10 min. The injection port was held at 280  C and the transfer line from GC to MS was held at 300  C. Samples were injected in split (1:20), using helium flow of 1 mL min1. The EI-voltage was 70 eV and full scan mode was used (m/z 50e650) with the ion source temperature at 200  C and the quadrupole temperature at 100  C. The GC-internal standard was 1phenyldodecane at 5 mg L1. Blank controls of cartridges and filters were analyzed and showed the absence of artifacts. The use of standards and the determination of the chemical ionizationspectrum confirmed the proposed structure.

a stream of purified air. Chlorpyrifos and chlorpyrifos-oxon were gently heated to accelerate their introduction into the chamber by air stream. After the reactants were mixed for 10 min, the chamber was exposed to natural sunlight at midday under a clear sky in summer (JNO2 z 6.6  103 s1) and the reaction started. The value of NO2 photolysis constant e measured during all the sunlight exposition by a filter radiometer (Meteorologie Consult GMBH, Glasshütten, Germany) e is a function of the latitude, the day of the year, the local time and the cloud cover allows to normalize the experimental photolysis rate constant in any region, season and time. The onset of aerosol formation was considered to occur when the first significant particle concentration was registered (signal > 3sbackground). Reactants and products are diluted during the experiment and to determine the correct concentration values, they must be corrected. The dilution rate in the chamber is calculated from the decay of SF6 by FTIR areas in the IR range of 762e956 cm1. The specific dilution process was determined by FTIR adding 120 mg m3 of SF6 as a non-reactive tracer (value of 1.1  105 s1) to the reaction mixtures at the start of the experiments.

2.3. Experiments 3. Results A blank chamber experiment, described in Borr
as and TortajadaGenaro (2012a), was performed to assure the absence of possible simulator artifacts due to the off-gassing of compounds from reactor walls. The photoreactor was filled with air from a purification system. Non-detectable hydrocarbons and nitrogen oxides were measured and the aerosol background was 0.010 ± 0.005 mg m3 (60 part cm3). Specific experiments were performed for guaranteeing the correct injection of chlorpyrifos and chlorpyrifos-oxon into the reaction chamber, because generally low-vapor pressure hydrophobic compounds show important problems (e.g. electrostatic effects with connections). An injection of chlorpyrifos and chlorpyrifosoxon e pressure vapor 2  105 mm Hg and 7  106 mm Hg, €en, Germany) respectively (Mackay et al., 2014)e (99%, Riedel de Ha via heated air stream (flow 10 L min1) through a short PTFE tube connection was selected. Under these conditions, losses or decomposition processes were negligible before photochemical degradation reactions started after the dome covers opening. The oxidation experiments e carried out by duplicate e consisted of photolysis and photo-oxidation under dry conditions (<2% RH, 295e298 K) and in the absence of initial inorganic seeds and low concentrations of nitrogen oxides and under the presence of an excess of cyclohexane (8 ppmV) e used as OH radical scavenger for both photolysis experiments to avoid a possible OH reaction competition and a misunderstanding for OH-rate constant and products concentrations e (Table 1). For photo-oxidation activated by NOx, OH radicals were generated by photolysis of nitrous acid (HONO). HONO was generated by a liquid-phase reaction between a 0.5% NaNO2 solution and a 30% H2SO4 solution and transferred via

3.1. Chlorpyrifos and chlorpyrifos-oxon consumption The typical profiles of the normalized concentration of gas phase chlorpyrifos and chlorpyrifos-oxon recorded during the experimental measurement of atmospheric reactivity under photolysis and under photo-oxidation in the presence of high NOx are shown in Fig. 1. A delay between the onset of oxidation after sunlight exposure and pesticide consumption, was observed for the photolysis experiments of both chlorpyrifos and chlorpyrifos-oxon. However, for photo-oxidation experiments under the addition of HONO e fast OH radical generation, pesticide and its oxon were immediately consumed after the exposition to sunlight radiation. From the decay curves of chlorpyrifos and chlorpyrifos-oxon (Fig. 1), the average OH concentrations generated during the experiment from NOx source were calculated using the rate constant of OH-radicals towards chlorpyrifos and chlorpyrifos-oxon. The average OH concentration, present in the smog chamber during the photooxidation reaction of chlorpyrifos, was (5.49 ± 0.14)  106 radicals cm3 using theoretical e based upon the structureeactivity relationship (SAR) methods developed by Kwok and ~ oz et al., 2014a,b) Atkison (1995)-and experimental kOH (Mun respectively 9.1  1011 cm3 molecule1 s1. The average OH concentrations, present in the smog chamber during the photooxidation reaction of chlorpyrifos-oxon, were (1.39 ± 0.12)  106 radicals cm3 and (3.18 ± 0.19)  106 radicals cm3 using theoretical (3.9  1011 cm3 molecule1 s1) and experimental (1.7 ± 0.3)  1011 cm3 molecule1 s1 kOH,

Table 1 Experimental conditions of atmospheric pesticide degradations. Standard error < 5%. Experiment type

Solar radiation

JNO2 (s1) Precursor (mg m3)

HONO (mg m3 min1)

% Pesticide consumption

Aerosol yield (%)

Aerosol density (g cm3)

Chamber background A Chlorpyrifos wall losses Photolysis of chlorpyrifos Photo-oxidation of chlorpyrifos (presence NOx) Photolysis of chlorpyrifos-oxon Photo-oxidation of chlorpyrifos-oxon (presence NOx)

No No Yes Yes

e e 6.4  103 6.9  103

e e 1344 1665

e e e 3.8 (global 490)

e e 53 95

e e 6 23

e e 1.8 1.7

Yes Yes

6.6  103 1808 6.4  103 2325

e 3.8 (global 228)

59 58

59 43

e e


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Fig. 1. Profiles of chlorpyrifos and chlorpyrifos oxon degradation under photolysis and NOx photo-oxidation conditions.

respectively. These values agreed with previous ones reported for urban locations (Martínez et al., 2003). Half-life times (50% consumption) under our experimental conditions varied between 4.9 to 5.4 h (photolysis) and 0.5 h to 3.2 h (OH-photo-oxidation), indicating the atmospheric residence of both organophosphorus compounds is relatively short in the troposphere at urban conditions. The half-life time was calculated under our experimental conditions e average [OH] concentrations of 5.5  106 radicals cm3 and 1.4  106 radicals cm3 for chlorpyrifos and chlorpyrifos-oxon, respectively. If we consider the average global atmospheric [OH] concentration by 2  106 radicals cm3, the OH-photo-oxidation half-life times of both compound were quite similar (0.3 h and ~ oz et al. (2014a) (0.6 h and 3.3 h)). 3.1 h previously reported by Mun Finally, the reaction reached the stationary state after <1 h of solar exposition for chlorpyrifos photo-oxidation and 5e6 h of solar exposition for chlorpyrifos photolysis, chlorpyrifos-oxon photolysis and chlorpyrifos-oxon photo-oxidation. The consumptions varied from 53 to 59%, in the photolysis experiments and from 58 to 95% in the photo-oxidation experiments. 3.2. Analysis of aerosol formation A significant amount of particulate matter between 22 and 563 mg m3 was obtained in all experiments at the stationary state (Fig. 2a). The steady state for aerosol production during the chlorpyrifos-oxon photo-oxidation is observed after more than 3 h reactions whereas no steady state is observed for the chlorpyrifosoxon consumption (Fig. 1). The aerosol steady state for chlorpyrifos oxidation is reached in 2 h whereas its consumption is quasi total within less than 1 h. It may be explained by the presence of secondary and ternary reactions. The aerosol yield (Y), or the capacity of chlorpyrifos and its main degradation product to produce particles, was calculated from the equation developed by Odum et al. (1996).

Y¼

M0 DHC

(1)

where Mo (mg m3) is the aerosol mass concentration formed and

DHC (mg m3) is the mass concentration of pesticide reacted. Yields, reported in Table 1, were thus calculated using the precursor concentration from FTIR data and the aerosol concentration obtained from wall losses corrected TEOM data, between the start and the maximum of aerosol formation. The Y-values ranged between 6.0 ± 0.3% and 59 ± 3% for chlorpyrifos and chlorpyrifos-oxon photolysis experiments, respectively, and between 23 ± 1% and 43 ± 4% for chlorpyrifos and chlorpyrifos oxon photo-oxidation experiments. Results indicated that the atmospheric photolysis as well as the photo-oxidation reaction led to formation of products with a low vapor pressure (particulate products). Besides, a higher aerosol formation was observed (2 or 3 folds) at high NOx photooxidation conditions for chlorpyrifos experiments. Compared to other organic aromatic pesticides, the aerosol yields obtained under photo-oxidation conditions were higher than the one of ~ oz propachlor (15%), a chloride-substituted aromatic ring (Mun et al., 2012), and hymexazol (4.8%), a N-heterocycle compound, (Tortajada-Genaro et al., 2013). However, the aerosol yields were similar than those calculated from the degradation of diazinon ~ oz et al., 2011b). The (40%), an organophosphorus compound (Mun results implied that organophosphorus pesticides are relevant aerosol precursor under OH photo-oxidation conditions in the presence of nitrogen oxides. The aerosol yields determined were confirmed by calculating the curves of the aerosol mass concentration (DMo) as a function of the chlorpyrifos or chlorpyrifos-oxon reacted after the onset aerosol formation (DHC) (Fig. 2b). A correlation (R2 average > 0.90) was observed with slopes of 4.6 ± 0.2%, 22.5 ± 0.8%, 59.3 ± 1.2%, and 48.7 ± 1.5%, for photolysis of chlorpyrifos, photo-oxidation of chlorpyrifos, photolysis of chlorpyrifos-oxon and photo-oxidation of chlorpyrifos-oxon, respectively. Statistical test (t-test) confirmed that calculated Y-values from both methods were comparable (p-value < 0.05). Then, aerosol yields have been determined in their low and high-NOx conditions. The SOA yields under low-NOx conditions were significantly larger than those under high-NOx conditions for the chlorpyrifos-oxon. This is likely a result of the competition between RO2 þ NO and RO2 þ HO2 reactions, similar to what has been observed in other studies as Ng et al.

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Fig. 2. a. Concentration profiles for particle matter from chlorpyrifos and chlorpyrifos-oxon photolysis and photo-oxidations under high NOx oxidative conditions. b. Plot of aerosol mass concentration against the reacted chlorpyrifos and chlorpyrifos oxon.

(2007), However, for chlorpyrifos experiments, the behavior was the opposite, as was observed for benzene in Borr
as and TortajadaGenaro (2012a). Since the particles deposition in human lung, and thus their toxicity, is related to their diameter, the particle size distributions were determined. The variation of oxidant conditions endorsed changes on the nucleation, coagulation and growth processes of aerosol, consequently promoting different particle sizes. A rapid OH generation from HONO photolysis promoted an immediate particulate matter formation and the aerosol size distribution showed an initial growth controlled by condensation or homogeneous/binary nucleation process. Later, the weighted average particle diameter increased to 71e216 nm (see Fig. 3). This observation can be explained by an incessantly condensation, promoting bigger particles but in lower particle number concentration. But, in all cases, the particle diameters corresponded to the fine particle fraction (diameters < 550 nm). Finally, particle densities were determined

from the aerosol mass linear regression slope from TEOM data as a function of aerosol volume from SMPS data. Density average value was 1.7 ± 0.3 g cm3. Most of the papers dealing with aromatic systems theoretically assumed an SOA density of 1 or 1.5 g cm3
s and Tortajada-Genaro, 2012b). Our study demonstrates that (Borra SOA density is a function of the precursor and, thus, must be specifically selected in order to develop reliable aerosol models. This is the first time that density parameter has been determined for organophosphorus pesticides. 3.3. Degradation products The next step of the study was focused on the determination of the chemical composition of the degradation products formed during photolysis and photo-oxidation reactions. The major gaseous products, measured by FTIR technique and monitors, were high oxidized compounds such as ozone (5e250 mg m3), formic

Fig. 3. Average particle diameter size for chlorpyrifos photolysis, chlorpyrifos photo-oxidation, chlorpyrifos oxon photolysis and chlorpyrifos oxon photo-oxidation.


s et al. / Atmospheric Environment 123 (2015) 112e120 E. Borra

acid (1e60 mg m3), sulfur oxide (1e60 mg m3), nitric acid (4e245 mg m3), nitrogen monoxide (0.1e105 mg m3), nitrogen dioxide (5e464 mg m3), hydrochloric acid (1e50 mg m3), methyl glyoxal (1e50 mg m3) and formaldehyde (0.3e20 mg m3) (see Fig. 4). For chlorpyrifos, significant differences were observed. For photolysis conditions, concentration of hydrochloric acid and sulfur oxide were 2 folders lower than for photo-oxidation conditions. Also, the presence of nitrogen oxides e mainly HONO e during the OH-photo-oxidation reaction, allowed the formation of ozone, nitrogen dioxide and nitric acid in high concentration. Regarding particle formation, an “induction period”, a delay between the beginning of the pesticide consumption and production of aerosol, was observed on photo-oxidation experiment of chlorpyrifos. It could be explained by the role of NOx in the peroxy radical chemistry (Borr
as and Tortajada-Genaro, 2012b). For chlorpyrifos-oxon, significant differences were observed for particle formation. For photo-oxidation conditions, particles were 2 folders higher than for photolysis conditions. Also, pyrimidol e the main ring retaining product- was also detected during photooxidation conditions but for both, chlorpyrifos and chlorpyrifosoxon experiments. Also, the degradation by OH radicals of chlorpyrifos and chlorpyrifos-oxon, as well as other volatiles and semi-volatile precursors, generated multi-oxygenated products as a result of a

117

partial oxidation process. For that, a GCeMS technique with PFBHAMSTFA derivatization was applied to gas-phase and particulate samples collected during reaction. No multi-oxygenated compounds were detected in photolysis experiments. However, a total of 15 products were identified for the photo-oxidation of chlorpyrifos and 9 products were identified for the photo-oxidation of chlorpyrifos-oxon. Identifications were done based on chemical properties and taking into account their ion fragments, retention time and expected polarity, being most of them detected for first time (see Fig. 5). The most abundant products, 3,5,6trichloropyridin-2-ol and ethyl 3,5,6-trichloropyridin-2-yl dihydrogen phosphate, were detected in both gaseous and particulate phases and in both chlorpyrifos and chlorpyrifos oxon photooxidation reactions. The rest of degradation compounds were all detected in the particulate phase and some of them in gas phase (see Table 2). Hence, most of the multi-oxygenated degradation products identified were ring-retaining products. That means that the partially oxidized molecules maintained the central skeleton of chlorpyrifos. The phosphorothioate structure was only modified by replacing one of its atoms (double bond P]S for P]O) or some of the OeCH3 substituent by OH. A reaction mechanism for the oxidation of chlorpyrifos and chlorpyrifos-oxon is proposed (see Fig. 6). The previous reaction ~ oz et al., 2014a), has been scheme, described by our group (Mun reinforced including new reaction routes and degradation products

Fig. 4. Gaseous and particulate profiles of main degradation products from a) Photolysis of chlorpyrifos; b) Photo-oxidation of chlorpyrifos; c) Photolysis of chlorpyrifos-oxon; d) Photo-oxidation of chlorpyrifos-oxon.


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Fig. 5. a. Extracted ion chromatograms (m/z ¼ 73 and m/z ¼ 181) from particulate sample collected during chlorpyrifos photo-oxidation. Codes included in Table 2. b. Extracted ion chromatograms (m/z ¼ 73 and m/z ¼ 181) from particulate sample collected during chlorpyrifos-oxon photo-oxidation.

supported by the experimental evidences obtained in the present study. All of the multi-oxygenated molecules proposed have been identified on samples collected during the atmospheric degradation process. The reaction mechanism for chlorpyrifos is based on the initial OH attack directed to P]S bond group and H abstraction. Molecular models indicated that the nucleophilic attack can be performed on both phosphorus atom and sulfur atom. If the OH forms an adduct with S atom, it would imply mainly the formation of 3,5,6-trichloropyridin-2-yl dihydrogen phosphate (8), diethyl 3,5,6-trichloropyridin-2-yl phosphate (10) and ethyl 3,5,6trichloropyridin-2-yl hydrogen phosphate (12). On the other hand, the formation of an adduct with P atom yields products such as diethyl hydrogen phosphate (1), o,o-diethyl hydrogen phosphorothioate (2), 3,5,6-trichloropyridine-2,4-diol (3), ethyl

dihydrogen phosphate (4), 3,5,6-trichloropyridin-2-ol (5), ethyl 2oxoethyl hydrogen phosphate (7), o-ethyl o-(2-oxoethyl) hydrogen phosphorothioate (14) and, o,o-bis (2-oxoethyl) hydrogen phosphorothioate (15). Additionally, we would like to point out that the OH can form an adduct with P atom of chlorpyrifos-oxon, that leads to the formation of some of the products identified of adduct with S atom of chlorpyrifos. If the H abstraction is produced o-ethyl o-(3,5,6-trichloro-4-hydroxypyridin-2-yl) hydrogen phosphorothioate (6) and o-(3,5,6-trichloropyridin-2-yl) dihydrogen phosphorothioate (13) are formed. Gaseous products such as diethyl 3,5,6-trichloropyridin-2-yl phosphate (10), 3,5,6-trichloropyridin2-ol (5), diethyl hydrogen phosphate (1), PAN and SO2 were pre~ oz et al., 2014a). New viously identified by FTIR technique (Mun products were detected in both gas and particulate phases by

Table 2 Multi-oxygenated compounds formed in the chlorpyrifos and chlorpyrifos-oxon photo-oxidations with main chromatographic characteristics (retention time in minutes, intensity and main fragments observed), phase in which the compound is detected and experiment type. Code r.t

Name

Formula

MW

Intensity Phase Type of expa

1

8.2 Diethyl hydrogen phosphate

C4H11O4P

154.1 (þþ)

2

8.3 O,O-Diethyl hydrogen phosphorothioate

C4H11O3PS

170.2 (þþ)

3

9.7 3,5,6-Trichloropyridine-2,4-diol

C5H2Cl3NO2

214.1 (þ)

4

9.9 Ethyl dihydrogen phosphate

C2H7O4P

126.1 Trace

C5H2Cl3NO

198.4 (þþþ)

5

12.1 3,5,6-Trichloropyridin-2-ol

6 7

15.7 O-Ethyl O-(3,5,6-trichloro-4-hydroxypyridin-2-yl) hydrogen phosphoro- C7H7Cl3NO4PS 338.5 (þþ) thioate 168.1 (þþ) 15.9 Ethyl 2-oxoethyl hydrogen phosphate C4H9O5P

8 9

16.6 3,5,6-Trichloropyridin-2-yl dihydrogen phosphate 17.7 Methylglyoxal

C5H3Cl3NO4P C3H4O2

10

18.1 Diethyl 3,5,6-trichloropyridin-2-yl phosphate

C9H11Cl3NO4P 334.5 (þþ)

11

18.3 O,O-Diethyl O-(3,5,6-trichloropyridin-2-yl) phosphorothioate

C9H11Cl3NO3PS 350.6 (þþ)

12 13 14 15

18.9 19.2 19.7 22.1

C7H7Cl3NO4P C5H3Cl3NO3PS C4H9O4PS C7H7O5PS

Ethyl 3,5,6-trichloropyridin-2-yl hydrogen phosphate O-(3,5,6-trichloropyridin-2-yl) dihydrogen phosphorothioate O-Ethyl O-(2-oxoethyl) hydrogen phosphorothioate O,O-bis(2-oxoethyl) hydrogen phosphorothioate

Trace; (þ) Low; (þþ) Medium; (þþþ) High. a A: Chlorpyrifos experiments; B: Chlorpyrifos-oxon experiments.

278.4 (þþ) 72.0 (þþ)

306.5 294.5 184.2 198.1

(þþþ) (þþ) (þþ) (þ)

g and p g and p g and p g and p g and p g and p g and p p g and p g and p g and p p p p p

Main fragments (m/z)

A,B

73, 75, 155, 183, 198, 211, 226

A

73,75, 147, 227, 242

A,B

73,75, 147, 270, 285

A,B

73, 211, 227, 255, 270

A,B

73, 209, 257, 258, 259, 272

A

73, 147, 395, 410

A,B

73, 75, 181, 195, 420, 435

A,B A,B

72, 77, 407, 422, 423 181, 462

A,B

109, 197, 199, 270, 298, 333, 335 109, 258, 260, 314, 316, 351, 353 73,75, 147, 364, 379 73, 147, 423, 438 75, 181, 211, 381, 436, 451 75, 181, 450, 465

A A,B A A A


s et al. / Atmospheric Environment 123 (2015) 112e120 E. Borra

119

Fig. 6. Proposed reaction mechanism for the oxidation of chlorpyrifos and chlorpyrifos-oxon with OH radical.

GCeMS derivatization technique: 3,5,6-trichloropyridine-2,4-diol (3), ethyl dihydrogen phosphate (4), o-ethyl o-(3,5,6-trichloro-4hydroxypyridin-2-yl) hydrogen phosphoro-thioate (6), ethyl 2oxoethyl hydrogen phosphate (7), 3,5,6-trichloropyridin-2-yl dihydrogen phosphate (8) and, o-(3,5,6-trichloropyridin-2-yl) dihydrogen phosphorothioate (13). This allowed us to improve and reinforce the reaction pathway for chlorpyrifos and chlorpyrifos-

~ oz et al. oxon under atmospheric conditions proposed in Mun (2014a) (Fig. 6). 4. Conclusions The extensive use of organophosphorus pesticides presents an important environmental effect, and there is a concern about the

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s et al. / Atmospheric Environment 123 (2015) 112e120 E. Borra

subsequent changes of chemical atmosphere dynamics and their toxicological hazards. In recent years, many studies have demonstrated that organophosphorus pesticides and some of their degradation products are mutagenic, carcinogenic, cytotoxic, genotoxic, teratogenic, and immunotoxic (Wang et al., 2013). The present study, based on the use of EUPHORE high volume photoreactor, provides useful data about atmospheric degradation processes of one of the most released pesticides to the atmosphere. Knowledge of the specific degradation products, including the formation of secondary particulate matter, could complete the assessment of their potential impact (Rathore and Nollet, 2012; Atkinson et al., 1999). In fact, the fingerprint chemical composition analysis has indicated that chlorpyrifos and chlorpyrifos-oxon are a relevant source of multi-oxygenated molecules. The formation of those types of degradation products is important because they play a significant role in the atmospheric chemistry. Multioxygenated compounds are related to health effects, global
s and Tortajada-Genaro, climate change and radiative force (Borra 2012b; Jaoui et al., 2004; Ramanathan et al., 2001). Hence, the results of this study can contribute to the selection of environmentally sustainable strategies against plagues. Thus, the understanding of atmosphere reactions should help to estimate the expected formation of gas and/or particulate products in the troposphere for each pesticide. Just then, the design and selection between related molecules will be correctly performed based on criteria of efficient action and low environmental impact of the pesticide and their residues. Acknowledgments The research leading to these results received funding from the Spanish Ministry of Economy and Competitivity (project IMPLACAVELES: CGL2013-49093-C2-1-R), Generalitat Valenciana for the DESETRES-Prometeo II project and by Dow AgroSciences (USA). We especially thank Steve Knowles from DOW for his valuable contri
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