Atmospheric oxidation of a thiocarbamate herbicide used in winter

27 molecule-1 s-1 at 288±10 K and atmospheric pressure by a conventional ... exposed and remote areas.1, 2, 3 In 2016, about 234 000 tons of pesticid...
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Environmental Processes

Atmospheric oxidation of a thiocarbamate herbicide used in winter cereals Amalia Munoz, Esther Borras, Milagros Ródenas, Teresa Vera, and Hans Albert Pedersen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02157 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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Atmospheric oxidation of a thiocarbamate herbicide used in

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winter cereals

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Amalia Muñoz*(1), Esther Borrás (1), Milagros Ródenas(1), Teresa Vera(1),

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Hans Albert Pedersen (2)

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(1)

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Fundación CEAM. C/Charles R. Darwin, 14. Parque Tecnológico 46980 Paterna (Valencia), Spain.

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(2)

Dept. of Agroecology. Aarhus University

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*Corresponding author:

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Amalia Muñoz

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Fundación Centro de Estudios Ambientales del Mediterráneo (Fundación CEAM)

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C/Charles R. Darwin, 14 46980 Paterna – Valencia - Spain

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Corresponding author’s e-mail: [email protected]

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Phone: +0034 609644051

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Abstract

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The gas-phase atmospheric degradation of prosulfocarb (a widely used thiocarbamate

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herbicide in winter cereals) at different NOx concentrations was investigated at the large

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outdoor European Photoreactor (EUPHORE) in Valencia, Spain. Photolysis under sunlight

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conditions and reaction with ozone were shown as unimportant. The rate constant for the

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reaction of prosulfocarb with OH radicals was determined as k = (2.9±0.5) × 10-11 cm3

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molecule-1 s-1 at 288±10 K and atmospheric pressure by a conventional relative rate

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method. Significant ozone and aerosol formation was observed following the reaction of

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prosulfocarb with OH radicals, and the main detected carbon-containing gas phase

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products

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benzylpropanoyl(propyl)carbamo-thioate.

were

benzaldehyde,

S-benzylformyl(propyl)carbamothioate,

and

S-

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Key Words

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Prosulfocarb, atmospheric degradation, photodegradation, atmospheric fate, rate constant,

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products formation

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1. Introduction

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Pesticides are among the more extensively used chemicals worldwide. Their intensive use

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has led to the contamination of not only water and soil, but also the atmosphere, in both

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exposed and remote areas.1, 2, 3 In 2016, about 234 000 tons of pesticide active ingredients

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were used in Europe (EU-28).4 The potentially adverse effects of exposure to pesticides in

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the general population are of much concern.5 Once a plant protection product is applied to

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the field, the active ingredient can be partitioned into soil, water, biota and the atmosphere.

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In the atmosphere, pesticides are distributed among gas, particle and aqueous phases,

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depending on their physicochemical properties and the environmental conditions.6, 7

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Prosulfocarb

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(C3H7)2NC(O)SCH2C6H5, is a thiocarbamate herbicide used in both pre- and post-

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emergence stages in winter cereals. It can be released to the atmosphere directly while

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being sprayed. It has been detected in air, with an average concentration

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over 4 years at levels higher than 1 ng m−3: in France reaching even week average

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concentration around 14–15 ng m−3.,8 and was also detected in the largest quantities in wet

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deposition studies at two different sampling locations.9 Various vapor pressures are

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reported in the literature, which range from 0.7 mPa at 20ºC 10 to 5-7 mPa at 25ºC 11-13. A

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vapor pressure of 6.4 mPa at 20°C, which was theoretically estimated

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that prosulfocarb could exist in both the gas and particulate phases in the atmosphere.

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Concentrations up to 3.6 µg/L have also been detected in rainwater in Sweden from 2002-

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2012 15. Prosulfocarb is often applied to winter crops early in autumn. In recent years the

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herbicide has been detected in several batches of organically and conventionally grown

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apples harvested near the prosulfocarb application time. This has led to its rejection at a

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significant cost to apple growers, and is also a matter of concern for consumers.

(CAS:

52888-80-9,

S-benzyl

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dipropylcarbamothioate,

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, would indicate

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Like other organic compounds, the main pathways for the tropospheric degradation of

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prosulfocarb could involve photolysis and reactions with ozone, hydroxyl and nitrate

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radicals, although no previous studies on the degradation of prosulfocarb in air have been

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found in the literature. Kwok et al.

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several thiocarbamates, including S-ethyl N,N-dipropylthiocarbamate, which has several

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similar structural features to prosulfocarb. They calculated the atmospheric lifetime of S-

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ethyl N,N-dipropylthiocarbamate in relation to the reaction with OH radicals to be close to

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6 h using an OH concentration of 1.5 x 106 molecules cm-1, for a 12-h-average daytime.

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. In that study the main detected metabolite was S-ethyl N-formyl-N-propylthiocarbamate.

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Photodegradation of prosulfocarb has been previously studied in the liquid phase and on

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soil and leaf surfaces of barley. Dipropylaimne and propylamine have been detected in

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studies carried out in methanol

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and sulfonic acid have also been detected in other studies

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systems, prosulfocarb sulfoxide and the complete transformation into CO2 have been

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observed 19.

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The present series of experiments was carried out to determine the major reaction

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pathways of prosulfocarb degradation in the troposphere. Studies were performed at

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EUPHORE (European PHOto-Reactor). The results provided information on the

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atmospheric lifetime of prosulfocarb. The main primary products of the OH radical

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initiated oxidation of prosulfocarb were also determined, and mechanisms for their

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formation were proposed. To the best of our knowledge, the experimental determination of

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the rate constant of OH with prosulfocarb in the gas phase at ambient temperature and the

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identification of the main degradation products have not been previously published in the

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peer-reviewed literature.

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investigated the gas-phase atmospheric chemistry of

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, while sulfoxide, despropyl prosulfocarb, benzoic acid 18

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. In soil and sediment-water

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2. Experimental Section

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2.1 Photoreactor and online instruments.

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The experiments were carried out in the EUPHORE high-volume outdoor smog chamber

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(Valencia, Spain) late in autumn. This simulation chamber enables reactions to be carried

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out using natural sunlight and by minimizing losses and wall-interaction effects by

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following similar procedures to those previously employed in this laboratory to investigate

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the atmospheric fate of a number of pesticides 20-23.

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The chamber consists of a half spherical fluoropolymeric bag (200 m3 volume) with

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integrated measuring systems for different compounds and parameters. Humidity and

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temperature were measured with a dew point hydrometer (TS-2, Walz, Effeltrich,

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Germany). A TAPI NOx monitor (T200UP, Teledyne, San Diego, USA) was used to

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measure NO, NO2 and NOx. Ozone was measured with a Serius 10 ozone analyzer

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(Echotec, Knoxfield, Victoria, Australia). A White-type multi-reflection mirror system

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(path length 553.5 m), coupled to a Fourier transform infrared (FTIR) spectrometer

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equipped with an MCT detector (NICOLET Magna 6700, Thermo Scientific, Waltham,

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MA, USA), was used to record the concentrations of prosulfocarb (1073-1196 cm-1),

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nitrous acid (762-956 cm-1), SF6 (762-956 cm-1) and benzaldehyde (725-1220 cm-1). The

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IR region analysis bands are shown in parentheses. Concentration profiles were calculated

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by a specific software

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products were also experimentally determined and validated by automated online SPME-

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GC-MS (solid-phase microextraction, gas chromatography-mass spectrometry) and by

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offline SPME-GC-FID (gas-chromatograph with a flame ionization detector). In the on-

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line active sampling, the air was forced through the fiber, by means of a pump, through a

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sampling cell for 5 min, while the off-line

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that compounds are retained in the fiber by direct exposition to EUPHORE chamber due to

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. The concentrations of prosulfocarb and other degradation

technique was passive sampling, that means

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diffusion processes. The optimization of exposure time was made in previous separate

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experiments where known quantities of prosulfocarb were introduced at EUPHORE

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photoreactor. The automatic online SPME-GC-MS consisted of a robot with an SPME

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adapted to air sampling. Air from the chamber was sampled through an inert Silconert-

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coated steel tube heated to 80ºC and connected to a sampling cell, where the SPME fiber -

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coated with polydimethylsiloxane/divinylbenzene (PDMS/DVB) from Supelco (Madrid,

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Spain)- was exposed. Air was passed through a cell at 10 L min-1 for 5 min. Finally, the

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sample was thermally desorbed at 250ºC in the injection port of an Agilent GC-MS (Santa

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Clara, CA, USA) equipped with an HP-5MS (Agilent) column of 30 m × 0.25 mm i.d. ×

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0.25 mm film thickness. The chromatograph was programmed at 170ºC for 1 min, then

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ramped at 25 ºC min-1 to 280 ºC, and held for 2 min. Samples were injected into the

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splitless mode using helium as a carrier gas at a flow of 1 mL min-1. The EI voltage was 70

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eV and the full-scan mode was used (m/z 45-650).

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J(NO2) was measured with a calibrated JAZ Spectroradiometer (Ocean Optics Inc., Largo,

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FL, USA). J(NO2) represents light intensity and is defined as the photolysis rate coefficient

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for NO2 calculated from the actinic flux measurements of the spectroradiometer and the

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recommended values for the absorption cross-section and quantum yield 25,26.

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A proton transfer reaction mass spectrometer (PTR-MS) instrument for monitoring organic

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compounds in the gas phase was used (Ionicon Analytik GmbH, Innsbruck, Austria).

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Reagent ions, H3O+, were produced from a pure water vapor flow in a hollow cathode

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discharge ion source. Data were continuously recorded in the PTR-MS instrument’s scan

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mode (m/z 21-140 with 500 ms data collection in each step). A 1/4” sulfinert© tube (4.0

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mm ID, 1m length), covered by a warming blanket set at 80ºC, was used as a sampling

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line.

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The aerosol mass concentration was measured with a scanning mobility particle sizer

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(SMPS), model 3080 (TSI, Shoreview, MN, USA). This system measured the size

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distributions within the 11-789 nm diameter range in real time at a 5-minute scan rate, and

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provided aerosol concentrations by assuming spherical shapes and multi-charge correction

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for the condensed organic material. The sheath and aerosol sampling flows were 3 L min-1

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and 0.3 L min-1, respectively.

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2.2 Offline analysis: SPME and filters.

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Manual SPME was also used to monitor reactants and products. The SPME device

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consisted of a holder assembly with 65-µm fibers. The SPME fiber was introduced into the

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chamber through a septum in the chamber floor. The methodology for prosulfocarb was

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validated by a comparison with FTIR in the dark. Samples were taken for 10 min at a time

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and were analyzed by GC-FID by inserting the fiber directly into the GC injector. A

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Hewlett-Packard 6890 Gas Chromatograph, equipped with an HP-5MS column of 30 m ×

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0.25 mm i.d. × 0.25 mm film thickness, was used. The chromatograph was programmed at

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150ºC for 2.5 min, then ramped at 15ºC min-1 to 235ºC and held for 2.5 min.

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For the fingerprint analysis, particles were collected at maximum aerosol formation at a

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flow rate of 23 L min-1 for 1 h on quartz fiber filters, which had been pre-baked at 500oC

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for 12 h. The analysis of the multi-oxygenated compounds was carried out by GC-MS after

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derivatization following the methodology described in Borrás and Tortajada-Genaro 27. O-

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(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA) (Sigma-Aldrich,

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Barcelona, Spain) was used as the derivatization agent to determine carbonyl compounds.

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N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA) (Sigma-Aldrich, Barcelona, Spain)

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was used as the derivatization reagent to analyze the compounds with OH groups (See

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Supporting Information S.1 for more details)

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2.3 Experiments.

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Prosulfocarb (99%, Sigma Aldrich quimica, Spain, address) was injected into the

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EUPHORE chamber via a heated air stream (flow rate, 10 L min-1).

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The oxidation experiments, which were carried out in duplicate, consisted of photolysis,

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ozonolysis under dry conditions (