Heterogeneous degradation of pesticides by OH radicals in the atmosphere: Influence of humidity and particle type on the kinetics

Abstract

Pesticides can be adsorbed on the surface of atmospheric aerosol, depending on their physicochemical properties. They can be degraded by atmospheric oxidants such as OH radicals but the influence of some environmental parameters on the degradation kinetics, especially relative humidity and particle surface type, is not well understood. Heterogeneous degradation by OH radicals of eight commonly used pesticides (i.e., difenoconazole, tetraconazole, cyprodinil, fipronil, oxadiazon, pendimethalin, deltamethrin, and permethrin) adsorbed on hydrophobic and hydrophilic silicas at a relative humidity ranging from 0% to 70% was studied. Under experimental conditions, only cyprodinil, deltamethrin, permethrin, and pendimethalin were degraded by OH radical in atmospheric relevant concentration. Second-order kinetic constants calculated for the pesticides degraded by OH radicals ranged from (1.93 ± 0.61) x 10-13 cm 3 molecule-1 s-1 (permethrin, hydrophobic silica, 30% RH) to (4.08 ± 0.27) x 10-12 cm 3 molecule-1 s-1 (pendimethalin, hydrophilic silica, 0% RH). Results obtained can contribute to improve the understanding of the atmospheric fate of pesticides and other semi-volatile organic compounds in the particulate phase and they highlight the importance of taking humidity and particle type into account for the determination of pesticides atmospheric half-lives.

Photolytic degradation of molecular iodine adsorbed on model SiO2 particles

“…Since atmospheric particles vary both in their physical nature and chemical complexity, to better simulate heterogeneous reactivity on atmospheric aerosols in a laboratory setting the choice of particle type is crucial. In this work, AEROSIL R812 silica particles were used to study the photolytic degradation of molecular iodine (Mattei et al, 2019(Mattei et al, , 2018Socorro et al, 2015). Of course, these commercial silica particles are model particles and are not necessarily representative of all the particles found in the real atmosphere.…”

“…The SiO2 particles were loaded using a liquid/solid adsorption method that has been reported previously (Mattei et al, 2019(Mattei et al, , 2018Socorro, 2015;Socorro et al, 2015). To minimize any photo-degradation of molecular iodine, particle loading was performed in the dark.…”

Influence of pesticide concentration on their heterogeneous atmospheric degradation by ozone

“…These four pesticides were selected based on their presence in the atmosphere up to a dozen of ng m -3 (Désert et al, 2018), on their toxicity M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 6 (APVMA, 2017), and on their distribution between the gas and particle phases (AOPWIN TM , Meylan and Howard 1993). Moreover, they have been the subject of numerous studies of reactivity towards ozone (Socorro et al 2015;Mattei et al 2018) and OH radicals (Socorro et al 2016;Mattei et al 2019). The two particle types were chosen as models for mineral atmospheric particles.…”

Photodegradation of Molecular Iodine on SiO2 Particles: Influence of Temperature and Relative Humidity

“…under given experimental conditions of temperature and relative humidity. As shown in an earlier study, heterogeneous reactivity of pendimethalin adsorbed on AEROSIL R812 particles was observed to decrease with increasing relative humidity[38]. To account for this "hydrophobic" effect, experiments were carried out at T = 298K and relative humidity (RH) values ranging from ≤ 2% to 75%RH.…”