Atmospheric OH Oxidation of Three Chlorinated Aromatic Herbicides

Abstract

Chlorinated phenoxy acids are a widely used class of herbicides and have been found in remote regions far from sources. However, the atmospheric chemistry of these compounds is poorly understood. We use an oxidative flow reactor coupled to chemical ionization mass spectrometry to investigate OH oxidation of two chlorinated phenoxyacid herbicides (2-methyl-4-chlorophenoxyacetic acid (MCPA) and mecoprop-p) and one chlorinated pyridine herbicide (triclopyr). OH radicals add to the aromatic rings of the three herbicides, produce peroxides via hydrogen abstraction, or fragment at the ether bond. OH oxidation of MCPA produced two potentially toxic compounds: chlorosalicylaldehyde and chlorosalicylic acid. We use standards to validate the detection of these oxidation products by acetate CIMS and quantify the reaction rate. Oxidation of triclopyr produced a known endocrine disruptor, 3,5,6-trichloro-2-pyridinol. Thus, while some OH oxidation products are less toxic than the parent molecules (e.g., C- carboxylic acids), others may be as or more toxic than the parent herbicide. We determine effective rate coefficients for OH addition to the aromatic ring ( k) for mecoprop-p of 1.5 (±0.7) × 10 cm molecules s and for MCPA of 2.6 (±0.3) × 10 cm molecules s. The atmospheric lifetimes with respect to OH are thus long enough that photochemistry may be relevant to the environmental fate of these pesticides.

Atmospheric OH Oxidation Chemistry of Particulate Liquid Crystal Monomers: An Emerging Persistent Organic Pollutant in Air

“…In the atmosphere, the persistence of many airborne pollutants (including LCMs) is often determined by OH-initiated oxidation reactions. − However, there are currently no published experimental reports of the atmospheric OH oxidation chemistry of LCMs. Thus, the reaction kinetics, oxidation products, degradation mechanisms, and the associated atmospheric lifetimes of airborne LCMs are unknown.…”

Real-Time Measurement of Herbicides in the Atmosphere: A Case Study of MCPA and 2,4-D during Field Application

“…Thus, while the sensitivity of acetate-CIMS is likely insufficient for eddy covariance flux analysis, longer averaging times of 1 or 5 min should yield detection limits adequate for regional agricultural studies during periods of application. The rapid and moderately sensitive measurements are adequate for field applications, such as investigating airborne pesticide drift during and immediately after application by measurements immediately adjacent to application areas, as well as laboratory measurements to determine chemical kinetics and oxidation chemistry [36,37].…”

“…Anthropogenic organic compounds, including the phenoxy herbicides described herein, can be oxidized by OH radicals and other oxidants in the atmosphere [36,37]. The calculated OH reactivity from observed concentrations and known rate constants for the two herbicides is small, <0.01 s −1 with MCPA dominating the reactivity.…”

“…Thus, while the atmospheric sources had a minor effect on ozone production in the local area, volatilization from larger-scale applications could impact local air quality. Oxidation products from phenoxy herbicides include an array of peroxide, carbonyl and carboxylic acid moieties, and may contribute to secondary organic aerosol [36].…”

Herbicide Drift from Genetically Engineered Herbicide-Tolerant Crops

“…In the atmosphere, chemical transformation by photolysis , or oxidants , in the gas and particle phases limits the persistence of agrochemicals like dicamba and 2,4-D. Day-time oxidants in the atmosphere include the hydroxyl radical ( • OH) and, in some environments, ozone (O 3 ), while the nitrate radical (NO 3 ) may be relevant at night. , Among these reactions, rate constants for gas-phase reactions of • OH with 2,4-D analogs mecoprop-p and 2-methyl-4-chlorophenoxyacetic acid have been experimentally measured to be 1.5 × 10 –12 cm 3 molecule –1 s –1 and 2.6 × 10 –12 cm 3 molecule –1 s –1 , respectively, in good agreement with a modeled rate constant for the gas-phase reaction of • OH with dicamba (3 × 10 –12 cm 3 molecule –1 s –1 ). − Given typical atmospheric • OH concentrations, these chemicals have estimated half-lives of several days by this pathway, which is sufficiently long to allow mesoscale transport . In the particle phase, dicamba and 2,4-D may also undergo heterogeneous oxidation, which should be considered when determining their persistence in the atmosphere. , …”