Abstract. Volatilization and subsequent processing in the atmosphere are an important environmental pathway for the transport and chemical fate of pesticides. However, these processes remain a particularly poorly understood component of pesticide lifecycles due to analytical challenges in measuring pesticides in the atmosphere. Most pesticide measurements require long (hours to days) sampling times coupled with offline analysis, inhibiting observation of meteorologically driven events or investigation of rapid oxidation chemistry. Here, we present chemical ionization time-of-flight mass spectrometry with iodide reagent ions as a fast and sensitive measurement of four current-use pesticides. These semivolatile pesticides were calibrated with injections of solutions onto a filter and subsequently volatilized to generate gas-phase analytes. Trifluralin and atrazine are detected as iodide-molecule adducts, while permethrin and metolachlor are detected as adducts between iodide and fragments of the parent analyte molecule. Limits of detection (1 s) are 0.37, 0.67, 0.56, and 1.1 µg m −3 for gas-phase trifluralin, metolachlor, atrazine, and permethrin, respectively. The sensitivities of trifluralin and metolachlor depend on relative humidity, changing as much as 70 and 59, respectively, as relative humidity of the sample air varies from 0 to 80 %. This measurement approach is thus appropriate for laboratory experiments and potentially near-source field measurements.
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.
Herbicides react with atmospheric OH, producing multiple oxidation products, including HNCO; these products contribute little to secondary organic aerosol.
Gas-phase organic acids are ubiquitous throughout the atmosphere and are tracers for volatile organic compound oxidation, although their abundance and sources are poorly characterized. Several studies have pointed to the oxidation of isoprene and monoterpenes as major sources of formic and acetic acids. Here, we quantify >100 gas-phase organic acids produced from isoprene and α-pinene oxidation in a series of laboratory chamber experiments. Overall, gas-phase organic acids constitute 1−28% of the initial organic carbon reacted in the experiments. Isoprene is a precursor for detected organic acids in amounts at least 2−3× greater than α-pinene from OH oxidation. We compare laboratory observations with two summertime field sitesone dominated by isoprene emissions (Southern Oxidant and Aerosol StudySOAS) and one by monoterpene emissions (Seasonal Particles in Forests Flux studYSPiFFY). Daytime organic acid mixing ratios were 4× higher at SOAS than SPiFFY, likely because of the substantial isoprene OH oxidation. In contrast to SOAS, factor analysis reveals separate daytime and nighttime organic acid sources at SPiFFY not obviously originating from photochemical oxidation. When available, isoprene is likely a dominant precursor for gas-phase organic acids.
Atmospheric sources of herbicides enable short- and long-range transport of these compounds to off-target areas but the concentrations and mechanisms are poorly understood due, in part, to the challenge of detecting these compounds in the atmosphere. We present chemical ionization time-of-flight mass spectrometry as a sensitive, real-time technique to detect chlorinated phenoxy acid herbicides in the atmosphere, using measurements during and after application over a field at Colorado State University as a case study. Gas-phase 2,4-dichlorophenoxyacetic acid (2,4-D) mixing ratios were greatest during application (up to 20 pptv), consistent with rapid volatilization from spray droplets. In contrast, atmospheric concentrations of 2-methyl-4-chlorophenoxyacetic acid (MCPA) increased for several hours after the initial application, indicative of a slower source than 2,4-D. The maximum observed gas-phase MCPA was 60 pptv, consistent with a post-application volatilization source to the atmosphere. Exposure to applied pesticides in the gas-phase can thus occur both during and at least several hours after application. Spray droplet volatilization and direct volatilization from surfaces may both contribute pesticides to the atmosphere, enabling pesticide transport to off-target and remote regions.
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