Measurements of current-use pesticides and oxidation products using chemical ionization mass spectrometry

2018 Spring.Includes bibliographical references.Pesticides are both naturally occurring compounds found within a variety of plant species and also synthetic chemicals that are used to protect vulnerable organisms against disease carriers, harmful pests, and intrusive or undesirable vegetation. Pesticide use has large agricultural, economic, and health benefits which include increased staple food production, protection of susceptible ecosystems and wetlands, increased productivity of the labor force via disease control, and the creation of a booming chemical industry. In the decades following the discovery of DDT's anti-insecticidal properties, organochlorine pesticides (OCPs) were generously applied across the globe. OCPs appeared to have low toxicity to mammals, chiefly humans, but had adverse effects to non-target species like fish and predatory birds. OCPs persisted in soil, air, and water, and were transported atmospherically, as far as the Arctic. The prohibition of OCPs by most nations spurred research into less harmful and persistent pesticides. These current-use pesticides (CUPs) have mostly replaced OCPs and are applied world-wide. However, recent studies revealed the transport of CUPs to remote areas, including isolated Pacific islands, high alpine mountain lakes, and, again, the Arctic. Once in the atmosphere, these pesticides undergo physical and chemical processes that affect atmospheric lifetimes and transport, and potentially change the toxicity of the parent pesticides, which can have unforeseen impacts on sensitive ecosystems and organisms. With pesticide use perpetually linked to negative health questions and concerns, atmospheric monitoring, understanding of chemical processes, and improving analytical methods is necessary. Presented in this dissertation is work towards understanding pesticides and their chemistry in the atmosphere using real time mass spectrometry. A new calibration and measurement method for four CUPs, atrazine, metolachlor, permethrin, and trifluralin is shown in Chapter 2. Iodide chemical ionization mass spectrometry (CIMS) offers a real-time, sensitive measurement technique for herbicides, as well as other low volatility species. Presented in Chapter 3, ambient pesticide spray volatilization and post-application volatilization of two chlorophenoxy acid herbicides, 2,4-D and MCPA, were measured using acetate CIMS. Concentrations of 2,4-D were highest during the application period, while MCPA concentrations increased with increasing ambient temperature. Henry's Law constants and vapor pressure were found to be predictors for spray volatilization and post-application volatilization, respectively. OH radical chemistry of three aromatic herbicides are presented in Chapter 4, along with proposed oxidation mechanisms and products. Experiments were performed in an Oxidative Flow Reactor (OFR) coupled to a switching reagent ion CIMS, for a non-targeted approach for pesticide oxidation product detection. Pesticide oxidation followed typical OH oxidation mechanisms (OH abstraction with subsequent peroxide formation, OH addition to aromatic systems). MCPA and Mecoprop-p reaction rate constants with OH radical were estimated and used to calculate their atmospheric lifetimes (3 and 5 days, respectively). Newly identified products from MCPA and triclopyr oxidation are potentially harmful to the environment and to humans. Lastly, Chapter 5 covers oxidation of two nitrogen containing herbicides, trifluralin and acetochlor and mechanisms with proposed products are shown. Trifluralin photolyzed to produce NOx, and both herbicides produced isocyanic acid (HNCO) upon OH oxidation, an atmospheric toxin

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

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

Atmospheric OH Oxidation of Three Chlorinated Aromatic Herbicides

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., C₁-₅ 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 (<i>k</i>_OH) for mecoprop-p of 1.5 (±0.7) × 10^–12 cm³ molecules^–1 s^–1 and for MCPA of 2.6 (±0.3) × 10^–12 cm³ molecules^–1 s^–1. The atmospheric lifetimes with respect to OH are thus long enough that photochemistry may be relevant to the environmental fate of these pesticides