Suspect screening for pesticides in rain and snow using liquid chromatography high-resolution mass spectrometry

Pesticides are one of the only chemical contaminants that are purposefully introduced to the environment in large quantities. After application to crops, pesticides can be volatilized into the atmosphere and undergo medium to long-range transport. Through wet deposition, pesticides can be removed from the atmosphere far from their initial application site, posing a variety of environmental and health hazards, such as toxicity to terrestrial and aquatic species, crop damage, and poor air quality. Identifying pesticides and their degradation products in wet deposition is crucial to understanding how they are transported in the atmosphere. High-resolution mass spectrometry is a useful tool to carry out suspect screening on complex mixtures. However, suspect screening for pesticides in atmospheric waters is lacking. In this work, liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (LC-QToF-MS) is used for the first time to screen for suspect pesticides in wet deposition. The composition of precipitation samples collected in the central United States from 2018 to 2021 was screened against a library of thousands of compounds, and the presence of 23 pesticides was confirmed using reference standards, including common herbicides such as atrazine, metolachlor, and alachlor, as well as three species that had never before been quantified in wet deposition: diphenamid, octhilinone, and desthio-prothioconazole. A significant increase in the deposition flux of many pesticides was observed at the start of planting season, suggesting local agricultural sources to the atmosphere. In addition, pesticides not commonly used locally were also detected, suggesting regional atmospheric transport. Using statistical analyses and air mass back trajectory calculations, the seasonal trends, physicochemical properties, and potential origins of pesticides were examined. Overall, this work highlights the power of liquid chromatography high-resolution mass spectrometry for identifying both known and unexpected pesticides in precipitation, allowing for more accurate modeling of the atmospheric transport and fate of these compounds.