Assessing the Atmospheric Fate and Transport of Pesticides Used to Control Mosquito Populations Post-Hurricane Harvey

Harris County Public Health authorities in Houston, TX release pesticides directly in to the atmosphere to control adult mosquito populations and reduce the threat of diseases (Zika). Pesticides registered with the EPA for this purpose are called adulticides. Permethrin and Malathion are the primary adulticides used in Houston and are typically sprayed as aerosols at night using ULV (ultra low volume) sprayers mounted to a truck. In 2017, Hurricane Harvey totaled >50 inches of rainfall in Houston, a new record from a U.S. tropical cyclone. The rainfall resulted in widespread flooding that created vast areas of mosquito habitat for mosquito population growth. Federal efforts to control mosquito populations resulted in aerially spraying Naled over 600,000 acres of the county via cargo planes, and was in addition to the ground-based county applications of Malathion and Permethrin. Research conducted by Baylor University in 2013 determined the nighttime atmospheric half-lives of Malathion in Harris County were 40-90% lower (<24 h) than Malathion half-lives measured in agricultural settings (1.6 to >9 days), which is driven by the high NO_x pollution in Houston that supports NO₃ and O₃ chemistry known to oxidize Malathion to Malaoxon. Analysis of 2016 PM_2.5 filter samples shows higher atmospheric concentrations of Malaoxon than Malathion, suggesting Malathion is undergoing transport from the ULV aerosols to the fine PM_2.5. Unlike Malathion, Permethrin is sprayed in solution with a mineral oil and PBO, a synergistic compound used to increase the toxicity of Permethrin. Atmospheric concentrations of Permethrin in PM_2.5 samples are an order and half lower, suggesting Permethrin undergoes less transport as a mixture. Little is known regarding the fate and transport of aerial-based applications of Naled in an urban atmosphere. Atmospheric PM filter samples (n=130) were collected post-Harvey at 3 different sites (urban and suburban), including both application and downwind sites; this increases our spatial understanding of adulticide oxidation and their ability to undergo long-range transport in an urban atmosphere. Higher resolution sampling periods improves our temporal understanding of adulticide oxidation. Lastly, the plume concentrations of ground vs. aerial-based adulticide applications are compared.