Assessing the atmospheric fate of pesticides used to control mosquito populations in Houston, TX

Highlights

• Pesticides were measured in multiple particulate matter size fractions.

• Atmospheric fate and transport was impacted by the use of pesticide carriers.

• The atmospheric half-life of malathion in Houston was estimated at <12 h.

Abstract

During the summer months, urban areas are literal hot spots of mosquito-borne disease transmission and air pollution. Public health authorities release aerosolized pesticides directly into the atmosphere to help control adult mosquito populations and thereby reduce the threat of diseases, such as Zika Virus. The primary adulticides (i.e. pesticides used to control adult mosquito populations) in Houston, TX are permethrin and malathion. These adulticides are typically sprayed at night using ultra-low volume sprayers. Particulate matter (PM) samples including total suspended and fine PM (PM < 2.5 μm in aerodynamic diameter) were collected at four ground-based sites across Houston in 2013 and include daytime, nighttime, and 24 h samples. Malathion is initially sprayed as coarse aerosol (5–25 μm), but is measured in fine aerosol (<2.5 μm) and coarse aerosol in the urban atmosphere. Particle size is relevant both for deposition velocities and for human exposure. Atmospheric permethrin concentrations measured in nighttime samples peak at 60 ng m⁻³, while malathion nighttime concentrations peak near 40 ng m⁻³. Malaoxon, an oxidation product of malathion, was also frequently detected at concentrations >10 ng m⁻³, indicating significant nighttime oxidation. Based on the loss of malathion and the increase in malaoxon, the atmospheric half-life of malathion in Houston was estimated at <12 h, which was significantly shorter than previous half-life estimates (∼days). Importantly, malaoxon is estimated to be 22–33 times more toxic to humans than malathion. Both the aerosol size and the half-life are critical for mosquito control, human exposure, and risk assessment of these routine pesticides.

Introduction

Pesticides are heavily used chemical compounds with applications in agriculture and urban settings. Due to their semi-volatile nature, many pesticides undergo atmospheric dispersion and deposition from both the gas and particulate phase and are subject to varying degrees of atmospheric degradation (e.g. oxidation and ultraviolet degradation). In ambient air, pesticides with vapor pressures above 10⁻⁵ Pa (7.5 × 10⁻⁷ mmHg) are predominantly present in the gas phase, while pesticides with vapor pressures below 10⁻⁵ Pa are predominantly present in the particle phase, and a specific pesticide may be associated with either fine (particulate matter with an aerodynamic diameter <2.5 μm) or coarse (particulate matter with an aerodynamic diameter >2.5 μm) particulate matter (PM) depending on the route into the atmosphere (Clark et al., 2016; Bidleman, 1988). Removal processes (such as atmospheric oxidation and deposition velocity) are impacted by particle size (Clark et al., 2016; Jacobson, 2002). Recently, there has been a significant number of chamber-based studies focusing on understanding pesticide oxidation through reactions with atmospheric oxidants including ozone (O₃), hydroxyl radical (OH), and nitrate radical (NO₃⁻) (Borrás et al., 2017; Socorro et al., 2014, 2016a; Mattei et al., 2018, 2019a, 2019b, 2019c; Meng et al., 2010; Beduk et al., 2012; Murschell and Farmer, 2018; Muñoz et al., 2011a, 2011b, 2014; Vera et al., 2015). However, very few real-world measurements have been conducted. This is particularly true for urban areas which use pesticides to control mosquito populations (commonly referred to as adulticides) and can experience relatively high concentrations of both daytime and nighttime oxidants (Clark et al., 2016; Brown et al., 1993; Wang et al., 2018).

Houston, TX is centered in Harris County and is the fourth-largest city in the US with approximately six million inhabitants in the metropolitan area. Mosquito population control at the community level is performed by the Harris County Public Health Mosquito Control Services (HCPHES) (Harris County Public Health). As part of their Integrated Mosquito Management program, HCPHES utilizes adulticides to control adult mosquito populations, typically spraying permethrin and malathion (the two most commonly used adulticides) on a rotating basis (HCPHES, personal communication) via ground-based vehicles equipped with ultra-low volume (ULV) sprayers that release the adulticide directly to the atmosphere as aerosols (5–25 μm) which target flying mosquitoes (McConnell et al., 1998; Bonds, 2012). Permethrin is typically applied as a mixture (e.g. 31–66; 31% permethrin, 66% piperonyl butoxide (PBO) as a synergist component, and 3% other ingredients). Malathion is typically applied as a technical mixture at 95–97% purity (Brown et al., 1993). Prior to an application, HCPHES provides proposed application maps as well as specific application data including when, where, and what is being sprayed (Harris County Public Health). During the mosquito season in Houston adulticides are typically applied at sunset and throughout the night between 5 and 8 months of the year with up to 20 to 25 application days per summer month (i.e. July, August, and September) (Harris County Public Health). Permethrin and malathion were applied using a ULV sprayer by HCPHES during the summer of 2013 (Harris County Public Health).

Concurrently, during the summer months, Houston has significant sources of nitrogen oxides (NO_x; NO + NO₂) and other air pollutants including biogenic and anthropogenic volatile organic compounds (VOCs) (Seinfeld and Pandis, 2016; Lu et al., 2015; Brown et al., 2013; Subedi et al., 2015). Many of these air pollutants are the precursors of atmospheric oxidants including O₃, OH, and NO₃⁻. NO₃⁻ is considered the dominant nighttime oxidant (rapidly photolyzes during the day) in urban areas and its formation is typically initiated by reaction of NO₂ with O₃ (Nah et al., 2016). It has been previously reported that malathion can oxidize to malaoxon in the presence of atmospheric oxidants including OH, O₃ and NO₃⁻ (Meng et al., 2010; Socorro et al., 2016b; Imgrund, 2003; Gervais et al., 2009; Kim et al., 2018). Based on ambient measurements collected post-aerial application the atmospheric half-life of malathion has been estimated to be 1.6–9 days (Brown et al., 1993). A study found that permethrin could undergo ozone-induced oxidation to phosgene, however, the photolysis rates of permethrin have been found to be slow (≤10⁻⁶ s⁻¹) (Socorro et al., 2014, 2016a, 2016b). Permethrin is believed to drift when applied aerially, such as ULV applications, but the low vapor pressure (2.87 × 10⁻⁶ Pa) and low Henry’s Law constant (1.42 × 10⁻¹ Pa m³ mol⁻¹) make it unlikely to volatilize once bound to soils (Imgrund, 2003). For reference, malathion has a vapor pressure and Henry’s Law Constant of 1.07 × 10⁻³ Pa, and 4.95 × 10⁻⁴ Pa (Jacobson, 2002) mol⁻¹, respectively (Gervais et al., 2009; Kim et al., 2018; Vogue, Kerle, and Jenkins). There are still uncertainties regarding the oxidation of malathion and permethrin following ground-based ULV applications in urban atmospheres.

When applying pesticides to urban areas, health concerns arise from long-term and widespread exposure to the environment and public (McCormick and Whitney, 2013; Hansen et al., 2008). Reported environmental and human health effects associated with current-use pesticide exposure include reduced biodiversity, developmental, neurological, immunological, skeletal, muscular, cardiovascular, respiratory and reproductive damages, and carcinogenic and genotoxic effects (Mattei et al., 2019a; Mattei et al., 2019b; Mattei et al., 2019c; Wang et al., 2018; Gervais et al., 2009; Hansen et al., 2008; Lipton, 2019; Estellano et al., 2015; Carratalá et al., 2017; Melgarejo et al., 2015; Chrustek et al., 2018; Piperonyl Butoxide (O), 2006; Coleman et al., 2010). Additionally, the oxidation product, malaoxon, is estimated to actually be 22–33 times more toxic to humans than malathion (EPA, 2000).

To estimate the human health risk of adulticide exposure associated with mosquito abatement, it is critical to improve our understanding of the environmental fate and oxidative potential of adulticides in an urban atmosphere. Houston, Texas, and the surrounding suburban areas serve as a natural laboratory for assessing adulticide oxidation as its mosquito season overlaps with its summertime oxidant chemistry. The goals of this study were to 1) collect PM samples of multiple size fractions (TSP, or total suspended particulate, and PM_2.5, or PM that is < 2.5 μm in aerodynamic diameter) in real-world adulticide applications, 2) measure the atmospheric concentrations of permethrin and malathion (and its oxidation product malaoxon) in PM samples, and 3) examine the degradation of malathion to malaoxon after spraying in an urban atmosphere.