Assessing the Influence of Polymer-Based Anti-Drift Adjuvants on the Photolysis, Volatilization, and Secondary Drift of Pesticides after Application

Abstract

One practice to reduce spray drift during pesticide application is the addition of certain chemical adjuvants to spraying solutions, which change their physicochemical properties and result in larger droplets. The environmental impact of these agrochemicals continues however also after application, depending on surface processes occurring upon treated surfaces. While the impact of anti-drift adjuvants has been studied regarding spray drift, their impact on the fate of deposited pesticides has received little attention. Here, the effect of a polymer-based adjuvant (polyacrylamide) on the photolysis and evaporation rates of pyrimethanil (common fungicide) from dry films were investigated under controlled laboratory conditions and during two field studies. The laboratory results indicate that the adjuvant enhances the volatilization and photolysis rate both on hydrophobic lemon leaves and hydrophilic glass substrates. These results can be attributed to an increase in the geometrical area of residual film and a widening of its circumference rim, where solutes are likely to concentrate, when generated from adjuvant-containing droplets. Such morphological differences may enhance the exposure of deposited pesticides to interact with the overlaying atmosphere and incident radiation. The field data was less conclusive, suggesting a small impact of the anti-drift adjuvant on the fungicide’s secondary drift from crops and an even lower effect on volatilization from bare soil.

1. Introduction

Pesticides are an essential part of modern agriculture, helping to keep a high crop yield and to address the increasing food demand. Yet, a significant portion of the applied material may escape the target areas via leaching, runoff, and atmospheric transport. The latter occurs during application due to spray droplets drift (i.e., primary drift) as well as after application, due to pesticide evaporation from treated surfaces (i.e., secondary drift) [1]. Drift intensity and transport distances depend on the physicochemical properties of the pesticide and spraying solution [2], environmental conditions [3,4], and the application methodology [5,6,7].

A common practice for reducing pesticide drift is the addition of drift control adjuvants (DCAs), often high molecular weight polymers, to the spray solution [8]. Such adjuvants increase the viscosity of the solution [6], which affects its breakup after exiting the nozzle [9] and shifts the size distribution of generated droplets to bigger diameters that are considered less drift prone [10].

In semi-arid areas, where pesticide applications occur mainly at the beginning of the hot and dry summer season, applied pesticides remain upon treated surfaces for months before being washed away by the first winter rains [11,12]. In such regions the environmental impact of pesticides (including secondary drift) depends on their post-application evaporation as well as surface degradation (i.e., photo-oxidation) rates.

Pesticide evaporation from plant foliage and soil surfaces is a complex process that can be affected by multiple parameters (e.g., [13,14,15]). Previous studies have demonstrated that pesticides’ evaporation rate from treated surfaces depends on both environmental conditions (wind speed and temperature) as well as on the physicochemical properties of the applied solution and of the treated surface [14,16]. Pesticides with a higher volatility and a lower octanol-water partitioning coefficient (K_ow) are more prone to evaporation [14]. However, the viscoelasticity of the sprayed solution may also affect the evaporation rate and the degradation rate of the active pesticide, as it affects the spreading of spray droplets on the treated surface [17] as well as the solutes’ distribution within the sessile droplet. Furthermore, in multi-component droplets, differences in physicochemical properties (e.g., mass density, viscosity, and surface tension) of the liquid components in the droplet can affect its liquid–air interface and evaporation rate [18,19]. Such heterogeneity may generate a Marangoni flow due to composition gradients at the interface, which may affect the evaporation rate, or change in droplet shape and causes a residual amount of the more volatile component to be entrapped on the surface at later stages of the drying process [20,21]. Hence, the addition of a liquid component with high viscosity (such as polymeric anti-drift adjuvants) is likely to affect the complex physical processes occurring within the droplet and at its interface, impacting the fate of incorporated pesticides.

Polymer-based drift control additives are commonly used in US and Australia to decrease spray drift [22]. The general mode of action of these adjuvants is the increase in liquid viscosity that induces the formation of coarse sprays, which is less likely to be carried away by crosswinds [2]. Nevertheless, increasing the solution’s viscosity can result in more complex outcomes. We recently showed that while the addition of the polyacrylamide adjuvant to the spraying solution shifts the droplets’ mass distribution to larger sizes, it also causes increases in the number density of small droplets [23]. The addition of such anti-drift adjuvants is likely to also alter the distribution of the active material on the treated surfaces and its penetration into foliage, which will affect the susceptibility of the active compounds to volatilization and photodegradation which result in changes in secondary drift rates. The current study investigates the impact of polyacrylamide adjuvant addition on the evaporation and photodegradation rates of pyrimethanil (a broad-spectrum fungicide) in residual film, both in controlled laboratory experiments and in-field applications.

2. Materials and Methods

2.1. Chemicals

Analytical-grade (>99%) pyrimethanil (PYR) was obtained from Sigma-Aldrich (Rehovot, Israel), and technical-grade solutions of this fungicide were prepared using the commercial product Mythos^TM (Bayer CropScience, St. Louis, MO, USA). The commercial polyacrylamide (PAM)-based anti-drift adjuvant (Control^TM) was obtained from Garrco Inc. (Converse, IN, USA). All aqueous solutions for laboratory experiments were prepared with double deionized water (DIW) (Millipore, 18.2 MΩ), and all solvents used were of analytical or HPLC grade.

2.2. Laboratory Evaporation Experiments

The effect of PAM on the pesticide evaporation rate was tested for two different surface types: glass slides and lemon leaves. Experiments were carried out in a fume hood under different wind speeds, controlled by the fume hood door opening, and measured using a handheld anemometer (Skywatch^® Meteos, JDC Electronica SA, Yverdon-les-Bains Switzerland) that was fixed next to the tested surfaces and at their corresponding heights. Temperatures during the glass slides’ and leaves’ experiments were in the range of 24.3–26.9 °C and 25.6–26.9 °C, respectively, with a maximal gap of 1.1 °C in each experiment.

Each experiment included four sample series, two with PAM and two without it, that were exposed simultaneously to the same environment conditions (Figure S1). Each sample contained dry films that were generated by depositing ten 1 µL droplets of 1200 μg/mL of technical-grade (i.e., commercial formulation) PYR in a DIW solution on the selected surface and allowing the solvent to evaporate before exposure to direct wind. In samples containing PAM, it was added to the PYR solution at a concentration of 600 μL/L.

Each experiment was repeated twice under constant wind speeds, and at selected time intervals (up to 10 h), a sample from each series was removed for analysis, giving a total of four repetitions for each solution (with and without PAM) at each wind speed and time. Leaf petioles were wrapped in cotton balls that were kept wet for the entire experiment’s duration.

Following the exposure, each sample was placed in a 30 mL glass vial with 1 mL of HPLC-grade Acetonitrile (ACN) as the solvent and sonicated for 30 min at 50 °C (ultrasonic cleaner, MRC Ltd., Holon, Israel). Pesticide concentration in the extract was then analyzed using GC-MS.

2.3. Laboratory Photochemical Experiments

The effect of PAM on the pesticide photodegradation rate was evaluated using a Xenon lamp (66924-450XF-R1 Oriel^® Arc Lamp, Newport Corporation, Irvine, CA, USA) as a light source. For these experiments, four solutions were prepared: two of the technical-grade PYR in DIW as the solvent and two of the analytical-grade ones in a DIW:ACN mixture (60:40 v/v) as the solvent. PYR concentrations in all solutions was 500 mg/L, and PAM was added to one out of each grade’s solutions at a concentration of 600 μL/L. Two samples of each solution, each containing ten 1 µL films pre-deposited on glass slides as described above for the evaporation experiments, were placed under the light source for selected irradiation time intervals (Figure S2). All experiments were repeated twice, yielding 4 replicates for each solution and exposure time. During irradiation, sample slides were held at a constant temperature (20 °C) using a circular cooling bath. After irradiation, samples underwent extraction, as described in the evaporation section, followed by a chromatographic analysis to quantify residual PYR levels. The lamp irradiation spectrum and intensity at the samples’ location were measured using a spectroradiometer (USB2000+UV-VIS-ES, Ocean Optics Inc., Orlando, FL, USA), equipped with a cosine collector. The measured total fluence was 149 W/m² for 280 < l < 340 nm (Figure S3). The absorption spectra of PYR and PAM were measured using a UV-vis spectrophotometer (Genesys 10UV, Thermo Fisher Scientific, Waltham, MA, USA).

The statistical significance of the impact of PAM’s addition on the photodegradation and evaporation of PYR in these laboratory experiments was evaluated using the nonparametric right-tail Wilcoxon rank sum test [24] for independent paired populations. Specifically, at each exposure interval (evaporation or photolysis), PYR concentrations in four replicate films without PAM were compared to the corresponding group of four replicate films containing PAM (all exposed to the same conditions). The analysis was performed using the “ranksum” function in MATLAB R2019a (The MathWorks, Inc., Natick, MA, USA). The alternative hypothesis stated that the median of the group not containing PAM is greater than the median of the group containing PAM, with a significance level of 10%.

2.4. Morphological Imaging

For the morphological examination of the films, 1 µL droplets of PYR solutions (as used for photochemistry experiments) were deposited on P-doped silica wafers. Microscopy micrographs were acquired using a secondary electron detector in the high-resolution scanning electron microscope (HRSEM, Zeiss Ultra Plus) at an acceleration voltage of 2 kV. The films deposited were carbon coated prior to electron radiation exposure.

The contact angle of 5 µL sessile droplets of DIW with and without PAM’s addition was measured on both glass and lemon leaf substrates. Measurements were conducted at 25 °C using a drop shape analyzer (DSA25, KRUSS) equipped with ADVANCE software for Drop Shape Analyzers (version 1.6–03).

2.5. Analytical Chemical Analysis

PYR concentrations in samples were analyzed using a gas chromatograph (Focus GC, Thermo Scientific, Milan, Italy) coupled with a single quadrupole mass analyzer (ISQ, Thermo Scientific, Milan, Italy). DB-5 column (30 m × 0.25 mm I.D. and 0.25 µm film thickness) was employed, and ultrapure He (99.999%) was used as the carrier gas at a flowrate of 1.2 mL/min. The injected volume was 1 μL, and a split-less method was used. The injector temperature was 280 °C. The oven temperature was held at 60 °C for 1 min, then raised to 220 °C at a 10 °C/min rate, followed by an increase to 280 °C at a rate of 20 °C/min, where it was held for 2 min. Ionization was obtained by Electron Impaction with an electron energy of 70 eV. The MS detector was closed for the initial 5 min (solvent delay time), then opened to an m/z ratio of 100–400. To overcome higher background noises in the field samples, the detector in these runs was opened to the selected ions characteristic of PYR (198,199 m/z) around its expected retention time (12.5–16 min). PYR quantification was performed based on the GC-MS peak area, using external calibration curves that were generated by the direct injection of analytical-grade PYR solutions at different concentrations.

2.6. Field Campaigns

Field measurements of secondary drift (SD) following aerial pesticide applications were conducted in two field campaigns, near the Kedma landing strip, Israel (31°42′04″ N/34°46′18″ E) (Figure S4). The first campaign (July 2016) was conducted over two consecutive days. On its first day, a cotton field was sprayed with a commercial solution of PYR, and on the following day, it was sprayed again using a similar solution with the addition of the PAM-based anti-drift adjuvant. In the second campaign (September 2017), PYR applications with and without PAM’s addition were conducted almost in parallel in two adjacent fallow fields (Figure S4b). These fields were sprayed 5–10 min apart, where the PAM adjuvant was added to the spraying solution only during application over the southern field. In both campaigns, PAM and PYR concentrations were approximately 600 µL/L and 15 gr/L, respectively. All applications were conducted using Ayres SR2-T45 Turbo Thrush 4X-AWG (Albany, GA, USA) that was equipped with a straight boom containing 56 hollow-cone-type nozzles (TeeJet D12-46, Spraying Systems Co., Glendale Heights, IL, USA). The swath width was 23 m, and the spraying flow rate was 345 L·min⁻¹ under a pressure of 50 psi.

Air sampling: Secondary drift was actively sampled using air samplers placed at three different heights along telescopic pneumatic masts (Clark-Mast^®, Binstead, UK). At each height, air was drawn at a flow rate of 30 L/min through a precleaned polyurethane foam (PUF) plug (2.7 cm long, with a diameter of 5.6 cm) placed within a glass holder covered with aluminum foil, using a centrifugal air pump (GAST R-4110-2) located near each sampling mast. Secondary drift sampling began about 10 min after the aircraft seized spraying and lasted 120 min in the 2016 campaign and 80 min in the 2017 campaign. Sampled PUF plugs were wrapped in aluminum foil, placed in sealed plastic bags, and kept refrigerated (at −3 °C) until analysis.

All PUF plugs underwent a cleaning process beforehand, using the Accelerated Solvent Extraction system (ASE 150, Thermo Scientific Dionex). Each PUF was held in a 90:10 (v/v) mixture of a n-Hexane:Diethylbenzene solvent inside a 66 mL cell for two static cycles, under 100 °C and 100 Bar. Each cycle lasted 20 min, and the rinse volume between two cycles was 50%. The clean PUFs were kept (till field deployment), wrapped in aluminum foil and in sealed plastic bugs. Extraction of the PUF plugs after field sampling was conducted following the same procedure used for cleaning, with the addition of a final volume reduction of the extract to 1ml. The latter was performed using TurboVap-II (Zymark corporation) with nitrogen as the inert gas and with a water bath temperature of 30 °C. The extracts were then centrifuged (Biofuge primo, Heraeus Instruments) for 10 min at 3000 rpm to remove any particulates and were analyzed by GC-MS.

To estimate pesticides’ recoveries from the PUF plugs, GC-MS signals of analytical solutions of PYR (in a 90:10 v/v n-Hexane:Diethyl ether mixture) were compared between direct injection and after application on PUF plugs and extraction following the same procedure for the field samples. PYR showed decent recoveries of 93–107% at an extract concentration range of 2.5–20 μg/mL.

3. Results and Discussion

3.1. Laboratory Experiments

Evaporation experiments: The influence of PAM’s addition on the evaporation rate of active ingredients from residual films of the technical-grade PYR solution was tested under different wind velocities.

The evaporation of PYR from films deposited on glass, in the presence and absence of PAM, is illustrated as the remaining mass (C/C₀) versus the exposure time in Figure 1a,b, respectively. The analog results for the PYR films deposited on the lemon leaves are presented in Figure 1c,d. In all cases, the data fit first order kinetics, and the obtained evaporation rate constants for PYR on glass and leaf surfaces are depicted in Figure 2a,b, respectively.

As demonstrated in Figure 2, the evaporation rate increases with an increasing wind velocity, with a stronger response for films deposited on smooth glass surfaces. Under all wind speeds, the evaporation rates were substantially higher for films deposited on glass surfaces than on leaves. These results are not surprising considering that unlike the uniform and smooth glass surfaces, the leaf’s upper surface is waxy and contains features (such as grooves, veins, and stomata), that can enhance surface sorption, act as dead volume, or reduce the air flow rate near the air–surface interface. The obtained results (Figure 2) suggest that the presence of PAM enhances the evaporation of PYR from the dry films deposited on both selected surfaces. This enhancement in the evaporation rate, which became more significant at higher wind speeds and longer exposure times, may result from changes in the morphology of the residual film and/or the distribution of PYR in it due to the presence of PAM in the solution. While much of pesticide volatilization from treated surfaces occurs after spray droplets are evaporated, available data regarding the impact of polymeric additives on pesticide spray evaporation focus on the behavior of the water droplets (e.g., [8,25,26,27]) and not on the volatilization of the active pesticide. Nevertheless, these studies clearly show that the presence of polymeric adjuvants (beyond critical micelle concentration) enhance droplets’ spreading and alter their evaporation dynamics on both hydrophilic and hydrophobic leaves. Increasing droplets’ spreading upon the surface will cause the residual film generated after its evaporation to be thinner, with the active material in it more likely to be exposed to the overlaying atmosphere [18,28]. Geometrical measurements of the residual films’ diameters indicate that on both tested substrates, the droplets of the PYR solution containing PAM spread slightly more than those without this additive (an 8% increase for analytical PYR on glass and 14% and 11% for the technical-grade one on glass and on leaves, respectively; Table S1).

Contact angle (CA) measurements of the sessile water droplets with and without PAM’s addition showed a lower contact angle for the PAM-containing droplets on the hydrophobic lemon leaves (90° ± 4° vs. 78° ± 8°), in line with enhanced wettability in the adjuvant’s presence. An opposite trend was observed on the hydrophilic glass surface, where the addition of the polymer to the solution increased the droplets’ initial CA (63° ± 2° for PAM-containing water droplets vs. 47° ± 2° for water alone). Considering that the residual films from the PAM-containing droplets depicted a larger diameter also on the glass surfaces, this suggests that the polymer’s addition also affects the evaporation process. These findings are consistent with prior studies indicating that the inclusion of polymeric additives leads to stronger adsorption onto solid surfaces, prolonging the phase during which the evaporating droplet maintains its contact diameter with the surface. (e.g., [8,25,26]). This, in turn, enhances wettability, even on hydrophilic surfaces, and even when the initial contact angle was higher in the presence of the additive [26]. Such alterations in the evaporation dynamics may also affect the distribution of the pesticide within the generated film (see further discussion in the morphology section below), further effecting its sensitivity to evaporation and its interaction with incident radiation.

Photolysis experiments: As described in the method section, the effect of PAM’s addition on the photodegradation rate of the residual film of PYR was evaluated by irradiating simultaneously analog PYR samples (i.e., films generated from solutions with and without PAM’s addition) on inert glass surfaces.

Table 1. Experimental photodegradation rate constants [minute⁻¹] for PYR on glass surfaces. Letters next to photolysis rate coefficients stand for groups of same statistical significance.

Grade	PAM Addition	Irradiated	Evaporation	Photolysis
Technical	no	$(7.8\pm 0.2)\times {10}^{-4}$	$(1.9\pm 0.1)\times {10}^{-4}$	$(5.2\pm 0.2)\times {10}^{-4}$ (a)
	yes	$(1.3\pm 0.04)\times {10}^{-3}$	$(2.9\pm 0.2)\times {10}^{-4}$	$(8.1\pm 0.4)\times {10}^{-4}$ (b)
Analytical	no	$(6.8\pm 0.4)\times {10}^{-3}$	$(1.5\pm 0.1)\times {10}^{-3}$	$(3.7\pm 0.2)\times {10}^{-3}$ (c)
	yes	$(7.3\pm 0.4)\times {10}^{-3}$	$\left(1.1\pm 0.1\right)\times {10}^{-3}$	$\left(4.7\pm 0.2\right)\times {10}^{-3}$(c)

presents the obtained first order degradation rate coefficients for PYR in films exposed to lamp irradiation (“Irradiated”), and for analog films, that were exposed to identical conditions apart from radiation (“Evaporation”). The coefficients for PYR direct photolysis (designated “Photolysis” in Table 1) were calculated from the difference between the two (i.e., k_irradiated − k_evaporation).

The faster photolysis of analytical-grade PYR might result from the interaction of the non-active ingredients in the technical-grade PYR with the incident light that interferes with the photodegradation of the active ingredients. Unfortunately, the identity of these non-active ingredients remains unknown due to commercial confidentiality. Furthermore, while the PAM solution was diluted in water, for the insoluble analytical-grade PYR, a DIW:ACN mixture (60:40 v/v) was used as the solvent. The lower surface tension of a binary mixture of H₂O:ACN at the given ratio compared to water (36.8 mN/m vs. 72.1 mN/m, respectively) [29] can lead to an improved spreading of the droplets containing the analytical-grade PYR when compared to the droplets containing the technical-grade one. This better spreading can contribute to higher exposure of analytical-grade PYR to incident radiation and its resulting photodegradation [18,28].

The addition of the PAM adjuvant increased the PYR photodegradation rate for both purity grades, with a less pronounced enhancement in its analytical-grade (+27%) form than in its technical-grade (+56%) one. A comparison of the degradation rates at each exposure time, separately, indicates that the enhancement in photolysis rate is statistically significant only for the technical-grade PYR (p < 0.05, in right-tailed Wilcoxon rank sum test). Spectral measurements of the PYR solutions (in both purity levels) in the presence and absence of PAM revealed that dissolved PAM has negligible absorption in the wavelength range of interest (280–350 nm), and its presence does not lead to a significant change in the absorption spectra of PYR (Figure S3). As discussed above in relation to evaporation rates, a plausible rationale for the accelerated photodegradation of PYR in films produced from solutions containing PAM could be attributed to their greater surface spreading and alterations in the distribution of the active material within the resulting film.

Effects of PAM on film morphology: In addition to enhancing droplets’ spreading, the PAM additive may also alter the residual film’s surface morphology on the macro- and microscopic scales, affecting the active ingredient distribution and reactivity. Figure 3 depicts a leaf on which films were deposited from 1 µL droplets of the PYR solution containing PAM (on its left side) and without PAM (on its right side). The difference between the groups is visible even to the naked eye, where films not containing PAM are more homogeneous and quite opaque, while for those containing PAM, most of the material seems to be concentrated at the circumference.

High resolution scanning electron microscopy (SEM) imaging of films formed from the technical-grade PYR solution, with and without PAM’s addition, reveals several morphological differences (Figure 4): The circumference rim observed in both films is significantly wider in the presence of PAM (Figure 4a vs. Figure 4d). A larger (×2000) magnification of the films’ rim shows it is much denser and sharper in the film without PAM (Figure 4b), whereas in the film containing PAM, matter seems to be distributed more sparsely at its edge (Figure 4e). Images taken at a random place near each film’s center (Figure 4c,f) also demonstrate differences in both the surface density and the size of the scattered elements.

The evaporation of a macroscopic sessile droplet can take place while maintaining a constant radius (i.e., pinned contact line) and reducing its contact angle, or at a constant contact angle while the contact line is receding [30]. In a multicomponent droplet, where only the water evaporates, the solutes help pinning the liquid contact line to the substrate [31]. The evaporation close to the contact line induces outward flow to replace the evaporating water and to maintain a constant radius. This flow transports the solutes to the edge where they are deposited (i.e., the coffee ring stain model). Baldwin and co-workers [32] showed that the evaporation of sessile droplets containing Polyethylene oxide (a linear polymer) proceeds initially at the pinned contact line, during which the polymer’s concentration at the edge of the droplet increases and starts depositing when it reaches saturation. The deposits cause evaporation through the surface at the contact line to cease, which in turn stops the radial flow. With the lack of radial flow, the contact line cannot remain pinned and starts receding. The nature of the final stage of evaporation and deposition depends on the initial concentrations, but for many, droplets occur in a flat rough puddle [32]. An extension of the initial evaporation stage, wherein the evaporating droplets keep a constant contact line with the surface, and the concentration of the solutes at the droplet’s circumference were reported also for other types of polymeric additives [8,26]. Based on these previous observations, [8,26,30,31,32] it can be assumed that PYR and PAM are concentrated in the deposits at the outer rim of the residual dry film. As seen in Figure 4, the outer rim of the PAM-containing film is wider and less dense, potentially making PYR in that area more exposed to ambient air and incident radiation. Understanding the exact mechanism for the spread of particle deposition at the droplet circumference in the presence of PAM is beyond the scope of this study. Yet, it is in line with previous findings that showed alterations in the liquid contact line’s behavior when nanoparticles or viscous materials were added to the base solution [19,31]. Overall, the observed differences in film morphologies indicate microscale changes in the droplets’ evaporation dynamics in the presence of PAM and support observed variations in pesticide reactivity after its deposition upon the tested surfaces. Such differences may result in changes in the secondary drift of the active ingredients, with faster evaporation leading to higher air concentrations short after pesticide application. Under ambient conditions, direct photolysis of semi-volatile pesticides (such as PYR) is often slower than their volatilization [33]. The observed differences in the photochemical decay rates are not likely to significantly affect the secondary drift in the first few hours after application but can be important regarding pesticides’ environmental fate with longer time scales. Nevertheless, if indeed the active pesticide in the film remaining at the air–leaves interface is more exposed, as suggested by the evaporation and photochemical results, it is also more likely to react with atmospheric oxidants (such as ·OH and O₃).

3.2. Field Campaigns

Pesticide volatilization from treated surfaces (plant and soil) is related to the physicochemical properties of the active ingredients and the treated surface as well as to the formulation of the spraying solution [14,34]. While multiple field studies have examined the impact of polymeric additives on spray droplets’ drift (e.g., [2,23,35]), the effect of these adjuvants on post-application pesticide emissions under field conditions has not received proper attention. Complimentary to the above laboratory work, the effect of PAM’s addition to the spraying solution on the secondary drift (SD) was measured following real field aerial pesticide applications. Figure 5 depicts the average air concentrations of PYR measured during the first 2 h following its application over a cotton field in the first campaign (two applications in consecutive days, with PAM being added to the spraying solution only on the second day). The results suggest higher SD concentrations on the second application when the PAM adjuvant was present in the spraying solution than in its absence. A comparison of the meteorological conditions that prevailed during the two applications in this campaign (Table S2) indicates a comparable irradiance, temperature, and wind direction with a slightly higher wind speed during the first application, when PAM was not added. Hence, the evaporative effect of the wind should have resulted in higher SD values in the absence of PAM, in contrary to the trend observed. As the air concentrations, shown in Figure 5, represent a single measurement at each height and mast, their error bar cannot be estimated reliably. Yet, the fact that in eight out of these nine measurements, the PYR concentration was higher when PAM was added to the spraying solution suggests that the effect is statistically significant (a < 0.05, in Wilcoxon rank sign test [24]). These field results agree with the laboratory observations reported by Zhou et al. (2018), showing higher wettability and a faster evaporation of the spray droplets containing a polymeric adjuvant upon cotton leaves [26]. It is interesting to note that while a faster evaporation of water droplets can enhance pesticide retention on the leaves and reduce loss via droplets rolling off the plant, it may enhance the susceptibility of pesticides remaining at the leaf–air interface to volatilization, photolysis, and oxidation by atmospheric oxidants.

As mentioned above, to minimize possible interferences from temporal variation in meteorological conditions, in the second campaign, the pesticide solution was applied almost parallelly in two adjacent fallow fields, with PAM’s addition to the spraying solution occurring only in one field. This experiment was repeated twice during the campaign and the SD was measured for 80 min following each application. As opposed to the observations from the first campaign over the cotton field, the average concentrations of the airborne PYR measured during the second campaign did not reveal any clear impact of PAM’s addition on the amplitude of the SD (Figure 6). This difference in the observed PAM effect on the SD in the two campaigns may result from a smaller impact on pesticide evaporation when deposited on soil rather than on leaves. This outcome is not unexpected given the increased roughness and absorptivity of soil as well as the laboratory experiments, which have demonstrated a stronger effect of PAM’s addition on the evaporation rate when the fungicide is applied to a smooth glass surface (Figure 2). It is worth noting that the pesticide load in several samples of the second campaign was relatively low and close to the range where recovery yields were lower; hence, these data should be considered with caution.

4. Conclusions

Polymeric adjuvants that are used as anti-draft additives for pesticide applications can lead to changes in the spreading of deposited droplets as well as alter the morphology of the residual films and solute distribution within them. Such alterations affect the evaporation and photolysis rates of the active materials present in the films. The presented laboratory experiments demonstrated enhanced surface loss rates of PYR, via both processes, when a PAM adjuvant was added to the parent solution. Differences were larger when PYR was deposited on a smoother glass surface than on lemon leaves. These trends are consistent with the observed morphological variations (on both tested surfaces) when PAM is present, including an improved droplet spreading and a less compact outer rim in the residual film.

The field measurements of secondary pesticide drift, which is initiated by pesticide evaporation from the treated surface, showed slightly higher air concentrations when the polymeric adjuvant was added to the spraying solution applied over a cotton field. The difference became less clear when the application took place over a fallow field, from which evaporation is usually lower. Overall, the obtained laboratory and field data indicate that the effect of polymeric adjuvants’ addition to spraying solutions is quite complex, and further research is needed to fully understand its environmental impact.