**1. Introduction**

Grape production represents one of the most important agricultural businesses worldwide, with 7.5 million hectares and five countries representing 50% of the world vineyard harvested area: Spain (13%), China (11%), France (10%), Italy (9%) and Turkey (7%) [1]. However, considering the total grape production, Italy ranks second after China, with 8.4 and 13.7 Mt respectively, followed by USA, France and Spain [1]. Considering wine grapes, only the EU ranks first with 61.3% of total world production [1].

Producing high-quality food in a safe manner is one of the most important goals in modern agriculture [2]. Indeed, the high number of spray applications during the vegetative season in intensive production systems determines a massive Plant Protection Product (PPP) use that can cause undesirable effects related to pesticide residues on food and adverse e ffects to the environment.

At present, PPP spraying is a commonly used technique to control pests and diseases in commercial crops. Nevertheless, spraying is a complex process in which several factors interfere [3]. For that reason, controlling all factors is nearly impossible, and as a result PPP applications could be ine fficient. This fact led the European Authorities to develop Directive 2009/128/EC on sustainable use of pesticides [4]. This Directive specifies that pesticide applications should always consider the principles of integrated pest control strategies. Among these, e fficiency in the pesticide application necessarily plays a major role that is not completely matched nowadays. Among the main parameters to be considered regarding spraying e fficiency, liquid flow rate, airflow rate and droplet size are three of the most important [5–14], along with the forward speed [15,16].

Indeed, during spray applications in bush/tree crops with conventional sprayers, only a limited fraction of the total amount of PPP is deposited on the intended target according to the canopy characteristics [7,17]. Therefore, part of the applied PPP can be transported outside the sprayed area by the action of air currents during the application process as spray drift [18]. Some authors have quantified that, during a spray application, up to 50% of the total applied PPP spray mixture can be lost to the air from the targeted site to a non-target receptor site [19–27]. In addition to the more localized movement of agrochemical residues in turbulent air masses downwind of the application area, residues can also become concentrated in inversions or stable air masses and be transported at long distances [28]. Another fraction could end up as spray deposition on the ground, directly in the field tractor path and underneath target tree rows or indirectly in the adjacent area [5–7,20,29,30]. Among spray losses, spray drift remains the most troubling, as it is really di fficult to control [23,31–36], especially in 3D crops such as vineyards [37]. Due to the importance to minimize PPP spray drift generation, strong e fforts have been undertaken to properly study this phenomenon and to give adequate advice to farmers around Europe [38].

Among the di fferent factors influencing spray drift, wind speed and direction are the main ones [39]. The higher the wind speed, the higher carrying e ffect it will have for droplets; therefore, the spray drift risk increases [40,41]. A solution to avoid spray drift is necessary, and it relies on not spraying when the wind is present. Nonetheless, reality is not so simple, as there are many situations in which pesticide needs to be applied a certain day or in a very narrow time window, so the farmer needs to spray even if the wind conditions are not favorable. Consequently, they must act on technical factors (those controlled by the applicator) through a proper adjustment of the spraying equipment [19,42].

Droplet size has proven to be the most e ffective factor in reducing spray drift [12,20,23–25,28,33,43]. The main reason why this factor a ffects spray drift is the weight of the emitted drops: the bigger the size, the higher the weight and, therefore, the lower the drift, as the carrying e ffect of wind will be lower with heavier drops [34]. This was the way followed to develop drift-reducing hydraulic nozzles decades ago. These nozzles produce coarser droplets than the conventional ones do. This result is achieved either by (i) decreasing liquid pressure in the nozzles chamber (drift-guard (DG) nozzles) or (ii) by generating air-inflated drops (air-inclusion (AI) nozzles). As it is the most common strategy to reduce PPP drift, recently di fferent studies have been focused to demonstrate that drift-reducing nozzles generate important spray savings while keeping the necessary spray deposition to ensure the biological e fficacy of treatments [44,45]. Nevertheless, some farmers still prefer conventional nozzles as low-size drops generate higher coverage on the leaf surface. As a consequence [46], it is commonly believed they can achieve a higher e fficacy against pest and diseases. Several studies have instead

showed that droplet size could be increased to reduce spray drift risks without compromising the treatment e fficacy [35,45,47], especially in unfavorable environmental conditions [48].

Pneumatic spraying is a well-known technique for its fine droplet size generation. As aforementioned, small droplets are generally requested by the farmers to achieve a uniform target spray coverage that is essential for contact PPP; thus, this technique is very widespread, especially among large-farm vine growers in America and Europe [49,50]. In Italy there are large areas in which vineyard treatments are performed almost exclusively with pneumatic sprayers [51]. The small droplet size produced by this type of nozzle increases the drift risk with respect to the hydraulic nozzles [52]. Thus, whilst the droplet volume median diameter (VMD) generated by conventional hydraulic hollow-cone nozzles typically ranges from 100 to 200 μm, for pneumatic nozzles the VMD is generally below 100 μm [53–55]. This threshold of 100 μm is broadly considered as the minimum diameter that droplets should have to reasonably limit the spray drift risk [56,57]. Therefore, a droplet population with diameters below this value is subjected to losses by drift even with slow environmental air currents.

In order to better understand the droplet population generated by pneumatic nozzles, Balsari et al. [53] investigated and quantified the e ffect of the main operational parameters, namely the liquid flow rate and the airflow rate, on di fferent variables related to the droplet population, mainly their size and homogeneity. They also assessed the driftability of the drops according to the aforementioned parameters. In order to do so, they developed a laboratory test bench to simulate a real pneumatic sprayer and to test di fferent combinations of parameters, measuring the droplet diameter with a laser device. Once the pneumatic spray behavior was well understood, the same authors planned a strategy to increase droplet size in case of high-speed environmental wind conditions [55]. The idea was to alter the liquid release position inside the air spout, taking advantage of the air speed decrease across the outermost section of the spout [55]. They obtained very promising results, recently confirmed by field trials [58], that demonstrated this strategy could have an important potential to properly reduce spray drift in pneumatic spraying thanks to the droplet size spectra increase.

The main objective of this work was to develop, for the first time, drift-reducing spouts, namely cannon and hand spout types, for vineyard pneumatic sprayers. In detail we evaluated i) the possibility to increase droplets size by adjusting the water income position out of the nozzle spouts and ii) its e ffect on the droplet size population and homogeneity. Furthermore, the feasibility of introducing alternative positions for the liquid hose in order to achieve di fferent droplet size spectra populations and then di fferent capabilities of drift reductions according to the liquid hose positions were investigated.

#### **2. Materials and Methods**

#### *2.1. Trial Location and Spraying Equipment*

The laboratory trial was carried out in the facilities of the Department of Agricultural, Forest and Food Sciences (DiSAFA) of the University of Turin located in Grugliasco (Turin, Italy). A test bench was used to simulate the working conditions of the pneumatic sprayer. This test bench consisted of both pneumatic and hydraulic circuits mounted over three di fferent spaces, as shown with detail in Miranda-Fuentes et al. [55]. These spaces included a spraying area, a droplet size measurement area and a control and data acquisition room.

The test bench liquid circuit simulated those mounted in real pneumatic sprayers, including a water tank, a membrane pump AR 202 (Annovi Reverberi S.p.a, Modena, Italy) driven by an electric engine equipped with pressure regulation valves, and a manometer characterized by 0.02 MPa resolution to precisely control the liquid pressure. The test bench ended up in the liquid hose inserted in the air spout of the pneumatic circuit. The liquid flow rate was controlled through a plastic disc with calibrated holes in its perimeter. The air assistance to achieve the spray in the test bench consisted of a centrifugal fan 500 mm diameter (CIMA S.p.a., Pavia, Italy) controlled by a central unit, in which the electric intensity flowing to the driving electric engine, and proportionally the rotary speed of the fan, could be manually adjusted. This control box could set the rotary speed (rev min-1) of the electric engine, measured by a laser tachometer integrated in the system. Thus, even when the control parameter was the amperage, the rotary speed was the indicator to manually regulate the system. As it was previously mentioned, both circuits merged in the air spout, in which the liquid was released through the liquid hose at constant pressure. The air spouts tested included two different types: a cannon-type spout and a hand-type spout (Figure 1a) generally installed on the spray head TC.2M2C.50P (Cima S.p.a., Pavia, Italy). These two kinds of spouts are usually simultaneously employed in the commercial pneumatic sprayers operating in vineyards, as shown in Figure 1b.

**Figure 1.** (**a**) Cannon-type and hand-type spouts mounted on the spray head TC.2M2C.50P for (**b**) multiple-row spray application in a vineyard.

These spouts were conveniently modified to alter the insertion position of the liquid hose, as it showed to have a major importance in the generated droplet size in cannon spouts [55]. The insertion position was, consequently, established as the main variable of the study, and it was called "hose position" (HP). It presented four possible levels in each spout type, called "conventional position" (CP), "alternative position" (AP), "out position" (OP) and "extreme position" (XP), as shown in Figure 2a. The CP position corresponds to that established by the manufacturer for the commercial equipment (Figure 2b,c). The other hose positions, namely AP, OP and XP, varied according to the spout type, as shown in Figure 2b for the cannon and in Figure 2c for the hand spouts. In particular, in both spout-types, the AP corresponds with the one in which the outermost part of the hose is placed coincident with the border of the air spout (Figure 2b,c), and the XP was experimentally set as the maximum distance in which a uniform liquid atomization could be achieved. This distance value was experimentally found by moving the liquid hose out of the air spout along its longitudinal axle through a rail in steps of 5 mm and the droplet size spectra measured in each position. Once this maximum distance was exceeded, the spray cloud changed drastically, appearing as extremely coarse

droplets and losing the normal distribution. The OP was set at an intermediate distance between the AP and the XP (Figure 2b,c).

**Figure 2.** (**a**) Liquid hose positions from the inner to the outer part of spouts (green arrow): conventional position (CP) (0 cm reference) passing through alternative position (AP), out position (OP) and extreme position (XP). Distances (cm) from the reference position were different for (**b**) the cannon-type and (**c**) the hand-type spouts.

#### *2.2. Droplet Size Spectra and Airflow Speed Measurements*

The droplet size spectra were measured with a Malvern Spraytec® laser diffraction system STP5342 (Malvern Instruments Ltd., Worcestershire, UK) (Figure 3a). The instrument has a maximum measurement frequency of 10 kHz and a measurement range of 0 to 2000 μm. The instrument includes software (SprayTec Software v3.30, Malvern) for managing the data acquisition and charting. This software directly acquired the droplet size parameters D50 (or Volumetric Mean Diameter, VMD), D10 and D90. It also calculated V100, the fraction of the spray with droplets below 100 μm in diameter, which easily can be blown away by the wind according to different authors [56,57]. The droplet homogeneity could also be drawn from the aforementioned droplet size parameters, if expressed as the RSPAN Factor (RSF), which can be calculated as shown in Equation (1) [12,59].

$$RSF = \frac{D90 - D10}{D50} \tag{1}$$

where RSF is dimensionless, and D90, D10 and D50 are expressed in μm.

**Figure 3.** (**a**) Malvern Spraytech® laser diffraction system STP5342 for the measurements of droplet size spectra and (**b**) Testo 400 Pitot-tube-based anemometer for the measurements of airflow speed generated by both cannon-type and hand-type spouts.

The conventional hydraulic hollow cone nozzle Albuz® ATR lilac and the air-induction Albuz® TVI 8001 (CoorsTek Inc., Evereux, France) both operated at 0.7 MPa pressure, were selected as reference conventional spray technology for the comparison with the droplet size spectra investigated using the modified pneumatic spouts. In particular, the ATR lilac nozzle is well known to produce very fine (VF) spray quality, likewise the TVI 8001 nozzle is well known as a drift-reducing nozzle for the ultra-coarse (UC) spray quality generated [43]. Furthermore, the Albuz® ATR lilac was studied by other authors, and it is widely used by farmers with conventional hydraulic air-assisted sprayers both in vineyards and orchards [60]. The droplet size spectra generated by the reference nozzle were measured using the same laser diffraction system device described above and the methodology usually applied for the measurements of droplet size spectra generated by hydraulic nozzles, fully detailed in Grella et al. [25].

The measurement of the airflow speed was performed with a Pitot-tube-based anemometer Testo 400 (Testo SE & Co. KGaA, Lenzkirch, Germany) (Figure 3b) at 1Hz frequency with a measurement resolution of ±0.01 hPa, a differential pressure corresponding to 1.28 m s<sup>−</sup>1, and a measurement range of up to +2000 hPa (571.43 m s<sup>−</sup>1). The instrument was fixed to an ad hoc support developed by the researchers with some modifications to properly adapt it to the experiment [55]. The main advantage of this support was the possibility to keep the air speed measurement instrument in the center of the air spout (Figure 3b), avoiding errors that could deeply affect the measurements when holding it manually. The modifications in the instrument were essentially aimed at an enlargement of the holding tube to measure speed in the outermost position of the liquid hose (XP). Thus, the airflow speeds were measured along the central axis in six positions, corresponding to 0, 5, 10, 16, 22 and 28 cm to the CP for the cannon-type spout and in five positions, corresponding to 0, 2.5, 5.0, 9.5 and 12.5 cm in a single spout for the hand-type nozzle. Preliminary trials were conducted to ensure the airflows generated individually by the four spouts that composed the hand-type nozzle were comparable. In each sampling position, the data were recorded for 60 s per each of the three replicates performed.

#### *2.3. Spray Parameters, Experimental Design and Settings Used During Trials*

Three different liquid flow rates (LFRs) were investigated, namely 1.00, 1.64 and 2.67 L min−1. The intended LFR were obtained setting the liquid circuit pressure at 0.1 MPa and using the liquid flow regulator disc (a plastic disc with calibrated holes in its perimeter) in positions 3, 5 and 7, respectively (Table 1). As mentioned above, four HPs were tested in both pneumatic nozzle types, namely CP, AP, OP and XP (Table 1). The intended exact positions of liquid hose, relative to the spout, were guaranteed thanks to a support integral with the spout body (Figure 3a). Concurrently, four AS were tested, namely 81.3, 90.0, 100.2 and 109.2 m s<sup>−</sup><sup>1</sup> for the cannon-type nozzle and 57.9, 64.6, 74.3 and 84.2 m s<sup>−</sup><sup>1</sup> for the hand-type nozzle. The tested AS values were referred to the reference position (CP), since the increase in spout section diameter deeply a ffects the decrease of AS along the spout body [55]. The intended AS were obtained properly setting the test bench fan rotary speed to simulate real conditions during spray application. Therefore, the main complexity was to adjust the test bench to make sure it matched the working parameters present in a real sprayer. There are many pneumatic circuit parameters that could be compared in both the bench and the sprayer, but the most representative and the one that ensures an equivalency in the working conditions is the air pressure. Thus, air pressure measurements were done to adjust both systems for every kind of pneumatic nozzle. The fully explained details of the calibration for both cannon- and hand-type nozzles along with the calibration results can be found in Balsari et al. [53] and Miranda-Fuentes et al. [55]. The air pressure measurements were performed with a manometer with a measurement resolution of ±1 mm H2O, which was attached to a plastic piece fixed to the nozzle in every case. These pieces were large enough to reach the central part of both air spouts. The air pressure was measured for di fferent Power Take O ff (PTO) speed values in order to match the intended AS values previously measured in the real sprayer. According to those measurements, for laboratory trials the test bench fan rotary speeds were set at 541, 598, 663 and 720 rev min−<sup>1</sup> for the cannon-type nozzle and at 488, 536, 609 and 677 rev min−<sup>1</sup> for the hand-type nozzle, respectively. The operating parameters assessed are listed in Table 1.



\* LFR: liquid flow rate, AS: Air speed, HP: liquid hose position. \*\* Distance from the conventional position (CP). \*\*\* CP:conventionalposition,AP:alternativeposition,OP: outposition,XP:extremeposition.

The experimental design was completely randomized, as no restrictions were present when measuring the droplet parameters (D50, D10, D90, RSF and V100) for a given combination of values of the independent variables (HP, LFR and AS; Table 1) in each studied pneumatic nozzle. The treatment order was randomized to avoid possible influence of external factors not considered in the design. The dependent variables were the droplet size parameters (i.e., D50, D10 and D90), the droplet homogeneity, given by the RSF, and the droplet driftability, given by the V100 parameter.

The trial began with the air spout placement, and it was conducted in two steps. In the first one, the airflow rate was established by adjusting the fan's rotary speed to the values obtained in the calibration [53,55]. The air speed was then measured in di fferent positions using the Pitot tube, as described before. In the second step, the spray was enabled, and a 30 s time was given to the system to stabilize. Once the time had passed, the laser di ffraction instrument was settled at the most appropriate distance from the spout (50 cm in accordance to previous trials [55]), and a total of 30 measurements were taken at 1 Hz acquisition frequency. The operational parameters (Table 1) were then combined to test every possible combination. In total, 48 configurations were tested for each spout type, deriving from the combination of di fferent LFR, AS and HP. Three test replicates for each spout configuration were done.
