Report from the Sub-comittee on the environment and health

Droplet size, volatilisation
and relative humidity
Height of spraying boom
above <strong>the</strong> ground
conditions with more wind or atmospheric instability, some of <strong>the</strong> spray
liquid is transported out of <strong>the</strong> sprayed area. The proportion transported
out of <strong>the</strong> area depends on <strong>the</strong> following factors (Jensen et al. 1998):
• <strong>the</strong> wind velocity and relative humidity
• <strong>the</strong> average droplet size and droplet size distribution, which depend on
<strong>the</strong> type of and size of <strong>the</strong> nozzle, <strong>the</strong> hydraulic pressure and <strong>the</strong>
surface tension and viscosity of <strong>the</strong> spray liquid
• <strong>the</strong> distance between <strong>the</strong> nozzle mouth and <strong>the</strong> spray target (boom
height)
• <strong>the</strong> spraying equipment used (conventional, air-assisted, screening,
size and design of <strong>the</strong> spraying equipment, electric charging of <strong>the</strong>
drops, etc.).
The spraying techniques that can be used to reduce spray drift are
discussed in section 9.6. In field spraying, for example, <strong>the</strong> farmer can
influence <strong>the</strong> drift through his choice of spraying equipment and its
setting, while <strong>the</strong> actual drift is heavily affected by <strong>the</strong> climatic
conditions - particularly <strong>the</strong> wind. Also drift is far greater when spraying
on bare soil or on soil with a low crop than when spraying in <strong>the</strong> later
stages of <strong>the</strong> crop, when <strong>the</strong> crop is dense.
Volatilisation <strong>from</strong> such small droplets simply reduces <strong>the</strong> size of <strong>the</strong>
droplets still fur<strong>the</strong>r and produces very small droplets that are transported
with wind over long distances. Droplets with a diameter of less than 50
micrometres have a critical size because <strong>the</strong>y remain suspended in <strong>the</strong> air
for a relatively long time. If <strong>the</strong> relative humidity is low, large droplets
can also become smaller, whereby <strong>the</strong> risk of drift increases
considerably. It has thus been calculated that <strong>the</strong> proportion of air-borne
droplets at a distance of 500 metres <strong>from</strong> <strong>the</strong> sprayed areas increases
more than tenfold if <strong>the</strong> relative humidity falls <strong>from</strong> 100% to 50% at
20 o C (Thomson, Ley 1982).
The height of <strong>the</strong> spraying boom has a considerable effect on drift. It is
primarily <strong>the</strong> small droplets – 100 micrometres or less – that are affected
by changes in <strong>the</strong> height of <strong>the</strong> boom. Drift thus doubles when <strong>the</strong> boom
height is increased <strong>from</strong> 50 cm to 70 cm with a traditional flat-spray
nozzle (Miller 1988).
As mentioned in section 9.6, spraying equipment is undergoing fur<strong>the</strong>r
development that could help to reduce drift.
4.6.4 Conclusions
Drift to <strong>the</strong> surrounding areas implies a risk of exposure of hedgerows,
dykes, dry stone walls and small biotopes in farmland and of natural
terrestrial and aquatic areas. Drift, toge<strong>the</strong>r with volatilisation of
pesticides, explains why pesticides are detected in precipitation, surface
water and on unsprayed areas. Drift depends particularly on <strong>the</strong> droplet
size and wind velocity. The droplet size depends on <strong>the</strong> spraying
equipment and spraying technique used.
The following specific conclusions can be drawn:
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