Report from the Sub-comittee on the environment and health

Models for volatilisation
46
• It is primarily droplets with a diameter of less than 100 micrometres
that are transported with <strong>the</strong> wind.
• Small droplets are formed when larger droplets evaporate in dry air.
• Pesticides with a very low vapour pressure cannot be transported to
<strong>the</strong> atmosphere by volatilisation. Even so, <strong>the</strong>y are found in <strong>the</strong>
atmosphere and in precipitation because <strong>the</strong>y can be transported as
small particles formed <strong>from</strong> droplets of spray liquid that are borne and
dried in <strong>the</strong> air.
• A low boom height reduces <strong>the</strong> quantity of small droplets that form
through volatilisation.
• In dry air, large droplets change into small droplets through
volatilisation. Spraying in moist air reduces <strong>the</strong> volatilisation. The air
is often moist and <strong>the</strong> wind velocity low in <strong>the</strong> morning hours, so <strong>the</strong>
risk of drift is generally lowest <strong>the</strong>n.
• Little is known about <strong>the</strong> magnitude of drift and <strong>the</strong> exposure of
terrestrial and aquatic biotopes. In current practice, terrestrial biotopes
are protected by means of spray-free zones and aquatic environments
are protected by means of substance-specific distance requirements in
<strong>the</strong> authorisation of pesticides.
4.6.5 Volatilisation
Pesticides evaporate both during and, especially, after spraying. Toge<strong>the</strong>r
with drift, see section 4.6.2, volatilisation conveys relatively large
quantities of pesticides to <strong>the</strong> atmosphere, which is thus, quantitatively,
<strong>the</strong> principal transport path for pesticides away <strong>from</strong> <strong>the</strong> sprayed area.
Lastly, it should be noted that considerable quantities of pesticides can be
transported by wind erosion – for example, if <strong>the</strong>re is a storm after
spraying of crops in <strong>the</strong> spring, when <strong>the</strong> plant density is low. The
volatilisation depends on <strong>the</strong> properties of <strong>the</strong> substance and, particularly,
on its vapour pressure. The temperature, water solubility and adsorption
to <strong>the</strong> soil and plant surfaces, <strong>the</strong> humidity of <strong>the</strong> soil, atmospheric
turbulence, and <strong>the</strong> concentration of <strong>the</strong> pesticide, including its
degradation, also play an important part. It is difficult to measure <strong>the</strong>
volatilisation because of its great natural variability (Løkke 1998).
Volatilisation of pesticides depends on <strong>the</strong> aforementioned factors and
can be calculated by means of ma<strong>the</strong>matical models that include <strong>the</strong>
main factors governing <strong>the</strong> volatilisation. Jansma and Linders (1995)
calculated <strong>the</strong> volatilisation <strong>from</strong> <strong>the</strong> surface of <strong>the</strong> earth by means of <strong>the</strong>
so-called “Dow method”, which takes account of <strong>the</strong> pesticides’ vapour
pressure, water solubility and adsorption to soil as <strong>the</strong> factors governing
<strong>the</strong> volatilisation. The method was validated by comparison with
measured values. This method can thus be used for crops grown in rows,
where <strong>the</strong> plant cover is small and most of <strong>the</strong> pesticides hit <strong>the</strong> surface
of <strong>the</strong> earth. The calculated values generally lay within a factor of 7 <strong>from</strong>
<strong>the</strong> measured values. However, this model has a tendency to overestimate
<strong>the</strong> volatilisation. There are as yet no suitable models for estimating <strong>the</strong>
volatilisation of pesticides that are mixed with <strong>the</strong> soil during harrowing
and ploughing. It is also difficult to make a general model for <strong>the</strong>
volatilisation of pesticides <strong>from</strong> <strong>the</strong> surface of <strong>the</strong> plants.