Pesticide Residues in the Atmosphere - Pesticides: Evaluation of Environmental Pollution - page 206

7.2.1 Structure of the Atmosphere
The surface of the Earth plays a critical role in the thermodynamic processes that take
place in the lower layers of the atmosphere; as a matter of fact, it is responsible for the
temperature decrease with increasing heights within the lowest layers of the atmosphere.
From a thermodynamic point of view, a perfectly stable atmosphere would be stratified
into layers, with the warmer layers closer to the surface and the colder ones aloft as a result
of conductive and radiative warming from the surface. Considering the temperature varia-
tion with height, the atmosphere can be divided into several regions. The troposphere, or the
region of the atmosphere closest to the Earth's surface, is where the temperature decreases
with increasing height. This evolution of temperature with height changes around 11 km
above the surface, in the so-called tropopause. From this height, and through the next region,
called the stratosphere, temperature increases with height. The stratosphere extends up to,
roughly, 50 km above sea level. Over the stratosphere, in the mesosphere, the temperature
variation changes again, decreasing with height up to 80 km. Above this and over the meso-
pause, the thermosphere begins. In this outer region of the atmosphere, the temperature
again increases with height.
The part of the troposphere that is directly influenced by the Earth's surface, on a time
scale of an hour or less, is called the boundary layer. The region of the troposphere over
the boundary layer is the free atmosphere (and over it, the tropopause). The boundary
layer responds to the changes in the physical and thermodynamic characteristics of the
Earth's surface (roughness, heating and cooling, etc.); thus, the diurnal variation in the
temperature of the air near the ground is one of the differential characteristics of the lower
troposphere or boundary layer with respect to the upper regions of the atmosphere (Stull
1989). Boundary layer depth varies frequently and ranges from a few meters under highly
stratified (stable) atmospheric conditions to more than 3 km under strong convective con-
ditions (Holton 1990).
All the exchanges of momentum between the atmosphere and the surface are made
through the boundary layer by turbulent motions with spatial scales of the order of the
depth of the boundary layer or less. The same applies to all the exchanges of water vapor
and many heat exchanges. This is why the concept of boundary layer is crucial in the study
of both the dynamics and the thermodynamics of the atmosphere.
On the other hand, most anthropogenic and biogenic atmospheric pollutants are released
into the boundary layer. Therefore, boundary layer meteorology will determine whether
the pollutants will be dispersed and diluted, exported to the free atmosphere, or settled on
the ground (Stewart 1979).
Under strong convective conditions, generally produced by the heating of the Earth's
surface, heat and momentum exchanges are dominated by convective turbulence, and the
boundary layer is frequently referred to as the mixed layer (Garratt 1999). An unstable
boundary layer usually increases its depth due to the entrainment of air from the free
atmosphere aloft toward the mixed layer. This drag process is responsible for the so-called
fumigation of the atmospheric pollutants aloft to the ground.
7.2.2 Meteorological Scales
Meteorological processes, driving the meteorology and the atmospheric pollutant disper-
sion, are traditionally classified according to their characteristic spatial (or temporal) scale.
In 1975, Orlanski introduced a classification describing the following three meteorological
scales.