Environmental Engineering Reference
In-Depth Information
atmospheric biogeochemistry provides important information for understanding
global biogeochemical cycles.
Transport in Atmosphere
The distance a chemical travels from its source region is highly dependent on its
atmospheric lifetime. Many of the biogeochemically relevant gases discussed
above are long-lived gases, which will reside in the atmosphere for several months
to hundreds of years (e.g., CO
2
,CH
4
,N
2
O). Other, reactive species (such as the
hydroxyl radical OH), can have very short lifetimes (
1 day), and thus will not
travel far from the sources. Aerosol species tend to have lifetimes between 1 day
and a few weeks, and this lifetime is dependent on the size of the particle as well as
whether precipitation has occurred, as this will remove particles.
For chemical species that have a relatively long lifetime (
<
a few days), long-
range transport can occur. Once emitted into the atmosphere, constituents mix
quickly into the atmospheric boundary layer (the lowest 1-3 km of the atmosphere,
where mixing is strongest), and can be transported long distances. Air parcels stay
on a constant buoyancy surface (called isentropes), unless they are heated or cooled
[
116
]. These surfaces of constant buoyancy tilt upward between the tropics and the
high latitudes, because the high latitudes are colder (
Fig. 2.4a
from [
109
]). An air
parcel emitted in the tropical boundary layer can reach the stratosphere (
>
10 km
high) in the high latitudes without being heated by moving along an isentropic
surface. On average, most of the atmosphere is radiatively cooling slightly during
non-stormy conditions, so, over time, parcels of air will become heavier and move
downward. If the parcel encounters a storm system with clouds and precipitation,
the parcel can experience large heating or cooling and move vertically very quickly,
perhaps traveling from the boundary layer up to 10 km in less than 1 h [
117
]. At the
same time, the precipitation within the storm system can cause wet deposition of
soluble gases or aerosols. Based on atmospheric heating rates, the residence time
for a parcel on a given isentropic surface varies between
<
1 day for the tropics to
3-6 days in midlatitudes (based on 75 percentile heating rates) (
Fig. 2.4a
). If a parcel
moves away from the pole, the tilting of the neutral buoyant surfaces will direct the
parcel downward vertically and vice versa.
Geographically, there tend to be particular regions in midlatitudes that have
more storms (
Fig. 2.4b
), called the storm tracks. Winds in the midlatitudes (30-60
)
tend to be from the west, while winds in the tropics and highlatitudes (
>
60
) are
less uniform. Tropical aerosols are more likely to encounter storms, resulting in
vertical mixing or quick removal (
Fig. 2.4b
). Transport across latitudes tends to
occur less often than transport along a latitude [
118
]. Thus, in the midand high
latitudes, one can think of a parcel containing a constituent moving along
isentropes, following the local winds, slowly cooling, and moving to a lower
isentropic surface until it encounters a storm system, at which time it can be quickly
removed or vertically transported.
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