Air
Microbiology
Of
all
environments,
air
is
the
simplest
one
and
it
occurs
in
a
single
phase
gas.
The
relative
quantities
of
various
gases
in
air,
by
volume
percentage
are
nitrogen
78%,
oxygen
21
%,
argon
0.9%,
carbon
dioxide
0.03%,
hydrogen
0.01
%
and
other
gases
in
trace
amounts.
In
addition
to
various
gases,
dust
and
condensed
vapour
may
also
be
found
in
air.
Various
layers
can
be
recognized
in
the
atmosphere
upto
a
height
of
about
1000km.
The
layer
nearest
to
the
earth
is
called
as
troposphere.
In
temperate
regions,
troposphere
extends
upto
about
11
km
whereas
in
tropics
up
to
about
16km.
This
troposphere
is
characterized
by
a
heavy
load
of
microorganisms.
The
temperature
of
the
atmosphere
varies
near
the
earth's
surface.
However,
there
is
a
steady
decrease
of
about
1
DC
per
150m
until
the
top
of
the
troposphere.
Above
the
troposphere,
the
temperature
starts
to
increase.
The
atmosphere
as
a
habitat
is
characterised
by
high
light
intensities,
extreme
temperature
variations,
low
amount
of
organic
matter
and
a
scarcity
of
available
water
making
it
a
non
hospitable
environment
for
microorganisms
and
generally
unsuitable
habitat
for
their
growth.
Nevertheless,
substantial
number
of
microbes
are
found
in
the
lower
regions
of
the
atmosphere.
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