<s>
In	O
atmospheric	O
chemistry	O
,	O
differential	B-General_Concept
optical	I-General_Concept
absorption	I-General_Concept
spectroscopy	I-General_Concept
(	O
DOAS	O
)	O
is	O
used	O
to	O
measure	O
concentrations	O
of	O
trace	O
gases	O
.	O
</s>
<s>
Typical	O
setups	O
allow	O
for	O
detection	O
limits	O
corresponding	O
to	O
optical	B-Algorithm
depths	I-Algorithm
of	O
0.0001	O
along	O
lightpaths	O
of	O
up	O
to	O
typically	O
15km	O
and	O
thus	O
allow	O
for	O
the	O
detection	O
also	O
of	O
weak	O
absorbers	O
,	O
such	O
as	O
water	O
vapour	O
,	O
Nitrous	O
acid	O
,	O
Formaldehyde	O
,	O
Tetraoxygen	O
,	O
Iodine	O
oxide	O
,	O
Bromine	O
oxide	O
and	O
Chlorine	O
oxide	O
.	O
</s>
<s>
Rather	O
,	O
what	O
is	O
done	O
is	O
to	O
take	O
the	O
ratio	O
of	O
two	O
measurements	O
with	O
different	O
paths	O
through	O
the	O
atmosphere	O
and	O
so	O
determine	O
the	O
difference	O
in	O
optical	B-Algorithm
depth	I-Algorithm
between	O
the	O
two	O
columns	O
(	O
Alternative	O
a	O
solar	O
atlas	O
can	O
be	O
employed	O
,	O
but	O
this	O
introduces	O
another	O
important	O
error	O
source	O
to	O
the	O
fitting	O
process	O
,	O
the	O
instrument	O
function	O
itself	O
.	O
</s>
<s>
where	O
we	O
call	O
the	O
differential	O
optical	B-Algorithm
depth	I-Algorithm
(	O
DOD	O
)	O
.	O
</s>
<s>
What	O
this	O
means	O
is	O
that	O
before	O
performing	O
the	O
inversion	O
,	O
the	O
continuum	O
components	O
from	O
both	O
the	O
optical	B-Algorithm
depth	I-Algorithm
and	O
from	O
the	O
species	O
cross	O
sections	O
must	O
be	O
removed	O
.	O
</s>
<s>
Obviously	O
,	O
this	O
will	O
not	O
produce	O
an	O
exact	O
equality	O
between	O
the	O
measured	O
optical	B-Algorithm
depths	I-Algorithm
and	O
those	O
calculated	O
with	O
the	O
differential	O
cross-sections	O
but	O
the	O
difference	O
is	O
usually	O
small	O
.	O
</s>
