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Abstract:
In order to promote the development of the passive
DOAS technique the Multi Axis DOAS – Comparison cam-
paign for Aerosols and Trace gases (MAD-CAT) was held at
the Max Planck Institute for Chemistry in Mainz, Germany,
from June to October 2013. Here, we systematically compare
the differential slant column densities (dSCDs) of nitrous
acid (HONO) derived from measurements of seven differ-
ent instruments. We also compare the tropospheric difference
of SCDs (delta SCD) of HONO, namely the difference of
the SCDs for the non-zenith observations and the zenith ob-
servation of the same elevation sequence. Different research
groups analysed the spectra from their own instruments us-
ing their individual fit software. All the fit errors of HONO
dSCDs from the instruments with cooled large-size detectors
are mostly in the range of 0.1 to 0.3
×
10
15
molecules cm
−
2
for an integration time of 1 min. The fit error for the
mini MAX-DOAS is around 0.7
×
10
15
molecules cm
−
2
. Al-
though the HONO delta SCDs are normally smaller than
6
×
10
15
molecules cm
−
2
, consistent time series of HONO
delta SCDs are retrieved from the measurements of different
instruments. Both fits with a sequential Fraunhofer reference
spectrum (FRS) and a daily noon FRS lead to similar con-
sistency. Apart from the mini-MAX-DOAS, the systematic
absolute differences of HONO delta SCDs between the in-
struments are smaller than 0.63
×
10
15
molecules cm
−
2
. The
correlation coefficients are higher than 0.7 and the slopes of
linear regressions deviate from unity by less than 16 % for
the elevation angle of 1
◦
. The correlations decrease with an
increase in elevation angle. All the participants also analysed
synthetic spectra using the same baseline DOAS settings to
results from their re-
spective fit programs. In general the errors are smaller than
0.3
×
10
15
molecules cm
−
2
, which is about half of the sys-
tematic difference between the real measurements.
The differences of HONO delta SCDs retrieved in the
selected three spectral ranges 335–361, 335–373 and 335–
390 nm are considerable (up to 0.57
×
10
15
molecules cm
−
2
)
for both real measurements and synthetic spectra. We per-
formed sensitivity studies to quantify the dominant system-
atic error sources and to find a recommended DOAS set-
ting in the three spectral ranges. The results show that wa-
ter vapour absorption, temperature and wavelength depen-
dence of O
4
absorption, temperature dependence of Ring
spectrum, and polynomial and intensity offset correction
all together dominate the systematic errors. We recom-
mend a fit range of 335–373 nm for HONO retrievals. In
such fit range the overall systematic uncertainty is about
0.87
×
10
15
molecules cm
−
2
, much smaller than those in the
other two ranges. The typical random uncertainty is es-
timated to be about 0.16
×
10
15
molecules cm
−
2
, which is
only 25 % of the total systematic uncertainty for most of
the instruments in the MAD-CAT campaign. In summary
for most of the MAX-DOAS instruments for elevation an-
gle below 5
◦
, half daytime measurements (usually in the
morning) of HONO delta SCD can be over the detection
limit of 0.2
×
10
15
molecules cm
−
2
with an uncertainty of
∼
0.9
×
10
15
molecules cm
−
2
.
