Introduction
Overview of some recently published water vapour cross sections
convoluted to a spectral resolution of 0.5 nm in the spectral interval from
330 to 500 nm. Also indicated is a typical MAX-DOAS detection limit for a
differential optical density (OD) of 10-4 at a water vapour column density
of 4×1023 (purple line, top panel). The middle
panel shows the O4 absorption cross section and the lowermost
panel shows other absorbers of atmospheric relevance (HONO, OClO, SO2,
HCHO, and BrO) in this spectral range.
The most important greenhouse gas is water vapour. It plays a key role in the
radiative balance of the Earth's atmosphere
e.g.. Due to the large temperature range
covered by observations on Earth but also on exoplanets, and due to the
spectral extend of observed water vapour absorption, accurate water vapour
line lists covering different temperatures over a wide range of wavelengths
are necessary. Since water vapour absorption is present in many wavelength
regions, precise knowledge of their properties is also required for assessing
greenhouse effects. In addition it is required for spectroscopic detection of
other trace gases, since their absorption structures often overlap with water
vapour absorption. The number of laboratory measurements of water vapour
absorption spectra at different temperatures is limited due to technical
reasons: experimental measurements of water vapour absorption are not
straightforward, as water vapour cannot be compressed to increase its optical
depths in a measurement volume at any temperature. Moreover, the absorption
cross section is relatively small in certain wavelength ranges, e.g. in the
blue and near-UV spectral ranges that concern us here. The gap between
observed absorptions and the available literature absorption cross sections
from laboratory measurements can be addressed by means of ab initio
models for water vapour absorption lines, which can provide energy (i.e.
wavelength), intensity and additional parameters for each absorption line.
This is done, e.g., in the HITRAN database , where
information from measured absorption lines is merged with information from
other sources such as ab initio models. In addition to HITRAN, other
line-list compilations are also available, such as the GEISA database
, which lists water vapour absorption lines up to 25 232 cm-1
(down to 396.3 nm).
found systematic structures in the fit residuals in this
spectral range below 370 nm with magnitudes of around 5×10-4 in
multi-axis differential optical absorption spectroscopy (MAX-DOAS)
atmospheric observations, which could point towards a tropospheric absorber
with absorption structures in this spectral range. The BT2
and HITEMP line lists could explain
some of the structures, but show inconsistencies. HITEMP is a synthesis of
the 2008 edition of HITRAN and BT2 with HITRAN lines
replacing BT2 ones where they were available.
Still, these two line lists show significant differences between each other,
mostly due to the individual line cut-off employed in the HITEMP database (see
also Fig. ). This cut-off removes weak absorption lines from
the line list and was introduced for the HITRAN and HITEMP line lists to
reduce the number of individual absorption lines for further processing as
described, e.g., in . It removes a large number of weak individual
lines below the line intensity cut-off of 10-27 cm molec-1 for
wavelengths shorter than 1 µm .
recently developed a computed line list (which we call
“POKAZATEL” here, according to the first letters of the name of each author)
containing water vapour lines in the spectral range below 400 nm. It is
independent of the other sources and is based on a number of theoretical
improvements compared to BT2. BT2 already listed absorptions in this spectral
region prior to the publication of POKAZATEL. The POKAZATEL line list
differs significantly from BT2 and thus also HITEMP below 380 nm (see
Fig. ). In general, only a few of these lines below 380 nm
have also been reported from laboratory measurements
. For a compilation of
spectroscopic data see . Previous publications, such as HITRAN
2012 , do not list water vapour lines below 388 nm.
Recently, deduced upper limits for the water vapour
absorption in the near-UV by incoherent broadband cavity enhanced absorption
spectroscopy measurements in the laboratory. They estimated the
water vapour absorption cross section to be smaller than
5×10-26 cm2 molec-1 at a spectral resolution of 0.5 nm
between 340 and 420 nm. This is significantly smaller than the water vapour
cross section measured by between 290 and 350 nm (see
Sect. ).
The POKAZATEL line list
Following up on previous high-quality-computed water line lists
, the POKAZATEL line list was
calculated for the purpose of producing a complete list of water lines
involving transitions between all the bound energy levels of H216O up
to dissociation. Until now the most complete water line list, called BT2
, only covered energy levels up to 30 000 cm-1 (333 nm)
and rotational quantum numbers, J, up to 50. POKAZATEL covers
the entire bound energies up to dissociation – 41 000 cm-1 (244 nm)
and the highest J considered is 72.
POKAZATEL extends BT2 3-fold. First, higher temperatures can be covered
by the line list, as higher energy levels are involved and more hot
transitions are calculated. Second, for room temperature the spectral range
is expanded in the UV region down to
about 244 nm.
Third, the predictions of the line positions and intensities by POKAZATEL
should be considerably more accurate. In particular, POKAZATEL is based
on variational nuclear motion calculations performed with the DVR3D
program suite .
In order to calculate the line positions and line intensities of the water
lines, two inputs into DVR3D are necessary – a water potential energy surface
(PES) for the ground electronic state and a dipole moment surface (DMS). A
global water PES, covering geometries up to dissociation, is available only
from ab initio calculations and is not accurate enough for our
purposes. POKAZATEL is therefore based on the semi-empirical PES obtained
by the fitting to the experimental data up to 41 000 cm-1 . The
details of the fit are given by . In particular, the rms
(root mean square) deviation for levels below 25 000 cm-1, calculated by this
fitted PES, is about 0.03 cm-1, and the levels from 25 000 to 41 000 cm-1
are reproduced to within about 0.1 cm-1 on average, using measured data from
.
A very accurate, ab initio, global DMS was computed by
and was used without modification for the POKAZATEL line-list calculation.
This DMS has been used to successfully construct comprehensive line lists for
H217O and H218O , which were included in their
entirety in the most recent, 2012, release of HITRAN. A recent laboratory
investigation has verified the accuracy of these line lists in the
near-infrared . However, as discussed below, the
intensities predicted by the various line lists have yet to be validated in
the near-UV.
Impact on DOAS measurements of atmospheric trace gases
The absorption lines listed in the UV range in POKAZATEL, BT2 and
HITEMP,
which are to our knowledge presently not included in DOAS retrievals, could
have an effect on the overall measurement errors of several trace-gas
retrievals and could lead to systematic biases in the spectral evaluation of
tropospheric absorbers in these spectral regions, such as the oxygen dimer
O2–O2 (or for short: O4), nitrous acid (HONO), chlorine
dioxide (OClO), sulfur dioxide (SO2), formaldehyde (HCHO) and
bromine monoxide (BrO). In Sect. , we discuss these
potential interferences. In Fig. , absorption cross sections
of these species are shown in the two lowermost panels.
In particular, spectral structures at around 360 nm have been observed in
atmospheric DOAS measurements before and were explained by erroneous
O4 literature cross sections, e.g. an incorrect spectral calibration of
the respectively used cross section data e.g.. In any
case, it could be possibly explained by an unaccounted tropospheric absorber.
Outline
Based on our field measurements combined with the POKAZATEL water vapour
line list, which yields new information about water vapour absorption below
390 nm, we make an attempt to answer the following questions:
Are the water vapour absorption bands near 335, 363 and 376 nm found in atmospheric DOAS measurements?
Is the magnitude of these absorptions in agreement with measurements in other wavelength ranges? cf. for the blue spectral range
How well is the shape and the magnitude of the measured absorption bands reproduced by the line lists?
What are the consequences for the spectral retrieval of other trace gases in the same spectral region (as, e.g., O4, HONO and OClO)?
The DOAS Method
The DOAS method
relies on attenuation of light with a wavelength
λ from suitable light sources (intensity I0) by absorbers within
the light path according to Lambert–Beer's law I(λ)=I0(λ)⋅exp(-τ(λ)).
The optical density (OD) τ(λ) is calculated
from a reference spectrum I0(λ) and a measurement spectrum
I(λ), τ(λ)=-lnI(λ)I0(λ). The
measured OD of the broadband extinction and scattering by molecules and
particles is represented by a broadband polynomial p(λ), or the
measured OD is filtered into a broadband and a narrow-band contribution.
Characteristic absorption features of different absorbing trace-gas species
with the total cross section σi(λ) are then used to determine
their respective concentrations ci(l) along the light path L:
τ(λ)=∑iσi(λ)∫0Lci(l)dl+p(λ).
The column density Si=∫0Lci(l)dl
is calculated
by a fitting routine, which is applied to data from a given wavelength
interval with a width of several nanometres to several tens of nanometres. The absorption path
L
is known for LP-DOAS measurements and can be estimated or calculated from radiative transfer
models for MAX-DOAS measurements. The high-resolution literature cross sections
σL,i are convoluted with the measured instrument function H of the respective
set-up to obtain σi=H⊗σL,i, the absorption cross section as
it would be determined by the instrument.
The instrument slit function is usually
measured by observing individual atomic
emission lines of mercury, which have a
spectral width that is 2 orders of magnitude smaller than the resolution of the instrument .
LP-DOAS measurements (Sect. ) have the advantage
of a well-defined light path and the possibility of measurements at night, but
typically do not yield the small measurement errors of slant column densities (SCDs) like in MAX-DOAS
(Sect. ) observations. The disadvantage of MAX-DOAS
measurements is that their effective light-path length depends on
various factors, such as atmospheric state (aerosols, clouds), which are often not
known precisely. This needs to be explicitly considered in the data evaluation
(Sect. ).
The spectral analysis was done using the DOASIS software package
.
LP-DOAS Measurements
The LP-DOAS instrument is based on an artificial light source (here
a laser-driven light-source Energetiq LDLS-EQ-99). The light is sent by a
telescope through the atmosphere to a retroreflector and reflected back to
the same telescope. Thus, the measured atmosphere is in-between the telescope and
retroreflector. The received light is transferred to a spectrograph. A
measurement sequence consists of four spectra: atmospheric spectrum over the
distance to the retroreflector, light-source spectrum, atmospheric background spectrum
(i.e. measurements with the light source switched off or
blocked) and light-source background spectrum. The correction of the
atmospheric and light-source spectra with background spectra ensures
independence from external sunlight, dark current and other instrumental
properties .
A description of the LP-DOAS instrument used here can be found in
and . The total light path used for the
measurements reported was 6.12 km long: above the city of Heidelberg from
the roof of the Institute of Environmental Physics to retroreflectors
mounted at the train station “Molkenkur” and back to the institute.
The optical density τ(λ) is calculated from a background-corrected
light-source spectrum and a background-corrected atmospheric spectrum and
filtered by a binomial high pass with 1000 iterations. The convoluted and
high-pass-filtered literature cross sections listed in
Table are then fitted in the respective
fitting interval to the corrected OD.
MAX-DOAS Measurements
Retrieval wavelength intervals and reference spectra
for the MAX-DOAS and LP-DOAS measurements. Literature cross sections
listed in brackets were used for sensitivity studies.
MAX-DOAS
LP-DOAS
T [K]
O4 / H2O
O4 / H2O
HONO
BrO
OClO
H2O
H2O
Wavelength interval [nm]
Start
340
452
337
332
332
356
441
End
380
499
375
358
370
370
450
H2O vapour
298
×
×
HITEMP
×
×
×
×
×
O4
293
×
×
×
×
×
×
×
273
(×)
(×)
203
(×)
287
(×)
296
(×)
O3
223
×
×
×
×
×
243
×
×
×
293
×
HCHO
×
×
×
×
×
HONO
×
×
BrO
×
×
×
×
OClO
×
SO2
(×)
NO2
293
×
×
×
×
×
(×)
NO2
293
×
×
NO2 absorption cell
293
(×)
Ring spectrum at
273
×
×
×
×
×
DOASIS
243
×
×
×
×
which uses
Ring spectrum ⋅λ4
×
×
×
×
Polynomial degree
3
3
5
3
4
3
3
Add. polynomial degree
1
1
1
1
1
0
0
described the method of MAX-DOAS measurements,
which improve the sensitivity of passive DOAS
observations at altitude ranges close to the instrument (i.e. up to a few
kilometres). It uses scattered sunlight collected by a telescope pointing towards the
sky at different elevation angles α. The horizon is here defined as
α=0∘ and the zenith viewing direction as α=90∘. Each
elevation has a different sensitivity for absorptions in different heights of
the atmosphere. Low-elevation angles have a higher sensitivity to absorbers
close to the surface because the additional light path compared to a zenith
spectrum recorded at the same time and location is mostly located within the
lowermost layers of the atmosphere .
The SCD is defined as the integral over the
concentration ci along the light path L and is hence given in units of
molecules cm-2.
S=∫Lci(s)ds
From MAX-DOAS measurements dSCDs can be
calculated for each fitted trace gas: a Fraunhofer reference spectrum (we
follow the customary nomenclature by referring to a spectrum Fraunhofer spectrum although it
also contains spectral features from Earth's atmosphere) I0(λ) is chosen from one of
the measurement spectra and the dSCD(α)=SCD(α)-SCDref is
obtained from the DOAS fit for each elevation angle α relative to the Fraunhofer
reference.
In the measurements reported here, the DOAS fit includes the cross sections
listed in Table . By choosing references
recorded shortly before and after the measurement spectrum, the influence of
instrumental instabilities on the result was minimised as well as the
influence of stratospheric absorbers.
The MAX-DOAS instrument during ANT XXVIII/1-2
The MAX-DOAS instrument operated during Polarstern cruise ANT XXVIII/1-2 consists of
a telescope unit mounted on the deck of Polarstern on port side, which
actively corrects for the roll movement of the ship, and a spectrometer unit
with two temperature stabilised OMT (Optische Messtechnik GmbH) spectrometers (f=60 mm, |ΔT|<0.1 ∘C, Δλ<0.01 nm), which have both been modified to
minimise instrumental stray light . Both spectrometers
use back-thinned and peltier-cooled Hamamatsu S10141 CCD detectors in order
to have a high quantum efficiency in the UV range. The optical resolution of
the instrument during this campaign was 0.7 and 0.9 nm and it covered a
spectral range from 277 to 413 nm and 390 to 617 nm, respectively. Spectra were
recorded for 2 min each at seven elevation angles of 90∘ (zenith),
40, 20, 10, 5, 3, 1∘, as long as solar zenith angles
(SZA) were below 85∘. Glyoxal data from this campaign were published
in .
The MAX-DOAS Instrument during M91
A description of the instrument operated during SOPRAN cruise M91 can be
found in . The optical resolution of the instrument
during this campaign was 0.45 nm. It covered a spectral range from 324
to 467 nm. The telescope elevation control unit actively compensated the
ship's roll movement. Spectra were recorded for 1 min each at eight elevation angles of 90∘ (zenith), 40, 20, 10, 6, 4, 2, 1∘,
as long as SZA were ≤85∘.
Spectral retrieval (MAX-DOAS)
The fit settings are summarised in
Table and example fits are shown in
Fig. . As Fraunhofer reference spectra, the sum of
the two 40∘ elevation angle spectra closest in time were used. Spectra
recorded at a telescope elevation of 90∘ were not used as reference
spectra, since they could have been influenced by direct sunlight during each
of the MAX-DOAS campaigns close to the Equator. The wavelength calibration
was performed using recorded mercury discharge lamp spectra. On ANT XXVIII/1-2 these
were recorded automatically each night together with offset and dark-current
spectra; during M91 they were recorded manually.
An additional intensity offset polynomial was used in the spectral evaluation
to compensate for instrumental stray light, as described, e.g., in
.
Measurement errors of dSCDs are calculated as twice the DOAS fit error,
according to . This estimate is justified, because the
standard deviation of the residual of the linear fit of H2O / O4
ratios at 363 and 477 nm shown in Fig. amounts to 2.1
times the average DOAS fit error, and the residual spectra from the DOAS fit
are dominated by noise in the UV. This estimate disregards possible
systematic errors, but these are estimated to be small compared to the water
vapour absorption (< 2×10-4) as the residual spectra are dominated
by random noise (see Fig. ).
For the water vapour absorption near 363 nm, the wavelength interval was
chosen using the technique described in on spectra recorded
on 1 individual day (15 November 2011 at about 6∘ N,
17∘ W) of the ANT XXVIII/1-2 data set using the O4 cross section at 298 K
by ; for narrower wavelength ranges beginning above 345 nm
and ending below 375 nm, lower H2O dSCDs were observed during the day.
However, the standard deviations of the H2O dSCDs for these retrieval
intervals are 5–6 ×1023 (uncorrected) as large as the
mean dSCDs. For the larger fit intervals, the standard deviation is
significantly smaller (1–2 ×1023), and the ratio of standard
deviation of H2O dSCDs and the average fit error is close to 2, as
expected from . For the broader fit intervals, the
H2O dSCD varies for fit intervals within 330–390 nm with a standard
deviation of 16 % of mean H2O dSCD. We thus estimate the error due to
the choice of fit settings to be below 20 %.
We assume that the small absorption structures of BrO and HCHO, which
are not sufficiently constrained within fit intervals beginning above
345 nm,
cause this effect and/or possible compensation of the relatively broad O4
absorption by the DOAS polynomial.
When including HONO in the DOAS analysis for this day with low-NO2
concentrations and thus presumably low-HONO concentrations, enhanced HONO
and H2O dSCDs are observed simultaneously for fit intervals ending above 382 nm.
The blue spectral range
The effective centre of the respective absorptions of O4 and H2O
can be calculated for each fit interval [λ1,λ2] using
λm=1∫λ1λ2σ(λ)dλ∫λ1λ2λσ(λ)dλ.
In the wavelength interval from 452 to 499 nm, the effective centre of the
water vapour absorptions of λmH2O=479 nm is close to the
effective centre of the O4 absorptions at λmO4=476 nm.
The fit range was chosen to have similar effective centres of absorptions of
O4 and H2O in order to have comparable conditions for radiative
transfer at both wavelengths.
HITEMP was chosen for the water vapour absorption cross section in the blue
wavelength region. The differences in the blue wavelength region to HITRAN
2012 are negligible at a spectral resolution of 0.5 nm. HITEMP was chosen
instead of POKAZATEL in the blue wavelength range, as already a couple of
previous publications used this cross section in the blue wavelength range
see, e.g.,and references therein. As described in
Sect. better agreement with observations was found for
HITEMP than for POKAZATEL from 452 to 499 nm.
The near-UV spectral range
In the analysed wavelength interval of 340–380 nm the absorption structures
of O4 and H2O are centred around λmO4=361 nm and
λmH2O=364 nm.
As the observed OD in the fit ranges around 360 nm are small, except for the
absorption of O4 and the OD related to the Ring effect, it was
necessary to include, in addition to the normal Ring spectrum, the
temperature dependence of the Ring spectrum. The Ring spectrum itself
compensates the measured apparent optical density due to inelastic scattering
of sunlight on air molecules , which
leads to an effective filling-in of Fraunhofer lines in the measured spectrum
of scattered sunlight e.g. and references therein. The
temperature dependence originates from the temperature dependence of the
population of rotational states of the air molecules. It was calculated from
the difference of Ring spectra R(T) calculated at T=273 K and T=243 K
using DOASIS (which is based on the work from , parts
of which can also be found in ): ΔR/ΔT=(R(T-ΔT)-R(T))/ΔT. The OD associated with the Ring
spectrum temperature dependence amounts up to 5×10-4 for the M91
data set when using a Ring spectrum calculated at 273 K. For a Ring signal
of 2.5×1025 (which is typical for MAX-DOAS observations), the Ring
for a temperature difference of 30 K. We found that warmer effective Ring
temperatures were found at low telescope elevation angles, which agrees with
the lower tropospheric temperature height profile. The temperature dependence
of the derivative of the Ring spectrum with respect to temperature was found
to be smaller than 0.5 %/1 K; therefore, it was sufficient to use one
individual spectrum to linearise this effect.
The contribution of vibrational Raman scattering of air on measurements in
this spectral range not only could be correlated to the size of the Ring effect
but also
agreed in its magnitude with the calculations given in .
Its effect on the results presented here was, however, negligible and was only
consistently observed when co-adding spectra from more than four elevation
sequences and for a rms of the resulting residuals of less than
1×10-4. The effect of the wavelength dependence of the air mass factor (AMF) for the
O4 absorptions at 344, 361 and 380 nm was found to be negligible for
the spectral retrieval of water vapour absorption in this spectral range.
Results and discussion
Starting with the largest absorption band below 380 nm listed in POKAZATEL
at around 363 nm, we show first experimental evidence of water vapour
absorption in the UV from LP-DOAS measurements (Sect. ), which
have the advantage of a well-defined light-path length. These are
complemented by an even clearer detection of this absorption band by MAX-DOAS
observations (Sect. ). The magnitude of the absorption is
quantified by comparison to water vapour absorption in the blue spectral
range. From these results based on MAX-DOAS observations, a correction of the
strength of the water vapour absorption band listed in POKAZATEL is
derived. We then also estimated the magnitude of the weaker water vapour
absorption bands to be at 335 nm (Sect. ) and 373 nm
(Sect. ).
LP-DOAS: detection of water vapour absorption at 363 nm
Measurements between 22 August and 24 September 2015 were used for this
analysis, when optimal instrumental performance could be guaranteed.
Measurement spectra were co-added in order to reduce the rms of the
residual in the UV fit interval to values of 1.5± 0.3×10-4
along the total light path of 6.12 km, which resulted in a time resolution of
2 h. This corresponds to an exposure time of about 15 min for each
measurement spectrum. Due to the need to change the wavelength setting of the
spectrometer between the different spectral windows around 440 and 360 nm, the
time for each measurement sequence is shorter than the total time resolution.
A LP-DOAS-fit result for the fitting intervals around 363
and 442 nm. The spectra were recorded on 29 August 2015 between 20:58
and 21:45 UTC. Top left panel: at 442 nm the H2O dSCD
(3.0± 0.04)×1023 molec cm-2 (O4 dSCD
(2.7± 0.6)×1043 molec2 cm-5).
Top right panel: at 360 nm the H2O
dSCD (8.4± 0.6)×1023 molec cm-2 (O4 dSCD (1.85± 0.03)×1043 molec2 cm-5).
Left: correlation of H2O slant column densities (SCDs) from LP-DOAS
measurements near 363 and 442 nm. Also shown is the result from Table
line (1) from MAX-DOAS observations. Right: time series of H2O column densities
from LP-DOAS measurements near 363 and 442 nm. Values near 363 nm were corrected by
the scaling factor determined from the correlation plot on the left.
A weak correlation of the water vapour absorption around 363 nm to the
absorption at 442 nm was found with a correlation coefficient of R2=0.25
(Fig. ) for individual measurements. The rather weak
correlation is due to the large individual measurement errors. This can be
directly seen by the large variations from one measurement to the next in the
time series shown in Fig. on the right. For daily
averaged values the correlation amounts to R2=0.61. Further co-adding of
spectral measurement data could not reduce the measurement errors further, as
systematic residual structures appear (see Fig. ).
Furthermore, large NO2 concentrations of up to 20 ppb led to additional
residual structures. Selecting measurement spectra according to the NO2
concentration or rms did not improve the correlation.
As the measurement period was in late summer with temperatures
between 9 and 36 ∘C and relative humidity between 20 and 96 % leading to a water
vapour VMR between 0.4 and 1.3 % (5–16.5 g m-3), low as well as high VMRs
are not well represented in this data set. This increases the error in the
correlation of water vapour column densities determined in both wavelength
intervals (see Table ). Linear regression yields a
relative magnitude of the absorption near 363 nm of 2.31± 0.25 and an offset
of 1.6± 4.5×1022 molec cm-2. Fixing the offset to zero yields a
scaling factor for the absorption cross section near 363 nm of 2.39± 0.05.
This means, the POKAZATEL line lists underestimate the observed absorptions
near 363 nm by a factor of 2.39. The measurement error will
contribute significantly to the error of the scaling factor, as it is about
30 % of the maximally measured column density near 363 nm. Thus, we
estimate the overall scaling factor from LP-DOAS measurements
to be 2.4± 0.7.
MAX-DOAS: detection of water vapour absorption near 363 nm
The absorption of water vapour was detected at about 363 nm
(27 548 cm-1) in measurements from ANT XXVIII/1-2 and M91, using a fit interval
from 340 to 380 nm (26 316–27 548 cm-1) according to
Table . The maximum signal-to-noise ratio
during both cruises (ratio between fitted H2O dSCD and measurement
error) were 14 and 10 (15 and 20, for 16 co-added
elevation sequences). The corresponding dSCD values showed the typical
separation for each elevation angle as observed for water vapour absorptions
in the blue wavelength range. The corresponding spectra are shown in
Fig. .
Fit results from ANT XXVIII/1-2 and M91 showing the detection
of water vapour absorptions at 477 and 363 nm; in red, the modelled
absorptions according to the cross sections listed in
Table ;
in grey, the measured values. In blue, the residual is shown if no water vapour absorption was
included in the fit. The fits from ANT XXVIII/1-2 use a spectrum (exposure time:
120 s;
spectral resolution: 0.7 nm) from 16 November 2011 at 13:20 UTC
at 3∘59′06′′ N, 14∘44′40′′ W at a telescope elevation angle of 3∘.
At 477 nm the O4 dSCD is (2.47± 0.01)×1043 molec2 cm-5 and the H2O dSCD
(6.27± 0.06)×1023 molec cm-2. At 360 nm the O4 dSCD
is (2.18± 0.04)×1043 molec2 cm-5, the H2O dSCD (1.13± 0.16)×1024 molec cm2.
The fit from M91 is using one spectrum (exposure time: 60 s; spectral resolution: 0.45 nm)
recorded on 5 December 2012, 19:44 UTC at 7∘24′29′′ S, 81∘30′18′′ W at a
telescope elevation of 3∘. It shows an O4 dSCD of
(3.43± 0.02)×1043 molec2 cm-5
and a H2O dSCD of (1.18± 0.16)×1024 molec cm-2. All fits
used the O4 cross section by .
The retrieved water vapour dSCDs at 363 nm were compared to the 20 times
stronger water vapour absorptions between 452 and 499 nm
(20 040–22 124 cm-1) for the ANT XXVIII/1-2 data set. To correct for possible
influences of varying radiative transfer conditions (which may result in
different light-path lengths and thus different dSCDs), the H2O dSCDs
retrieved from both spectral windows were divided by the respective O4
dSCD from the same fitting window. These fitting intervals were selected in a
way that the wavelength of the main absorptions of O4 and H2O are
at similar wavelengths. This needs to be done in order to have approximately
the same radiative transfer properties for both absorbers (see
Sect. ). The wavelength ranges are listed in
Table . The absorption of O4 is an
indicator for the light-path length, since the O4 concentration is
proportional to the square of the concentration of molecular oxygen, which
has a well-defined and sufficiently constant concentration profile.
For Fig. , measurements at an elevation angle of
3–5∘ with an rms of less than 8×10-4 (UV) and
4×10-4 (VIS) were used; in addition, the error of the
H2O / O4 ratio calculated from the fit errors of both trace gases had
to be below 5×10-21 cm3 molec-1 (UV) and 3×10-22 cm3 molec-1 (VIS).
This implicitly removes all measurements with low-O4 dSCDs, which is the case for fog and very low clouds. These
conditions lead to different numbers of valid observations in
Table for different spectral retrieval settings.
Top left panel: ratio of water vapour
dSCD and O4 dSCD at 363 and 479 nm for a telescope
elevation angle of 3 and 5∘ during ANT XXVIII/1-2, using the O4
cross section by . Error bars represent typical
measurement errors and are calculated from fit errors of both absorbers.
Error bars for the ratios at 479 nm are omitted. They are more than 1
order of magnitude smaller than those at 363 nm. A ratio of 10-20 cm3 molec-1
corresponds to an absolute water vapour-mixing ratio of 0.01 at ground level or a vertical
column density of 5×1022 molec cm-2 or 15 kg H2O m-2, assuming a scale height
of 2 km. Top right panel: the residual of the linear fit shows a Gaussian distribution
and agrees with respect to its width of σ=6.12×10-21 cm3 molec-1 with
the mean measurement error (2 times DOAS fit error,
2.75± 0.92×10-21 cm3 molec-1) obtained from the DOAS fit. The
contribution of the statistical error of the linear fit is negligible. The individual
correlations of H2O and O4 dSCDs are shown in the lower panels,
which show individually smaller correlation coefficients than their respective ratios at
363 and 479 nm.
The scale height of O4 is 4 km, the scale height of water vapour is
typically 2 km . MAX-DOAS measurements of trace-gas
dSCDs are most sensitive to the lowermost 2 km e.g..
Thus, for a given surface volume-mixing ratio of water vapour, an almost
constant ratio of H2O and O4 dSCD is expected.
Figure shows that this approximation is valid for the
ANT XXVIII/1-2 measurements, as the correlation coefficients R2 for the individual
O4 and H2O dSCDs are smaller (0.81 and 0.74) than the correlation
coefficient R2=0.89 for their ratio.
However, the different profile shapes can introduce deviations, which were
investigated by radiative transfer modelling using the Monte Carlo radiative
transfer model McArtim . Assuming different water
vapour surface concentrations (0.1–3 %), water vapour scale heights of
1, 2 and 3 km, an aerosol layer with an extinction of 0, 0.2, 1, 2 and
10 km-1,
with a thickness of 1 and 3 km at an altitude of 0, 1, 2 and 3 km were
the resulting simulated H2O / O4 dSCD ratios correlate for both
wavelengths 363 and 477 nm, with an R2=0.98 and a slope of
1.00± 0.02.
The intercept was fixed to zero, elevation angles were
3,5,90∘ and 6480 individual simulations were performed. A significant
systematic dependence of the ratios on ground albedo, solar zenith angle and
relative azimuth angle was not observed, each of them resulting in less than
a 1 % change of the simulated O4 / H2O ratio. Simulations with small
O4 dSCDs, which result in a large simulation error for the
H2O / O4 dSCD ratio, were removed analogously to the measurements.
The Ångström exponent was varied using values of 0.0, 0.5 and 1.0
according to AERONET aerosol optical depth (AOD)
measurements during ANT XXVIII/1
.
The effect on the ratio was however also smaller than 1 %.
As for the measured data, the correlation of the simulated O4 or
H2O dSCDs individually is significantly worse with RO42=0.74
and RH2O2=0.91 compared to the correlation of their respective
ratios. The slope of a linear polynomial fit to the O4 dSCDs at 360
and 470 nm is similar to the observed values.
As seen from Fig. , the H2O / O4 dSCD ratios
from ANT XXVIII/1-2 correlate well for the wavelength ranges around 360 nm and
around 477 nm with an R2=0.89. However, the absolute magnitude of the
absorption cross section near 363 nm is underestimated by a factor of
2.6± 0.3 (see also Table ).
In Fig. the ratios of H2O and O4 dSCDs at 3∘
telescope elevation were converted to H2O vertical column densities (VCDs) assuming a light path at
ground level under normal conditions, a water vapour scale height of 2 km
and using the correction factor of 2.6. Qualitatively the latitudinal
variation of the ANT XXVIII/1-2 and GOME-2 data agree. For a quantitative comparison
further radiative transfer modelling to obtain tropospheric water vapour
profiles from the ship-based data would be needed.
Results from Fig. to determine
the relative magnitude of the water vapour absorption at 363 nm compared to
477 nm, using the HITEMP cross section for different retrieval settings using
different O4 cross sections. Values in brackets denote the error of the last
digits of the respective value calculated from the error-weighted linear regression.
For LP-DOAS measurements (see Sect. ), the correlation was done for
slant column densities (SCDs) instead of H2O / O4 dSCD ratios because the light path was constant.
The offset (LP-DOAS) was, however, normalised by the mean O4 dSCD at 360 nm in order
to have comparable values. The systematic error of the slope was determined by using the
typical relative measurement error of water vapour for measurements at a dSCD of
3×1023 molec cm-2 determined in the respective blue wavelength range.
Type
O4 cross section
R2
Slope
Syst. error [%]
Offset [cm3 molec-1]
n
1
MAX-DOAS
Thalman 273 K
0.89
2.63(1)
8
0.16(4)×10-21
2621
2
MAX-DOAS
Thalman 273 K free shift
0.88
2.61(1)
8
0.34(4)×10-21
2634
3
MAX-DOAS
Thalman 273 + 293 K
0.83
2.39(1)
8
7.25(5)×10-21
2562
4
MAX-DOAS
Hermans
0.86
2.62(1)
8
4.22(4)×10-21
2630
5
MAX-DOAS
Greenblatt
0.84
2.55(1)
9
21.1(1)×10-21
2183
6
MAX-DOAS
Greenblatt (shifted by 0.2 nm)
0.89
2.58(1)
11
10.1(1)×10-21
2586
7
LP-DOAS
Thalman 293 K
0.25
2.31(25)
30
1(3)×10-21
320
The O4 cross section is known to change its shape with changing
temperature . As this effect could potentially
introduce similar dependencies as the water vapour distribution, the spectral analysis
was run in addition to the original analysis including two O4 cross sections at
293 and 273 K. This changed the slope of the correlation shown in Fig.
by -10 % from 2.63 to 2.39 (see Table ). In addition, an increase is
observed for the offset of the linear fit, which should be ideally zero. Fixing the linear
regression line for high water vapour content at the observed values, this increase in the
offset of the linear fit corresponds to the observed change in the slope. We therefore
conclude that the observed absorption structure is not caused by the temperature
dependence of the O4 absorption cross section, but indeed by water vapour
absorption, as this offset is observed in polar regions, where almost no water vapour absorption
is expected. Note that this offset is still small and amounts to 10 % (7.25×10-21 cm3 molec-1)
of the observed maximum ratio of H2O / O4 dSCDs shown in Table .
A spectral shift of the O4 literature cross section can effectively
compensate parts of the water vapour absorption cross section at 363 nm.
This is discussed in Sect. . However, stable results were even
obtained when the shift of the set of literature cross sections was
determined by the Levenberg–Marquardt algorithm of the DOAS fit, as shown in
the second row in Table .
As seen from Table , the resulting slopes from
Fig. agree within their respective errors for
different O4 cross sections. The O4 absorption by
shows a systematic shift for the absorption at
360 nm and was therefore analysed once with the original wavelength
calibration and once shifted by 0.2 nm (used e.g. in ).
The results of the shifted O4 cross section include more measurements
but still show a significant offset of the linear regression. The results
using the O4 cross section seem more reliable, as more
data points can be used and the offset of the slope is smaller. The most
consistent results are obtained when using the O4 cross section by
, showing a small offset and the highest correlation
coefficient.
Differences using different dipole moment surfaces
The POKAZATEL line list employs the DMS from , while the
POKAZATEL (CVR, core valence relativistic) line list employs the DMS from , using the
same PES. This leads to significant differences in the intensities of the
resulting line lists in the near-UV spectral region. The magnitude of the
absorption between 362 and 365 nm in POKAZATEL (CVR) is on average 2.9 (ranges
between 2.3 and 4.6) times larger than in POKAZATEL, and might therefore
explain the observed discrepancy in the magnitude of the cross section shown
in Sect. .
However, the shape of the absorption band in the atmospheric measurements is
significantly better predicted by POKAZATEL. Fitting POKAZATEL (CVR) to measured
spectra from M91 leads to 20 % higher rms for the residual (see Fig. ) at low-elevation angles.
The additional absorption structures around 354 nm listed in POKAZATEL (CVR) are not
found in observations (cf. Fig. ).
These findings are consistent with the spectral analysis of data from ANT XXVIII/1-2.
POKAZATEL (CVR) also predicts water vapour absorption between 330 and 360 nm, which should be above our detection limit.
These could however not be identified for either of the two POKAZATEL nor the BT2
line lists during the M91 cruise (see also Sect. ).
Two MAX-DOAS fits of the same measurement spectrum from
M91 showing the detection of water vapour absorptions at 363 nm using
two different DMSs (see Sect. ). In order to reduce residual noise,
the fit is using four spectra with a total exposure time of 240 s recorded
on 22 December 2012, starting at 17:59 UTC at 15∘31′ S, 75∘36′ W
at a telescope elevation of 3∘. The POKAZATEL (CVR) line list shows a 20 %
larger residual than POKAZATEL, whose shape fits the observed optical density better.
Comparison to other line lists
As shown in Fig. , other water vapour line lists also
contain lines in the spectral range below 390 nm, which should be
theoretically above typical detection limits of our measurements (often
better than 10-4 along a light path of 10 km). However, in this spectral
range BT2 and HITEMP are based on calculations only and have not yet been
confirmed by laboratory or atmospheric measurements. The absorption at
380 nm should be clearly above the detection limit of the instrument used
during M91, but as reported in , it was not unambiguously
found and showed inconsistencies. These two line lists show further
absorption lines between 330 and 360 nm, which could also not be identified in
.
Fitting simultaneously a cross section based on POKAZATEL and a
cross section based on HITEMP or BT2 to the measurements (M91), the optical
density (from 340 to 380 nm) attributed to BT2 and HITEMP remained below
(3± 12) and (2± 8)%, respectively, of the optical density of the
water vapour absorption attributed to the POKAZATEL cross section. The
optical density attributed to BT2 and HITEMP was (-1± 6)×10-5 and
(-1± 4)×10-5, respectively, while the POKAZATEL cross section
showed absorptions of (4.5± 4.3)×10-4 for all spectra at all
elevation angles of the M91 data set with an rms of the residual of less than
4×10-4.
These findings demonstrate that the shape of the water vapour absorption
appears to be better predicted in the POKAZATEL line list than in the BT2
and the HITEMP line list. For HITEMP this was expected, since HITEMP is
partly based on BT2, but the individual line intensity cut-off leads to
changes in absorption-band shape and the significantly smaller water vapour
absorption cross section in HITEMP compared to BT2 as shown in
Fig. .
Compensation of H2O absorption by O4 absorption near 363 nm
Since the water vapour absorption is found at the red flank of the O4
absorption band at 361 nm, the absorption can be partly compensated by
shifting the O4 absorption band towards longer wavelengths. This effect
is more clearly observed for the ANT XXVIII/1-2 data set than for the M91 data set,
due to the lower spectral resolution, which seems to match better the widths
of the spectral absorption structures of O4.
Error-weighted daily averaged DOAS-fit results for the shift of
the O4 cross section for measurements with a signal-to-noise ratio
for the O4 dSCD of more than 50. For this evaluation, the shift of
the O4 cross section was freely determined by the DOAS fit and not
linked to the other absorption cross sections. Error bars denote the standard
deviation during 1 day. The shift of the instrumental calibration was determined
from the fit of the measured spectra to data from a convolved solar atlas.
When evaluating the ANT XXVIII/1-2 data set using the same settings as listed above
in Table , but allowing for a spectral
shift of the O4 cross section by , a systematic shift
of the O4 cross section of up to 0.20 nm relative to a Fraunhofer
reference calibrated using the solar atlas of is observed
in tropical regions (shown in Fig. ). A systematic shift of
the O4 cross section of up to 0.15 nm relative to a freely shifting
O4 cross section from a fit including the POKAZATEL water vapour
absorption cross section is observed. When the water vapour absorption is
included, the free shift of the O4 cross section shows a standard
deviation of 0.035 nm for measurements with a signal-to-noise ratio of more
than 50 for the O4 dSCD. The instrument calibration shows a standard
deviation of 0.007 nm due to a slow drift of 0.3 pm d-1.
It was found that a small shift of O4 with temperature (e.g. 0.05 nm as
in from 273 to 293 K) cannot explain the apparent shift of
the O4 absorption when not considering the water vapour absorption.
As described in , a spectral shift can be linearised for
small shifts by the derivative of the absorption cross section with respect
to the wavelength using Taylor expansion. Turning the argument around,
a correlation of the size of the absorption structure of water vapour and the
product of O4 absorption and spectral shift (from a DOAS fit where water
vapour absorptions are not considered) is therefore expected. This correlation is found
for ANT XXVIII/1-2 data with R2=0.89 and a slope of
aS=6.78×1018 nm molec cm-3. For this instrument with a
spectral resolution of 0.7 nm, it thus means effectively that a water vapour
dSCD of SH2O=5×1023 molec cm-2 and an O4 dSCD of
SO4=2.5×1043 molec2 cm-5 lead to a shift of the O4
cross section by aS⋅SH2O/SO4=0.14 nm, which was indeed
observed in tropical regions as shown in Fig. . The change in
overall O4 dSCD is discussed in Sect. .
Upper limit for water vapour absorption at 335 nm
The water vapour absorption band at 335 nm in the POKAZATEL line list would
amount to an OD of 1.2×10-4 for a water vapour dSCD of
4×1023 molec cm-2 at a spectral resolution of 0.5 nm.
Analogous to the procedure described in Sect. , the water
vapour absorption band at 335 nm (fit range 332–358 nm) was compared to
the water vapour absorption within the interval from 452 to 499 nm for the
ANT XXVIII/1-2 measurements, divided by the dSCD of the respective O4 absorption
band. A clear correlation was not observed (R2<0.2) due to too large fit
errors to detect water vapour in the BrO–HCHO fit range (for fit settings see
Table ). The water vapour dSCD (at
335 nm) stayed below the average detection limit of 7×1023 molec cm-2.
For the M91 MAX-DOAS measurements, the detection limit was reduced by
co-adding 16 elevation sequences. However, the correlation of water vapour
dSCDs at 335 and 442 nm was small (R2=0.2) and the 2σ
detection limit of 6.5×1023 molec cm-2 was only exceeded for 10 % of
all spectra.
We therefore conclude that the predicted magnitude of the absorption at
335 nm is correct or overestimated, as we could not find it in our MAX-DOAS
observations; if the shape of the water vapour absorption is correctly
predicted by POKAZATEL, the magnitude of the differential water vapour
cross section from 332 to 358 nm at a spectral resolution of 0.45–0.70 nm
is smaller than 2.5×10-28 cm2 molec-1.
Water vapour absorption around 376 nm
The literature values for the water vapour absorption cross sections based on
POKAZATEL and BT2 (and thus also HITEMP) differ by about 1 order of
magnitude in the spectral region between 370 and 380 nm (cf.
Fig. ). Using the M91 MAX-DOAS measurements the absorptions
listed in BT2 could not be unambiguously identified or its predicted
absorption shape did not match the observed absorptions. We therefore apply
here the POKAZATEL line list on data from the M91 campaign.
We use a fit range from 370 to 386 nm and the settings for the water vapour
absorption at 363 nm without considering the absorption cross section of
O3, HONO, BrO and HCHO. Co-added spectra based on four elevation
sequences were used in order to reduce the average fit error to
2×1023 molec cm-2 (average rms of the residual: 1.1×10-4). The
water vapour dSCD was compared to the water vapour dSCD from 340 to 380 nm,
retrieved in Sect. . The resulting correlation of
dSCDs at 363 and 376 nm is significant with R2=0.6 and a slope of
1.2± 0.3. A DOAS-fit result is shown in Fig. . As both
absorption bands are at similar wavelengths and the absorptions are small,
the difference in expected dSCDs introduced by differences in radiative
transfer are negligible compared to the measurement error itself.
This shows that the water vapour absorptions at 376 nm is found in MAX-DOAS
measurements and its magnitude is predicted in agreement with the absorption
at 363 nm. It underestimates the absorption inferred from measurements by a
factor of 3.1± 0.7.
Fit result for the same MAX-DOAS spectrum as that used in Fig.
to show the water vapour absorption at 376 nm, which correlates for the M91 data set
with R2=0.6 and a slope of 1.2 ± 0.3, with the water vapour
absorption at 363 nm. The measurement error of this individual fit amounts to 20 %.
Water vapour absorption below 330 nm
reported significant water vapour absorptions of up to
2.94×10-24 cm2 molec-1 at 330 nm and up to
2.19×10-24 cm2 molec-1 at around 315 nm.
could not confirm these findings and found upper limits
for the differential absorption of water vapour from 332 to 370 nm of
3×10-27 cm2 molec-1 from the M91 data set, which are 2
orders of magnitude smaller. also could not confirm the
values published by between 325 and 420 nm. They estimated the
water vapour absorption cross section at a spectral resolution of 0.5 nm to
be less than 2×10-25 cm2 molec-1.
For a water vapour dSCD of 4×1023 molec cm-2, the findings
of would result in differential optical depths around unity, which
is unrealistic judging from observations of BrO and HCHO in the troposphere in
wavelength intervals within 330–360 nm (see references listed in
and ). The instrument operated during ANT XXVIII/1-2 covers a wider spectral
range than in , we therefore applied the BrO–HCHO fit settings
from Table to a fit interval from 310 to 350 nm.
We used the water vapour dSCD determined from POKAZATEL at 363 nm of
4.3×1023 molec cm-2 for the spectrum from ANT XXVIII/1-2 shown in
Fig. . For the calculation of the upper limit, we
used conservatively only half of the value of the dSCD in order to account
for the shorter light path at wavelengths between 310 and 350 nm. Polynomials
with degree 0–2 were applied in the fit in order to account for broadband
absorptions and scattering, and to estimate the dependence of the inferred
upper limits on the degree of the DOAS polynomial. The polynomial could
compensate for water vapour absorption if it is a rather broad
absorption in this spectral region as suggested by . The
resulting peak-to-peak (ptp) magnitudes of the residual are listed for an
example measurement spectrum at 3∘ elevation angle in
Table . To avoid unnecessary compensation of potential
water vapour absorption by other absorbers, their dSCDs were determined using
a DOAS polynomial of the third order, then the dSCDs of the trace gases in the
fit were fixed to these values.
The resulting upper limits for the water vapour absorption cross section in
the spectral range from 310 to 350 nm are thus 200–600 times smaller than the
maximum cross section values measured by , and are 14–33 times
smaller than the upper limit value presented in .
Magnitude peak-to-peak (ptp) residual sizes and upper
limits for water vapour absorption between 310 and 350 nm at a spectral
resolution of 0.7 nm for different polynomial degrees of the DOAS polynomial.
Polynomial
ptp residual
Upper limit diff. H2O XS
degree
cm2 molec-1
0
3.0×10-3
14.0×10-27
1
1.6×10-3
5.4×10-27
2
1.0×10-3
4.6×10-27
Estimation of the accuracy of the shape and wavelength calibration of the POKAZATEL H2O cross section
The DOAS fit provides dSCDs as mentioned above, but also residual spectra.
These residual spectra are the difference between the modelled and the
observed OD (cf. Fig. ). In order to disentangle
different contributions to the residual spectra, a multi-linear regression
was performed based on the retrieved dSCDs see. This
allows for the systematic identification of residual structures caused by each of
the absorbers considered in the fit (cf.
Table ). However, since potential
differences between modelled and observed absorptions can be compensated by
any of the other absorbers, this information cannot be used to correct a
given absorption cross section. It can though yield an estimate of the accuracy of
the cross section.
For ANT XXVIII/1-2, the resulting spectrum from 340 to 380 nm, which correlates with
the water vapour dSCD (shown in Fig. ) has an rms of
1.7×10-28 cm2 molec-1 and a maximum peak-to-peak amplitude
of 1.1×10-27 cm2 molec-1. The maximum magnitude of water
vapour absorption cross section at 363 nm is 2.5×10-27 cm2 molec -1
for this spectral resolution (see Fig. ). For
M91, a residual structure at 344 nm is found, which could not be attributed
to other absorbers and is correlated with the water vapour dSCD. The
variation of humidity during M91 is significantly less than during ANT
XXVIII/1-2;
therefore, this structure could have been caused by any tropospheric absorber
with a similar concentration height profile. As this residual structure is
not observed for both data sets, we do not attribute it to water vapour
absorption.
Using a multi-linear regression on the residual spectra
from the campaigns ANT XXVIII/1-2 and M91, the water vapour dSCD-correlated
residual structures were obtained. Negative values can be explained by
compensation of the missing water vapour absorption structures by other
absorbers included in the DOAS fit. The resulting spectrum, including water
vapour absorption, yields an estimate on the accuracy of the convolved cross section.
The maximum absorption of water vapour at 363 nm according to POKAZATEL
seems to be re-shifted by 0.5 nm relative to the maximum absorption listed
in BT2 (see inset in Fig. ). To test if the wavelength of
the water vapour absorption is correct, a spectral shift of the water vapour
absorption was allowed; i.e. the shift was determined by the
Levenberg–Marquardt algorithm of the DOAS fit. As the spectral resolution is
higher, this was done for the M91 measurements. The shift of the POKAZATEL
water vapour absorption was found to agree with observations within
0.02± 0.06 nm (corresponding to 1.5± 4.6 cm-1) for measurements
exceeding a signal-to-noise ratio for the water vapour dSCD of 5 for the 16 elevation sequence
co-added M91 data sets. This result is in agreement with the
estimate of the precision of the PES by , which was able to
reproduce energy levels from laboratory measurements within about 0.1 cm-1 on
average.
Further potential error sources
As the observed OD for water vapour absorption were small in the UV (< 2%
for individual absorption lines at high spectral resolution), no saturation
correction was applied during convolution of the line list
for the spectral retrieval of MAX-DOAS data. The POKAZATEL line list does
not provide line broadening parameters; therefore, the I0 correction
also was not applied. This correction would have resulted
in a change of the convolved cross section of less than 5 %.
In the visible (452–499 nm), the saturation effect for dSCD of 6×1023 molec cm-2 amounts to less than 2 % change of the
obtained dSCD.
Uncertainties of the H2O literature cross sections in the blue wavelength range
Since we compared the UV absorptions of H2O vapour to the values derived
in the blue spectral region, the errors in the latter spectral region, which
we analyse in the following, enter into the calculation of the total
uncertainty of the UV absorption cross sections of H2O.
The uncertainty of the absolute magnitude of water vapour cross section
(HITEMP) in the blue wavelength from 452 to 499 nm is less than 15 %: The
6ν absorption band around 490 nm seems to be overestimated by
(13± 3) % relative to the 6ν+δ absorption band around 470 nm
when fitting the absorption bands separately analogously to
. This is one of the main reasons for the strongly
structured fit residual in the visible fit range shown in
Fig. .
The magnitude of the 6ν+δ absorption band around 470 nm agreed with
the magnitude of the 7ν absorption band around 440 nm according to
LP-DOAS measurements by , for which in turn an agreement
within 10 % with independent measurements of humidity and temperature was
found in the same publication.
Uncertainties of the O4 literature cross sections
For constant atmospheric water vapour content, water vapour and O4 dSCDs
from MAX-DOAS observations are typically well correlated because the bulk of
the variations in the H2O dSCD is due to variations in the path length.
Therefore, it is important to disentangle potential problems of the water
vapour absorption cross section and O4 absorption cross section. The
three available O4 cross sections for the spectral range below 400 nm
were published by , and
. The POKAZATEL water vapour line list shows a local
maximum at 363 nm (at a spectral resolution of 0.45 nm), which is at the
slope of the O4 absorption peak at 360.8 nm (see
Fig. ).
Differences in differential OD from 340 to 390 nm between different
literature O4 cross sections amount to up to 2×10-3 for a
typical dSCD of O4 of 4×1043 molec2 cm-5. This is larger than the
OD of water vapour in this spectral range as listed in POKAZATEL.
A systematic error in the respective O4 cross section, which could lead to false
apparent water vapour absorption, is expected to scale with the column
density of O4. It would thus result in a constant offset of the correlation
of H2O / O4 ratios shown in Fig. . This was not
observed.
This also agrees with the observation that the wavelength dependence of the
O4 dSCDs was found to have no result on the water vapour dSCD at
363 nm. stated an absolute accuracy of 2–4 % for the
their integrated O4 absorption cross section at 361 and 476 nm.
For strong absorbers, the AMF of the observation also depends on the
magnitude of the absorption itself . However,
for an optical density of O4 at 360.8 nm of 2.5×10-3 we
estimate a reduction of the effective light path by less than 1.3 %. This is
an OD of less than 3.5×10-4 and would result in a reduction of the
apparent water vapour dSCD by 10 %. This effect would be smaller by a factor
of 4 in tropical regions due to a smaller contribution of the O4
absorption to the total optical depth. No correlation of the water vapour
dSCDs at 363 nm with the square term of the O4 absorption was found for
the ANT XXVIII/1-2 data set.
The differences between the cross sections published by ,
and did not allow to identify
any systematic differences similar to the water vapour absorption, which
could have pointed towards water vapour contamination during the acquisition
of the cross section data.
As seen in Table , it was possible to observe
good correlations for water vapour absorption at 363 nm and around 477 nm
for all available O4 literature cross sections. The smallest offset is
observed when using the O4 cross section by . The best
correlation coefficients R2 are found for and
.
Absolute maximum O4 absorption cross section values differ for the three
available cross sections at 293 K by less than 7 % at 360 nm and less than
5 % at 477 nm. This uncertainty could directly affect the
H2O / O4 ratios listed in Table .
Influence on DOAS retrievals of other trace gases
Neglecting the water vapour absorption around
363 nm not only increases the fit errors of several DOAS trace-gas
retrievals, but also could introduce a systematic bias in the trace-gas
concentrations obtained. Trace-gas species, which are potentially influenced,
are O4, HONO, OClO and SO2.
The effect may vary for different data sets, different DOAS fit intervals and
different instrumental parameters such as the respective spectral resolution.
Here the impact on trace-gas retrieval is investigated based on the M91 MAX-DOAS
data set using the settings listed in
Table . Only fit results with an initial
rms of the fit residual of less than 4×10-4 were considered.
Impact on spectral retrievals estimated
from DOAS evaluations with and without accounting for
the water vapour absorption from POKAZATEL for the
M91 MAX-DOAS data set (at a spectral resolution of
0.45 nm or 34 cm at 363 nm/27 548 cm). The typical
difference was estimated for a water vapour dSCD of 4×1023 molec cm-2 along a 10 km long light path.
Trace gas
Wavelength
rms
Rel. change of dSCD
Typ. diff.
(nm)
per H2O dSCD
O4
340–380
-25 %
+2.9×1018 molec cm-3
+5 %
HONO
337–375
-18 %
+1.4×10-9
+22 ppt
OClO
332–370
-20 %
+3.1×10-11
+0.5 ppt
SO2
337–375
-20 %
-2.3×10-7
-3.6 ppb
O4 (340–380 nm)
For MAX-DOAS observations, the effective light-path length needs to be
determined to convert observed slant column densities into concentrations of
the respective trace gas. The absorption of the oxygen dimer O4 can be
used to infer information about atmospheric light paths
e.g.. Atmospheric aerosol extinction profiles can be
estimated by constraining the input parameters of radiative transfer models
to match the observed O4 column densities. For MAX-DOAS measurements
this approach has been described, e.g., in .
However, for some observations of scattered sunlight, the absorption of
O4 had to be corrected by a correction factor in order to
explain the measured column densities as reported by e.g..
estimated a correction factor value of 1.2–1.5 for modelled
dSCD values to match observed dSCDs. The reason for this correction
factor is so far unknown. However, for direct-sun DOAS measurements and measurements in
the tropopause showed that a correction factor is not necessary to
explain the measurements.
Recently a possible explanation for a part of these previous observations was
provided by : elevated aerosol layers in heights above 2 km,
which affected the apparent O4 dSCDs but could not be resolved from
ground-based MAX-DOAS measurements due to their limited information content
for aerosol extinction in these altitudes. Another reason for this correction
factor could be an unaccounted tropospheric absorber, e.g. water
vapour absorption.
To estimate the effect of water vapour absorption, the same evaluation
for O4 according to Table
was performed once with and once without the POKAZATEL water vapour
absorption cross section.
An increase in O4 dSCD is observed when including the POKAZATEL water
vapour absorption cross section in the DOAS evaluation.
Using the correction factor of 2.63 determined in Sect. 4.2,
including the water vapour absorptions leads to an increase in O4 dSCD
per H2O dSCD of +(2.9± 0.3)×1018 molec cm-3,
independent of the settings whether a shift and/or squeeze is allowed for the
literature absorption cross sections.
For a typical H2O dSCD of 4×1023 molec -2 in summer at
mid-latitudes and a O4 dSCD of 2.5×1043 molec2 cm-5 (10 km light-path
length),
including the water vapour absorption, leads to an absolute
increase of O4 dSCD of 1.2×1042 molec2 cm-5, which corresponds to a
change of +5.0 %.
Thus, the water vapour absorption at 363 nm cannot explain the correction
factor for O4 dSCDs introduced in various publications, it even
increases the factor by +5.0 % for measurements during summer in
mid-latitudes.
HONO (337–375 nm)
Nitrous acid (HONO) is a key species in the atmospheric chemistry of urban
air masses e.g. because its photolysis leads to
the production of OH radicals, the “detergent” of the atmosphere. Due to its
high reactivity and fast daytime photolysis, HONO concentrations are low, in
particular during daylight hours , and
thus their measurements are difficult but can be performed, e.g., by
absorption spectroscopy. If all relevant absorbers are accounted for,
spectroscopic measurements have the advantage of being less affected by
interferences, which were observed for wet chemical methods, e.g.
LOPAP e.g..
Therefore it is important to account for all possible absorbing trace-gas
species in the respective wavelength range, e.g. 337–375 nm ,
in order to further reduce the detection limit and eliminate potential biases.
Adapting the wavelength range from and using the settings
listed in Table , neglecting the water vapour absorption
in the HONO fit has led to an decrease of HONO dSCDs. The decrease is clearly correlated
to the water vapour dSCD and amounts per corrected H2O dSCD to 1.4×10-9.
This corresponds to a H2O dSCD of 4×1023 molec cm-2 to a negative bias of
HONO dSCDs by 5.6×1014 molec cm-2, which corresponds to a HONO surface volume-mixing ratio of 22 ppt along a light path of 10 km.
The rms decreases for this water vapour dSCD by 0.4×10-4 at
a typical rms of 2.2×10-4, which is a decrease of 18 %.
This decrease of dSCDs explains negative HONO dSCDs around noon during M91,
when not considering water vapour absorption.
At an elevation angle of 3∘, we obtain a distribution of dSCDs around
(-3.9± 2.4)×1014 molec cm-2 without including water vapour
absorption. Including the water vapour absorption, the HONO dSCDs are
distributed around (1.0± 2.3)×1014 molec cm-2. During the cruise
significant positive HONO dSCDs were observed close to NO2 plumes from
cities (HONO dSCDs of up to 2×1015 molec cm-2 at low telescope elevation
angles), when the cruise track was close to the Peruvian coast. Therefore, a
slightly positive average HONO dSCDs can be expected, but it is in agreement
with zero within the standard deviation of the observed values. Filtering the
results based on HONO dSCDs could have introduced a negative bias, as the
observed HONO values are generally close to the respective detection limits.
We therefore used the complete MAX-DOAS data set.
OClO (332–370 nm)
Stratospheric OClO has been observed in polar regions
e.g.. Recently, OClO
has also been observed in volcanic plumes . All of these measurements
were limited on one side of the retrieval interval close to 360 nm,
potentially indicating unaccounted absorptions or erroneous O4
cross sections.
and references therein suggested that so far
tropospheric OClO outside volcanic plumes has been observed only in polar
regions with small absolute tropospheric water vapour content.
Integrated absorption in [10-27 nm cm2] over each of the wavelength
intervals W0–W5 for different sources of cross section data. Not only for the largest
absorption structure W3 variations between the different compilations are
seen, but especially integrated absorption values relative to W3 vary. The
upper part of this table is adapted from . The bold column marks the wavelength
range, which was used as reference in Table 7 following the scheme from Lampel et al. (2015). These data are
visualised in Fig. .
Dominating polyad
8ν
7ν+δ
7ν
6ν+δ
name
W0
W1
W2
W3
W4
W5
Start of interval
[nm]
394.0
410.0
423.5
434.0
451.5
461.5
End of interval
[nm]
410.0
423.5
434.0
451.5
461.5
480.0
Source of cross section data
[10-27 nm cm2]
Integrated cross section
HITRAN 2000
0.00
0.00
0.00
69.02
0.00
31.03
HITRAN 2004
13.62
3.11
0.89
96.75
0.87
42.25
HITRAN 2008 v2009
13.71
3.13
0.90
97.07
0.88
42.46
HITEMP
21.01
15.73
4.01
106.90
4.50
51.44
BT2
26.05
23.84
7.86
116.50
8.46
62.67
HITEMP rescaled
22.06
9.91
3.09
106.90
1.62
52.98
POKAZATEL
15.98
5.26
2.00
95.7
1.48
40.26
Measured relative absorption-band strengths for the different cross sections
with respect to the absorption at W3 (the 7ν polyad; column in bold). Errors
obtained from the linear regression are shown for the last digits in brackets. The relative
DOAS fit errors are listed in Table . Results with typical DOAS fit errors
of more than 25 % of the measured values were put in square brackets. MAX-DOAS values are
corrected by the results of radiative transfer modelling .
Name
W0
W1
W2
W3
W4
Start of interval
[nm]
394.0
410.0
423.5
434.0
451.5
End of interval
[nm]
410.0
423.5
434.0
451.5
461.5
POKAZATEL
1.2605(6)
1.7052(13)
[0.8135(41)]
1
[2.1270(81)]
Typical relative DOAS fit errors in fitting windows W0–W4 at a
water vapour dSCD in W3 of 4×1023 molec cm-2 for an
individual spectrum integrated over 60 s. Values are given in % and are
corrected by the relative magnitudes given in Table .
[%]
W0
W1
W2
W3
W4
Start of interval
[nm]
394.0
410.0
423.5
434.0
451.5
End of interval
[nm]
410.0
423.5
434.0
451.5
461.5
POKAZATEL
MAX-DOAS
4
6
40
0.8
29
The 363 nm water vapour absorption band is located between two absorption
bands of OClO and thus neglecting the water vapour absorption leads to an
underestimation of OClO dSCDs and systematic residual structures.
Even when including water vapour absorption according to POKAZATEL, OClO was
not positively identified during M91 (332–370 nm) above a 2σ
detection limit of 1.6×1013 molec cm-2 at an elevation angle of
3∘, the dSCDs showed a distribution around
(-0.9± 8.0)×1012 molec cm-2. Without correction for water vapour
absorption, the dSCDs showed a distribution around
(-6.3± 8.9)×1012 molec cm-2.
Corrected by the scaling factor of 2.63 from Sect. , the
increase in OClO dSCD per H2O dSCD amounts to 3.08×10-11. The
difference in OClO is clearly correlated with the H2O dSCD with
R2=0.9. This corresponds to a H2O dSCD of 4×1023 molec cm-2 to
an increase of OClO dSCD by 1.2×1013 molec cm-2, which corresponds to a
OClO surface volume-mixing ratio of 0.5 ppt along a light path of 10 km.
Comparison of different available water vapour cross section data
in the blue spectral range, using different bands listed in Fig. . W3
was used as a respective reference in all cases and is therefore by definition unity.
All magnitudes were normalised with respect to the rescaled HITEMP absorption cross section
from to obtain relative magnitudes of each of the absorption bands W0, W1, W2, W4
and W5. A value of unity identifies good agreement with the relative magnitude of the absorption bands'
sizes according to MAX-DOAS and LP-DOAS measurements presented in .
Impact on the retrieval of other absorbers
In the spectral region below 360 nm, concentrations of HCHO and BrO can be
retrieved. For HCHO systematic problems were discussed in
and pointed towards uncertainties of the available O4 cross sections.
The absorptions listed within this fit range (336.5–359 nm) in BT2 are of
similar magnitude to BrO concentrations for the lower troposphere as reported
by . POKAZATEL also lists lines here.
So far, the absorption at 335 nm could not be unambiguously identified in
measurements but can potentially have an impact on the spectral retrievals of
tropospheric BrO and HCHO (see Sect. ).
For very high column densities of SO2, DOAS evaluation wavelength
intervals above 340 nm can be used in order to minimise saturation effects
due to large optical depths . If such
spectral evaluation schemes are applied to ground-based MAX-DOAS measurements
also using low telescope elevation angles for locations with high absolute
water vapour concentrations, water vapour absorption might need to be
also considered in the spectral evaluation of SO2. We estimated the
impact using the HONO (337–375 nm) fit settings with the additional
SO2 absorption cross section from in
Table . The overall change in dSCD was of the same magnitude
as the fit error (see Table ).
MAX-DOAS: relative water vapour absorption-band strengths in the blue spectral
range
The consistency of the POKAZATEL line list with other line lists and
measured absorption was checked in analogy to in the
blue spectral range for MAX-DOAS observations. The relative absorption
strength relative to the much stronger absorption band around 442 nm, which
is called W3 here, was determined for the POKAZATEL water vapour line list.
The different wavelength intervals are listed in Table . The
same MAX-DOAS data set (M91) and the same settings as described in
were applied. The magnitude of the absorptions W0 and W1
are underestimated compared to MAX-DOAS observations, leading to the
observation of water vapour dSCDs, which are 26 %(W0) and 71 %(W1) larger
than the dSCDs observed simultaneously for the stronger absorption W3. The
results are shown in Fig. and Table .
Overall, POKAZATEL predicts the integrated absorption cross sections in the
blue spectral range to the 480 nm range better than previous versions of
HITRAN and BT2, as seen from Table and summarised in
Fig. . It was however not used as a reference cross section in
the blue wavelength range, as HITEMP (and HITRAN2012) reproduced the observed
water vapour absorptions in the blue fit interval (452–499 nm)
significantly better. These differences, which are also seen from
Fig. , will require further investigation, as they do not
only involve a difference of the overall absorption strength of both bands
near 470 and 490 nm, but also differences in the shape of the absorption
bands were observed between HITEMP and POKAZATEL (see also
Sect. ).
Conclusions
The water vapour absorption structure predicted from calculations for
wavelengths around 363 nm by was found for the first time
in two different MAX-DOAS measurement data sets of tropospheric air masses
with optical depths of up to 2×10-3 at a spectral resolution of
0.45–0.7 nm. Additionally, it was observed for the first time in LP-DOAS
observations. Until now, to our knowledge these absorptions were neither
experimentally verified nor considered in the spectral analysis of DOAS
observations.
Comparing the strengths of the UV absorption lines of water vapour to the
water vapour absorptions listed in HITEMP between 452 and 499 nm showed that
the absorptions are indeed caused by water vapour, and that the cross section
calculated from the data provided by underestimates the
measured absorption by a factor of 2.6± 0.5. For MAX-DOAS, the different
light-path lengths in the two different wavelength windows were corrected by
normalisation with the respective O4 dSCD in the same wavelength
interval. The water vapour absorption feature at 363 nm in MAX-DOAS
measurements was identified and shown to be independent of the chosen
literature value of the O4 absorption cross section; i.e. it was found
to be at a similar magnitude for all three available O4 literature
absorption cross sections. It was also independent of the temperature-induced
broadening of the O4 cross section.
In contrast, a slight spectral shift of the O4 reference spectrum could
have compensated in previous evaluations (not including the 363 nm H2O
absorption) for the water vapour absorption, which is located on a slope of
the O4 absorption (Sect. ). This apparent shift might
have lead to wavelength calibration corrections of O4 literature
cross sections in previous publications for individual campaigns with
relatively constant H2O / O4 dSCD ratios.
Other predicted water vapour absorption features at 335 nm could not be
unambiguously identified in the measurements as these did not exceed the
respective detection limits. The absorption structure at 377 nm was slightly
above the detection limit and was found to correlate with the water vapour
absorption at 363 nm.
The identified water vapour absorption at 363 nm can have a significant
impact on the retrieval of trace gases, which absorb in the same wavelength
range, namely O4, HONO, OClO and SO2. For measurement locations
with high absolute water vapour concentrations, consideration of the water
vapour absorption at 363 nm, if included in the spectral analysis of MAX-DOAS
measurements, will lead to a reduction of measurement errors and will thus
lower the overall limit of detection. We showed that neglecting this
absorption introduces systematic biases in their spectral analysis.
During M91, for O4 dSCDs an increase of about 5 % was observed when
including the additional absorption in the DOAS analysis. Thus, the water
vapour absorption cannot explain the much larger correction factor for
O4 dSCDs introduced in various publications (it rather increases the
observed discrepancies).
For HONO the water vapour absorption explains negative HONO dSCDs of several
1014 molec cm-2 for mid-latitude absolute water vapour volume-mixing ratios.
Negative HONO dSCD at low-elevation angles were often observed around noon
during the SOPRAN M91 campaign in the Peruvian upwelling when not considering
water vapour absorption. In the same way negative OClO dSCDs in MAX-DOAS
observations at low-elevation angles of around -1×1013 molec cm-2 during
M91 could also be linked to water vapour absorption at 363 nm.
Future DOAS evaluations encompassing the spectral range around 363 nm will
require one to include these water vapour absorption features, if they aim at
residual spectra with an rms of less than 4×10-4 or try to fit
absorbers with measurement errors corresponding to optical densities of less
than 1×10-3 in mid-latitude to tropical regions.
The predictions of POKAZATEL do not yield complete agreement with the
observed absorption features. While, as discussed above, this line list
should give very accurate line positions, the situation regarding absorption
intensities is more problematic. This is indeed observed in the measurements
presented here, as the position of the absorption is found to be accurate
(shift of 0.02± 0.06 nm, or 1.5± 4.6 cm-1), while the magnitude
of the observed absorption bands differs relative to each other. This was
before also observed in the blue spectral range by .
While the ab initio dipole moment calculations
of cover an appropriate range of geometries and are expected
to be accurate, using them to construct a reliable DMS is not
straightforward. A number of studies
have shown
that it is difficult to produce analytic fits, which correctly reproduce the
intensity of weak transitions. Here we are dealing with very weak water
absorptions on the margins of detectability. For this reason we performed
some test calculations using the POKAZATEL methodology but utilising the
CVR DMS of . The results shown in Sect. indicate
that this DMS could explain the systematic underestimation
of the magnitude of water vapour absorption, but probably do not predict the
spectral shape of the absorption peak as accurately as POKAZATEL. Further
work is required on the precise representation of the ab initio DMS to
try to resolve these problems. Studies should also be performed to obtain a
more reliable representation of the water dipole moment for the purpose of
predicting absorption intensities in the near-UV. Laboratory studies on this
problem would also be very helpful.
The values for the absorption cross section of water vapour in the UV range
reported by cannot be confirmed. We derived upper limits, which
are at least 2 orders of magnitude smaller in the spectral range from
310 to 370 nm.