ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-1671-2018Detection of O4 absorption around 328 and 419 nm in measured atmospheric absorption spectraLampelJohannesjohannes.lampel@iup.uni-heidelberg.dehttps://orcid.org/0000-0001-7370-9342ZielckeJohanneshttps://orcid.org/0000-0002-6051-6813SchmittStefanhttps://orcid.org/0000-0002-7742-4990PöhlerDenishttps://orcid.org/0000-0003-0179-5194FrießUdohttps://orcid.org/0000-0001-7176-7936PlattUlrichWagnerThomasMax Planck Institute for Chemistry, Mainz, GermanyInstitute of Environmental Physics, University of Heidelberg, Heidelberg, Germanynow at: Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germanynow at: Faculty of Medicine, University of Heidelberg, Heidelberg, GermanyJohannes Lampel (johannes.lampel@iup.uni-heidelberg.de)6February2018183167116836July201711August201716December201718December2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/1671/2018/acp-18-1671-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/1671/2018/acp-18-1671-2018.pdf
Retrieving the column of an absorbing trace gas from spectral
data requires that all absorbers in the corresponding wavelength range are
sufficiently well known. This is especially important for the retrieval of
weak absorbers, whose absorptions are often in the 10-4 range.
Previous publications on the absorptions of the oxygen dimer O2–O2 (or
short: O4) list absorption peaks at 328 and 419 nm, for which no
spectrally resolved literature cross sections are available. As these
absorptions potentially influence the spectral retrieval of various trace
gases, such as HCHO, BrO, OClO and IO, their shape and magnitude need to be
quantified.
We assume that the shape of the absorption peaks at 328 and 419 nm can be
approximated by their respective neighbouring absorption peaks. Using this
approach we obtain estimates for the wavelength of the absorption and its
magnitude. Using long-path differential optical absorption spectroscopy
(LP-DOAS) observations and multi-axis DOAS (MAX-DOAS) observations, we estimate
the peak absorption cross sections of O4 to be (1.96±0.20)×10-47 cm5 molec-2 and determine the wavelength of its maximum
at 328.59±0.15 nm.
For the absorption at 419.13±0.42 nm a peak O4 cross-section
value is determined to be (5.0±3.5)×10-48 cm5 molec-2.
Introduction
The collision-induced absorption of the O2–O2 dimer (or
short: O4) needs to be considered in various wavelength regions for
in situ and remote sensing absorption spectroscopy of various atmospheric
trace gases. Furthermore, the O4 absorption governs the budget of
tropospheric singlet oxygen O2(1Δ) ,
which can potentially impact the oxidation of atmospheric trace gases,
like CO and SO2. In addition,
the absorption of O4 itself can be used to deduce information about the
actually observed light paths for passive remote sensing applications by
applying inverse modelling methods and radiative transfer models.
and
reported absorption
peaks observed for liquid oxygen at 328 and 419 nm (for an overview, see
Table ). Most of the absorption peaks in the UV–vis range of
liquid oxygen can be also found for gas-phase oxygen, as measured by
, , ,
, and others. It was first observed in the atmosphere
by . These absorptions are potentially shifted by
less than a nanometre compared to the liquid phase. A clear difference of the
relative magnitude for the absorption of O4 in the liquid and gas
phases cannot be seen from the available data.
Spectrally resolved cross-section data suitable for spectroscopy applications
exist for the absorption peaks at 344, 361, 380, 447, 477, 533 and 577 nm and
continue further into the red spectral range. Due to instrumental
limitations (detection limits and/or covered spectral range), spectrally
resolved cross-section data for the absorption peaks at 328 nm (transition
3Σg-+3Σg-→1Σg++1Σg+(ν=3); see also
Table ) and 419 nm (transition 3Σg-+3Σg-→1Σg++1Δg(ν=2)) are, to our knowledge, not reported in
literature.
measured the intensity of the absorption at 328 nm to
be 4.2 % of the O4 absorption at 360 nm, i.e. 15 % of the O4
absorption at 344 nm. For an O4 differential slant column
density (dSCD) of 4×1043 molec2 cm-5, as
e.g. found in MAX-DOAS observations under low elevation angles, this
corresponds to an optical depth of 1.7×10-2 at 360 nm
and thus an optical depth of 7×10-4 at
328 nm. This could introduce systematic biases in the spectral retrievals of
BrO, HCHO, OClO and SO2, when this wavelength range is included in the
respective spectral analysis, as e.g. in satellite retrievals of HCHO
, various observations of BrO in the boundary layer (see
, and references therein) and ground-based measurements of
SO2 (e.g. and ).
Convoluted to a spectral resolution of 0.5 nm, the overlaying absorption
structure of HCHO at 328 nm has a peak absorption cross section of
4.5×10-20 cm2 molec-1. Typical
ground-based HCHO dSCDs observed on the tropical open ocean are about
1–3×1016 molec cm-2e.g. and lower
at higher latitudes (e.g. ), which corresponds to an
optical depth of 4-14×10-4 at 329 nm. Therefore it is important
to consider the possible O4 absorption at around 328 nm if the retrieval
wavelength range for HCHO is extended towards shorter wavelengths below
330 nm. The recommended HCHO setting from suggests a fit
interval from 336.5 to 359 nm and thus does not include the O4 absorption
at around 328 nm. This recommendation is for an analysis with fixed
Fraunhofer reference spectrum. If sequential references are used (as it is
typically done today for the spectral retrieval of weak absorbers, as e.g. in
), then the spectral range can and should be
extended towards shorter wavelengths (e.g. 323 nm).
In addition to that, for the spectral retrieval of BrO,
found a lower limit for the choice of reliable fit intervals at about
330.6 nm, which could have been caused by the neglected O4 absorption at
around 328 nm – the FWHM (full width half maximum) of the known absorption
peaks at 344 and 360 nm amounts to 4 nm. The method from
was applied here on measured atmospheric spectra along different light path
lengths. This fit interval was then widely used, e.g. in ,
, , and
. Widening the spectral retrieval interval for both species
could reduce the fit error of BrO and HCHO dSCDs, as relatively large
absorption bands are found for both species below 330 nm.
For the absorption at 419 nm no estimate for its intensity is known.
However, this spectral region is also known for uncertainties of the
available water vapour cross section as reported in ,
which overlays potential O4 absorptions. Using the same ratio of peak
absorption cross sections of O4 at 477 and 446 nm, an absorption band at
419 nm could potentially have a peak optical depth (OD) of 2×10-4 for
an O4 dSCD of 4×1043 molec2 cm-5. This spectral range
is of interest, as it is typically included in the spectral analysis of IO
e.g.. The potential to obtain residual
spectra with an root mean square (RMS) below 1×10-4 has been
demonstrated e.g. and allows us to
measure IO at sub-ppt concentrations, corresponding to an OD along 10 km
light path of <7×10-4. Such concentrations have been reported
on the open ocean by and were found above their respective
limit of detection by in polar regions (0.2–0.4 ppt
using LP-DOAS and MAX-DOAS observations).
In this work, spectral data from three different instruments and
environmental settings, utilizing two different remote sensing geometries,
are
presented in order to estimate the magnitude of the O4 absorption peaks at
around 328 and 419 nm. The different techniques used and the different
locations of the measurements make it very unlikely that instrumental
artefacts or local atmospheric influences are responsible for the observed
absorption structures.
Measurement campaigns
In this section we shortly describe the measurement campaigns, during which
DOAS data were collected that are used in this paper.
Antarctica LP-DOAS 2012
In 2012, a measurement campaign was undertaken based at New Zealand's
research station Scott Base in Antarctica. The station is situated on Ross
Island at 78∘ S and the campaign lasted throughout austral spring,
from the end of August until the end of November.
The LP-DOAS instrument used in this campaign has been used in previous
studies and therefore has already been extensively described elsewhere, e.g.
in and .
In brief, light is coupled in and out of a telescope using a Y-shaped fiber
optic bundle consisting of seven individual fibers. The common end is mounted
near the focal point of the telescope, the single central fiber is connected
to the spectrometer and the six outer fibers are attached to a
800 µm mono fiber which leads to the light source. A diffuser
plate can be driven into the light path in front of the telescope end of the
fiber in order to record spectra of the lamp (so-called optical shortcut
spectra). For most of the campaign, a 75 W xenon arc lamp (Osram
XBO, not ozone-free) was used as the light source. During the last 2 weeks,
a 500 W arc lamp (PLI Hanovia HSAX5002) was used, which has a higher
luminous density and thus leads to a lower limit of detection in the deeper
UV region around 300 nm. Both lamps were coupled into the fiber using a
single fused silica lens. Besides several colour filters to reduce stray
light, the lamp housing also featured a shutter in front of the fiber
coupling to block the lamp in order to be able to record background spectra
with the telescope, i.e. spectra of the light entering the telescope due to
scattering on the surface or air and not from the dedicated light source. The
spectrometer, an Acton 300i, was used with a resolution of 0.50 nm and
spectra were recorded by a back-illuminated CCD camera from Roper Scientific
(Spec-10:2KBUV).
Two light paths were set up approximately 1.5 m over sea ice, for which two
retro reflector arrays were deployed. The retro reflector arrays reflecting
the light back into the telescope consisted of 12 (short path) and 50 (long
path) individual elements made of fused silica with a diameter of 63.5 mm
each. One array was located closer to the station, to be able to measure
during periods with fog or snow drift, the other one further away to achieve
lower detection limits. They were located at distances of 1.46 km (short)
and 4.01 km (long).
The measurement procedure was as follows: spectra were acquired alternatingly
on both light paths. For each light path, spectra were recorded at four
different wavelength intervals. The region of interest for this study is
between 271 and 355 nm. For each of those regions, 25 individual spectra
with a saturation of 70 % were averaged to one measurement spectrum, with
a maximum duration of 1 s per individual spectrum. Typical exposure times
for one individual spectrum were between 6 and 500 ms depending on the
wavelength range, visibility conditions and the used arc lamp. Thirty-two measurement
spectra are averaged afterwards to achieve an optimal signal-to-noise ratio.
MAX-DOASPolarstern 2014–2016
The MAX-DOAS instrument used during these cruises PS88-PS98 (ANT XXX, ARK
XXIX, ANT XXXI
The respective cruise reports can be found at
https://www.pangaea.de/expeditions/cr.php/Polarstern.
) on board RV
Polarstern from October 2014 to April 2016 is described in
and the same upper limits for the spectral stability
of the instrument also apply here for each leg. As during previous cruises,
the exposure time per spectrum was set to 2 min. Spectra were recorded
at seven elevation angles of 90∘ (zenith), 40, 20, 10, 5, 3 and 1∘ as long as solar zenith angles were below 85∘. To
reduce RMS, four elevation sequences were co-added before the DOAS analysis.
Penlee Point Atmospheric Observatory 2015–2016
MAX-DOAS measurements were performed from 3 April 2015 to 3 March 2016 at the
Penlee Point Atmospheric Observatory (PPAO) on the south-west coast of the UK
(e.g. ).
Similar to the instrument used in during the MAD-CAT
campaign, the EnviMeS
Now continued by Airyx GmbH, Eppelheim,
Germany, http://www.airyx.de.
MAX-DOAS instrument is based on an
Avantes ultra-low stray-light AvaSpec-ULS2048x64 spectrometer (f=75 mm)
using a back-thinned Hamamatsu S11071-1106 detector. The spectrometer is
temperature stabilized (ΔT<0.02∘C). The UV spectrometer
covered a spectral range of 296–459 nm at a FWHM spectral resolution of
≈ 0.55 nm (at 334 nm) or ≈ 6 pixel. The spectral
stability was sufficiently high with a maximum spectral shift within 1 day of
less than 3 nm after
correction for the tilt effect . During the night,
mercury (Hg) discharge lamp spectra were recorded automatically in order to
measure the instrument's spectral response function. No significant change of
the response function was observed during the campaign.
The elevation sequence included elevation angles of -2, -1, 1, 2, 3, 5,
10, 20, 40 and 90∘, heading towards an azimuthal south-westerly
direction of 245∘. After 22 January 2016, the azimuthal viewing
direction was changed to a south-easterly direction of 147∘. Spectra
with a total exposure time of 1 min were recorded at an adaptive
integration time per scan in order to obtain spectra with a maximum
saturation of 50 %. From 5 June to 27 August 2015 the total exposure time
per spectrum was reduced to 10 s.
The inherent non-linearity of the measured intensity values with respect to
the actual incoming intensity of the spectrometers was corrected for by
multiplying all intensities with a non-linearity correction polynomial. This
polynomial was determined from a set of spectra recorded at different
exposure times, which were recorded previous to the campaign using a
temperature stabilized “white” LED light source.
In order to have a coherent data set and to reduce the RMS noise of the fits,
spectra of subsequent elevation angle sequences during 1 day were co-added
during preprocessing in order to obtain a consistent MAX-DOAS data set with
spectra with a total exposure time of 4 min.
Retrieval wavelength intervals for DOAS measurements. Values in
parentheses were used for sensitivity studies
only.
MAX-DOAS LP-DOASO4 328 nmO4 419 nmO4 477 nmO4 328 nmWavelength interval322.5/311.5410450320nm358439490347.5O3223 K××××223 K(×)Taylor expansion terms 243 K×HCHO××BrO××NO2293 K××××O4××××SO2(×)HONO(×)OClO(×)H2O298 K××HITRAN 2012 corrected IO×Glyoxal(×)Ring spectrum at 273 K ×××DOASIS Ring spectrum at 243 K ×Ring spectrum ⋅λ4×××VRS (N2) (×)×Background and shortcut spectrum ×Polynomial degree 5534Add. polynomial degree 111Method
Two DOAS methods were applied to quantify the O4 absorption around
328 nm. The MAX-DOAS measurements have often longer effective light paths,
which are not initially known. LP-DOAS measurements have the advantage
that the absolute light path length is known, but they often yield larger fit
residuals.
MAX-DOAS
The MAX-DOAS elevation sequences were evaluated against a current Fraunhofer
reference using the sum of the two nearest zenith sky spectra in order to
minimize the effect of stratospheric absorbers. At the same time, this
approach minimizes the effect of instrumental instabilities on the data
evaluation.
The literature absorption cross sections listed in
Table were convoluted with the
measured Hg emission line at 334 nm. The Ring spectrum was
calculated using DOASIS , the correction spectrum for
vibrational Raman scattering of molecular nitrogen VRS;
was calculated from the Fraunhofer reference itself,
shifting the spectrum by the corresponding vibrational energy quantum.
Neither HONO and SO2 nor OClO were not detected in significant amounts.
Water vapour absorption in the UV as reported in is
small below 358 nm (σ<3×10-28 cm2 molec-1)
and thus negligible, especially in polar regions.
LP-DOAS
The optical depth was calculated by dividing atmospheric and optical shortcut
spectra after their respective background spectra were subtracted. The
optical densities were then high-pass filtered and afterwards the fit was
applied. The applied fit scenario settings are shown in
Table . As for the MAX-DOAS evaluation,
the literature cross sections were convoluted with a measured Hg emission
line at 334 nm.
Hypothetical O4 absorption cross sections
The absorption peak shape of the O4 absorption at 328 and 419 nm cannot
be deduced directly from field measurements, as a large number of other
absorbers (HCHO, BrO, SO2, HONO, OClO, NO2, O3, H2O and others)
potentially overlay the respective O4 absorption peak. Their abundance is
unknown in field observations. Additionally, their absorption cross sections
may not be known precisely enough (e.g. water vapour;
) to determine their abundances in
other spectral ranges and thus constrain their overall absorption in order to
obtain an extended O4 absorption cross section from MAX-DOAS or LP-DOAS
field measurements. Dedicated laboratory studies will be needed.
The shape of the O4 absorption peaks at 344, 360, 380 and at 446 and
477 nm is similar as shown in Fig. a when plotting the
absorption cross section (here from ) over the
difference in wave numbers to the peak absorption. We therefore guessed the
shape of the potential absorption bands at 328 and 419 nm by shifting the
O4 absorption peaks at 344 and 446 nm by 1414 and 1476 cm-1,
respectively. These shifts were chosen arbitrarily based on previous
publications , which
list the wavelengths of the respective absorption peaks.
This approach is reasonable, as the width of the O4 absorption peaks is
defined by the lifetime of the collision complex
and is thus related to the energy of the
respective absorption peak. The absorption peak shape at 477 nm was
parameterized by , but we are not aware of
parameterizations or quantum–mechanical calculations yielding absorption
peak shapes at other wavelengths at room temperature.
Shifted (a by wave number; b by wavelength) and
normalized O4 absorption cross sections at 446 and 477 nm according to
.
Relative intensities and peak wavelengths of O4 absorption peaks
below 500 nm: ElKn , SaSt
, DiKl intensities manually extracted from
Fig. 4 and ThVo . The values in
the column for relative intensities are normalized to the value of the
absorption peak at 344 or 446 nm, respectively. Transitions according to
.
As we assume the maximum of the O4 absorption peak at 328.2 nm
and at 419 nm
, its exact wavelengths needed to be
determined again from the measurements. This was done based on experimental
MAX-DOAS data, here from PPAO: if introduced as a free parameter in the a
DOAS fit, the individual shift of the hypothetical O4 absorption
cross section can be used in order to derive a more accurate estimate for the
spectral position of the absorption peak. Measurements, which showed a ratio
of fitted dSCD and dSCD fit error of more than 8 were considered for this
analysis. The resulting peak position was found at 328.59±0.15 nm
and thus results in an overall shift of the absorption at 344 nm of
+1359 ± 13 cm-1. In the blue wavelength range it was found at
419.13±0.40 nm and thus results in an overall shift of the
absorption at 446 nm of +1469 ± 23 cm-1. For the absorption
around 419 nm a larger scatter of shift values was observed, probably due to
uncertainties of the overlaying water vapour absorption.
Exemplary LP-DOAS fit result of a measurement from 22 November 2012
on the 8 km light path. The O4 absorption at 328 nm is clearly visible.
The retrieved column densities are O4(2.67±0.04)×1043 molec2 cm-5 and O4 328 nm (6.09±0.38)×1042 molec2 cm-5.
Absorption band at 328 nmLP-DOAS
The LP-DOAS data evaluation reveals the suspected absorption structure at
328 nm nicely, given the extensive averaging of the data as described in
Sect. . An exemplary DOAS fit is shown in
Fig. . Compared to the ozone and O4 absorption, the other
species feature only a negligible optical depth in this example. Besides
ozone and the O4 absorption band at 343.8 nm, the 328 nm absorption of
O4 is the most prominent absorption feature in this case and spectral
region.
LP-DOAS correlation of the suspected 328 nm absorption feature with
the known absorption at 343.8 nm. Data for the short light path are shown in
orange and for the long path in blue. Linear regression results are shown with
and without for a constant offset term.
Results for the O4 absorption at 328 nm for
different fit ranges and settings. MAX-DOAS data were selected according to
RMS <4×10-4 and LP-DOAS data below an RMS of 1.2×10-4,
which then results in a different value for n, the number of valid
observations. The correlation coefficient R2 was calculated in each case.
The peak magnitude of the absorption cross section at 328 nm was calculated
using the O4 cross section published by using the
maximum cross-section value of 9.5×10-47 cm5 molec-2 at
343.8 nm. The wavelength dependence of the O4 AMF for the MAX-DOAS
observations was not corrected here.
In order to compare the relative absorption strength of the 328 and 344 nm
bands, the entire data set was filtered for evaluations with a residual RMS
better than 1.2×10-4 since the structure of interest can only
then be retrieved with sufficient accuracy. Typical optical densities of the
328 nm absorption are between 4 and 6×10-4 along the 8 km light
path and column densities reach up to a maximum of around 20 times the
instrumental detection limit.
Figure shows the correlation of the LP-DOAS column
densities of the suspected 328 nm absorption feature with the well-known
absorption feature at 344 nm. Data for the short light path are shown in
orange, while data for the long path are shown in blue. A correlation between
the suspected and the known absorption is visible. As expected, higher column
densities for the longer light path and lower ones for the shorter light
path. A linear fit performed on the entire data set indicates a relative
absorption strength of 0.183±0.015 (see also
Table , where cross-section values are listed) at 328 nm
compared to the known 344 nm band when allowing for a constant offset.
Fixing this offset to zero results in a slightly larger relative absorption
strength of 0.212±0.006. The given error is the error of the linear
regression analysis and does not reflect the actual measurement error of each
individual observation.
A variation of the total O4
SCD along both
light paths can be observed, which is found simultaneously for both O4
absorption bands at 328 and 344 nm, leading to the distribution of the
measurements shown in Fig. . This points towards a
general property of the absorber O4 and can be explained by temperature
variations (up to 35 K within the measurement period) and pressure
variations. We estimate that almost two-thirds of the observed variation are
due to the changing absorption cross section with temperature
, one-third due to the change in number density according
to the ideal gas law. Pressure variations between 962 and 1006 hPa during
the observations also contribute. Variations depending on temperature and
pressure were also reported by and
for ground-based MAX-DOAS observations and
simulated for satellite observations by and
. A further and more detailed analysis of these
dependencies is not within the scope of this publication.
MAX-DOAS fit from PPAO, 6 July 2015, 11:18 UTC, at 3∘
elevation and a 90∘ elevation reference spectrum. Spectra were
co-added to obtain a total exposure time per spectrum of 4 min. The 328 nm
O4 absorption structure found here is 14 times as large as the DOAS fit
error. Without considering this absorption, the HCHO dSCD in this case is
larger (+2.5 ×1014 molec cm-2 or 1.4 %), and the
RMS of the residual increases by +22 %. “RS-temp” denotes the
difference between Ring pseudo-absorption spectra calculated at 273 and
243 K as listed in
Table .
Correlation of O4 dSCD at 344 nm and the fitted O4 OD at
328.5 nm assuming the same shape of the absorption cross section as at
344 nm, using the PPAO MAX-DOAS data set. The wavelength dependence of the
O4 AMF was not corrected here.
MAX-DOAS
For the PPAO data set, several sensitivity studies were performed in order to
estimate the effect of ozone absorption and the contribution of VRS on the results.
To account for non-linear effect of strong ozone absorption, we also included
a wavelength-scaled version of the ozone absorption as well as its square
term, as suggested in and . This was
mainly necessary when extending the fit interval below 320 nm to include
additional absorption bands of HCHO. One part of the absorption of ozone is
due to changes in the slant stratospheric ozone column during the measurement
of the elevation sequence and thus assumed to be symmetric and not introduce
a systematic bias on the evaluation of the O4 absorption band. The
tropospheric contribution is of similar magnitude.
The additional intensity caused by VRS did not significantly change the
result as seen from Table .
BrO was analysed in the spectral range from 332 to 358 nm and not found above
the detection limit (1×1013 molec cm-2) at PPAO. During
the Polarstern cruises the amount of HCHO (evaluated in the same spectral
range) was typically smaller than at PPAO; however, also significant amounts
of BrO were observed during Arctic spring (May–June; up to
1.4×1014 molec cm-2 at 3∘ elevation) and
Antarctic spring (December–January; up to
5×1013 molec cm-2 at 3∘ elevation). We therefore
exclude the possibility that the absorption at 328 nm is caused by erroneous
BrO or HCHO absorption cross sections.
As the MAX-DOAS measurements took place in regions without strong
anthropogenic pollution, tropospheric NO2 absorption does not
significantly contribute to the overall magnitude of the observed residual
spectra.
The resulting estimates for the absorption cross section of O4 at 328 nm
can be found in Table . Good correlations of the
absorptions at 328 and 344 nm can be found for LP-DOAS observations as well
as for both MAX-DOAS data sets. For MAX-DOAS different settings for the
spectral retrieval were tested and yielded similar results, but for the
larger fit interval slightly larger (+10 %) absorption cross-section
peak values are estimated. The low correlation for the setting not using the
Taylor expansion approach by on the large fit interval was
expected as here strong ozone absorption can produce significant residual
structures, which can interfere with other fitted species. Note that the
values in this table were not yet corrected for the influence of the
wavelength dependence of the air mass factors (AMFs), which is discussed shortly in the following
paragraph.
Uncertainties
For PPAO, the distribution of fit errors of the O4 absorption at 328 nm
has its maximum at 5×1041 molec2 cm-5 (similar as in
Fig. , corresponding to a peak OD of 4.6×10-5
using the shifted absorption peak from 344 nm), while the distribution of
O4 dSCDs at 328 nm has its maximum at 6×1042 molec cm-2. Thus most observations are above the detection
limit.
The influence of strong ozone absorption is largely compensated for by the
Taylor expansion approach and even allows fits at sufficiently low RMS down
to 311.5 nm. The deduced magnitude of the O4 absorption at 328 nm is
slightly larger for larger fit intervals, compare Table .
The effect of VRS is negligible especially at the lower end of the fit
interval, as can be also seen from Fig. . It is, however,
correlated with the Ring signal as previously reported for the blue spectral
range and increases slightly the number of valid observations, while the
deduced magnitude of the O4 absorption stays constant within the fit
error.
The difference in MAX-DOAS AMFs of O4 for low elevation
angles at of 328 nm compared to 344 nm is expected to lead to an
underestimation of the absolute O4 absorption cross section at 328 nm
with the approach presented above. Using a set of 10 representative aerosol
profiles with aerosol optical thicknesses ranging from 0 to 5 and simulating the
resulting O4 dSCDs at 328 and 344 nm using SCIATRAN
, an underestimation of 14 % is yielded. This is
slightly less than one could have expected for the pure Rayleigh case,
yielding ≈1-(328/343)4=16 %. Applying this correction of
14 % to the observed data from Table , this means that
the LP-DOAS result for the regression without allowing for an offset fits
best the corrected MAX-DOAS result of 1.72×1.14=1.96. This makes
sense as for the reference measurements (at a light path length of 0 m), the
column density is by definition zero. Estimating the potential systematic
measurement errors by the measurement error itself, LP- and MAX-DOAS results
agree with each other with and even without the correction for the wavelength
dependence of the AMF, as the measurement errors and therefore the scatter of
data points are of the same magnitude as the expected difference between the
two DOAS measurement types.
As seen from Table the relative intensity of the absorption
peak at 328 nm relative to 344 nm of 0.18 is found in between the values
listed in of 0.15 and
with a value of 0.22.
Absorption band at 419 nm
If assuming a large O4 dSCD of 1×1044 molec2 cm-5 and
using the ratio of the magnitudes of the other 3Σg-+3Σg-→1Σg++1Δg absorption bands at 446 nm and 477 nm of about 12.7
, the peak magnitude of the O4 absorption at 419 nm
could be expected to reach an optical density of 4×10-4. It is,
however, unclear whether this extrapolation is valid. For the
3Σg-+3Σg-→1Σg++1Σg+ absorption bands at 361, 344
and 329 nm, this is approximately the case, as seen in Sect. .
The absorption structure reported by
around 419 nm is overlayed by water vapour absorption at around 416 nm with
a peak absorption of 3×10-3 for a water vapour dSCD of 4×1023 molec cm-2 at a spectral resolution of 0.5 nm using the
HITRAN2012 line list . From MAX-DOAS and LP-DOAS
observations it was reported that these absorption lines are overestimated by
a factor of 2 in HITRAN2012 , while the overall shape
is relatively well reproduced. Thus the water vapour absorption contains some
uncertainty, which could have an effect on the detection of the O4
absorption band at 419 nm. This is the main reason why the wavelength of the
maximum O4 absorption around 419 nm is difficult to estimate.
Therefore the absorption band at 419 nm was fitted in different wavelength
intervals, including and excluding the larger water vapour absorption peaks
around 442 nm. The variation of the absolute O4 absorption at 419 nm in
these two intervals was less than 25 %.
To circumvent the potential influence of water vapour absorption in this
spectral region, an overdetermined system of linear equations was set up in
order to quantify the contribution of water vapour as well as O4
absorption on the apparent O4 absorption at 419 nm. The “true” dSCDs,
S, were determined at a wavelength close to the wavelength of interest, in
this case for the water vapour absorption bands and the O4 absorption band
at around 477 nm. These are both more than 10 times stronger than their
respective absorptions between 410 and 420 nm. This can be done as enough
variation in water vapour concentrations is found for all MAX-DOAS campaigns
included here, thus not leading to linearly dependent data points of water
vapour and O4 dSCDs.
cH2O442nm⚫SH2O+cO4⚫SO4470nm=SO4419nm
The resulting cO4 is 0.0785±0.0070. The correction for
wavelength dependence of the AMF is estimated for the pure Rayleigh case,
yielding ≈1-(419.0/446.4)4=22 %. In other words, the
absorption peak at 419 nm is 10.4±0.9 times smaller than the
absorption peak around 445 nm.
The resulting cH2O is (3.4±1.0)×1018 molec cm-3. For a H2O dSCD of 3×1023 molec cm-2, this results in a change of the O4 dSCD of 1×1042 molec2 cm-5; i.e. the main variation of the obtained O4
dSCD is indeed caused by the O4 dSCD variation. These show a mean value of
2.9×1043 molec2 cm-5 and a standard deviation of
2.1×1043 molec2 cm-5 at 3∘ elevation.
However, as the exact shift of the O4 absorption structure is only poorly
restricted from the MAX-DOAS measurements, the remaining uncertainty strongly
depends on the position of the absorption peak. Using different fit settings
and different positions of the absorption peak (±0.3 nm) yields results
for cO4 within 0.08±0.05. When
ignoring the possible interference with water vapour absorption, 0.12 is obtained. This then
results in a peak value of the O4 absorption cross section (after
correction of radiative transfer effects) of (5.0±3.5)×10-48 cm5 molec-2 at a wavelength of 419.13±0.42.
Despite the large error, this is significantly smaller than the relative
intensity listed by but agrees well with the
relative scaling of the larger absorption peaks at 477 and 447 nm as listed
in Table .
LP-DOAS observations
The O4 absorption around 419 nm is close to its detection limit. The
maximum of the distribution of O4 fit errors is 7×1041 molec2 cm-5, and the maximum of the distribution of O4
dSCDs is 6×1042 molec2 cm-5.
Using the available data sets, a reliable conclusion on the magnitude of the
O4 absorption is difficult to draw. With an estimated value of 0.07±0.05 the ratio of magnitudes to the next larger O4 absorption peak seems
to be similar as the ratio of the magnitudes of the other
3Σg-+3Σg-→Σg++1Δg absorption bands at 446 and
477 nm of about 0.079.
Conclusions
We analysed atmospheric measurements of LP- and MAX-DOAS setups
from different field campaigns in order to estimate the magnitude and
wavelength of previously reported O4 absorption peaks at 328 and 419 nm,
for which no spectrally resolved literature cross sections are currently
available and which have not been reported from atmospheric observations so
far.
The main conclusion is that both O4 absorption peaks at 328 and 419 nm
can be observed using current MAX-DOAS and LP-DOAS setups and therefore have
the potential to introduce biases in the spectral retrieval of weak
absorbers. Therefore, further laboratory studies are needed to quantify the
magnitude of these small absorption peaks and to obtain a continuous
absorption cross section in between these absorption peaks. We suggest
addressing at the same time uncertainties in recent water vapour absorption
cross sections (see e.g. and
) at around 419 and 360 nm in a controlled
laboratory environment with well-known light path lengths at different
pressure and humidity values. The availability of such a data set has the
potential to improve the spectral retrieval of various trace gases
substantially as pointed out below.
The O4 absorption peak at 328 nm was unambiguously identified. Its
magnitude agrees with a previous publication by and is
found to be 0.19±0.02 of the magnitude of the next absorption peak at
344 nm. This results in a maximum peak absorption cross section based on
of (1.96±0.20)×10-47 cm5 molec-2 at 328.59±0.15 nm.
The impact on incoming sun radiation in the spectral region from 323 to
331 nm is small and amounts to 7.8×10-4 Wm-1 for zenith sun
and an O4 vertical column density (VCD) of 1.3×1043 molec2 cm-5 using the solar atlas by .
It has therefore only a small impact on the radiative balance in the
atmosphere.
It is interesting to note that the potential SO2 oxidation by singlet
oxygen requires singlet oxygen molecules with the
vibrational eigenstate ν>2. In particular this weak absorption near
328 nm (and near 315) leads to the formation of singlet oxygen molecules in
the third (and fourth) vibrationally excited state (see Table )
and thus could play a role for SO2 oxidation in the atmosphere.
The impact on trace gas retrievals depends on the fit settings of the
respective trace gas and instrumental properties. For DOAS measurements in
pristine to semi-polluted regions, a significant impact is expected for
spectral retrievals of HCHO, BrO, SO2 and OClO, which could encompass this
spectral region. Previous publications often avoided this spectral region. We
suggest that the reason for the previously observed discrepancies was often
the O4 absorption, which was not accounted for, rather than the increasing
influence of tropospheric and stratospheric ozone absorption towards shorter
wavelengths.
Incorporating this O4 absorption in the spectral retrievals of the
above-mentioned absorbers will lead to a substantial improvement of the
respective detection limits as additional absorption bands can be included in
the spectral retrieval. In our evaluations, extending the fit range lower
limit from 332.5 to 322.5 nm led to a reduction of the fit error by
≈ 35 % for HCHO and BrO. It furthermore significantly reduced
the previously observed interferences between the BrO and HCHO absorption
structures .
The O4 absorption peak at 419 nm was difficult to identify using the
method presented here, as it is difficult to exclude the possible influence
of water vapour absorption which overlays the O4 absorption structure. For
water vapour absorption in this spectral range significant uncertainties were
reported by . Its magnitude is estimated to be about
0.08±0.05 of the absorption peaks at 446 nm. No published data are
available for the absorption at 419 nm. Based on , this
results in a peak absorption cross section of
(5.0±3.5)×10-48 cm5 molec-2 at a wavelength of
419.13±0.42.
Also in the case of the 419 nm absorption, the impact on trace gas
retrievals depends on the fit settings for the respective trace gas and
instrument parameters, but an influence can be expected for the spectral
retrievals of weak absorbers in pristine regions, such as IO and NO2.
However, the O4 absorption peak at around 419 nm cannot explain the
observed differences between different water vapour absorption cross sections
in recent literature (compare ) but could contribute
to previously observed systematic residual structures.
The spectra used for the data evaluation in this paper
(about 10 GB) can be obtained on request from the
author.
Information about the Supplement
According to the procedure described above, a merged absorption cross section
of O4 based on was calculated. The absorption peak at
344 nm was shifted by +1366 cm-1 to shorter wavelengths and scaled
by 0.1863 according to Table and additionally the
radiative transfer correction by 14 % (as in
Sect. ). These cross-section values were then
added to the original O4 absorption cross section at 293 K below 331 nm.
To avoid negative cross-section values below 337.5 nm after convolution to
instrument resolution, absorption cross-section values between 331 and
337.5 nm were set to zero. For the shifted absorption peak from 446.4 to
419.0 nm values between 409.25 and 426.0 nm were used, while the original
O4 absorption cross section at 293 K between 390.0 and 435.5 nm was set
to zero. The peak was scaled by 0.0785 and corrected for radiative transfer
dependence with wavelength by 22 %. The resulting file is provided as a
Supplement.
The Supplement related to this article is available online at https://doi.org/10.5194/acp-18-1671-2018-supplement.
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank Mingxi Yang and coworkers for operating the MAX-DOAS instrument at
the Penlee Point Atmospheric Observatory. We also thank Timothy Hay for the
help in performing the LP-DOAS measurements and NIWA and Antarctica NZ for
hosting our campaign (K084) and the received support. We thank the captain,
officers and crew of RV Polarstern for support during cruise
ANT XXVIII. Especially for the support by Johannes Rogenhagen/FIELAX/AWI and
technicians on board. We thank Jan-Marcus Nasse for doing maintenance of the
MAX-DOAS on RV Polarstern in the shipping yard before the campaign
listed above.
The article processing charges for this open-access
publication were covered by the Max Planck Society.
Edited by: Rainer Volkamer
Reviewed by: Ryan Thalman and Henning Finkenzeller
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