The Journal of Physical Chemistry
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Measurements of the absorption cross section of
13CHO13CHO at visible wavelengths and application to
DOAS retrievals
Journal:
Manuscript ID:
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Complete List of Authors:
The Journal of Physical Chemistry
Draft
Article
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Goss, Natasha; University of Colorado, Chemistry and Biochemistry
Waxman, Eleanor; University of Colorado, Chemistry and Biochemistry;
University of Colorado, CIRES
Coburn, Sean; University of Colorado, Chemistry and Biochemistry;
University of Colorado, CIRES
Koenig, Theodore; University of Colorado, Chemistry and Biochemistry;
University of Colorado, CIRES
Thalman, Ryan; University of Colorado, Chemistry and Biochemistry;
University of Colorado, CIRES
Dommen, Josef; Paul Scherrer Institute,
Hannigan, James; National Center for Atmospheric Research, Atmospheric
Chemistry Division
Tyndall, Geoffrey; National Center for Atmospheric Research, Atmospheric
Chemistry Division
Volkamer, Rainer; University of Colorado, Chemistry and Biochemistry;
University of Colorado, CIRES
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The Journal of Physical Chemistry
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Measurements of the absorption cross section of 13CHO13CHO at visible
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wavelengths and application to DOAS retrievals
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Natasha R. Goss1,a, Eleanor M. Waxman1,2, Sean C. Coburn1,2, Theodore K. Koenig1,2, Ryan
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Thalman1,2,b, Josef Dommen3, James W. Hannigan4, Geoffrey S. Tyndall4, and Rainer
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Volkamer1,2,*
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1
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80309-0215, USA.
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2
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at Boulder, Boulder, CO, 80309-0215, USA
Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO,
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado
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3
Paul Scherrer Institute, Villigen, Switzerland.
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4
NCAR/ACD, Mesa Lab 041, Boulder, CO, 80307, USA.
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a
now at: Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, MA, 02138,
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USA.
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b
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Department, Upton, NY, 11973, USA.
now at: Brookhaven National Laboratory, Biological, Environmental and Climate Sciences
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*
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Rainer Volkamer, Department of Chemistry and Biochemistry & CIRES, University of Colorado
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at Boulder, Boulder, CO, 80309-0215, USA. Phone: +1 (303) 492-1843. Fax: +1 (303) 492-
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5894. email: [email protected]
Corresponding author address:
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Manuscript prepared for submission to J. Phys. Chem. (Mario Molina Festschrift)
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Last edited 8 November 2014
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Abstract
The trace gas glyoxal (CHOCHO) forms from the atmospheric oxidation of hydrocarbons
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and is a precursor to secondary organic aerosol. We have measured the absorption cross section
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of disubstituted 13CHO13CHO (13C glyoxal) at moderately high (1 cm-1) optical resolution
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between 21280-23260 cm-1 (430-470 nm). The isotopic shifts in the position of absorption lines
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were found to be largest near 455 nm (∆ν =14 cm-1; ∆λ = 0.29 nm), while no significant shifts
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were observed near 440 nm (∆ν < 0.5 cm-1; ∆λ < 0.01 nm). These shifts are used to investigate
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the selective detection of 12C glyoxal (natural isotope abundance) and 13C glyoxal by in situ
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Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) in a series of
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sensitivity tests using synthetic spectra, and laboratory measurements of mixtures containing 12C
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and 13C glyoxal, nitrogen dioxide and other interfering absorbers. We find the changes in
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apparent spectral band shapes remain significant at the moderately high optical resolution typical
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of CE-DOAS (0.55 nm FWHM). CE-DOAS allows for the selective online detection of both
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isotopes with detection limits of ~200 pptv (1 pptv = 10-12 volume mixing ratio), and sensitivity
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towards total glyoxal of few pptv. The 13C absorption cross section is available for download at
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http://chem.colorado.edu/volkamergroup/index.php/publications.
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Keywords: glyoxal, 12C, 13C, absorption spectroscopy, isotope shifts
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1. Introduction
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Glyoxal (CHOCHO) is an alpha-dicarbonyl product of hydrocarbon oxidation in the
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atmosphere and a useful indicator of volatile organic compound (VOC) photochemistry.1-3 The
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atmospheric degradation of biogenic VOCs such as isoprene and its oxidation products is the
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largest source of glyoxal worldwide but it also has anthropogenic and biomass burning sources.42
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hours.6, 7 Glyoxal can also be lost heterogeneously through aerosol uptake to form secondary
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organic aerosol (SOA).9
Photolysis and reaction with OH radicals limit glyoxal’s lifetime during daylight to a few
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The extent of glyoxal’s contribution to SOA formation is of current interest. 8-14
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Laboratory studies have shown that it can produce significant SOA in the presence of light,9, 15
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and such studies have detected a number of products from multiphase reactions of glyoxal in
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model aerosols. Liggio et al.12 reported organosulfate formation from glyoxal using a low-
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resolution mass spectrometer and Galloway et al.14 reported a number of oligomers, imidazoles,
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and organosulfates in a series of experiments using a high-resolution aerosol mass spectrometer.
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However, some products that were attributed to glyoxal-SOA in laboratory experiments could
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indeed have been the result of chamber background contamination.16 Isotopic labeling of VOC
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precursors injected into the chamber can be utilized to separate SOA products formed due to
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repartitioning of chamber background from products formed from VOC precursor oxidation in-
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situ. Knowledge of the 13C glyoxal absorption cross-section is prerequisite for time resolved
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measurements of glyoxal isotopes, which can serve as useful tools in laboratory studies of SOA
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formation.
Rotational assignments of the strong à 1Au ← ෩
X 1Ag (π* ← n) transition at 455 nm are
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available for 12C and 13C glyoxal.17,18 The high-resolution absorption cross-section spectrum of
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observations,22 as well as calculations of glyoxal photolysis in the atmosphere.21 However, to
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date there is no UV-visible absorption cross section spectrum of 13C glyoxal available in the
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literature. We present absorption cross section spectra for 12C and 13C glyoxal, and explore their
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use to measure both isotopes by cavity enhanced absorption spectroscopy.
C glyoxal19 has undergone detailed evaluations,20, 21 and is widely used for global glyoxal
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2. Experimental Section
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Generation of 13C glyoxal
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Glyoxal is often synthesized by heating glyoxal trimer dihydrate crystals in the presence
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of P2O5.19 However, these methods were not practical for this work because 13C glyoxal is not
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readily available as either the trimer dihydrate form or in aqueous solution. Gas phase glyoxal
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(12C and 13C) was synthesized from the reaction of acetylene with chlorine radicals (see Figure 1)
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in the presence of oxygen with a 21% yield.23 The reaction was done at room temperature and a
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total pressure of ~620 Torr, in a reaction mixture that contained the following partial pressures: 3
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Torr of chlorine (Matheson, 99% purity), 80 Torr of acetylene (12C from Air Products, 13C from
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Sigma Aldrich), 150 Torr of oxygen (US Welding), back filled with nitrogen (General Air,
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boiled off from liquid dewar). To conserve reagent during the production of 13C glyoxal, the 13C
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acetylene was added to the cell first, then the excess was drawn off into the original vessel for
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later use with a liquid nitrogen trap. A UV blacklamp was used to initiate the reaction.
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Figure 1: Mechanism of chlorine-radical-initiated formation of glyoxal from acetylene.
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Recording of High-Resolution Visible Spectra
Spectra were collected using a Bruker 120 HR FTS equipped with an external lightemitting diode (LED) light source centered at 459 nm (FWHM 27 nm, LEDEngin). The LED
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light was collimated through a 1.00 m gas cell equipped with UV blacklamps for photolysis of
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chlorine gas (Figure 2). The LED was chosen as it yields higher photon flux and thus better
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signal-to-noise ratios (SNR) than Xe-arc or halogen lamps at blue wavelengths.24, 25 The
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instrument was configured for moderately high resolution (1 cm-1) and boxcar apodization was
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applied to the data.
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Figure 2: Experimental setup. A Light Emitting Diode (LED, 1) is collimated through the
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glyoxal-containing gas cell, and reflected by mirrors (2-4), a focusing lens (5) onto the FTS
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aperture (6).
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In addition to benefiting from higher photon flux, SNR is increased due to the smaller
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spectral range emitted by the LED. The SNR with FTS is inversely proportional to the number of
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points N in a spectrum, and benefits from examining narrower bandpasses according to:
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SNR = (2/N)1/2*(B(λ)/Bmean)*SNRx
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where B(ߣ) is the signal at the wavelength of interest and Bmean is the mean spectral signal. The
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interferogram noise SNRx is constant for a given instrument optical setup. The LED was
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temperature controlled to minimize baseline drift, which can be as low as 0.05% over 3 hours.25
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For comparison, Xe-emission lines that are superimposed on the thermal emission spectrum of
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Xe-arc light sources. These lines vary in shape and intensity with gas temperature and pressure.
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The lamp intensity can drift by up to 5.4% below 30,000 cm-1 or 20% above 30,000 cm-1 in 3
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hours, and is typically actively controlled.19, 26 Use of the LED was straightforward, but not free
Equation (1)
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of drift either (see below).
Prior to collecting glyoxal spectra, we took a one-hour reference spectrum of the gas cell
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containing pure nitrogen at atmospheric pressure. Once reagents were added and the reaction
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was initiated, we typically obtained glyoxal optical densities near 10% and were able to collect
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spectra for approximately 5 hours. Each spectrum had a one hour integration time and after
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collection was analyzed to determine whether glyoxal was still present. When the amount of
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glyoxal had substantially diminished, the cell was pumped out and flushed with nitrogen. A
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second one-hour reference was then collected. The attainable glyoxal SNR was optimized by
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varying photolysis lamp filtration, reactant concentrations, and flow cell surface-area-to-volume
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ratio. The optical density was most sensitive to chlorine concentration and was also affected by
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acetylene concentration. Under the final protocol, glyoxal loss to the reactor walls was limiting
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the attainable SNR in our setup.
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Spectra containing significant glyoxal were averaged and converted to absorption spectra
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using Beer’s Law:
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I
A = ln  0
 I
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where I0 is the average of the reference spectra, I is the spectrum to be analyzed, σ is the
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wavelength-dependent absorption cross section in cm2 per molecule, c is the analyte
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concentration in molecules per cm3, and l is the path length in cm. These absorption spectra were
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converted from optical density units to a cross-section independent of pathlength or
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concentration using:
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
 = σ cl

Equation (2)
I 
ln  0 
I
σ=  
cl
Equation (3)
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Normalization to the integrated absorption cross section from Volkamer et al.19 provided the
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absolute calibration, as is described in the next section.
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Baseline drift correction
The 12C and 13C glyoxal spectra were affected by baseline drift because the LED light
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source was not perfectly stable. The averaged spectra were corrected for baseline drift following
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the procedure described by Volkamer et al.19. First, the average 12C spectrum recorded here, and
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the literature high-resolution cross-section spectrum19 were both convolved with a Gaussian line
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function of 0.3 nm FWHM common resolution. Then the slant column density (SCD) (units
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molecules cm-2) of glyoxal in the 12C spectrum was determined by ordinary least-squares fitting
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using the literature spectrum as a cross-section. The original high-resolution 12C spectrum
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(optical density units) was then divided by this SCD to convert to units of cm2/molecule, and this
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cross-section spectrum was integrated over fixed 50 cm-1 intervals. Now the high-resolution
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literature spectrum was convoluted to 1 cm-1 optical resolution to match the resolution of our
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measurements. In the following, integral absorption cross-sections were calculated using the
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same 50 cm-1 intervals for both measured and literature spectra, and the difference taken in
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wavenumber space. A 5th-order polynomial was fit and used to interpolate the differences
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between integral values to the wavenumber scale of the 1 cm-1 resolution spectrum using spline
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interpolation, and the resulting polynomial was subtracted from the 12C spectrum to produce a
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baseline corrected absorption cross-section spectrum.
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A similar process was followed to correct the 13C glyoxal spectrum. After convoluting to
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0.3 nm, second-order spectral stretching and shifting were used to account for isotopic
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distortions to the locations of 13C absorption peaks compared to the Volkamer et al.19 spectrum.
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Isotope shifts caused residual structures during spectral fitting, and those parts of the spectrum
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with high residual structures were avoided during integration to perform the baseline correction;
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the point-to-point integration was performed only over wavelength segments where no
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significant glyoxal absorption was observed to determine the polynomial. A second-order
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polynomial was fit through the regions of low absorption to force the integral correction to be
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performed as similarly as possible to the 12C baseline correction described above. The corrected
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1 cm-1 resolution spectra span a wavenumber range from 23250-21250 cm-1 (430-470 nm).
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Simulation Chamber Experiments
Simultaneous detection of 12C and 13C glyoxal was tested from absorption spectra
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recorded at the simulation chamber located at the Paul Scherrer Institut in Villigen,
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Switzerland.10 Glyoxal was produced under high NOx conditions via the following reactions:
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R1: HONO + hν → OH + NO
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R2: C2H2 + O2 + OH → CHOCHO + OH
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Glyoxal is formed from acetylene (99% 13C acetylene, Aldrich, CAS 35121-31-4) with ~65%
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yield, the remaining 35% forming CO and formic acid.27
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The primary objective of these chamber experiments was to investigate SOA formation
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from glyoxal that will be described elsewhere.28 Here, we use two individual spectra recorded at
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PSI by the University of Colorado Light Emitting Diode Cavity Enhanced DOAS;25 one
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spectrum each from an experiment that used 12C acetylene and 13C acetylene (to form 12C and
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13
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and NO2, as well as water vapor. The chamber had a 5 week history of exposure only to 12C
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glyoxal (12C spectrum); the 13C glyoxal spectrum was recorded after two consecutive
C glyoxal), respectively. At the time of recording, the chamber contained few ppbv of glyoxal
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experiments and cleaning cycles using 13C acetylene.
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Sensitivity studies using synthetic and chamber spectra
The 12C and 13C cross-section spectra show significant differences in the position of the
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strong absorption feature near 455 nm. We performed sensitivity studies to evaluate the potential
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of spectral cross-correlations between 12C and 13C spectra in a DOAS retrieval, using the two
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spectra from chamber experiments and synthetic spectra with added noise that contain well-
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known mixtures of 12C and 13C glyoxal (see Supplemental Information for analysis of synthetic
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spectra). The conditions in the synthetic spectra resemble those of the chamber spectra with
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respect to species and concentrations of absorbers (SI text), i.e., the spectral fits included cross
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sections for O4,29 two NO2 cross sections (low and high concentration30), water,31 O3,32 12C
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glyoxal,19 13C glyoxal (this work), and a 4th order polynomial. The 12C and 13C cross sections
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were orthogonalized to minimize spectral cross-correlation using a routine for orthogonalization
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that is internal to WinDOAS and based on the Gram-Schmidt orthogonalization algorithm.33, 34
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The cross section of the isotope with the highest concentration was kept unchanged, i.e., the 13C
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spectrum was orthogonalized to the 12C spectrum in the 12C experiment (and 12C was
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orthogonalized in the 13C experiment) during spectral fitting. All cross sections were convolved
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to the spectrometer slit function (0.55 nm FWHM), and fitted simultaneously in the spectral
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window from 438-465 nm (‘standard’ glyoxal window).
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We performed sensitivity studies to understand the stability of the glyoxal retrieval with
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respect to variations of the fitted spectral window. These tests were conducted over a range of
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wavelengths using a step-form algorithm in which the same spectrum was analyzed multiple
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times.35 The upper and lower limits of the spectral range used for analysis were systematically
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varied for all spectral fit intervals 438-465 nm using interval steps of 0.5 nm; the width of the fit
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window thus varied between 2.5 and 27 nm. The results were calculated as a percent deviation
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from the fit in our standard fit window, since the true concentration of glyoxal is initially not
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known. However, this fit window is identical to that used to measure glyoxal during a detailed
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instrument comparison exercise (nine instruments, two separate simulation chambers), where
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CE-DOAS results obtained with a similar setup were found both precise and accurate.20
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3. Results and Discussion
The line positions of the rich rovibronic structure at visible wavelengths agree very well
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between the two 12C spectra (Figure 3a, c, see also supplementary information), indicating that
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the wavenumber calibration is well known (< 0.005 cm-1). A 13C shift is present near 455 nm
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(3b), but none is visible at 440 nm (3d). The consistency in transferring the calibration between
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the 12C and 13C cross section spectra was tested. Both spectra were convoluted to 0.3nm FWHM
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optical resolution, and the 13C cross section was fit to the 12C spectrum allowing for a second-
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order stretch and shift in wavelength. A fit factor of unity would indicate that the calibration
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from the high-resolution 12C spectrum19 was indeed successfully transferred. The observed fit
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factor was found to be 1.01 ± 0.04. Figure 3 compares the baseline corrected high-resolution
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absorption spectra recorded in this study (1 cm-1 resolution) to the literature cross-section (0.06
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cm-1 resolution).
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Contamination of the 12C spectrum with 13C glyoxal is not significant at the SNR
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explored here. Such glyoxal originates from acetylene that contains 13C in its natural 1.109%
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abundance; further 13C glyoxal forms from disubstituted 13C acetylene with an abundance of
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0.012% (1.109%2). For our spectra the optical density of the averaged 12C and 13C glyoxal
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spectrum is smaller 10% (base e), and we expect the 13C glyoxal contribution to the 12C spectrum
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to be ~2 orders of magnitude smaller than the noise level of our measurements. The absorption
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of strong 12CHO13CHO lines would thus be near or below the noise level of our measurements.
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Some of the stronger absorption lines of 12CHO13CHO are probably visible at the higher
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resolution and SNR used to record the 12C literature spectrum.19 However, the absorption
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spectrum of 12CHO13CHO is currently unmeasured to investigate this further. We consider our
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12
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glyoxal. 12CHO13CHO further does not present an error for the calibration of our 13C spectrum as
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natural abundance of 13C glyoxal is accounted by the pressure measurement that underlies the
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calibration of the literature spectrum.19
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C spectrum to be free of any obvious absorption features of mono- and/or di-substituted 13C
The isotopic shift is dictated by symmetry elements and the involvement of carbon in a
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particular rovibronic effects. The observed isotopic shifts were not uniform throughout the
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observed range (Table 1). The maximum shift is observed near 455 nm, and at 440 nm no shift is
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discernible within measurement precision. This lends added importance to the decision about the
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wavelength range selection for DOAS retrievals. Birss et al.17, 18 provide evidence for isotopic
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shifts in glyoxal through their examination of bending and stretching modes in the visible range.
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In combination with deuterated and 18O enriched forms of glyoxal, 13C glyoxal has been used to
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determine the geometry of the glyoxal molecule.18 However, their work does not provide a
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quantitative cross-section, which is required for calibration. The active stretch at 440 nm, the v8
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H-wagging mode, undergoes a large shift in frequency from the ground state to the electronic
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state (from 1048 cm-1 to 742 cm-1 for the 12C isotopomer).
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Figure 3: a) Match between 12C glyoxal measured in this work and high-resolution measurement
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of Volkamer et al.,19 including at 455 nm b) shifted absorption cross section of 13C glyoxal (red)
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near 455nm c,d) No shift is apparent at 440 nm.
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Table 1: Wavelength dependent isotope shifts of 12C and 13C absorption lines
Line Position
12
C (nm)
436.386
440.267
444.903
453.018
455.145
C (cm-1)
22915.5
22713.5
22476.8
22074.2
21971.0
C (nm)
436.441
440.257
444.761
452.761
454.856
C (cm-1)
22912.6
22714.0
22484.0
22086.7
21985.0
∆λ (nm)
-0.055
0.010
0.142
0.257
0.289
∆ν (cm-1)
2.9
-0.5
-7.2
-12.5
-14
12
13
13
Line Shift
(12C-13C)
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Because this mode is not infrared active as a result of its symmetry, the frequency of this
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mode in the ground state for the 13C isotopomer is not known. However, we can infer from the
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visible spectrum that the frequency in the ground state must be about 1006 cm-1. When combined
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with the frequency of 729 cm-1 in the upper electronic state, this produces a smaller shift than in
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12
C, hence the coincidence of the bands.
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Spectral proof of fits from the chamber experiments are shown in Figure 4. Panel A has
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fits for the 12C experiment (Exp 15) and panel B has fits from the 13C acetylene experiment (Exp
260
17). The top row shows the residual, or the leftover noise after the polynomial and all other
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scaled cross sections have been subtracted. The RMS of the residual is compatible with photon
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counting statistics here, indicating the lack of unaccounted absorption features. The second row
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shows 12C glyoxal for Exp 15 and 12C orthogonalized to 13C for Exp 17, and the third row shows
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13
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much higher for 12C in Exp 15 than for 13C in Exp 17, showing that even at this moderately high
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resolution18 (0.55 nm FWHM), we are able to successfully differentiate both isotopes. There is
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likely a small amount of 12C contamination in this experiment, but not enough to be significant
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outside of the 3σ range. The lowest row shows NO2 (the sum of the high concentration cross
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section and low concentration cross section), and illustrates that two isotopes can be
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differentiated well even in the presence of other trace gases that absorb at the same wavelength.
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Interestingly, tests using synthetic spectra showed that the selectivity of the detection does not
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suffer much, even at much lower resolution (SI text). The orthogonalized 13C cross section is
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zero within 3σ fit error for Exp 15, when the chamber had not been exposed to 13C glyoxal in
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prior experiments. Any chamber background is expected to be 12C glyoxal. Experiment 16 was a
C glyoxal orthogonalized to 12C for Exp 15 and 13C glyoxal for Exp 17. The optical density is
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16 are expected to be 13C glyoxal. The orthogonalized cross section in this experiment contains
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information about the amount of 12C glyoxal, and again it is not different from zero within 3σ fit
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error.
C experiment, so glyoxal generated during Exp 17, and residual glyoxal carried over from Exp
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Figure 4: Spectral proof of the selective detection of 12C and 13C glyoxal. Panel A: Exp 15
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(12C experiment). Panel B: Exp 17 (13C glyoxal experiment). Dashed lines in orthogonal
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spectra show cross section enlarged and offset to show detail.
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Figure 5 shows sample wavelength-range sensitivity tests in which the upper and lower
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bounds of the fitted region were varied systematically for the chamber spectra. The green area in
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the graph indicates fit-windows that reproduce glyoxal results consistent with our standard fit
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Figure 5: Sensitivity tests using chamber spectra to assess the stability of typical DOAS
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retrievals for a fit window with a given upper and lower limit. The percent difference from the fit
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in the standard 438-465 nm window is shown for : A) 12C in the spectrum from the 12C
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experiment, B) 13C orthogonalized to 12C in the 12C experiment, C) 12C orthogonalized to 13C in
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the 13C experiment, and D) 13C in the 13C experiment.
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The non-orthogonalized spectra (panels A and D) show stable fits over large regions, i.e.,
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fit windows that contain the weak glyoxal band at 440 nm, the strong glyoxal band at 455 nm, or
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both. This is expected, as these are the regions with the largest differential cross sections.
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Outside of these regions, the fit becomes unstable as it changes rather erratically from a higher
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concentration than the standard fit to lower concentrations than the standard fit. The
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orthogonalized cross sections (panels B and C) are non-zero in the vicinity of both the weak
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bands and the strong bands. The greatest selectivity, however, is observed when the strong band
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is included in the fit. Variation is minimized when the fit window extends beyond 455 nm. The
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differential cross-section of the orthogonalized 13C cross-section near 455nm is 2.7×10-19
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cm2/molec, compared to 5.3×10-19 cm2/molec of the 12C spectrum at 0.55 nm FWHM optical
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resolution. This orthogonal cross-section suggests that online CE-DOAS measurements should
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be able to distinguish both isotopes rather well. In practice, a slightly negative offset is observed
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for 13C glyoxal during Exp 15, while no such offset was observed for 12C glyoxal during Exp 17
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(Fig. 4). A CE-DOAS instrument with ~10 pptv detection limit for glyoxal, can thus selectively
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measure 13C glyoxal with a detection limit of ~20 pptv; however, the accuracy of 13C glyoxal
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measurements is conservatively estimated to be on the order of 200 pptv.
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The analysis of mixtures of 12C and 13C glyoxal in the synthetic spectra with added noise
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give accurate results within 3σ fit error when the strong and/or weak bands at 440 nm and ~455
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nm are fitted simultaneously. A robust retrieval is observed over a wide range of optical
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resolutions and noise conditions (SI text). Introducing different forms of noise (structured and
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white noise) in the synthetic spectra does not significantly seem to affect the selectivity in the 12C
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and 13C glyoxal detection. Including NO2 absorptions, which are located at similar wavelengths,
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in the synthetic spectra caused only insignificant perturbations in the overall amount of glyoxal
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and the individual amounts of 12C and 13C glyoxal that were detected. Including water vapor in
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the synthetic spectra had no noticeable effect.
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4. Conclusions
Visible absorption spectra of 12C and 13C glyoxal were collected at moderately high
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resolution. A significant isotope shift was detected at the 455 nm strong band, but not at 440nm,
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suggesting that the two isotopes of glyoxal have sufficiently different spectra to facilitate
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separation by means of in situ Cavity Enhanced DOAS. This was confirmed for two laboratory
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spectra recorded at moderately high optical resolution from experiments using 12C and 13C
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glyoxal, as well as numerous synthetic spectra which simulated mixtures of both compounds.
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5. Acknowledgements
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EMW, TKK, and RMV acknowledge funding from Eurochamp proposal E2-2013-04-10-0088 to
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record the spectra at PSI. The authors thank the entire PSI team, especially Jay Slowik, Nivedita
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Kumar, Felix Klein, Andre Prevot, and Urs Baltensperger for access to the PSI chamber facility.
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6. Supporting Information Available
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Supplemental information is provided on work done with synthetic spectra containing both 12C
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and 13C glyoxal. This information details the creation of synthetic spectra with a range of 12C
339
and 13C concentrations. It includes sensitivity studies performed on the effect of optical
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(instrumental) resolution, fit range, and deviation from known (input) concentration. This
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material is available free of charge via the Internet at http://pubs.acs.org.
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