Article pubs.acs.org/JPCA
Measurements of the Absorption Cross Section of 13CHO13CHO at Visible Wavelengths and Application to DOAS Retrievals Natasha R. Goss,†,⊥ Eleanor M. Waxman,†,‡ Sean C. Coburn,†,‡ Theodore K. Koenig,†,‡ Ryan Thalman,†,‡,# Josef Dommen,§ James W. Hannigan,∥ Geoffrey S. Tyndall,∥ and Rainer Volkamer*,†,‡ †
Department of Chemistry and Biochemistry and ‡Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado at Boulder, Boulder, Colorado 80309-0215, United States § Paul Scherrer Institute, 5232 Villigen, Switzerland ∥ NCAR/ACD, Mesa Lab 041, Boulder, Colorado 80307, United States S Supporting Information *
ABSTRACT: The trace gas glyoxal (CHOCHO) forms from the atmospheric oxidation of hydrocarbons and is a precursor to secondary organic aerosol. We have measured the absorption cross section of disubstituted 13CHO13CHO (13C glyoxal) at moderately high (1 cm−1) optical resolution between 21 280 and 23 260 cm−1 (430−470 nm). The isotopic shifts in the position of absorption features were found to be largest near 455 nm (Δν = 14 cm−1; Δλ = 0.29 nm), whereas no significant shifts were observed near 440 nm (Δν < 0.5 cm−1; Δλ < 0.01 nm). These shifts are used to investigate the selective detection of 12C glyoxal (natural isotope abundance) and 13C glyoxal by in situ cavity enhanced differential optical absorption spectroscopy (CE-DOAS) in a series of sensitivity tests using synthetic spectra, and laboratory measurements of mixtures containing 12C and 13C glyoxal, nitrogen dioxide, and other interfering absorbers. We find the changes in apparent spectral band shapes remain significant at the moderately high optical resolution typical of CE-DOAS (0.55 nm fwhm). CE-DOAS allows for the selective online detection of both isotopes with detection limits of ∼200 pptv (1 pptv = 10−12 volume mixing ratio), and sensitivity toward total glyoxal of few pptv. The 13C absorption cross section is available for download from the Supporting Information.
1. INTRODUCTION Glyoxal (CHOCHO) is an α-dicarbonyl product of hydrocarbon oxidation in the atmosphere and a useful indicator of volatile organic compound (VOC) photochemistry.1−3 The atmospheric degradation of biogenic VOCs such as isoprene and its oxidation products is the largest source of glyoxal worldwide but it also has anthropogenic and biomass burning sources.4−8 Photolysis and reaction with OH radicals limit glyoxal’s lifetime during daylight to a few hours.6,7 Glyoxal can also be lost heterogeneously through aerosol uptake to form secondary organic aerosol (SOA).9 The extent of glyoxal’s contribution to SOA formation is of current interest.8−14 Laboratory studies have shown that it can produce significant SOA in the presence of light,9,15 and such studies have detected a number of products from multiphase reactions of glyoxal in model aerosols. Liggio et al.12 reported organosulfate formation from glyoxal using a low-resolution mass spectrometer, and Galloway et al.14 reported a number of oligomers, imidazoles, and organosulfates in a series of experiments using a high-resolution aerosol mass spectrometer. However, some products that were attributed to glyoxal-SOA in laboratory experiments could indeed have been the result of chamber background contamination.16 Isotopic labeling of © XXXX American Chemical Society
VOC precursors injected into the chamber can be utilized to separate SOA products formed due to repartitioning of chamber background from products formed from VOC precursor oxidation in situ. Knowledge of the 13C glyoxal absorption cross-section is prerequisite for optical spectroscopic measurements of glyoxal isotopes, which can serve as useful tools in laboratory studies of SOA formation. Rotational assignments of the strong à 1Au ← X̃ 1Ag (π* ← n) transition at 455 nm are available for 12C and 13C glyoxal.17,18 The high-resolution absorption cross-section spectrum of 12C glyoxal19 has undergone detailed evaluations20,21 and is widely used for global glyoxal observations,22 as well as calculations of glyoxal photolysis in the atmosphere.21 However, to date there is no UV−visible absorption cross section spectrum of 13C glyoxal available in the literature. We present absorption cross section spectra for 12C and 13C glyoxal, and explore their use to measure both isotopes by cavity enhanced absorption spectroscopy. Special Issue: Mario Molina Festschrift Received: November 12, 2014 Revised: December 31, 2014
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DOI: 10.1021/jp511357s J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
2. EXPERIMENTAL SECTION Generation of 13C Glyoxal. Glyoxal is often synthesized by heating glyoxal trimer dihydrate crystals in the presence of P2O5.19 However, these methods were not practical for this work because 13C glyoxal is not readily available as either the trimer dihydrate form or in aqueous solution. Gas phase glyoxal (12C and 13C) was synthesized from the reaction of acetylene with chlorine radicals (Figure 1) in the presence of oxygen with
points N in a spectrum, and benefits from examining narrower bandpasses according to SNR = (2/N )1/2 (B(λ)/Bmean )SNR x
(1)
where B(λ) is the signal at the wavelength of interest and Bmean is the mean spectral signal.26 The interferogram noise SNRx is constant for a given instrument optical setup. The LED was temperature controlled to minimize baseline drift, which can be as low as 0.05% over 3 h.25 For comparison, Xe-emission lines that are superimposed on the thermal emission spectrum of Xearc light sources vary in shape and intensity with gas temperature and pressure. For standard (non-laser-driven) Xe-arc lamps the intensity can drift by up to 5.4% below 30 000 cm−1 or 20% above 30 000 cm−1 in 3 h and is typically actively controlled.19,27 Use of the LED was straightforward, but not free of drift either (see below). Prior to collecting glyoxal spectra, we took a 1 h reference spectrum of the gas cell containing pure nitrogen at atmospheric pressure. Once reagents were added and the reaction was initiated, we typically obtained glyoxal optical densities near 10% and were able to collect spectra for approximately 5 h. Each spectrum had a 1 h integration time and after collection was analyzed to determine whether glyoxal was still present. When the amount of glyoxal had substantially diminished, the cell was pumped out and flushed with nitrogen. A second 1 h reference was then collected. The attainable glyoxal SNR was optimized by varying photolysis lamp filtration, reactant concentrations, and flow cell surface-areato-volume ratio. The optical density was most sensitive to chlorine concentration and was also affected by acetylene concentration. Under the final protocol, glyoxal loss to the reactor walls was limiting the attainable SNR in our setup. Spectra containing significant glyoxal were averaged and converted to absorption spectra using the Lambert−Beer Law:
Figure 1. Mechanism of chlorine-radical-initiated formation of glyoxal from acetylene.
a 21% yield.23 The reaction was done at room temperature and a total pressure of ∼620 Torr, in a reaction mixture that contained the following partial pressures: 3 Torr of chlorine (Matheson, 99% purity), 80 Torr of acetylene (12C from Air Products, 13C from Sigma-Aldrich), 150 Torr of oxygen (US Welding), backfilled with nitrogen (General Air, boiled off from liquid dewar). To conserve reagent during the production of 13 C glyoxal, the 13C acetylene was added to the cell first, then the excess was drawn off into the original vessel for later use with a liquid nitrogen trap. A UV blacklamp was used to initiate the reaction. Recording of High-Resolution Visible Spectra. Spectra were collected using a Bruker 120 HR FTS equipped with an external light-emitting diode (LED) light source centered at 459 nm (fwhm 27 nm, LEDEngin). The LED light was collimated through a 1.00 m gas cell equipped with UV blacklamps for photolysis of chlorine gas (Figure 2). The LED
⎛I ⎞ A = ln⎜ 0 ⎟ = σcl ⎝I⎠
(2)
where I0 is the average of the reference spectra, I is the spectrum to be analyzed, σ is the wavelength-dependent absorption cross section in cm2 per molecule, c is the analyte concentration in molecules per cm3, and l is the path length in cm. These absorption spectra were converted from optical density units to a cross-section independent of path length or concentration using
( II ) 0
ln σ=
cl
(3)
Normalization to the integrated absorption cross section from Volkamer et al.19 provided the absolute calibration, as is described in the next section. Baseline Drift Correction. The 12C and 13C glyoxal spectra were affected by baseline drift because the LED light source was not perfectly stable. The averaged spectra were corrected for baseline drift following the procedure described by Volkamer et al.19 First, the average 12C spectrum recorded here, and the literature high-resolution cross-section spectrum19 were both convolved with a Gaussian line function of 0.3 nm fwhm common resolution. Then the slant column density (SCD) (units molecules cm−2) of glyoxal in the 12C spectrum was determined by ordinary least-squares fitting using the literature spectrum as a cross-section. The original high-resolution 12C
Figure 2. Experimental setup. A light emitting diode (LED, 1) is collimated through the glyoxal-containing gas cell, reflected by mirrors (2−4), and passed through a focusing lens (5) onto the FTS aperture (6).
was chosen as it yields higher photon flux and thus better signal-to-noise ratios (SNR) than Xe-arc or halogen lamps at blue wavelengths.24,25 The instrument was configured for moderately high resolution (1 cm−1) and boxcar apodization was applied to the data. In addition to benefiting from higher photon flux, SNR is increased due to the smaller spectral range emitted by the LED. The SNR with FTS is inversely proportional to the number of B
DOI: 10.1021/jp511357s J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
glyoxal (see Supporting Information for analysis of synthetic spectra). The conditions in the synthetic spectra resemble those of the chamber spectra with respect to species and concentrations of absorbers (Supporting Information text); i.e., the spectral fits included cross sections for O4,29 two NO2 cross sections (low and high concentration30), water,31 O3,32 12 C glyoxal,19 13C glyoxal (this work), and a fourth-order polynomial. The 12C and 13C cross sections were orthogonalized to minimize spectral cross-correlation using a routine for orthogonalization that is internal to WinDOAS and based on the Gram−Schmidt orthogonalization algorithm (see Cross Section Orthogonalization section below).33,34 The cross section of the isotope with the highest concentration was kept unchanged; i.e., the 13C spectrum was orthogonalized to the 12C spectrum in the 12C experiment (and 12C was orthogonalized in the 13C experiment) during spectral fitting. All cross sections were convolved to the spectrometer slit function (0.55 nm fwhm) and fitted simultaneously in the spectral window from 438 to 465 nm (“standard” glyoxal window). We performed sensitivity studies to understand the stability of the glyoxal retrieval with respect to variations of the fitted spectral window. These tests were conducted over a range of wavelengths using a step-form algorithm in which the same spectrum was analyzed multiple times.35 The upper and lower limits of the spectral range used for analysis were systematically varied for all spectral fit intervals 438−465 nm using interval steps of 0.5 nm; the width of the fit window thus varied between 2.5 and 27 nm. The results were calculated as a percent deviation from the fit in our standard fit window, because the true concentration of glyoxal is initially not known. However, this fit window is identical to that used to measure glyoxal during a detailed instrument comparison exercise (nine instruments, two separate simulation chambers), where CEDOAS results obtained with a similar setup were found both precise and accurate.20 Cross Section Orthogonalization. Orthogonalization is a mathematical concept that is widely used in linear algebra to identify a maximally unique “basis set” of vectors. The WinDOAS software uses the Gram−Schmidt orthogonalization34 algorithm, which is described below. The initial basis set is made up of two vectors Ai,
spectrum (optical density units) was then divided by this SCD to convert to units of cm2/molecule, and this cross-section spectrum was integrated over fixed 50 cm−1 intervals. Now the high-resolution literature spectrum was convoluted to 1 cm−1 optical resolution to match the resolution of our measurements. In the following, integral absorption cross sections were calculated using the same 50 cm−1 intervals for both measured and literature spectra, and the difference taken in wavenumber space. A fifth-order polynomial was fit and used to interpolate the differences between integral values to the wavenumber scale of the 1 cm−1 resolution spectrum using spline interpolation, and the resulting polynomial was subtracted from the 12C spectrum to produce a baseline corrected absorption crosssection spectrum. A similar process was followed to correct the 13C glyoxal spectrum. After convoluting to 0.3 nm, second-order spectral stretching and shifting were used to account for isotopic distortions to the locations of 13C absorption peaks compared to the Volkamer et al.19 spectrum. Isotope shifts caused residual structures during spectral fitting, and those parts of the spectrum with high residual structures were avoided during integration to perform the baseline correction; the point-topoint integration was performed only over wavelength segments where no significant glyoxal absorption was observed to determine the polynomial. A second-order polynomial was fit through the regions of low absorption to force the integral correction to be performed as similarly as possible to the 12C baseline correction described above. The corrected 1 cm−1 resolution spectra span a wavenumber range from 23 250 to 21 250 cm−1 (430−470 nm) and are reported as vacuum (not air) wavenumber/wavelength. Simulation Chamber Experiments. Simultaneous detection of 12C and 13C glyoxal was tested from absorption spectra recorded at the simulation chamber located at the Paul Scherrer Institut in Villigen, Switzerland.10 Glyoxal was produced under high NOx conditions via the following reactions: (R1)
HONO + hν → OH + NO C2H 2 + O2 + OH → CHOCHO + OH
(R2)
13
Glyoxal is formed from acetylene (99% C acetylene, Aldrich, CAS Registry No. 35121-31-4) with ∼65% yield, the remaining 35% forming CO and formic acid.28 The primary objective of these chamber experiments was to investigate SOA formation from glyoxal that will be described elsewhere. Here, we use two individual spectra recorded at PSI by the University of Colorado Light Emitting Diode Cavity Enhanced DOAS;25 one spectrum each from an experiment that used 12C acetylene and 13C acetylene (to form 12C and 13C glyoxal), respectively. At the time of recording, the chamber contained few ppbv of glyoxal and NO2, as well as water vapor. The chamber had a 5 week history of exposure only to 12C glyoxal (12C spectrum); the 13C glyoxal spectrum was recorded after two consecutive experiments and cleaning cycles using 13C acetylene. Sensitivity Studies Using Synthetic and Chamber Spectra. The 12C and 13C cross-section spectra show significant differences in the position of the strong absorption feature near 455 nm. We performed sensitivity studies to evaluate the potential of spectral cross-correlations between 12C and 13C spectra in a DOAS retrieval, using (1) the two spectra from chamber experiments and (2) synthetic spectra with added noise that contain well-known mixtures of 12C and 13C
Ai = (ai1 , ..., ain)
i.e., the 12C and 13C cross sections, where n is the number of data points (wavelengths) and ain is the cross-section value at the nth wavelength. The orthogonal basis set is made up of two vectors Bi that have the same dimension n. The first vector is unchanged, and the second vector identifies those peaks that are unique to the orthogonal cross section. The orthogonal basis set is calculated according to eqs 4 and 5:33 B1 = A1
B2 = A 2 −
(4)
(A 2 , B1) B1 n(B1)
(5)
where (A2, B1) is the inner product of A2 and B1 and n(B1) is the inner product of B1 with itself. For convenience we define B1 as the cross section of the more abundant glyoxal isotope. C
DOI: 10.1021/jp511357s J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 3. (a) Match between 12C glyoxal measured in this work and high-resolution measurement of Volkamer et al.,19 including at 455 nm. (b) Shifted absorption cross section of 13C glyoxal (red) near 455 nm. (c, d) No shift is apparent at 440 nm.
Table 1. Wavelength Dependent Isotope Shifts of 12C and 13C Absorption Bands Line Position 12 C (nm) 12 C (cm−1) 13 C (nm) 13 C (cm−1) Line Shift (12C−13C) Δλ (nm) Δν (cm−1)
436.386 22915.5 436.441 22912.6
440.267 22713.5 440.257 22714.0
444.903 22476.8 444.761 22484.0
453.018 22074.2 452.761 22086.7
455.145 21971.0 454.856 21985.0
−0.055 2.9
0.010 −0.5
0.142 −7.2
0.257 −12.5
0.289 −14
3. RESULTS AND DISCUSSION The line positions of the rich rovibronic structure at visible wavelengths agree very well between the two 12C spectra (Figure 3a, c, see also Supporting Information), indicating that the wavenumber calibration is well-known (
