The FTIR instrument is integrated as part of the NCAR chamber, and measures along the long-axis of the chamber (2 m long, 16 passes, giving a total light path of 32 m). The spectrometer consists of a BOMEM DA3.01 FTIR, and was operated at 1 cm-1 resolution and collected and averaged 200 spectra between 800 and 4000 cm-1 over a period of 4 min. Standard spectra used for spectral subtraction were obtained using the same conditions as above, from scans of samples prepared via injection of known quantities of analyte into the chamber. Absorption cross sections quoted are derived from these standard spectra.

The NCAR chamber experiment involved measurements of VOCs by using a high resolution PTR-ToF-MS (Ionicon Analytik GmbH, Innsbruck, Austria; Jordan et al., 2009). For a detailed review of the instrumentation, refer to de Gouw and Warneke (2007). During the experiment, the PTR-ToF-MS was operated under H3O+ mode, which uses hydronium ions (H3O+) as the primary reagent ions to protonate VOC species. The ionization conditions in the drift tube were controlled by setting the drift voltage at 542 V, drift temperature at 60 ∘C and drift pressure at 2.3 mbar, resulting in an E/N value of about 120 Td (with E being the electric field strength, and N the gas number density; 1 Td = 10-17 V cm2). The integration time was set to 1 s. A 1/16 inch OD capillary PEEK inlet (∼1 m length) heated to 60 ∘C was used as a transfer line, with a flow rate of 100 standard cm3 min-1. The transfer line was connected to an unheated 1/8 inch OD PTFE line (∼1 m length), which was connected to the chamber outlet through a dilution system. Standard gas calibration was performed by using a custom built calibration system. Zero air was produced by pumping ambient air through a catalytic convertor heated to 400 ∘C. A gravimetrically prepared gas standard containing isoprene (7.25 ppmv) and camphene (4.87 ppmv) was dynamically diluted by the zero air and analyzed by the PTR-ToF-MS.

The University of Colorado, Boulder, light-emitting diode Cavity Enhanced Differential Optical Absorption Spectrometer (CE-DOAS) consists of a high-power blue Light Emitting Diode (LED) coupled to a high finesse optical cavity (highly reflective mirrors, measured R=0.999972 at 460 nm, cavity length, d0=92 cm, useable range 430–490 nm, corresponding to an absorption path of 18.4 km in dry air; Thalman and Volkamer, 2010). The CE-DOAS instrument was present for both the experiments at NCAR as well as those at EUPHORE and is here used as the comparative standard for purposes of cross-comparison. In the NCAR experiments 5 standard cm3 min-1 of sample flow was sampled from the chamber through a mass flow controller (MKS) and diluted with 500 standard cm3 min-1 of dry air before flowing through the optical cavity. At EUPHORE, the same CE-DOAS setup was connected directly to the chamber. The instrument sampled at 500 standard cm3 min-1 from the chamber without dilution through 1 m long Teflon tubing with a 1 µm size 25 mm diameter Teflon filter (Pall) in a Teflon filter holder (Entegris) at the beginning of the line to remove aerosol. Spectra were acquired for 1 min and evaluated against a 5-min reference spectrum in pure nitrogen.

Analysis of CE-DOAS spectra was performed for the retrieval of glyoxal, methyl glyoxal, NO2 and O4 as described in Thalman and Volkamer (2010). The mirror reflectivity was calibrated from the differential Rayleigh scattering of helium and nitrogen (Washenfelder et al., 2008) using the Rayleigh scattering cross-section values as described in Thalman et al. (2014). The mirror reflectivity curve was then used to calculate the absorption path in the cavity using the following equation: L(λ)= ds1-R(λ)+αRayAir(λ)d0+σO4(λ)Nd2O2,mixing,ratio2ds+σi(λ)cids.

If this equation is evaluated at 477 nm, knowledge of the O2 volume mixing ratio and cavity pressure confirm control over cavity alignment and R(477 nm) by measuring of the O4 SCD under atmospheric conditions: L(477nm)=O4SCDNd2O2,mixing,ratio2, where L(λ) is the effective path length with respect to wavelength (cm), ds is the sample length (cm), R(λ) is the mirror reflectivity with respect to wavelength, α is the extinction due to the Rayleigh scattering in air (cm-1), d0 is the cavity length (cm), σi is the absorption cross section of the corresponding gas, Nd is the density (molecules cm-3), ci is the concentration of the corresponding gas (molecules cm-3), and O4SCD is the slant column density (concentration × pathlength of O4, cm-5 molecule2). Absorption cross sections are scaled by a wavelength dependant scaling factor based on the path length, that is close to unity and normalized relative to a single wavelength to account for the wavelength dependence, as is described in detail in Thalman and Volkamer (2010). The Windoas software (Fayt and Van Roosendael, 2001) was used to adjust literature cross sections to the instrument resolution, and perform DOAS fitting of multiple reference spectra simultaneously. Glyoxal, methyl glyoxal, NO2, H2O and O4 were retrieved by non-linear least squares fitting (O4 fit window: 457–487 nm, 4th order polynomial; glyoxal fit window: 442–465 nm, 4th order polynomial and Greenblatt et al. (1990) for O4 in this window (see discussion of σO4 baseline noise in Thalman and Volkamer, 2013); glyoxal weak band fit: 458.5–475 nm, 4th order polynomial). The DOAS output in units of slant column density (SCD = concentration × L) was then divided by the path length to get concentration. Measurements of O4 SCDs as part of each spectrum at high signal-to-noise facilitate online control over cavity alignment and/or R. The path length calculated from Eq. (1) agreed with the O4 calibration gas within 1 %. Equation (1) was solved iteratively to account for self-limitation until the concentrations converge. NO2 was self-limiting during E3, E4, E7, E9 and E10 and glyoxal during E1 and E8. For experiments with high glyoxal concentrations, data were retrieved in two ways: (1) fitting of two cross sections bounding the absorption range or (2) fitting of the weak absorption structures in the wavelength range 458.5–475 nm. For experiments N3, E9 and E10 (see Table 2) a NO2 residual is fitted to account for systematic structures arising from extremely high NO2 concentrations leading to a more stable retrieval of the glyoxal or methyl glyoxal concentrations.

The University of Leicester broadband cavity-enhanced absorption spectroscopy (BBCEAS) instrument is based on predecessor BBCEAS instruments used to detect NO2 in urban air (Langridge et al., 2008) and iodine in the marine atmosphere (Ball et al., 2010). In its present form, it has been deployed as the reference instrument for glyoxal and methyl glyoxal quantification in experiments at the EUPHORE chamber to test a micro-fluidic derivatisation instrument (Pang et al., 2014) and to investigate glyoxal uptake onto ammonium sulphate aerosol (Hamilton et al., 2013). The instrument uses a high power LED peaking around 455 nm to pump an optical cavity constructed from two high reflectivity plano-concave mirrors separated by 110.5 cm (measured peak reflectivity =0.999817 at 462 nm, light paths of 5 km when flushed with dry nitrogen). Gas mixtures were sampled from the EUPHORE chamber into the cavity through a PFA inlet line (1.2 m length, 6.35 mm outside diameter, 2 L min-1 flow rate) that passed through a bulkhead compression fitting in a flange in the chamber floor, close to the centre of the chamber (see Fig. 1b). The inlet line protruded 40 cm above the chamber floor in order to sample well-mixed gas. Because the BBCEAS instrument shared the same flange used to inject samples into the chamber, the instrument often measured elevated trace gas concentrations during and shortly after injections. Hence data within 5 min of any such trace gas injection have been excluded from the comparisons in this paper.

Spectra of the light intensity transmitted through the cavity and gas sample were recorded using a miniature spectrometer (Ocean Optics HR2000) housed inside a temperature stabilised enclosure. For this work, spectra were integrated for 10 s, and six spectra were averaged together and combined with I0(λ) reference spectra (obtained whilst flushing the cavity with dry synthetic air; averaged for 10 min) to produce BBCEAS spectra at a 1 min time resolution. Absorber concentrations were retrieved by fitting the molecular absorption features in the spectra between 430 and 486 nm using the same reference absorption cross sections as the other spectroscopic instruments (references in Sect. 2.1). Spectra were routinely fitted for glyoxal, methyl glyoxal, NO2, oxygen O2–O2 collision complex and a high order polynomial function (typically 6th order) to account for all remaining unstructured extinction contributions, such as extinction by secondary organic aerosol formed from VOC oxidation in the EUPHORE chamber. Spectra were also fitted for water absorption bands whenever water vapor had been admitted into the chamber (e.g., the ambient air experiment E6). The highly structured glyoxal, NO2 and water cross sections (Rothman et al., 2009) were degraded to the instrument's spectral resolution (between 0.09 and 0.13 nm half width at half maximum) using asymmetric line shape functions deduced at some 20 wavelengths across the spectrometer's bandwidth by recording and fitting atomic emission lines from argon and krypton calibration lamps. Spectra were not explicitly fitted for ozone or biacetyl absorption, even for experiments where these species were known to be present (see Sect. 4.3); both these molecules have broad, relatively unstructured absorptions within the instrument bandwidth, and their absorptions were adequately fitted by the polynomial function.

Allan variance tests conducted on a long time series of BBCEAS spectra obtained whilst flushing the cavity with dry nitrogen showed that the measurement precision is dominated by random noise components for averaging times up to several hundred seconds. The instrument was subject to small long-term drifts over the ∼12 h duration of the Allan tests that degraded the achievable precision. However these drifts were always smaller than the 1σ measurement precision for each molecular absorber for the 1 min averaging time, as also evidenced by the modest departures of the BBCEAS data's means from zero in the histograms in Figs. 9 and 10 below. For this deployment at the EUPHORE facility, I0(λ) reference spectra were obtained only at the start and the end of each experiment, whereas more frequent re-acquisitions of the reference spectra during experiments themselves, at time intervals informed by the Allan tests, would reduce the effects of instrument drift. The overall accuracy of the BBCEAS concentration measurements is estimated to be 7 % for glyoxal and NO2 and 10 % for methyl glyoxal. Three main factors (which are comparable in size) control the accuracy: uncertainties in the reference absorption cross sections used to fit the molecular absorbers, uncertainties in determining the reflectivity of the cavity mirrors (this work used a combination of Rayleigh scattering in helium and nitrogen, and absorption by the O2–O2 dimer in pure oxygen samples), and uncertainties in determining the proportion of the cavity occupied by the gas sample (the cavity mirrors were flushed with synthetic air to prevent degradation of the mirror reflectivity during experiments). Daniels and Ball (2015) provide a full discussion of the BBCEAS instrument and its performance.

A PTR-ToF-MS instrument (Series I, Kore, UK) was employed to detect methyl glyoxal during the EUPHORE experiments. The PTR-ToF-MS technique is based on the chemical ionization of trace VOCs present in atmospheric samples by proton transfer reactions with the hydronium reagent ion (H3O+) (Blake et al., 2009). The product is a protonated molecular ion (VOC-H)+ for each VOC of suitable proton affinity which is then separated and quantified by time-of-flight mass spectrometry (Wyche et al., 2007; Blake et al., 2009)

The PTR-ToF-MS method can also measure oxygenated VOCs such as glyoxal and methyl glyoxal. However, one drawback to PTR-ToF-MS, common to mass spectrometric techniques, is isobaric interference between VOC species being sampled; glyoxal is isobaric with acetone and propanal while methyl glyoxal is isobaric with several oxidized C4 species and also the protonated water cluster (H2O)4⋅ H+. A full discussion of the challenges and interferences for measuring glyoxal and methyl glyoxal are given in Pang et al. (2014).

With methyl glyoxal detection, moisture within an air sample can lead to an interference from water cluster adducts. As sample humidity increases, the background signal from the protonated water cluster (H2O)4⋅ H+ increases, elevating background noise on the m/z= 73 mass channel and changing the methyl glyoxal limit of detection. With calibration of the instrument response to changing chamber temperature and humidity it is possible to correct for interference from isobaric water clusters. In this study the m/z= 73 Da signal for methyl glyoxal-H+ was used to analyse the concentration of methyl glyoxal. The linear range for methyl glyoxal is 1.5–172 ppbv by PTR-ToF-MS measurement with a limit of detection of 1.51 ppbv (3σ for 3 min averaging) using dry nitrogen as a carrier. The instrumental error on the methyl glyoxal measurement is ±0.86 ppbv.

The Mad-LIP light source is a pulsed, narrow bandwidth (< 0.00078 nm), doubled Ti:Sapphire laser (Photonix Ind.) that is operated at 3 kHz and 20–70 mW. It is further capable of rapid and reproducible wavelength tuning on the scale of the vibro-rotational absorption spectral features of glyoxal (∼0.06 nm, 440.138 and 440.104 nm on and off band center wavelengths) that are exploited for its detection as discussed below. The emitted laser light is then directed through a White-type multi-pass cell, typically operated at 32 passes and 100 Torr. Gas is drawn through the cell via a scroll pump (Edwards) orthogonal to the laser beam path. During ambient operation, the gas flow is nominally ∼20 standard L min-1 that was reduced to ∼3 standard L min-1 for the first half of the comparison to be increased to ∼13 standard L min-1 in the later half for operational reasons. As a result of the initial flow being very different from standard field operating conditions, operational problems occurred during calibrations. These were accounted for after the fact but resulted in extensive instrument maintenance, which resulted in variability of the alignment of the multi-pass cell not observed during standard field operation. The variability of the alignment is reflected in variability of the calibration factors as changes in alignment effect (1) throughput how much light is scattered by the mirrors or absorbed/scattered by the baffles as opposed to exciting glyoxal, and (2) the position of the beams relative to the focal point of the PMT (photo-multiplier tube) assembly (changing the collection efficiency of the detector relative to the excitation light and is not well captured by the normalization to laser power). The detection axis is orthogonal to both the laser and gas axis. The detector for phosphorescence photons is a single photon counting PMT guarded by a 520±20 nm bandpass filter (Barr Associates). The interior of the detection cell was optically baffled to reduce laser and ambient light scattering and/or reflecting into the detector.

The Mad-LIP instrument detects both glyoxal and methyl glyoxal by phosphorescence. This is initiated in either analyte by absorption of the laser light, after which, they relax by emission of a phosphorescent photon or are quenched collisionally. As a result, the amount of phosphorescent photons emitted by either is linearly proportional to the optical cross section, which is a function of wavelength described by their respective absorption spectra, the intensity of light, and analyte number density. Both glyoxal and methyl glyoxal signals are normalized by laser power to account for its variation. The photons between 2.5 and 37.5 µs after each laser pulse during a period of integration are summed and recorded as the signal during this time. Due to this gate and delayed photon counting, combined with a 520±5 nm bandpass filter, the effect from laser scatter and fluorescent photons are diminished, minimizing the signal background, and, in particular eliminating any detection of NO2 fluorescence.

The PMT signal (Stotal) is a linear combination of several components: dark counts (Sdark), light scatter (Sscatter), glyoxal phosphorescence (Sgly) and methyl glyoxal phosphorescence (Smgly). The glyoxal mixing ratio (Glyoxalmr) is proportional to the difference in Stotal at two different wavelengths: one at high glyoxal absorbance (λ1=440.138 nm) and another at low glyoxal absorbance (λ2=440.104 nm, Fig. S1 in the Supplement and Eq. 3). Stotal is expressed in Eq. (2), followed by the calculation of Glyoxalmr in Eq. (3).

S(λ)total=Sdark+Sscatter+S(λ)gly+Smgly, Glyoxalmr=[Sλ1total-Sλ2total]⋅ηgly=[Sdark+Sscatter+Sλ1gly+Smgly]-[Sdark+Sscatter+Sλ2gly+Smgly]⋅ηgly=[Sλ1gly-Sλ2gly]⋅ηgly, where ηgly is the calibration factor relating glyoxal mixing ratio to the net glyoxal signal (S(λ1)gly-S(λ2)gly). The intensity of dark counts is a characteristic of the PMT, and light scatter as well as methyl glyoxal absorption are the same at λ1 and λ2. The calibration factor is determined by introducing a known amount of glyoxal by diluting a calibration standard quantified by CRDS and introducing it into the White-type multi-pass cell. See the CRDS system description as well as theory of operation later in this section. A very high degree of selectivity for glyoxal is achieved using this wavelength dithering approach coupled with monitoring only phosphorescent emission. Only molecules that absorb at ∼440 nm, phosphoresce at ∼520 nm, and have similar absorption spectra to glyoxal would be able to interfere. To the authors' knowledge, the Mad-LIP instrument has not observed any interferences with glyoxal detection.

Because Sgly is proportional to the glyoxal optical cross section at λ1(1.02×10-18 cm2 molecule-1), and the net glyoxal signal is proportional to the difference in optical cross section at λ1 and λ2 (3.42×10-19 cm2 molecule-1, Volkamer et al., 2005b), the contribution of glyoxal at λ1 is calculated in Eq. (4). This is then substituted into Eq. (2), and is solved for Smgly, and related to the mixing ratio of methyl glyoxal (methyl glyoxalmr) by a calibration factor (ηmgly).

Sλ1gly=σ(λ1)glyσ(λ1)gly-σ(λ2)gly⋅Sλ1gly-Sλ2gly Methylglyoxalmr=S(λ)total-Sdark-Sscatter-σ(λ1)glyσ(λ1)gly-σ(λ2)gly⋅(Sλ1gly-Sλ2gly)⋅ηmgly

The calibration factor for methyl glyoxal is determined in an analogous way as glyoxal via CRDS. Due to the lack of structured absorption of methyl glyoxal (Meller et al., 1991; also see Fig. S1 in the Supplement), Mad-LIP does not possess as high of selectivity for methyl glyoxal as for glyoxal. Additionally, the maximum absorption of methyl glyoxal is about 3 times lower than the maximum of glyoxal absorption at λ1. Furthermore, the quantum yield of phosphorescence for methyl glyoxal is lower than that of glyoxal. Due to these three reasons, methyl glyoxal has a much higher limit of detection and is susceptible to interferences due to small concentrations of glyoxal.

Instrumental calibrations were performed using cavity ring-down spectroscopy (CRDS), an absolute quantification method in that it relies only on well-documented absorption cross sections. Further details about the theory of this method are described elsewhere (O'Keefe and Deacon, 1988).

A cavity 62 cm long and 0.635 cm in diameter was formed between two parallel, highly reflective mirrors with a radius of curvature of 1 m (99.995 % reflectance, Los Gatos Research Inc.). The bulk of the cavity was encased in a 3/8 inch OD (outside diameter), 1/4 inch ID (inside diameter) PTFE tube. Halfway along the cavity, a PTFE tee was used as an inlet for the calibrant gas. On each end of the cavity, the mirror mounts were coupled via metal bellows to a Teflon PTFE tee which coupled the cell to exhaust ports for the cell. The dead volumes between the exhaust ports and the mirrors were flushed with zero air through a 200 standard cm3 min-1 flow controller (1779A, MKS Instruments) to prevent optics fouling as well as bias. This purging did not allow any sample gas to mix beyond the exhaust ports, fixing the physical absorber path length to 42 cm. This cell design is based on to the NOAA NO3 ring-down cell design (Dubé et al., 2006; Osthoff et al., 2006). The entire cavity length between (and including) the exhaust tee fittings was enclosed in a 1.5×1.5 inch block of aluminum which was maintained at a constant temperature (35 ∘C) to discourage analyte deposition inside the cavity.

A 10 standard cm3 min-1 flow controller (MKS Instruments) supplied calibrant gas that was then diluted by zero air. The zero air was delivered by a 200 standard cm3 min-1 flow controller (MKS Instruments) at a rate which made up the remainder to a total flow of 100 standard cm3 min-1 of diluted calibrant. The purge was held at 100 standard cm3 min-1 using a 200 standard cm3 min-1 flow controller (MKS Instruments). To maintain a constant cell pressure and therefore achieve a stable baseline, both the purge and the diluted calibrant flows were held constant. Laser pulses were introduced into the cavity through one of the high-reflectivity mirrors. A beamsplitter placed between the light source and the White-type multipass cell supplied light to the CRDS cell. With each reflection of a laser pulse, a small quantity of light escaped through the mirrors. On the opposite side of this cavity, a PMT (Hamamatsu), guarded by a 440 nm bandpass filter, detected this escaped light. Loss of photons within the cavity is a first-order process; thus, the light leaking from the cavity has the characteristics of an exponential decay. The number density of a chemical absorber (molecules cm-3) can be determined by relating two determined lifetimes, those determined with and without the presence of the absorber, by the following equation: Nd=1-Rσlaτ0-ττ, where Nd is the number density of the absorber, R is mirror reflectivity, σ is the absorption cross section (either Volkamer et al., 2005b, or Meller et al., 1991) for glyoxal and methyl glyoxal, respectively), la is the path length of the absorber, τ and τo are the lifetimes with and without the absorber, respectively (Zalicki and Zare, 1995).

A Differential Optical Absorption Spectroscopy device using a white multi-reflection cell (W-DOAS) of 8 m base path-length is deployed at EUPHORE. The optical system employed a Xenon high pressure short-arc lamp (XBO-550W) as the light source, coupled to a telescope that collimates the light into a narrow beam and sends it into the chamber. The multi-reflection cell used during these experiments consisted of a set of prisms and mirrors dielectrically coated, allowing an optical path of 1154 m with reflection of the beam in the range 389–469 nm, for the detection of glyoxal, methyl glyoxal and NO2. Two laser diodes and web-cameras were used to adjust the path-length of the system. The beam is finally driven outside of the chamber where it is focused by a telescope onto the entrance slit of a spectrograph equipped photodiode array detector. A detailed description can be found in Becker (1996).

The system collected spectra every 80–110 s by co-addition of 100 samples. A blank spectrum taken at the beginning of each day in the clean chamber was used as background I0(λ). Also, during the experiment, the stray-light was corrected by subtracting a spectrum recorded by introducing an edge filter in the light beam. The resolution was set to 0.35 nm FWHM. The analysis of the data was performed using a fitting routine (Rodenas, 2008) adapted to process DOAS data; this fitting routine was successfully tested in previous intercomparison exercises (Rodenas, 2008). The same literature cross sections for glyoxal, methyl glyoxal and NO2 as the other instruments were used.

The EUPHORE chamber is equipped with a Fourier Transform Infrared system (FTIR). The spectrometer (NICOLET 550, MCT/B-detector) is coupled to a white-type multi-reflection cell installed into the chamber for the detection of gaseous reactants and products in the IR spectral range (400–4000 cm-1). The gold-coated mirrors of the cell allow a total path length of 616 m (8.3 m base path). With FTIR, it is possible to calculate the concentration of a wide range of compounds and reaction products using absorption reference spectra previously collected and the corresponding calibration thereof. A detailed description of the instrument is given in Becker (1996).

The spectra were derived from the co-addition of 280 scans, collected over a 5 min period, with a resolution of 1 cm-1. During the experimental campaign, concentration profiles of glyoxal and methyl glyoxal were determined using improved analysis software developed at CEAM (Rodenas, 2008) adapted to analyze infrared spectra, and applied to the region of 2700–2900 cm-1. This program is based on a classical least squares fitting which also removes the spectral interfering broadband (formed due to the presence of aerosols, equipment instabilities or unknown broadband products) by including a curve that models and subtracts it locally. The software has been tested and used in previous works (Muñoz et al., 2011, 2012).

Reference spectra were previously collected with the instrument, and calibrated with the references used by the W-DOAS system (Sect. 2.1). Water, formaldehyde, methanol and other compounds show absorption bands in the same spectral region as glyoxal and methyl glyoxal. Together with these compounds, the instrument was used to report the evolution of most of the reactants and products forming the complex mixture in the experiments preformed. These compounds were present in the samples to a greater or lesser degree depending on the experiment carried out. The fitting was done using both the aldehydic C-H band and the region 770–1140 cm-1. The list of compounds analyzed includes ozone, isoprene, nitric acid, o-xylene, and formic acid. SF6 was also monitored by FTIR to quantify the dilution range of the chamber.

Solid-phase microextraction (SPME) methodology was used to determine glyoxal and methyl glyoxal through PFBHA on-fiber derivatization. A detailed description of the methodology used at the EUPHORE chambers can be found in the literature (Gómez Alvarez et al., 2007; Alvarez and Valcárcel, 2009). Briefly, the SPME device used in this work consisted of a holder assembly with 65 µm fibers coated with Polydimethylsiloxane/Divinylbenzene (PDMS/DVB), from Supelco, Bellefonte, PA (USA). These fibers were conditioned following the manufacturer's recommendations for at least 0.5 h at 250 ∘C to eliminate any impurities. Fibers were loaded with PFBHA derivatization reagent, for 2 min, through the headspace of a 4 mL opaque amber vial containing a 17 mg mL-1 PFBHA water solution.

Exposing the fiber to the air of the chamber was achieved by means of an aluminum adapter located in one of the flanges in the chamber floor. In the exposed position, fibers extend into the chamber by a few millimeters.

Samples were taken for several minutes and were subsequently analyzed by GC-FID by injecting the fiber directly into the GC injector. Sampling time ranged depending on the dicarbonyl concentrations. Whenever possible, identification of the peaks was also cross-checked using GC-MS. The chromatographic conditions were as follows: a 6890 HP gas chromatograph was used, coupled to a flame ionization detector (FID), equipped with a HP5-MS capillary column (30 m × 0.25 mm ID × 0.25 µm) and an inlet liner with a narrow internal diameter 0.75 mm ID pre-drilled Thermogreen LB-2 septa for SPME were used. The chromatograph was programmed at 80 ∘C for 2 min, then ramped at a rate of 20–280 ∘C and held at 280 ∘C for 3 min. The injection port was held at 270 ∘C and detector at 300 ∘C. Samples were injected in splitless mode, using on column constant helium flow of 1 mL min-1.

Experiments E1 and E8a consisted of the injection of pure glyoxal into the chamber followed by stepped dilution. Correlations of data segregated between high (0–15 ppbv) and low (0–2 ppbv) mixing ratio data are shown in Fig. 3. It should be noted that the W-DOAS instrument is affected by the distortion of the light beam during the flushing of the chamber (the air input of the flushing is in the center of the chamber and intersects the W-DOAS light path). Table 3 compares individual instruments to CE-DOAS; correlation matrices that compare each instrument pair-wise to each other instrument for experiments E1 and E8a can be found in Tables S1–S2 in the Supplement. The slopes varied between 0.76 and 1.09 between all instruments and both experiments. Mad-LIP defines the highest and lowest slopes observed, reflecting ∼33 % difference in separate calibrations between both experiments as well as different operating conditions (see Sect. 2.1.6). All other instruments agreed within 15 %.

Experiment E9 investigated the possible interference of a large amount of NO2 on detection of glyoxal for instruments using visible (430–490 nm) light spectroscopy and found no scalable bias due to NO2. In this experiment the Mad-LIP was used as the glyoxal reference to evaluate effects of NO2 with the UV-visible absorption techniques as previous work by Huisman et al. (2008) had shown Mad-LIP measurements of glyoxal to be insensitive to NO2 interferences, tested up to 1 ppmv. Figure 4 shows the time series of glyoxal and NO2 concentrations for E9. The initial glyoxal amount (0.6 ppbv) was diluted and left to stabilize around 0.3 ppbv in absence of NO2, followed by stepped NO2 additions up to ∼180 ppbv. SF6 was added and measured by FTIR as a tracer for dilution. The SF6 signal in Fig. 4 has been scaled to the initial glyoxal and shows that the theoretical decay according to dilution of the glyoxal glyoxal signal is in good agreement with the Mad-LIP glyoxal data. Note that the error bars increase at high NO2, more for CE-DOAS than for BBCEAS due to larger light losses at the longer absorption path in CE-DOAS. Deviations in glyoxal however were small for all instruments; they are marginally significant for BBCEAS and insignificant for CE-DOAS during periods when MAD-LIP data is available (see Fig. 5). Deviations in the SPME concentrations were large but appear to be unconnected to the high NO2 levels in the chamber. For both CE-DOAS and BBCEAS (Figs. 4 and 5) we do not find significant bias; i.e., an upper-limit change in glyoxal due to NO2 is derived as ±200 pptv glyoxal in the presence of 200 ppbv NO2 (or bias of 1 pptv glyoxal/1 ppbv NO2).

Experiment E2 compared methyl glyoxal measurements in a pure compound system. Approximately 25 ppbv of methyl glyoxal was injected into the chamber, and diluted in 6 discrete steps to less than 1 ppbv. Figure 6 shows correlation plots of data segregated into high and low (< 3 ppbv) concentrations, and the regression lines (see Table 3). The slopes varied between 0.97 and 1.40, with generally larger differences in slopes between instruments than for glyoxal. Mad-LIP showed the highest slope, while W-DOAS had the lowest slope. Experiment E10 tested the interference of NO2 on methyl glyoxal, as previously described for glyoxal (Sect. 3.2.1). The initial level of methyl glyoxal was 5.3 ppbv (Fig. S3), and the SF6 dilution signal was used as the dilution reference. For FTIR the concentrations were high enough to obtain good signal, and the methyl glyoxal and NO2 absorption are well separated at IR wavelengths. No significant deviations in methyl glyoxal were observed in CE-DOAS and BBCEAS, and excellent agreement is observed even in excess of 200 ppbv NO2 (see Fig. 5b). In the presence of 200 ppbv we found a maximum bias of ±1 ppbv on a sample of 5.3 ppbv methyl glyoxal (or a bias of 5 pptv methyl glyoxal/1 ppbv NO2; see Figs. 5 and S3).

Experiment E3 investigated o-xylene photo-oxidation by OH radicals in the presence of NOx (added as HONO), as a source for highly variable concentrations of glyoxal, methyl glyoxal, biacetyl and NO2. Figure 7 illustrates the time series and correlation plots. Table 3 gives the results of regression fits (correlation plots include data from before and after HONO addition and chamber opening). The slopes varied between 0.83–1.1 (glyoxal), 0.86–1.7 (methyl glyoxal), and 0.95–1.01 (NO2), and most instruments agreed within 12, 30, and 5 %, respectively. These differences were similar or slightly larger than those observed in the pure compound experiments (Sects. 3.2.1, and 3.2.2.). Notably, differences of up to 8 % between BBCEAS and CE-DOAS for methyl glyoxal are observed despite excellent agreement (better than 1 %) for both glyoxal and NO2. While Mad-LIP data show excellent correlation (R2 > 0.95 for both α-dicarbonyls, Table 3) they also mark the largest (1.66 methyl glyoxal) and smallest (0.83 glyoxal) slopes for both α-dicarbonyls. Although FTIR performed well for methyl glyoxal, concentrations of glyoxal were close to the detection limit of the FTIR, and the measured concentrations did not scatter around zero as expected (Fig. 7a) most probably due to unknown interfering products formed because the chamber was exposed to light (NO2, ozone and HCHO formation were observed from walls). Hence, FTIR data were only considered for further discussion if values exceeded detection limits by at least a factor of 2 for constructing correlations and 10 for calculation of accuracy (Table 4).

In the isoprene/NOx system (experiment E7, see Fig. S4) results were generally similar. However, the variations in slopes were somewhat higher, i.e., 0.94–1.54 (glyoxal), and 0.7–2.2 (methyl glyoxal), while most instruments agreed within 30 % for both species (see Table 3). Again, Mad-LIP data show excellent correlation (R2 > 0.98) indicating the shape of the Mad-LIP profile agreed with the other instruments, but there were systematic differences in the slopes (up to a factor of 2.2 for methyl glyoxal). This is indicative of the calibration and stability issues present in the Mad-LIP instrument during the campaign (see Sect. 2.1.6) as well as the difficulty in differentiating the methyl glyoxal signal from a large glyoxal background.

Experiment E4 consisted of a higher NOx isoprene oxidation experiment (NO2 concentrations up to 170 ppbv) and has been excluded from these comparisons for operational reasons. The NOx control system failed to maintain a stable NOx concentration in the chamber and a dilution valve failed in the CE-DOAS system which prevented the retrieval of the data to compare with other instruments (dilution of the chamber flow into the CE-DOAS system was not attempted on any of the other experiments).

For experiment E6, ambient air was added to a cleaned chamber, to assess possible interferences from other species. For example, water vapor absorbs light at blue visible wavelengths, and can create problems with the molecular spectroscopy in UV-visible absorption techniques. Further, the transfer of α-dicarbonyls through sampling lines can become complicated in presence of ambient levels of relative humidity. Aerosols can reduce path length with BBCEAS, and other species in ambient air may create further interferences. Figure 8 shows the time series: a clean chamber was exposed to sunlight, and ambient air was added (see Fig. S5 for correlations of BBCEAS and Mad-LIP with respect to CE-DOAS); the chamber's roof was closed and ∼100 ppbv O3 was added. Some of the accelerated decrease in NO2 during the following hour may indicate formation of NO3 radicals, and subsequent N2O5 hydrolysis on aerosols and chamber walls. The roof was then opened, and after 1.5 h HONO was added in a defined way such that NOx (NO + NO2) remained constant. Finally, a small amount of isoprene (18 µL, ∼25 ppbv in the chamber) was injected while NOx was controlled via the HONO source. The RH varied between 30 and 50 %, and NO2 levels were below 16 ppbv at all times, while concentrations of α-dicarbonyls varied between 0.1 < glyoxal < 1 ppbv, and 50 pptv < methyl glyoxal < 5 ppbv, with average concentrations of 380 pptv glyoxal, and 1.7 ppbv methyl glyoxal. The slopes of correlations (Table 3) varied between 0.95–1.5 (glyoxal), 0.68–1.83 (methyl glyoxal), and 0.995 (NO2) – agreement between most instruments was on the order of 10 % for glyoxal, and 30 % for methyl glyoxal, with extreme slopes showing differences of 50 % in case of SPME-glyoxal, and 83 % in the case of Mad-LIP methyl glyoxal. Interestingly, CE-DOAS and BBCEAS slopes agreed within 2 % for NO2, 5 % for glyoxal, but differed by 32 % for methyl glyoxal. We note that the range of methyl glyoxal concentrations is fairly limited (correlations are driven by essentially two groups of points one near 0 and the other near 2 ppbv). The cause for this difference is not clear to the authors. A possible partial explanation may exist in the difference in sampling location from the chamber, as CE-DOAS sampled close to the wall of the chamber, and the rise in methyl glyoxal after 3 p.m. that drives the CE-DOAS vs. BBCEAS correlation away from 1:1 could be an artefact of wall interaction (and not an instrumental difference). Generally, correlations are slightly more variable in humid air than in dry air, and were found to be slightly lower for methyl glyoxal (0.58 < R2 < 0.68) than for glyoxal (0.79 < R2 < 0.99). For discussion of the effect of H2O on measurements of low glyoxal and methyl glyoxal concentrations see Sects. 4.5 and 4.6. The effect of ambient levels of NO2 on the glyoxal retrieval from CE-DOAS in E6 is shown in Fig. S6 and discussed more at length in Sect. 4.4.

Experiment E5 tested the interference of O3 directly via spectral interference with absorption measurements and/or indirectly by either production of glyoxal on reaction with Teflon (walls of the chamber or sampling lines) or other VOCs in the chamber. In the first half of the experiment O3 was injected into the chamber to three stable levels (0–2.5 ppmv) and then flushed out of the chamber (see Fig. S8). During these stable periods, the CE-DOAS and BBCEAS instruments changed the lengths of their sampling lines (adding new clean lengths of Teflon) to attempt to observe any change in the measured concentration. The only effect observed from longer Teflon lines was an increased amount of NO2 (an increase of 60 pptv from 30–60 pptv NO2 background) with longer sample lines caused by the reaction of O3 with NO trapped at the surface of the tubing. In the second half of the experiment, attempts were made to observe glyoxal production in a dark, NOx-free environment via reaction of OH with acetylene. The intention was to generate OH in the dark from the reaction of O3 with 2,3-dimethyl-2-butene (tetramethylethylene, TME; ozonolysis OH yield of 0.90 – IUPAC recommendation). The chamber was left filled with 200 ppbv of O3, acetylene (20 ppmv) was added and TME was shortly to be injected into the chamber (with the chamber roof closed). However, before the TME could be injected, rapid glyoxal production ensued with the glyoxal concentration reaching 45 ppbv over the following 3 h before the chamber was flushed clean (see Fig. S7a). The glyoxal is thought to have come from the reaction of O3 with an impurity in the C2H2 (since several ppmv of C2H2 were added to the chamber an impurity with a relatively moderate yield of glyoxal would only have need to be 1 % of the C2H2 added). Several of these impurities were detected by FTIR in the 20 ppmv acetylene mixture inside the chamber, including 60 ppbv of ethene and 160 ppbv of acetone. The ozonolysis of ethene is known to produce OH (12–18 % yield, Chew and Atkinson, 1996) which then likely reacted with acetylene, which produces glyoxal as well as regenerate OH (30 % yield, Siese and Zetzsch, 1995; Bohn and Zetzsch, 1998).

The data set presents an opportunity to assess precision, accuracy and detection limits of the various instruments while observing the same air mass. Experiment E8b investigated the overnight dilution of initially ∼60 ppbv of glyoxal (4000 L min-1 flush rate) with all of the instruments measuring continuously in their normal operating set up until the following morning. A time series of the data is shown in Figs. S8b and c. Several hours of data were collected in a flushed chamber where glyoxal and methyl glyoxal were both well below the detection limits of all of the instruments, but there may have been very small amounts of NO2 and aerosol present. The expected glyoxal concentration during the targeted period (02:00–06:00 UTC; see gray area in Fig. S8c) was below 3 pptv based on the theoretical dilution. From these data histograms were constructed. From the Gaussian distributions of the histograms the standard deviation and mean were calculated for each instrument (see Fig. 9 for glyoxal and Fig. 10 for methyl glyoxal, the latter also using data from E8b after 02:00 UTC). The limit of detection (LOD) is defined as follows: LODexp=3⋅σGaussian+background, where the 1-σ variability was calculated during a period when the sensor signal is expected to be constant (E8b), and multiplied by 3; the “background” is taken as the absolute offset from the zero reference spectrum of the same time period (Figs. 9, 10 and S8). LOD as defined by Eq. (7) is widely used in analytical chemistry (IUPAC, 2006).

The experimental LODs are listed in Table 4 together with LOD values submitted with their measurement data by the operators of the various instruments. We find excellent agreement between the experimental LODs determined here and the reported LODs, once a common definition is applied. As seen in Figs. 9 and 10, the distributions are Gaussian (except for FTIR) and yielded LODs lower than or similar to the values reported for each instrument (see Table 4). For FT-IR the spread of data did not form a Gaussian distribution, and instead a simple average and standard deviation were calculated. All instruments performed within their specifications.

Accuracy represents the measurement uncertainty at high signal to noise (see Ryerson et al., 2013; Thalman and Volkamer, 2010). We assess accuracy from the variability in slopes relative to CE-DOAS, using only data from experiments where the maximum concentration is at least 10 times larger than the 1-σ variability deduced from E8b (LOQ, limit of quantification). The accuracy of the instruments was assessed from the difference between different methods at the 95 % CI level (see Table 4). See Sect. 4.2 for further discussions on LODs.

Both CE-DOAS and BBCEAS were considered as reference techniques. We chose CE-DOAS to assess relative differences to other instruments for the following reasons: (1) the instrument participated in both campaigns, (2) had excellent data coverage, and (3) high time resolution. Use of CE-DOAS yields the maximum number of data points to calculate correlations between different instruments at EUPHORE. Further, (4) CE-DOAS demonstrated the lowest LODEq.(7), and concentration offsets for both glyoxal and methyl glyoxal among all available instruments (see Table 4, Figs. 9 and 10); (5) CE-DOAS benefits from inherent path length calibration through O4 at very high signal-to-noise to demonstrate control over cavity alignment with very little error (2 %). (6) The comprehensive coverage and consistent performance from CE-DOAS in context with the other instruments that we compared at both chamber facilities provides strong evidence to suggest CE-DOAS is precise, and accurate. (7) The size of white-noise residuals observed by CE-DOAS can be understood in terms of the measured photon fluxes, and provides additional information to assess LOD and accuracy (Sect. 3.2.6, Fig. S9). The Supplement contains a discussion of potential sources for systematic bias with CE-DOAS measurements. At high concentrations the resulting error of 3.5 % is dominated by the uncertainty in the absorption cross sections. A discussion of the factors that influence accuracy at low concentrations is provided in Sect. 4.6.