Field measurements of methylglyoxal using proton transfer reaction time-of-flight mass spectrometry and comparison to the DNPH–HPLC–UV method

Abstract

Abstract. Methylglyoxal (MGLY) is an important atmospheric α-dicarbonyl species for which photolysis acts as a significant source of peroxy radicals, contributing to the oxidizing capacity of the atmosphere and, as such, the formation of secondary pollutants such as organic aerosols and ozone. However, despite its importance, only a few techniques exhibit time resolutions and detection limits that are suitable for atmospheric measurements. This study presents the first field measurements of MGLY by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) performed during the ChArMEx SOP2 field campaign. This campaign took place at a Mediterranean site characterized by intense biogenic emissions and low levels of anthropogenic trace gases. Concomitant measurements of MGLY were performed using the 2,4-dinitrophenylhydrazine (DNPH) derivatization technique and high performance liquid chromatography (HPLC) with UV detection. PTR-ToF-MS and DNPH–HPLC measurements were compared to determine whether these techniques can perform reliable measurements of MGLY. Ambient time series revealed levels of MGLY ranging from 28 to 365 pptv, with a clear diurnal cycle due to elevated concentrations of primary biogenic species during the daytime, and its oxidation led to large production rates of MGLY. A scatter plot of the PTR-ToF-MS and DNPH–HPLC measurements indicates a reasonable correlation (R2=0.48) but a slope significantly lower than unity (0.58±0.05) and a significant intercept of 88.3±8.0 pptv. A careful investigation of the differences between the two techniques suggests that this disagreement is not due to spectrometric interferences from H3O+(H2O)3 or methyl ethyl ketone (or butanal) detected at m∕z 73.050 and m∕z 73.065, respectively, which are close to the MGLY m∕z of 73.029. The differences are more likely due to uncorrected sampling artifacts such as overestimated collection efficiency or loss of MGLY into the sampling line for the DNPH–HPLC technique or unknown isobaric interfering compounds such as acrylic acid and propanediol for the PTR-ToF-MS. Calculations of MGLY loss rates with respect to OH oxidation and direct photolysis indicate similar contributions for these two loss pathways.

An IBBCEAS system for atmospheric measurements of glyoxal and methylglyoxal in the presence of high NO<sub>2</sub> concentrations

“…From aspects of miniaturization and improving time resolution, Min et al optimized Washenfelder et al's IBBCEAS for aircraft GLY measurements. The demonstrated precision (2σ ) for retrievals of GLY is 34 ppt over 5 s (Min et al, 2016). The IBBCEAS developed by Fang et al has a precision (1σ ) of 28 ppt for GLY at 1 min averaging time.…”

“…While proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) is considered to be a good mass spectrometry technique for measuring volatile organic compounds (VOCs), its sensitivity is still too low to monitor the ambient concentration of GLY (Thalman et al, 2015;Stonner et al, 2017) and both (H 2 O) 3 H 3 O + and acrylic acid (C 3 H 4 O 2 ) can interfere with MGLY results because they have the same m/z ratio (Thalman et al, 2015;Yuan et al, 2017;Zarzana et al, 2018). There are also many types of optical methods, including long path differential optical absorption spectroscopy (LP-DOAS) (Volkamer et al, 2005a), cavity-enhanced differential optical absorption spectroscopy (CE-DOAS) (Thalman and Volkamer, 2010), multi-axis differential optical absorption spectroscopy (MAX-DOAS) (Li et al, 2014), laser-induced phosphorescence (LIP) (Henry et al, 2012), incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) (Min et al, 2016), and methods using satellite techniques like the global ozone monitoring experiment (GOME) (Vrekoussis et al, 2010) and ozone monitoring instrument (OMI) (Chan Miller et al, 2014).…”

“…Since the first application of IBBCEAS by Fiedler (Fiedler et al, 2003), it has been widely used in laboratory studies and field campaigns to measure atmospheric trace gases such as H 2 O, O 3 , O 4 , IO, I 2 , OIO, SO 2 , NO 2 , NO 3 , N 2 O 5 , HONO, HCHO, GLY, and MGLY (Vaughan et al, 2008;Washenfelder et al, 2008;Thalman and Volkamer, 2010;Axson et al, 2011;Kahan et al, 2012;Min et al, 2016;Duan et al, 2018). The core part of the instrument is a high-finesse cavity that holds of a pair of highreflectivity mirrors, which typically have reflectivity greater than 0.9999.…”

“…1, the IBBCEAS flow system includes an inlet tube, aerosol filter, optical cavity, temperature sensor, pressure sensor, rotary vane pump (50104, Thomas, Gardner Denver, Germany), rotameter, three mass flow controllers (MFCs), and three solenoid valves (6014, Bürkert, Ingelfingen, Germany). Since Teflon has the best GLY transmission efficiency (Min et al, 2016), the inlet tube, aerosol filter, and optical cavity were all constructed from Teflon.…”

“…Compared to the methods that produce GLY or MGLY by either heating GLY trimer dihydrate powder or MGLY powder (Gen et al, 2018) or using a temperature-controlled Teflon bubbler filled with solution (Min et al, 2016;Zarzana et al, 2018), the standard gas produced by our generator is more stable and can be maintained at a relatively constant concentration; the 50 L mix chamber used in this study can provide a constant concentration of GLY or MGLY for approximately 20 min. Furthermore, our generator can produce GLY or MGLY concentrations on the order of parts per trillion to parts per million by adjusting the flow rate of the elution gas and the dilution ratio of the gas in the mix chamber.…”