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.
Highlights
Methylglyoxal (MGLY, CH3C(O)CHO) is an important αdicarbonyl species in the atmosphere
While the lower limit of the tested range is larger than observed ambient concentrations (0.05–0.3 ppbv, Fig. 2), it has to be noted that the PTR-MS response has always been observed to be linear with the analyte concentration and a linear response is expected for MGLY for mixing ratios below 0.6 ppbv
Washenfelder et al (2011) showed a breakdown of the photostationary state when applied to glyoxal, a dicarbonyl compound exhibiting a lifetime of the same order of magnitude as MGLY, and as a consequence the calculated loss rate reported in this study only provides a rough estimation of the local production rate
Summary
Methylglyoxal (MGLY, CH3C(O)CHO) is an important αdicarbonyl species in the atmosphere. It is mainly produced during the oxidation of volatile organic compounds (VOCs) of which isoprene and acetone are the main contributors. Other precursors of MGLY are C3-C5 isoalkanes (Jacob et al, 2002), aromatic compounds (Volkamer et al, 2001; Pan and Wang, 2014; Wu et al, 2014), and monoterpenes (Fick et al, 2003; Nunes et al, 2005). Due to the anthropogenic and biogenic natures of MGLY precursors, this compound can be found at significant levels (low tens to hundreds of pptv) in urban, rural or even remote and marine environments (Henry et al, 2012 and references therein)
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