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