Diurnal Variations in Partitioning of Atmospheric Glyoxal and Methylglyoxal between Gas and Particles at the Ground Level and in the Free Troposphere

This work presents diurnal variations of gas- and particle-phase dicarbonyls (glyoxal (Gly) and methylglyoxal (Mgly)) in the atmosphere, which are important compounds that contribute to the formation and growth of atmospheric particulate matter. To obtain variations in partitioning, continuous collection of gaseous dicarbonyls was performed using a parallel plate wet denuder, and at the same time, the dicarbonyls in particle were collected using a spray-type particle collector downstream. Hourly samples were analyzed by high performance liquid chromatography–electrospray ionization–tandem mass spectrometry. This method is advantageous to monitor the gaseous and particulate carbonyls separately without loss during sampling. Sampling was performed in summer and winter in a midsize city (Kumamoto, Japan). The concentrations of the dicarbonyls increased in the summer daytime, which suggests that they are mostly formed by secondary production in the local atmosphere. The dicarbonyls and formaldehyde (HCHO) were found in both gas and particle phases, and partitioning to the particle phase was highest for Gly, followed by Mgly and HCHO. It was observed that the compounds moved to the particle phase in the midnight and early morning hours according to the growth of hygroscopic aerosols in summer. The particle/gas ratio also increased in the presence of high PM_2.5, which is transported from the Chinese Continent in winter. The dicarbonyls were also observed on Mt. Fuji (3776 m) in the free troposphere. From back trajectory data and information on volatile organic compounds, they were most likely produced from relatively long-lifetime organic compounds from the Chinese Continent and biogenic volatile organic compounds emitted in the Japan Alps mountain range. Higher particle/gas ratios at the Mt. Fuji station indicate that low temperatures and high humidity precede the partition. The estimated effective Henry’s law constants for the dicarbonyls, 10⁸ order in mol/kgH₂O/atm for summer data, were much higher than those for ideal liquid/vapor equilibrium but close to reported results obtained by chamber experiments. In the proposed method, oligomers in particle were also counted as the compounds. The dicarbonyl compounds existed up to submolar levels in real atmospheric aerosols, which suggests they undergo further reactions in the particle phase