Mechanisms of VOC oxidation and aerosol formation: Atmospheric organic chemistry of glyoxal.

摘要

Secondary organic aerosol (SOA) has impacts on both global climate and human health, but its formation and properties are not well characterized. To understand the formation and relative importance of individual compounds to SOA formation, we must understand their gas-phase production and their potential to form aerosol. Glyoxal, a product of volatile organic carbon oxidation, is a potentially critical component of atmospheric aerosol. As current atmospheric models overestimate glyoxal mixing ratios, chamber studies of OH- initiated, high-NOx isoprene oxidation were performed. The Master Chemical Mechanism v. 3.1 was updated with first generation yields which had not previously included. Also, glyoxal production via secondary production via C5 carbonyls was attenuated to account for the MCM's overprediction of higher-generation production. The updated mechanism enables improved modeling of gas -phase production of glyoxal.;In order to gain insight into processes controlling SOA formation from glyoxal, chamber studies of glyoxal uptake onto ammonium sulfate aerosol were performed under dark and irradiated conditions and in the presence of OH. The chemical composition of aerosol formed from glyoxal and the glyoxal uptake rate were determined to be independent of OH. The glyoxal uptake coefficient is directly proportional to relative humidity and decreases exponentially as a function of glyoxal exposure. Glyoxal monomers and oligomers were the dominant organic compounds formed in the aerosol; their formation was reversible under dark conditions. However, the formation of 1H-imidazole-2-carboxaldehyde from glyoxal reacting with the aerosol seed was irreversible. To our knowledge, this is the first time carbon-nitrogen compounds resulting from condensed phase reactions with ammonium sulfate seed have been detected in aerosol. An organosulfate, previously assigned as glyoxal sulfate in ambient samples and chamber studies of isoprene oxidation, was observed only in irradiated experiments. Through a laboratory standard, this organosulfate has been shown to be glycolic acid sulfate, an isomer of the previously proposed glyoxal sulfate. This work highlights the importance of aerosol organic chemistry and improves our understanding of formation and chemical and optical properties of aerosol as a means to improving atmospheric models.