Monday, 10 January 2005
Secondary organic aerosol formation by reactive condensation of glyoxal, water vapor and other aldehydes
The formation of secondary organic aerosol particles by particle-phase reactions is currently of great interest. Glyoxal and other volatile aldehydes been identified as significant components of the particle phase in recent smog chamber aromatic oxidation studies. This is surprising because phase partitioning theory predicts that compounds with high vapor pressures remain almost entirely in the gas phase. Growth of inorganic seed aerosol in a particle chamber was monitored by scanning mobility particle sizing during addition of gas-phase aldehydes and small amounts of water vapor. Glyoxal was observed to condense on inorganic seed aerosol when relative humidity exceeded 25 %. Glyoxal condensed in the presence of a second aldehyde (citral, nonanal, actetaldehyde or benzaldehyde) at even lower humidity levels. This behavior can be explained by a chemical reaction: glyoxal is known to polymerize when exposed to water vapor.1-3
We show by MS/MS that in the presence of a second aldehyde (“A
”), mixed glyoxalx
polymers form. This glyoxal-induced polymerization may be a general mechanism for secondary aerosol formation in urban airsheds.
The reactivity of hydrated and polymerized forms of glyoxal during analysis by gas chromatography was assessed. Hydrated glyoxal was found to convert to glyoxal at even slightly elevated temperatures in GC injection ports. We then showed that breakdown of solid-phase glyoxal trimer dihydrate, forming gas phase glyoxal and water vapor, occurs at temperatures just above 50 C, the boiling point of glyoxal. These observations suggest that reports of particle-phase glyoxal are likely caused by GC sampling artifacts, and that the actual particulate species are instead mixed glyoxal/aldehyde polymers. It does not appear that chemical derivatization protects glyoxal polymers from thermal breakdown during GC analysis. The existence of mixed glyoxal polymers (with negligable vapor pressures) in the particle phase, rather than volatile glyoxal, is consistent with phase partitioning theory.
(1) Jang, M.; Kamens, R. M. Environ. Sci. Technol. 2001, 35, 4758-4766. (2) Jang, M.; Carroll, B.; Chandramouli, B.; Kamens, R. M. Environ. Sci. Technol. 2003, 37, 3828-3837. (3) Liggio, J.; Li, S.-M.; McLaren, R. Environ. Sci. Technol. 2004, submitted.