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Oligomers Formed Through In-cloud Metylglyoxal Reactions: Chemical Composition, Properties, and Mechanisms Investigated by Ultra-high Resolution FT-ICR Mass Spectrometry
Citation:
Altieri, K. E., S. SEITZINGER, A G. Carlton, B. TURPIN, G. C. Klein, AND A. G. Marshall. Oligomers Formed Through In-cloud Metylglyoxal Reactions: Chemical Composition, Properties, and Mechanisms Investigated by Ultra-high Resolution FT-ICR Mass Spectrometry. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, 42(7):1476-1490, (2008).
Impact/Purpose:
The National Exposure Research Laboratory's (NERL's) Atmospheric Modeling Division (AMD) conducts research in support of EPA’s mission to protect human health and the environment. AMD's research program is engaged in developing and evaluating predictive atmospheric models on all spatial and temporal scales for forecasting the Nation's air quality and for assessing changes in air quality and air pollutant exposures, as affected by changes in ecosystem management and regulatory decisions. AMD is responsible for providing a sound scientific and technical basis for regulatory policies based on air quality models to improve ambient air quality. The models developed by AMD are being used by EPA, NOAA, and the air pollution community in understanding and forecasting not only the magnitude of the air pollution problem, but also in developing emission control policies and regulations for air quality improvements.
Description:
Secondary organic aerosol (SOA) is a substantial component of total atmospheric organic particulate matter, but little is known about the composition of SOA formed through cloud processing. We conducted aqueous phase photooxidation experiments of methylglyoxal and hydroxyl radical to simulate cloud processing. Oligomer formation from methylglyoxal-hydroxyl radical reactions was detected by electrospray ionization mass spectrometry (ESI-MS). The chemical composition of the oligomers and the mechanism of their formation were investigated by ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and LCQ DUO ion trap mass spectrometry (ESI-MS-MS). Reaction products included 415 compounds detected in the mass range 245-800 Da and the elemental composition of all 415 compounds were determined by ultra-high resolution FT-ICR MS. The ratio of total organic molecular weight per organic carbon weight (OM:OC) of the oligomers (1.0-2.5) was lower than the OM:OC of the organic acid monomers (2.3-3.8) formed, suggesting that the oligomers are less hygroscopic than the organic acid monomers formed from methylglyoxal-hydroxyl radical reaction. The OM:OC of the oligomers is consistent with that of aged atmospheric aerosols and atmospheric humic like substances (HULIS). A mechanism is proposed in which the organic acid monomers formed through hydroxyl radical reactions oligomerize through esterification. The mechanism is supported by the existence of series of oligomers identified by elemental composition from FT-ICR MS and ion fragmentation patterns from ESI-MS-MS. Each oligomer series starts with an organic acid monomer formed from hydroxyl radical oxidation, and increases in molecular weight and total oxygen content through esterification with a hydroxy acid (C3H6O3) resulting in multiple additions of 72.02113 Da (C3H4O2 )to the parent organic acid monomer. Methylglyoxal is a water soluble product of both gas phase biogenic (i.e., isoprene) and anthropogenic (i.e., aromatics, alkenes) hydrocarbon oxidation. The varied and multiple sources of methylglyoxal increase the potential for these low volatility cloud processing products (e.g., oxalic acid and oligomers) to significantly contribute to (SOA). Aqueous phase oligomer formation investigated here and aerosol phase oligomer formation appear to be more similar than previously realized, which may simplify the incorporation of oligomers into atmospheric SOA models.
