Maillard chemistry in clouds and aqueous aerosol as a source of atmospheric humic-like substances

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The reported optical, physical, and chemical properties of aqueous Maillard reaction mixtures of small aldehydes (glyoxal, methylglyoxal, and glycolaldehyde) with ammonium sulfate and amines are compared with those of aqueous extracts of ambient aerosol (water-soluble organic carbon, WSOC) and the humic-like substances (HULIS) fraction of WSOC. Using a combination of new and previously published measurements, we examine fluorescence, X-ray absorbance, UV/vis, and IR spectra, complex refractive indices, 1H and 13C NMR spectra, thermograms, aerosol and electrospray ionization mass spectra, surface activity, and hygroscopicity. Atmospheric WSOC and HULIS encompass a range of properties, but in almost every case aqueous aldehyde-amine reaction mixtures are squarely within this range. Notable exceptions are the higher UV/visible absorbance wavelength dependence (Angström coefficients) observed for methylglyoxal reaction mixtures, the lack of surface activity of glyoxal reaction mixtures, and the higher N/C ratios of aldehyde-amine reaction products relative to atmospheric WSOC and HULIS extracts. The overall optical, physical, and chemical similarities are consistent with, but not demonstrative of, Maillard chemistry being a significant secondary source of atmospheric HULIS. However, the higher N/C ratios of aldehyde-amine reaction products limits the source strength to ≤50% of atmospheric HULIS, assuming that other sources of HULIS incorporate only negligible quantities of nitrogen.