Appearance of strong absorbers and fluorophores in limonene‐O₃ secondary organic aerosol due to NH₄⁺‐mediated chemical aging over long time scales

Abstract

[1] This study investigated long-term chemical aging of model biogenic secondary organic aerosol (SOA) prepared from the ozonolysis of terpenes. Techniques including electrospray ionization mass spectrometry (ESI-MS), UV-visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, NMR, and three-dimensional fluorescence were used to probe the changes in chemical composition of SOA collected by impaction on substrates and also of aqueous extracts of SOA. The addition of ammonium ions or amino acids to limonene SOA reproducibly produced orange-colored species that strongly absorbed visible radiation and fluoresced at UV and visible wavelengths. Simultaneous addition of H 2 SO 4 to the SOA aqueous extracts inhibited this color transformation. These observations suggest the existence of aging processes leading to heavily conjugated molecules containing organic nitrogen. The presence of nitrogen in the chromophores was confirmed by the dependence of the absorption and fluorescence spectra on the amino acids added. In contrast to the strong change in the absorption and fluorescence spectra, there was no significant change in the ESI-MS, FTIR, and NMR spectra, suggesting that the chromophores were minor species in the aged SOA. Aqueous extracts of aged limonene + NH 4 + SOA were characterized by an effective base-e absorption coefficient of $3 L g À1 cm À1 at 500 nm. Assuming particulate matter concentrations typical of polluted rural air gives an upper limit of 0.2 M m À1 for the aerosol absorption coefficient due to the aged limonene oxidation products. Biogenic SOA can therefore become weakly absorbing if they undergo aging in the presence of NH 4 + -containing aerosol.

Brown carbon formation from ketoaldehydes of biogenic monoterpenes

“…stages of aqueous aging, which is obscured by the 500 nm band at longer timescales and upon evaporation. 26,35 Even this temporal evolution of chromophores in limonene/O 3 SOA can be reproduced by KLA (Fig. S4 in the ESI section †), offering strong evidence of similar chemistry.…”

“…25,26 Interestingly, SOA components generated from the ozonolysis of apinene, an isomeric monoterpene, do not form brown carbon when reacted with the same nitrogen species. 26,27 Both limonene and a-pinene are abundant volatile organic compounds, so it is important to understand their potential to modify aerosol optical properties. For example, limonene and a-pinene contribute 3-18% and 23-53%, respectively, to the total monoterpene emissions from forests in the United States.…”

“…[5][6][7]9,32,33 As such, the active brown-carbon precursors in the limonene/O 3 SOA mixture have yet to be identied. The study of and related studies 5,8,10,26,27,34 offer important insights on the nature of the chromophore structure and the NH 4 + -mediated reaction. These studies suggest that the chromophores likely contain a pC]N-(imine) motif in their structures.…”

Applications of Infrared Spectroscopy in Analysis of Organic Aerosols

“…In the study of Bones et al [11], with the aim to investigate chemical aging of SOA formed from ozonolysis of terpenes in the presence of ammonium sulfate aerosol, FTIR was used to confirm whether the "colored" products, which showed strong visible absorption measured by UV-Vis spectroscopy and fluorescence at UV and visible ranges by three-dimensional fluorescence, could lead to the changes in the chemical compositions of SOA. By comparing FTIR spectra of the fresh SOA to those of the chemically aged SOA, almost no difference was observed, which implied that the "colored" products from the long-term aging chemistry contributed insignificantly to the aged SOA.…”

Evidence for pyrazine-based chromophores in cloudwater mimics containing methylglyoxal and ammonium sulfate

“…Although careful work has suggested that organo-nitrogen species are largely responsible for the bulk absorptivity of these amine-aldehyde systems (Bones et al, 2010), only a handful of aqBrC chromophores have been positively identified (Kampf 25 et al, 2012;Nguyen et al, 2012;Laskin et al, 2014;Lin et al, 2015;Aiona et al, 2017a). Most of these are functionalized and conjugated imidazole products, summarized in detail in a recent review (Laskin et al, 2015).…”