title: Brown carbon formation from ketoaldehydes of biogenic monoterpenest.
abstract: Sources and chemical composition of brown carbon are poorly understood, and even less is known about the mechanisms of its atmospheric transformations. This work presents molecular-level investigations of the reactive compound ketolimononaldehyde (KLA, C9H14O3), a second-generation ozonolysis product of limonene (C10H16), as a potent brown carbon precursor in secondary organic aerosol (SOA) through its reactions with reduced nitrogen compounds, such as ammonium ion (NH4+), ammonia, and amino acids. The reactions of synthesized and purified KLA with NH4+ and glycine resulted in the formation of chromophores nearly identical in spectral properties and formation rates to those found in similarly-aged limonene/O3 SOA. Similar chemical reaction processes of limononaldehyde (LA, C10H16O2) and pinonaldehyde (PA, C10H16O2), the first-generation ozonolysis products of limonene and alpha-pinene, respectively, were also studied, but the resulting products did not exhibit the light absorption properties of brown carbon, suggesting that the unique molecular structure of KLA produces visible-light-absorbing compounds. The KLA/NH4+ and KLA/GLY reactions produce water-soluble, hydrolysis-resilient chromophores with high mass absorption coefficients (MAC = 2000-4000 cm2 g(-1)) at lambda - 500 nm, precisely at the maximum of the solar emission spectrum. Liquid chromatography was used to isolate the light-absorbing fraction, and UV-Vis, FTIR, NMR and high-resolution mass spectrometry (HR-MS) techniques were used to investigate the structures and chemical properties of the light-absorbing compounds. The KLA browning reaction generates a diverse mixture of light-absorbing compounds, with the majority of the observable products containing 1-4 units of KLA and 0-2 nitrogen atoms. Based on the HR-MS product distribution, conjugated aldol condensates, secondary imines (Schiff bases), and N-heterocycles like pyrroles may contribute in varying degree to the light-absorbing properties of the KLA brown carbon. The results of this study demonstrate the high degree of selectivity of organic compound structures on the light-absorbing properties of SOA.
authors.affiliation: Department of Chemistry, University of California, Irvine, California 92697, USA. tbn@calech.edu
authors.forename: Tran B
authors.initials: TB
authors.lastname: Nguyen
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
authors.forename: Alexander
authors.initials: A
authors.lastname: Laskin
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
authors.forename: Julia
authors.initials: J
authors.lastname: Laskin
authors.affiliation: Department of Chemistry, University of California, Irvine, California 92697, USA.
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Nizkorodov
mesh_terms: D000447:Aldehydes / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D013729:Terpenes / Q000737:chemistry*
chemical_list: D000447:Aldehydes; D013729:Terpenes; D002244:Carbon