Heterocyclic Product Formation in Aqueous Brown Carbon Systems

Abstract

Brown carbon in aerosol remains a significant source of error in global climate modeling due to its complex nature and limited product characterization. Though significant efforts have been made in the previous decade to identify the major lightabsorbing brown carbon chromophores formed through the reactions of carbonylcontaining compounds with ammonium, substantial work is still required to identify the main absorbing species resulting from reactions of glyoxal, glycolaldehyde, and hydroxyacetone with ammonium sulfate (AS). Using tandem mass spectrometry and 15 N experiments to confirm proposed structures and support their mechanistic pathways, compelling evidence is provided for the formation of pyrazines and imidazoles in the glyoxal + AS, glycolaldehyde + AS, and hydroxyacetone + AS systems. Through density functional theory calculations, the N-containing oligomers and aromatic heterocycles formed within these reaction systems are shown to contribute to brown carbon light absorption, thus holding significant relevance toward accurately predicting their effects on global climate.

Modeling of Carbonyl/Ammonium Sulfate Aqueous Brown Carbon Chemistry via UV/Vis Spectral Decomposition

“…Similar speciation studies using HPLC-MS in MG/AS aerosol mimic solutions also identified m/z peaks consistent with methylated analogues to these heterocyclic products [20,32]. These products have all been observed [20,32] or estimated [18,32] to absorb light in the UV/visible region, classifying these compounds as brown carbon (BrC) species that may contribute to overall aerosol direct forcing effects.…”

“…Glycolaldehyde (GA) and methylglyoxal (MG), which are similarly structured to glyoxal, appear in similar environments, though in smaller quantities [16,17]. GA and MG have been proposed to undergo analogous chemistry to glyoxal in aqueous conditions [18,19]. In addition, MG has also been identified to generate comparable or greater quantities of light-absorbing dark chemistry products compared to G [20,21] and can alter the bulk-phase physical properties of aerosol systems, even in trace amounts [20,22,23].…”

“…In addition, MG has also been identified to generate comparable or greater quantities of light-absorbing dark chemistry products compared to G [20,21] and can alter the bulk-phase physical properties of aerosol systems, even in trace amounts [20,22,23]. As a result, multiple studies have explored G [6,10,16,18,19,[24][25][26][27][28][29], GA [16,18,27,30,31], and MG [16,19,21,26,27,29] reaction systems, quantifying and speciating their potential contributions to aqSOA under a variety of laboratory conditions.…”

“…In dark conditions, observed higher-mass aqSOA products in G, GA, and MG mimic solutions have been attributed to Maillard-type or aldol condensation reactions between CVOCs, primary amines, and/or ammonium ions present in the aqueous phase [14,[17][18][19]21,24,28,29]. Recent work using supercritical fluid chromatography tandem mass spectroscopy (SFC-MS/MS) has unambiguously identified heterocyclic pyrazine and imidazole derived oligomerization products in mimic solutions containing G or GA and ammonium sulfate (AS) [18]. Similar speciation studies using HPLC-MS in MG/AS aerosol mimic solutions also identified m/z peaks consistent with methylated analogues to these heterocyclic products [20,32].…”

Engaging undergraduate students in computational chemistry research: A tutorial for new assistant professors

“…Heidi's Ph.D. work involved time-dependent density functional theory (TDDFT), so she was able to design a project in which the student carried out and analyzed TDDFT calculations to determine how reaction products identified by the Galloway lab contribute to absorption of visible light in atmospheric aerosols. [4,5] Likewise, another student wondered if it would be possible to computationally model aspects of the melanopsin protein he began studying in Prof. James Dearworth's biology lab at Lafayette. Heidi used her postdoc experience studying rhodopsin to develop a collaborative QM/MM project investigating the structure of turtle melanopsin.…”