Elucidating the critical oligomeric steps in secondary organic aerosol and brown carbon formation

Abstract

Abstract. Small α-dicarbonyls represent the major precursors of secondary organic aerosol (SOA) and brown carbon (BrC) in the atmosphere, but the chemical mechanisms leading to their formation remain unclear. Here we elucidate the fundamental kinetics and mechanisms for aqueous-phase oligomerization of glyoxal (GL) using quantum chemical and kinetic rate calculations. Our results identify several essential isomeric processes for GL, including protonation to yield diol / tetrol and carbenium ions, nucleophilic addition of carbenium ions to diol / tetrol as well as to free methylamine / ammonia (MA / AM), and deprotonation to propagate oligomers and N-heterocycles. Both protonation and nucleophilic addition occur without activation barriers and are dominantly driven by electrostatic attraction. Deprotonation proceeds readily via water molecules in the absence of MA / AM but corresponds to the rate-limiting step for N-containing cationic intermediates to yield N-heterocycles. On the other hand, the latter occurs readily via a catalytic process by acidic anions (e.g., SO42-). A carbenium ion-mediated reaction rate of GL is 4.62 × 10−3 s−1 under atmospheric conditions, in good agreement with the experimental data. Our results provide essential mechanistic and kinetic data for accurate assessment of the role of small α-dicarbonyls in SOA and BrC formation.

Oligomerization Mechanism of Methylglyoxal Regulated by the Methyl Groups in Reduced Nitrogen Species: Implications for Brown Carbon Formation

“…The rate constants, presented in Table S6, are all in the range of ∼10 9 M −1 s −1 for the MG + MA and MG + AM reaction systems. Combined with previous studies, 26,54 we can infer that the formation of IMIs and DIMIs in the two reaction systems is regulated by electrostatic attraction and steric hindrance.…”

“…The ΔG r value of the alde-R MGM-H+ 51 pathway is −134.26 kcal mol −1 , which includes the proton affinity of water with −110.87 kcal mol −1 . 41,54 The subsequent deprotonation of alde-CI MGM 51 proceeds via a prereactive complex (alde- As shown in Figure 2a, the ΔG r value of the alde-R For the MG + AM reaction system, the formation of IMI MGA s and DIMI MGA s also proceeds via the following four vital steps (see the SI for a detailed discussion and Figure 3): (i) protonation of AAL MGA 1 at the alcohol hydroxyl group to yield IMI MGA s; (ii) isomeric conversion of IMI MGA s via protonation, hydration, and deprotonation; (iii) protonation and dehydration of IMI MGA s to N-CB MGA s; and (iv) formation of DIMI MGA 1 by the association reactions of N-CB MGA s with AM. Six IMI MGA s and one DIMI MGA 1 are formed in the MG + AM reaction system.…”

Continuous Measurement and Molecular Compositions of Atmospheric Water‐Soluble Brown Carbon in the Nearshore Marine Boundary Layer of Northern China: Secondary Formation and Influencing Factors

“…Atmospheric reduced nitrogen compounds such as ammonium, ammonia, amines, and amino acids serve as important precursors of BrC (Ji et al., 2022; Nguyen et al., 2013; Updyke et al., 2012). The liquid‐phase reaction of carbonyl compounds with ammonia or amines is an important pathway for the formation of heterocyclic nitrogen BrC, which could be promoted in the presence of acidic particles (De Haan et al., 2017, 2020).…”

Heterogeneous chemistry of methyl ethyl ketone on mineral oxide surfaces: impacts of relative humidity and nitrogen dioxide on product formation

“…Numerous laboratory and field observations indicate the formation of various oligomers of smaller atmospheric carbonyls and their derivatives. However, most of these studies are limited to photochemical and/or aqueous phase conditions, 40,64–66 while a limited understanding exists for small carbonyl-oligomerization in multiphase chemistry. 12,31,32,38 Thus, our study provides new evidence on mineral-specific heterogeneous reactions of MEK.…”

“…acetone, glyoxal, and formaldehyde, and subsequent heterogeneous chemistry, including oligomerization, dehydration, and aldol condensation, have been previously observed. [31][32][33][34]38,40 In previous studies with metal oxide surfaces, 31,32 the adsorption, chemistry and photochemistry of small carbonyl compounds have been investigated to understand heterogeneous chemistry. In the current study, we extend this aspect further to identify the products formed on these surfaces and investigate the detailed mechanism via isotope labeling experiments.…”