Unexpected Oligomerization of Small α-Dicarbonyls for Secondary Organic Aerosol and Brown Carbon Formation

Abstract

Large amounts of small α-dicarbonyls (glyoxal and methylglyoxal) are produced in the atmosphere from photochemical oxidation of biogenic isoprene and anthropogenic aromatics, but the fundamental mechanisms leading to secondary organic aerosol (SOA) and brown carbon (BrC) formation remain elusive. Methylglyoxal is commonly believed to be less reactive than glyoxal because of unreactive methyl substitution, and available laboratory measurements showed negligible aerosol growth from methylglyoxal. Herein, we present experimental results to demonstrate striking oligomerization of small α-dicarbonyls leading to SOA and BrC formation on sub-micrometer aerosols. Significantly more efficient growth and browning of aerosols occur upon exposure to methylglyoxal than glyoxal under atmospherically relevant concentrations and in the absence/presence of gas-phase ammonia and formaldehyde, and nonvolatile oligomers and light-absorbing nitrogen-heterocycles are identified as the dominant particle-phase products. The distinct aerosol growth and light absorption are attributed to carbenium ion-mediated nucleophilic addition, interfacial electric field-induced attraction, and synergetic oligomerization involving organic/inorganic species, leading to surface- or volume-limited reactions that are dependent on the reactivity and gaseous concentrations. Our findings resolve an outstanding discrepancy concerning the multiphase chemistry of small α-dicarbonyls and unravel a new avenue for SOA and BrC formation from atmospherically abundant, ubiquitous carbonyls and ammonia/ammonium sulfate.

Functionality-Based Formation of Secondary Organic Aerosol from m-Xylene Photooxidation

“…The measured SSA for m-xylene is lower than that previously reported for toluene under comparable experimental conditions (Li et al, 2021b). The difference in SSA/BrC formation between toluene and m-xylene is explained by a higher yield of methylglyoxal from m-xylene than from toluene (Jenkin et al, 2003;Nishino et al, 2010), since methylglyoxal produces BrC more efficiently via aqueous reaction than glyoxal (Li et al, 2021a).…”

“…products. While dicarbonyls undergo aqueous phase reactions to form oligomers (Figure S4) (Ji et al, 2020;Li et al, 2021a), organic acids engage in ionic dissociation and acid-base reaction to yield carboxylates (Figure 3b). In addition, the reaction of dicarbonyls with NH3 produce Nheterocycles/N-heterochains (Figure S5), which are light-absorbing (Marrero-Ortiz et al, 2019;Li et al, 2021a).…”

“…While dicarbonyls undergo aqueous phase reactions to form oligomers (Figure S4) (Ji et al, 2020;Li et al, 2021a), organic acids engage in ionic dissociation and acid-base reaction to yield carboxylates (Figure 3b). In addition, the reaction of dicarbonyls with NH3 produce Nheterocycles/N-heterochains (Figure S5), which are light-absorbing (Marrero-Ortiz et al, 2019;Li et al, 2021a). Oligomerization of dicarbonyls involves protonation, hydration, and deprotonation, which are mediated by carbenium ions via nucleophilic addition (Ji et al, 2020;Li et al, 2021a).…”

“…In addition, the reaction of dicarbonyls with NH3 produce Nheterocycles/N-heterochains (Figure S5), which are light-absorbing (Marrero-Ortiz et al, 2019;Li et al, 2021a). Oligomerization of dicarbonyls involves protonation, hydration, and deprotonation, which are mediated by carbenium ions via nucleophilic addition (Ji et al, 2020;Li et al, 2021a).…”

Chemical analysis of nitro‐aromatic compounds of secondary organic aerosol formed from photooxidation of p‐xylene with NOx

“…[ 1,2 ] BrC had attracted widespread attention due to its vital contributor to light absorption and climate forcing. [ 2,3 ] In addition to primary sources such as coal and biomass combustion, the nitrogen‐containing organic compounds formed from the reaction of volatile organic compounds (VOCs) and NO 3 radical, [ 4,5 ] VOCs and OH radical in the presence of NO x , [ 6,7 ] carbonyl compounds (glyoxal, methylglyoxal) and NH 3 or methylamine [ 8–10 ] are the main secondary source of BrC. Nitro‐aromatic compounds (NACs) of nitrophenols generated from the OH and NO 2 groups attached to the aromatic ring are an important class of nitrogen‐containing organic compounds in ambient aerosol particles.…”