An aldehyde as a rapid source of secondary aerosol precursors: theoretical and experimental study of hexanal autoxidation

Abstract

Abstract. Aldehydes are common constituents of natural and polluted atmospheres, and their gas-phase oxidation has recently been reported to yield highly oxygenated organic molecules (HOMs) that are key players in the formation of atmospheric aerosol. However, insights into the molecular-level mechanism of this oxidation reaction have been scarce. While OH initiated oxidation of small aldehydes, with two to five carbon atoms, under high-NOx conditions generally leads to fragmentation products, longer-chain aldehydes involving an initial non-aldehydic hydrogen abstraction can be a path to molecular functionalization and growth. In this work, we conduct a joint theoretical–experimental analysis of the autoxidation chain reaction of a common aldehyde, hexanal. We computationally study the initial steps of OH oxidation at the RHF-RCCSD(T)-F12a/VDZ-F12//ωB97X-D/aug-cc-pVTZ level and show that both aldehydic (on C1) and non-aldehydic (on C4) H-abstraction channels contribute to HOMs via autoxidation. The oxidation products predominantly form through the H abstraction from C1 and C4, followed by fast unimolecular 1,6 H-shifts with rate coefficients of 1.7×10-1 and 8.6×10-1 s−1, respectively. Experimental flow reactor measurements at variable reaction times show that hexanal oxidation products including HOM monomers up to C6H11O7 and accretion products C12H22O9−10 form within 3 s reaction time. Kinetic modeling simulations including atmospherically relevant precursor concentrations agree with the experimental results and the expected timescales. Finally, we estimate the hexanal HOM yields up to seven O atoms with mechanistic details through both C1 and C4 channels.

Indoor Air Quality Implications of Germicidal 222 nm Light

“…The products formed in the 222 nm irradiation of hexanal are also broadly consistent with OH-initiated oxidation (see Scheme S2). 35 Secondary Organic Aerosol Formation. In all experiments, dry ammonium sulfate seed particles are added to the chamber, providing surface area onto which low-volatility species may condense and enabling the assessment of potential SOA formation.…”

Quantitative kinetics reveal that reactions of HO2 are a significant sink for aldehydes in the atmosphere and may initiate the formation of highly oxygenated molecules via autoxidation

“…In contrast, the present investigations show that HO 2 may initiate the formation of RO 2 radials that are capable of undergoing autoxidation leading to formation of higher oxidized products. In a recent study, RO 2 (C 6 H 11 O 3 ) formed from OH + C 5 H 11 CHO 44 was shown to autoxidize at ∼ 0.17 s −1 and initiate higher oxidized products formation (Scheme 2). Here, we show that an analogous process can take place from HO 2 oxidation and produce an RO 2 (C 6 H 13 O 3 ) that can autoxidize at a faster rate constant (∼ 0.45 s −1 ).…”

“…, hexanal and decanal) have been shown to undergo a hydrogen shift followed by O 2 addition (widely referred to as “autoxidation” 35–40 ), leading to the formation of highly oxygenated molecules (HOMs) that can contribute appreciably to SOA formation because of their low volatility. 41–44 If HO 2 reacts rapidly enough with larger aldehydes, they could provide another SOA source in the atmosphere.…”

Gas-Phase Oxidation of Atmospherically Relevant Unsaturated Hydrocarbons by Acyl Peroxy Radicals

“…Autoxidation processes combined with dimerization reactions can rapidly produce low-volatility compounds, which in turn participate in particle formation and growth processes. In addition to regular peroxy radicals, the atmosphere contains on the order of 10 7 –10 8 cm –3 of short-chain acyl peroxy radicals [R­(O)­OO • , APR]. , Moreover, APRs are known to be more reactive than ROO • . , Bimolecular APR reactions with NO 2 , producing long-lived acyl peroxy nitrates, have been extensively studied. , Unimolecular H-shift reactions of acyl peroxy radicals have been found to have rate constants in the range of from 10 –8 to 10 5 s –1 , with larger and more functional radicals tending to have faster reaction rates. , Additionally, R­(O)­OO • + R­(O)­OO • dimerization reactions are suggested to lead to low-volatility products through a similar mechanism as the ROO • + ROO • reaction. , As APRs have higher reactivities than other peroxy radicals, different mechanisms might control their atmospheric chemistry. Specifically, APRs may be able to react directly with closed-shell species such as alkenes, opening up new potential pathways for accretion product formation.…”

“…21,22 Unimolecular H-shift reactions of acyl peroxy radicals have been found to have rate constants in the range of from 10 −8 to 10 5 s −1 , with larger and more functional radicals tending to have faster reaction rates. 23,24 Additionally, R(O)OO • + R(O)OO • dimerization reactions are suggested to lead to low-volatility products through a similar mechanism as the ROO • + ROO • reaction. 25,26 As APRs have higher reactivities than other peroxy radicals, different mechanisms might control their atmospheric chemistry.…”