Kinetics of Acid-Catalyzed Dehydration of Cyclic Hemiacetals in Organic Aerosol Particles in Equilibrium with Nitric Acid Vapor

Abstract

Previous studies have shown that 1,4-hydroxycarbonyls, which are often major products of the atmospheric oxidation of hydrocarbons, can undergo acid-catalyzed cyclization and dehydration in aerosol particles to form highly reactive unsaturated dihydrofurans. In this study the kinetics of dehydration of cyclic hemiacetals, the rate-limiting step in this process, was investigated in a series of environmental chamber experiments in which secondary organic aerosol (SOA) containing cyclic hemiacetals was formed from the reaction of n-pentadecane with OH radicals in dry air in the presence of HNO3. A particle beam mass spectrometer was used to monitor the formation and dehydration of cyclic hemiacetals in real time, and SOA and HNO3 were quantified in filter samples by gravimetric analysis and ion chromatography. Measured dehydration rate constants increased linearly with increasing concentration of HNO3 in the gas phase and in SOA, corresponding to catalytic rate constants of 0.27 h(-1) ppmv(-1) and 7.0 h(-1) M(-1), respectively. The measured Henry's law constant for partitioning of HNO3 into SOA was 3.7 × 10(4) M atm(-1), ∼25% of the value for dissolution into water, and the acid dissociation constant was estimated to be <8 × 10(-4), at least a factor of 10(4) less than that for HNO3 in water. The results indicate that HNO3 was only weakly dissociated in the SOA and that dehydration of cyclic hemiacetals was catalyzed by molecular HNO3 rather than by H(+). The Henry's law constant and kinetics relationships measured here can be used to improve mechanisms and models of SOA formation from the oxidation of hydrocarbons in dry air in the presence of NOx, which are conditions commonly used in laboratory studies. The fate of cyclic hemiacetals in the atmosphere, where the effects of higher relative humidity, organic/aqueous phase separation, and acid catalysis by molecular H2SO4 and/or H(+) are likely to be important, is discussed.

Tuning the Reactivity of Terminal Nickel(III)–Oxygen Adducts for C–H Bond Activation

“…23 In contrast the − ONO 2 ligand is much more electron-poor (aqueous scale p K a of HNO 3 = −1.2). 24 The most electrophilic Ni III -oxygen adduct is therefore likely to be 3b , based on the E 0 Ni(II/III) for 3a .…”

Modeling of the chemistry in oxidation flow reactors with high initial NO

“…Significant OA loss due to heterogeneous oxidation can only be seen at photochemical ages as high as weeks . The enhancement of heterogeneous oxidation due to NO is remarkable only at OH concentrations close to the ambient values but not at typical values in an OFR (Richards-Henderson et al, 2015).…”

“…6 are primary VOCs, except phenols and a dihydrofuran, which can be intermediates of the atmospheric oxidation of (alkyl)benzenes (Atkinson and Arey, 2003) and long-chain alkanes (Aimanant and Ziemann, 2013;Strollo and Ziemann, 2013;Ranney and Ziemann, 2016), respectively. Nevertheless, only the phenol production may occur in high-NO OFRs, as the particle-phase reaction in the photochemical formation of dihydrofurans from alkanes is too slow compared to typical OFR residence times (Ranney and Ziemann, 2016). Therefore, the impact of NO 3 oxidation on VOC fate needs to be considered only if the OFR input flow contains high NO mixed with biogenics and/or aromatics, e.g., (alkyl)benzenes and/or phenols.…”

“…Thus, even under the assumption of unity uptake coefficient, CS cannot compete with OHR ext (usually on the order of 10 s −1 or higher) in OH loss. Uptake of NO onto aerosols only occurs through the reaction with RO 2 on the particle surface (Richards-Henderson et al, 2015), which is formed very slowly (see below) compared to gas-phase HO x and NO x chemistry. Uptake of HO 2 , O 3 , NO 3 , etc.…”

Chemistry of hydroperoxycarbonyls in secondary organic aerosol

“…After 1,4-hydroxycarbonyls cyclize to form CHA they can undergo dehydration to form dihydrofurans, or oligomerization in which CHA undergo self-reactions to form acetals (Aimanant and Ziemann 2013b). Studies conducted on CHA in dry SOA in the presence of added HNO 3 showed that the dehydration of CHA is catalyzed by undissociated molecular HNO 3 (general acid catalysis) (Ranney and Ziemann 2016), whereas acetal formation is apparently catalyzed by H þ (specific acid catalysis) (Ziemann and Atkinson 2012). On the basis of these studies, CPHA is expected to undergo either dehydration or reaction with 1-propanol to form a peroxyacetal.…”