Kinetics, Mechanism, and Secondary Organic Aerosol Yield of Aqueous Phase Photo-oxidation of α-Pinene Oxidation Products

Abstract

Formation of secondary organic aerosol (SOA) involves atmospheric oxidation of volatile organic compounds (VOCs), the majority of which are emitted from biogenic sources. Oxidation can occur not only in the gas-phase but also in atmospheric aqueous phases such as cloudwater and aerosol liquid water. This study explores for the first time the aqueous-phase OH oxidation chemistry of oxidation products of α-pinene, a major biogenic VOC species emitted to the atmosphere. The kinetics, reaction mechanisms, and formation of SOA compounds in the aqueous phase of two model compounds, cis-pinonic acid (PIN) and tricarballylic acid (TCA), were investigated in the laboratory; TCA was used as a surrogate for 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a known α-pinene oxidation product. Aerosol time-of-flight chemical ionization mass spectrometry (Aerosol-ToF-CIMS) was used to follow the kinetics and reaction mechanisms at the molecular level. Room-temperature second-order rate constants of PIN and TCA were determined to be 3.3 (± 0.5) × 10(9) and 3.1 (± 0.2) × 10(8) M(-1) s(-1), respectively, from which were estimated their condensed-phase atmospheric lifetimes. Aerosol-ToF-CIMS detected a large number of products leading to detailed reaction mechanisms for PIN and MBTCA. By monitoring the particle size distribution after drying, the amount of SOA material remaining in the particle phase was determined. An aqueous SOA yield of 40 to 60% was determined for PIN OH oxidation. Although recent laboratory studies have focused primarily on aqueous-phase processing of isoprene-related compounds, we demonstrate that aqueous formation of SOA materials also occurs from monoterpene oxidation products, thus representing an additional source of biogenically driven aerosol formation.

Characterization of Highly Oxidized Molecules in Fresh and Aged Biogenic Secondary Organic Aerosol

“…Relative humidity can affect which products of the Criegee radical are dominant in fresh SOA produced by ozonolysis, 40 and also the extent to which aqueous phase aging occurs. 43 No significant relative humidity dependence was observed in this study between 35 and 70%. This work employed 254 nm radiation in the PC owing to the high photochemical yield of OH.…”

Aqueous Secondary Organic Aerosol Formation in Ambient Cloud Water Photo-Oxidations

“…Chamber pinonic acid-OH oxidations have resulted in highly functionalized, lowvolatility molecules (Müller et al, 2012) similar to unpublished pinonic acid aqueous photo-oxidations in our laboratory (identified via ESI-ToF-MS). Our observation of dominant carbonyl formation compares favorably to single-precursor pinonic acid photo-oxidations in our photoreactor (Schurman, 2014) and other aqueous cis-pinonic acid (Aljawhary et al, 2016) and α-pinene oxidations (Bleier and Elrod, 2013); however, α-pinene products appear to vary with OH exposures, with dominant carboxylic acid formation and little change in f43 under high-OH conditions (George and Abbatt, 2010), and carbonyl formation in the form of acetone, formaldehyde, formic acid, etc. under lower [OH] (Nozière et al, 1999); Lignell et al (2013) show that direct aqueous PA photolysis is a minor mechanism.…”

Evidence of 1991–2013 decrease of biogenic secondary organic aerosol in response to SO 2 emission controls

“…This deficit could be compensated by a decline in anthropogenic POA or by a dependence of monoterpene and sesquiterpene SOA formation on aqueous aerosol volume, not included in our simulation. The laboratory study of Aljawhary et al (2016) suggests that aqueous-phase processing may be an important SOA formation pathway for monoterpenes. Glyoxal is an oxidation product of monoterpenes (Fu et al 2008, Chan Miller et al 2016 and would also contribute to dependence of monoterpene SOA on aqueous aerosol volume.…”