Effect of temperature on the formation of highly oxygenated organic molecules (HOMs) from alpha-pinene ozonolysis

Abstract

Abstract. Highly oxygenated organic molecules (HOMs) are important contributors to secondary organic aerosol (SOA) and new-particle formation (NPF) in the boreal atmosphere. This newly discovered class of molecules is efficiently formed from atmospheric oxidation of biogenic volatile organic compounds (VOCs), such as monoterpenes, through a process called autoxidation. This process, in which peroxy-radical intermediates isomerize to allow addition of molecular oxygen, is expected to be highly temperature-dependent. Here, we studied the dynamics of HOM formation during α-pinene ozonolysis experiments performed at three different temperatures, 20, 0 and −15 ∘C, in the Aarhus University Research on Aerosol (AURA) chamber. We found that the HOM formation, under our experimental conditions (50 ppb α-pinene and 100 ppb ozone), decreased considerably at lower temperature, with molar yields dropping by around a factor of 50 when experiments were performed at 0 ∘C, compared to 20 ∘C. At −15 ∘C, the HOM signals were already close to the detection limit of the nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer used for measuring gas-phase HOMs. Surprisingly, comparing spectra measured at 0 and 20 ∘C, ratios between HOMs of different oxidation levels, e.g., the typical HOM products C10H14O7, C10H14O9, and C10H14O11, changed considerably less than the total HOM yields. More oxidized species have undergone more isomerization steps; yet, at lower temperature, they did not decrease more than the less oxidized species. One possible explanation is that the primary rate-limiting steps forming these HOMs occur before the products become oxygenated enough to be detected by our CI-APi-TOF (i.e., typically seven or more oxygen atoms). The strong temperature dependence of HOM formation was observed under temperatures highly relevant to the boreal forest, but the exact magnitude of this effect in the atmosphere will be much more complex: the fate of peroxy radicals is a competition between autoxidation (influenced by temperature and VOC type) and bimolecular termination pathways (influenced mainly by concentration of reaction partners). While the temperature influence is likely smaller in the boreal atmosphere than in our chamber, both the magnitude and complexity of this effect clearly deserve more consideration in future studies in order to estimate the ultimate role of HOMs on SOA and NPF under different atmospheric conditions.

Aerosol mass yields of selected biogenic volatile organic compounds – a theoretical study with nearly explicit gas-phase chemistry

“…The temperature dependence in PRAM is based on quantum chemical calculations wherein the autoxidation rates correspond to an activation energy of 24 kcal mol −1 . The activation energies vary for autoxidation of different RO 2 from α-pinene ozonolysis between 22 and 29 kcal mol −1 (Rissanen et al, 2015), leading to varying autoxidation rates at different temperatures (Roldin et al, 2019). It should be noted that the temperature dependence in PRAM is the first of its kind but needs further evaluation using recent measurements of HOM formation at different temperatures (e.g., Quéléver et al, 2019).…”

Observations of highly oxidized molecules and particle nucleation in the atmosphere of Beijing

“…Highly oxidized multifunctional molecules (HOMs), organic molecules with O : C ratios > 0.6, are the result of atmospheric autoxidation and have recently been subject to much investigation, in part because the extremely low volatilities arising from their high O : C ratios favour their condensation into the particulate phase. HOMs are most well characterized as the product of oxidation of the biogenic monoterpenoid compound α-pinene (Riccobono et al, 2014;Tröstl et al, 2016;Bianchi et al, 2017). Although globally biogenic volatile organic compound (BVOC) concentrations far exceed anthropogenic volatile organic compound (AVOC) concentrations, in the urban environment the anthropogenic fraction is far more significant.…”

“…Recent chamber studies have elucidated the contribution of individual species to particle nucleation, ammonia, and amines, greatly enhancing the rate of sulfuric acid nucleation (Kirkby et al, 2011;Almeida et al, 2013). In these studies, HOMs have been identified, formed through autoxidation mechanisms Riccobono et al, 2014;. These are key to early particle growth (Tröstl et al, 2016) and can nucleate even in the absence of sulfuric acid in chambers (Kirkby et al, 2016) and in the free troposphere (Rose et al, 2018).…”

Oligomer and highly oxygenated organic molecule formation from oxidation of oxygenated monoterpenes emitted by California sage plants

“…Formation of highly oxygenated organic molecules (HOM) has been identified as a pathway for new particle formation and growth of SOA (Bianchi et al, 2019). Recent work has shown that alongside HOM formation from ozonolysis (Quéléver et al, 2019), 35 hydroxyl radical oxidation of monoterpenes is a large source of HOM (Berndt et al, 2016). Oligomerisation is another key pathway to formation of SOA and has been identified as important for monoterpenes including α-pinene and limonene (Hall and Johnston, 2011;Kourtchev et al, 2016;Kundu et al, 2012;Putman et al, 2011;Tolocka et al, 2004).…”