Effect of temperature on the formation of Highly-oxygenated Organic Molecules (HOM) from alpha-pinene ozonolysis

Abstract

<p><strong>Abstract.</strong> Highly-oxygenated Organic Molecules (HOM) 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 (VOC), 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 alpha-pinene ozonolysis experiments performed at three different temperatures, 20 °C, 0 °C and −15 °C, in the Aarhus University Research on Aerosol (AURA) chamber. We found that the HOM formation, under our experimental conditions (50 ppb alpha-pinene, 100 ppb ozone), decreased considerably as temperature decreased, 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 HOM. Surprisingly, very little difference was seen in the mass spectral distribution of the HOM molecules of interest at 0 °C and 20 °C, with e.g. the ratios between the typical HOM products C<sub>10</sub>H<sub>14</sub>O<sub>7</sub>, C<sub>10</sub>H<sub>14</sub>O<sub>9</sub>, and C<sub>10</sub>H<sub>14</sub>O<sub>11</sub> remaining fairly constant. The 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 be that the rate-limiting step forming these HOM occurs 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 for 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, the magnitude and complexity of this effect clearly deserves more consideration in future studies in order to estimate the ultimate role of HOM on SOA and NPF under different atmospheric conditions.</p>

Aerosol Mass yields of selected Biogenic Volatile Organic Compounds – a theoretical study with near explicit gas-phase chemistry

“…This contrasts with high NOx conditions where the RO2+NO reactions dominate over reactions with HO2 or RO2, resulting in the formation of more volatile products such as aldehydes, ketones and organonitrates (Presto et al, 2005;Sarrafzadeh et al, 2016), and likely suppressing the autoxidation process leading to a decrease in SOA mass loadings (Ehn et al, 2014). PRAM (Roldin et al, 2018) which explains the lower contribution by PRAM compounds to SOA mass loadings with increasing NO. The PRAM contribution increases at low NO (<1 ppb) and then decreases thereafter.…”

“…Kinetic preprocessor (KPP) is used to generate a system of coupled differential equations to solve the gas-phase chemistry schemes (MCM+PRAM, Damian et al, 2002). The peroxy radical autoxidation mechanism (PRAM), (Roldin et al, 2018), formulated based on the oxidation of monoterpenes as described by Ehn et al (2014) was incorporated alongside MCMv3.3.1. PRAM describes the evolution of peroxy radicals (RO2) from the ozonolysis of monoterpenes driven by subsequent H-shifts and O2 additions.…”

“…At low NOx concentrations RO2 radicals undergo rapid autoxidation until they react with HO2 or RO2 resulting in production of low volatility hydro-peroxide products (Sarrafzadeh et al, 2016), closed shell monomers or dimers (Ehn et al, 201;Roldin et al, 2018), which increase SOA mass. This contrasts with high NOx conditions where the RO2+NO reactions dominate over reactions with HO2 or RO2, resulting in the formation of more volatile products such as aldehydes, ketones and organonitrates (Presto et al, 2005;Sarrafzadeh et al, 2016), and likely suppressing the autoxidation process leading to a decrease in SOA mass loadings (Ehn et al, 2014).…”

“…Again, it should be noted that the temperature dependence of peroxy radical autoxidation product formation still needs further validation based on recent experiments (e.g. Quéléver et al, 2018).…”

“…Further we study the effect of varying temperature (258.15 K -313.15 K) and NO concentrations (0 -5 ppb) on α-pinene oxidation mass yields. We use the master chemical mechanism (MCMv3.3.1) (Jenkin et al, 1997(Jenkin et al, , 2012(Jenkin et al, and 2015Saunders et al, 2003), a near explicit gas-phase chemical mechanism together with peroxy radical autoxidation mechanism (PRAM, Roldin et al, 2018) (PRAM + MCM). The aim is to understand the importance and contribution of peroxy radical autoxidation products to the SOA mass yields from terpenes.…”