New Particle Formation Promoted by OH Reactions during α-Pinene Ozonolysis

Abstract

Monoterpenes have been reported to rapidly convert to extremely low volatility organic compounds (ELVOCs), which can act as nucleation agents leading to new particle formation (NPF). The formation of highly oxygenated organic molecules (HOMs) via autoxidation is proposed to be a key process of the NPF in the monoterpene oxidation, but the mechanism has not yet been established. In this study, the size distribution of the number concentration of secondary organic aerosol (SOA) from α-pinene ozonolysis in the presence of seed particles was investigated. In the absence of the OH radical scavenger, this distribution was found to be bimodal. The peak arising from small particles was a result of NPF; however, when the OH radical scavenger was present in the experiment, this peak disappeared. Owing to the bimodal size distribution of SOA from limonene ozonolysis with the OH radical scavenger and seed particles, it was proposed that addition of the OH radical to the α-pinene double bond followed by isomerization, which accompanies ring opening, generating limonene-like compounds, could be one of the key steps in producing ELVOCs as nucleating agents during α-pinene ozonolysis.

The nitrate radical (NO<sub>3</sub>) oxidation of alpha-pinene is a significant source of secondary organic aerosol and organic nitrogen under simulated ambient nighttime conditions

“…No SOA formation was observed in the absence of seed aerosol, indicating that α-pinene + NO3 does not nucleate. This is in contrast to our ozonolysis control experiments where nucleation was observed, in agreement with other accounts (Burkholder et al, 2007;Hoppel et al, 2001;Inomata, 2021;Takeuchi et al, 2019). The lack of nucleation is consistent with the expectation that the α-pinene + NO3 reaction dominates the nighttime experiments over ozonolysis.…”

“…2). Caveats to this approach include: (1) in the controls, ozone and OH are larger sinks for a-pinene due to a lack of competition from NO3; thus, a larger fraction of a-pinene produces SOA from ozonolysis in the control compared to the experiment and the subsequent subtraction obtains a lower-limit SOA yield; (2) synergistic reactions between RO2 intermediates from the different oxidation pathways are not possible to isolate, and contributed roughly 20% to the analytical signal from the SOA composition analysis (Section 3.3); however, it is now appreciated that these synergies also occur in the ambient and are not realistic to ignore in laboratory and modeling studies (Kenseth et al, 2018;Inomata, 2021). Romer et al, 2018).…”

“…In light of the emerging appreciation for the importance of RO2 radical fate in designing chamber experiments (Nguyen et al, 2014a;Xu et al, 2019;Boyd et al, 2015;Teng et al, 2017;Crounse et al, 2013), we re-investigate this reaction to probe the SOA yield and organic nitrate formation from a-pinene + NO3 from each relevant nRO2 reaction channel. While a chamber experiment may never truly replicate the field, and ours certainly are no exception, the nRO2 fate distribution in this work was designed to approach those expected in the ambient nighttime (Ayres et all., 2015;Romer et al, 2018), including any reaction synergies that may occur (Kenseth et al, 2018;Inomata, 2021). Finally, relatively little information is available for the nRO2 compared to their hydroxylated counterparts; this work also constrains the rate coefficients and branching ratios of the a-pinene nRO2 through a combination of chamber reactions and modeling.…”

Secondary organic aerosol and organic nitrogen yields from the nitrate radical (NO<sub>3</sub>) oxidation of alpha-pinene from various RO<sub>2</sub> fates

“…Synergistic reactions between RO 2 intermediates from the different oxidation pathways are not possible to isolate and contributed roughly 20 % to the analytical signal from the SOA composition analysis (Sect. 3.3); however, it is now appreciated that these synergies also occur in the ambient atmosphere and are not realistic to ignore in laboratory and modeling studies (Kenseth et al, 2018;Inomata, 2021).…”

“…In light of the emerging appreciation for the importance of RO 2 radical fate in designing chamber experiments (Nguyen et al, 2014a;Xu et al, 2019;Teng et al, 2017;Crounse et al, 2013), we reinvestigate this reaction to probe the SOA yield and organic nitrate formation from α-pinene + NO 3 from each relevant nRO 2 reaction channel. While a chamber experiment may never truly replicate the field, and ours certainly are no exception, the nRO 2 fate distribution in this work was designed to approach those expected in the ambient nighttime (Ayres et all., 2015;Romer et al, 2018), including any reaction synergies that may occur (Kenseth et al, 2018;Inomata, 2021). Finally, relatively little information is available for the nRO 2 compared to their hydroxylated counterparts; this work also constrains the rate coefficients and branching ratios of the α-pinene nRO 2 through a combination of chamber reactions and modeling.…”