Fate of organic peroxy radicals in multicomponent VOC systems and their impact on secondary organic aerosol formation

Atmospheric aerosols play important roles in climate, air quality and human health (Shrivastava et al., 2017). Of these aerosols, a large fraction is comprised of submicron organic aerosol (Jimenez et al., 2009). They can be directly emitted into the atmosphere through processes such as incomplete combustion, or indirectly through the oxidation of volatile organic compounds (VOCs) from biogenic and anthropogenic sources by e.g., hydroxyl (OH) radicals, ozone (O₃) or nitrate (NO₃) radicals. Upon oxidation, these gas-phase species form low volatility oxidation products that will either directly nucleate particles, or condense onto existing seed particles, to form secondary organic aerosol (SOA; Hallquist et al., 2009).

Of particular interest for the formation of SOA are the reactions and fate of organic peroxy radicals (RO₂). RO₂ can undergo autooxidation resulting in the formation of highly oxygenated molecules (HOMs), which were recently identified as key contributors to aerosol formation (Bianchi et al., 2019). RO₂ can also cross-react with peroxy radicals (HO₂) and other RO₂, the latter of which result in extremely low volatility accretion products (termed dimers) which are also of interest for SOA formation (Mohr et al., 2017, Berndt et al., 2018). Furthermore, in the presence of NO_x (which is largely from anthropogenic sources), RO₂ can react through additional pathways leading to the formation of species such as peroxyl acyl nitrates (Rissanen, 2018). As such understanding the branching ratio of RO₂ reactions – especially in complex multicomponent VOC systems (Takeuchi et al., 2022) – is critical for understanding aerosol formation.

In this project, the fate of RO₂ radicals will be explored using a combination of laboratory approaches. Atmospheric simulation chamber and aerosol flow tube experiments will be used to explore the gas-phase oxidation chemistry surrounding VOC mixtures, under different NO_x regimes. Furthermore, gas-phase measurements will be coupled with aerosol sizing and composition measurements to elucidate how gas-phase composition ties to the formation and composition of secondary organic aerosol in the presence and absence of seed particles.

Project Objectives:

The project will use state-of-the-art mass spectrometry and spectroscopic instruments coupled with the atmospheric simulation chamber and aerosol flow tube experiments to address the following objectives:

(1) To identify key oxidation products and determine RO₂ branching ratios of mixed VOC systems (e.g., alpha-pinene and toluene) under low and high NO_x regimes;

(2) To identify the impact these oxidation products and branching ratios have on aerosol formation and initial SOA composition in the presence and absence of seed particles;

(3) To establish key differences and links between gas-phase oxidation chemistry and aerosol composition for single versus multicomponent VOC systems and use these data to guide improved parameterizations for models.

The research will lead to an improved understanding of gas-phase RO₂ cross-reactions in complex VOC systems and how they influence the formation and composition of SOA. You will acquire expertise in a range of instrumentation including mass spectrometry, laser-based spectroscopy, aerosol sizing instruments while conducting atmospheric simulation chamber and aerosol flow tube measurements. This will also introduce you to common lab practices in atmospheric sciences (e.g., safe handling of vacuum and gas systems, data acquisition, electronics), and engage in collaborations within the Atmospheric, Planetary and Theoretical Chemistry group in the School of Chemistry.

By working with expert investigators, you will receive advanced technical training and enhance your skills for your independent research career. This project will build upon existing expertise at Leeds in VOC oxidation chemistry, by connecting how it leads to SOA formation in the atmosphere. You will use state-of-the-art research infrastructure in the School of Chemistry, including HIRAC (and associated instruments; Glowacki et al., 2007), the aerosol flow tube and laser-based spectroscopy instruments for the detection of OH, HO₂ and RO₂, allowing for detailed understanding of the multiphase formation of SOA. In addition to collaborations at Leeds, there will be opportunities to collaborate with atmospheric scientists in the UK and abroad. Results from this project will be communicated to the scientific community through high-quality publications in leading journals, and at conferences both on the national and international level.

Training:

You will work under the supervision of Dr. Natasha Garner, Prof. Dwayne Heard and Prof. Paul Seakins from the School of Chemistry at Leeds. The supervisors have expertise in conducting laboratory-based experiments to study aerosol multiphase and VOC oxidation chemistry, in addition to expertise in NO_x and peroxy radical chemistry.

The PhD will provide you with expertise in state-of-the-art atmospheric instrumentation and conducting atmospheric simulation and aerosol flow tube experiments. In addition to gaining technical expertise in the design and conduction of experiments and instrumentation, you will be supported in writing publications and encouraged to attend both national and international conferences. This will help you build your communication skills and help establish a network to assist your career upon graduation. You will also have access to a broad spectrum of training workshops (e.g., scientific writing), in addition to training provided through the National Centre for Atmospheric Science.

Your Profile:

You should have an interest in atmospheric chemistry, air quality, climate and global environmental problems, with a strong background in chemistry or a similar discipline (e.g. physics, engineering, environmental science).

References:

Shrivastava, M., et al. (2017). “Recent advances in understanding secondary organic aerosol: Implications for global climate forcing.” Reviews of Geophysics 55(2): 509-559.

Jimenez, J. L., et al. (2009). “Evolution of Organic Aerosols in the Atmosphere.” Science 326(5959): 1525-1529.

Hallquist, M., et al. (2009). “The formation, properties and impact of secondary organic aerosol: current and emerging issues.” Atmospheric Chemistry and Physics 9(14): 5155-5236.

Bianchi, F., et al. (2019). “Highly Oxygenated Organic Molecules (HOM) from Gas-Phase Autoxidation Involving Peroxy Radicals: A Key Contributor to Atmospheric Aerosol.” Chem Rev 119(6): 3472-3509.

Mohr, C., et al. (2017). “Ambient observations of dimers from terpene oxidation in the gas phase: Implications for new particle formation and growth.” Geophysical Research Letters 44(6): 2958-2966.

Berndt, T., et al. (2018). “Accretion Product Formation from Self- and Cross-Reactions of RO(2) Radicals in the Atmosphere.” Angew Chem Int Ed Engl 57(14): 3820-3824.

Rissanen, M. P. (2018). “NO(2) Suppression of Autoxidation-Inhibition of Gas-Phase Highly Oxidized Dimer Product Formation.” ACS Earth Space Chem 2(11): 1211-1219.

Takeuchi, M., et al. (2022). “Non-linear effects of secondary organic aerosol formation and properties in multi-precursor systems.” Nature Communications 13(1): 7883.

Glowacki, D. R., et al. (2007). “Design of and initial results from a Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC).” Atmospheric Chemistry and Physics 7(20): 5371-5390.