Resolving the hydroxyl radical budget in the tropical marine boundary layer through field measurements of the total OH loss rate

Roughly a quarter of tropospheric methane removal (a key greenhouse gas, GHG), occurs in the tropical marine boundary layer (MBL) driven by the reaction with the hydroxyl radical, OH [1].  With a knowledge of the OH sources and OH sinks, OH concentrations in the atmosphere can be modelled and compared to observations. In the remote MBL, the OH sources that need to be considered are relatively few and are well understood, with ozone photolysis accounting for roughly 90 % of OH production [2]. In contrast, recent MBL observations of OH reactivity, kOH, which is a measure of the total OH loss rate, have demonstrated that there are unknown species which contribute to OH destruction in these environments [3]. This missing or unknown kOH implies that our understanding of the quantity of MBL volatile organic compound (VOC) emissions and the impact these emissions have on OH (and, therefore, methane lifetime) is incomplete. Missing kOH will also impact our ability to accurately predict peroxy radical concentrations (formed from VOC oxidation by OH) which can lead to the formation of another important GHG, ozone, or the formation of secondary organic aerosol (SOA) which influence cloud formation and the earth’s radiative balance.

To determine the full magnitude of kOH in the tropical MBL and assess and identify compounds which contribute to missing kOH, a large field and modelling project (Closing the budget in marine atmospheric Oxidative Capacity through the quantification of Oceanic VOC emissions, COCO-VOC) has been developed. As part of COCO-VOC, a comprehensive suite of VOC observations, alongside a measurement of kOH will be made 1) from the island archipelago of Cabo Verde (located in the tropical Atlantic ocean) and 2) on a research ship cruise, which will permit observations over wider spatial scales, including in and around the Mauritanian upwelling which stimulates enhanced biological activity (and VOC production).

The total OH loss rate is measured using the laser flash-photolysis, laser-induced fluorescence (LFP-LIF) [4] method, and this technique has been used successfully by the Leeds team on numerous field campaigns [e.g. 5, 6, 7] including recent observations at Cabo Verde in Feb 2023.

 

Specifically in this project you will:

• Participate in the field observations, operating the Leeds LFP-LIF instrument to determine the total kOH in and around Cabo Verde in the tropical MBL. Help to operate the Leeds fluorescence assay by gas expansion (FAGE) instrument for the detection of OH and peroxy radicals during the field observations.
• Perform analysis and interpretation of the field data by comparing the kOH observations to kOH calculated from the individually measured OH reactants observed during the field campaigns. Compare kOH observations made during COCO-VOC at Cabo Verde to the previous kOH observations at the site to identify if there is any seasonality in the magnitude of kOH.
• Develop a box model based on the Master Chemical Mechanism and constrained to the field observations to model kOH to assess if VOC oxidation products can close the observed kOH budget and to predict OH and peroxy radical concentrations.
• Where opportunities arise, take part in airborne observations of radicals and kOH on board the FAAM BAe-146 research aircraft to assess oxidation processes throughout the troposphere.

You will work under the supervision of Dr Lisa Whalley, Dr Daniel Stone and Professor Dwayne Heard from the School of Chemistry at Leeds, who are all members of the Atmospheric and Planetary Chemistry Group. The supervisors lead active and vibrant research groups exploring the role of gas-phase and aerosol chemical processes in the atmosphere, using experimental and modelling approaches. The project will provide opportunities to work with other atmospheric scientists in the UK as part of collaborative fieldwork.

[1] Bloss et al. Faraday Discussions, 2005, 130: p. 425-436

[2] Whalley et al. Atmospheric Chemistry and Physics, 2010, 10: p. 1555–1576

[3] Thames et al. Atmospheric Chemistry and Physics, 2020, 20: p. 4013-4029

[4] Stone et al. Atmospheric Measurement Techniques, 2016, 9: p. 2827-2844

[5] Whalley et al. Atmospheric Chemistry and Physics, 2016, 16: p. 2109–2122

[6] Whalley et al. Atmospheric Chemistry and Physics, 2021, 21: p. 2125–2147

[7] Woodward-Massey et al. Atmospheric Chemistry and Physics, 2023, 23: p. 14393–14424