PhD Studentship: CENTA 2027-B08 Less emission, more aerosol How elevated CO2 changes organic gas emissions from forests and their capacity to form particles

Forests emit large amounts of volatile organic compounds (VOCs). In the atmosphere these are oxidised to form large, oxygenated gases which then form secondary organic aerosol (SOA): tiny particles that scatter sunlight and form cloud droplets. The effect of aerosol on clouds is the largest single source of uncertainty in how much humans have changed the climate. Forest emissions therefore feed back on climate, but we do not know how this feedback will change as atmospheric CO2 rises. Trees respond to high CO2 by changing both how much they emit and what they emit. Isoprene is the most abundant VOC emitted by plants, and its emission from many species, including oak, falls under elevated CO₂, while monoterpene and sesquiterpene responses are more variable. These compounds differ enormously in how well they make particles. Isoprene makes little SOA itself, and it actively suppresses particle formation from monoterpenes by interfering with the chemistry that builds oxygenated gases. A forest that emits less isoprene but similar or more monoterpene could therefore make more aerosol, even though its total emissions have fallen (Figure 1). This project will test that idea in a real forest. The Birmingham Institute of Forest Research (BIFoR) FACE facility has exposed a mature oak woodland to CO2 150 ppm above ambient since 2017, alongside control plots at current CO2. It is one of very few places in the world where the emissions of a future forest can be measured directly. The doctoral researcher (DR) will measure the VOC mixtures emitted under current and elevated CO₂, recreate those mixtures in the laboratory to measure how much aerosol they form, and use a chemical box model to work out which compounds cause any difference. Part of the laboratory work will take place at the University of Eastern Finland (UEF) with Dr Angela Buchholz, within the ACE-Forest Centre of Excellence, a leading group in aerosol formation from real plant emissions. The outcome will be the first direct, mechanistic estimate of how CO2 fertilisation changes the aerosol-forming potential of a mature temperate forest, something climate models currently have to assume. For further information on this project and details of how to apply to it please visit https://centa.ac.uk/studentship/2027-b08-less-emission-more-aerosol-how-elevated-co2-changes-organic-gas-emissions-from-forests-and-their-capacity-to-form-particles/ Further information on how to apply for a CENTA studentship can be found on the CENTA website: https://centa.ac.uk/apply/ Funding notes: This project is offered through the CENTA3 DLA, funded by the Natural Environment Research Council (NERC). Funding covers: annual stipend, tuition fees (at home-fee level), Research Training Support Grant. Academic requirements: at least a 2:1 at UK BSc level or a pass at UK MSc level or equivalent. Further information: https://centa.ac.uk/apply/ International students are eligible for studentships to a maximum of 30% of the cohort. Funding does not cover any additional costs relating to moving or residing in the UK. International applicants must fulfil the University’s international student entry requirements, including English language. References: Hart et al. (2020) ‘Characteristics of free air carbon dioxide enrichment of a northern temperate mature forest’, Global Change Biology, 26(2). Rosenstiel et al. (2003) ‘Increased CO2 uncouples growth from isoprene emission in an agriforest ecosystem’, Nature, 421. Heinritzi et al. (2020) ‘Molecular understanding of the suppression of new-particle formation by isoprene’, Atmospheric Chemistry and Physics, 20. McFiggans et al. (2019) ‘Secondary organic aerosol reduced by mixture of atmospheric vapours’, Nature, 565.