Trees in the city and natural emissions of VOCs Exploring Tree Emissions of Volatile Organic Compounds (VOCs), interview to Dr. Glenn-Michael Oomen

Volatile Organic Compounds (VOCs) are organic molecules that reside in the gas phase. Although our atmosphere only contains trace amounts of VOCs, they play an important role in air quality and climate. Through various chemical reactions, they eventually can form aerosols and tropospheric ozone, which are harmful for human health. Wildfires and industrial activities release VOCs, but the most important source is emissions by trees. Dr. Glenn-Michael Oomen works at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) and his research focuses on modelling of VOC emissions, including the ones from trees.

So do trees pollute? Should we stop planting trees ?

Trees do emit large amounts of VOCs, but it is the combination of VOCs with anthropogenic pollution (NOx and SOx) which has detrimental effects on air quality through complex chemical processes leading to ozone and particulate matter formation. But by no means does this mean we should avoid planting trees. Firstly, growing trees has positive impacts on climate change: they are carbon sinks, and they contribute to reducing peak summer temperatures through evapotranspiration and shade. Besides, they also improve mental health (in cities) and lower the potential for flooding and landslides. The overall benefits of trees are still overwhelmingly positive!

Okay, so trees are beneficial, but they still emit VOCs. Why is it important to study these emissions ?

The extent of VOC emissions by trees is very uncertain, so studying them is essential for estimating their contribution to atmospheric pollution. Our research group specifically focuses on tropospheric chemistry modelling, or in other words, modelling the transport and reactions of different gas species in the lower atmosphere. Within SEEDS, we use a state-of-the-art chemistry transport model and an inversion framework to derive emissions of volatile hydrocarbons in Europe using satellite data.

Why use satellite data to estimate VOC production?

Satellites can provide us with global daily measurements of the most abundant VOC in the atmosphere: formaldehyde. This compound is the result of the oxidation of other VOCs, which come from both natural and anthropogenic origin. Using these daily measurements, a detailed chemistry-transport model and inversion techniques, we can link satellite measurements of formaldehyde to the emissions of parent VOCs. This way, we extract information on the total VOC emissions across Europe, and we can highlight areas that are prone to poor air quality due to anthropogenic pollution and tree emissions.

Can this data inform a more resilient urban planning?

The mere awareness of the existence of VOCs can be used for urban planning, even without data. It is recommended to use trees that are weak VOC emitters, such as beech, ash, hornbeam, cedar. It is also helpful to choose trees with a low production of pollen, which also negatively impacts air quality. As of now, the data in the SEEDS project is produced at a resolution of 10 kilometres. This type of emission inventory is suited to represent emissions at a regional scale, around different cities in a consistent and homogeneous way, but not at urban scale. Future satellites at finer spatial resolution, combined with high-resolution atmospheric models, could be used to develop biogenic emission inventories at city scale.

Which are your next steps for the last year of SEEDS?

So far, we have published the emission datasets for 2018 to 2020. In this last year, we will perform the analysis for the years 2021 and 2022. Additionally, we are making an extensive validation of our results by comparing with independent measurements from different instruments on the ground. We will further evaluate the impact of specific assumptions that are made in the model on the top-down VOC emissions produced in SEEDS. Our final biogenic emission product is planned to be published in September 2023.