Improved estimation of biogenic fluxes over Europe

2023-2024
Volatile organic compounds (VOCs) are key precursors of particulate matter and tropospheric ozone. The terrestrial biosphere is by far the largest VOC source, yet its emissions remain poorly constrained.

Using weekly TROPOMI formaldehyde data, we estimate that biogenic VOC emissions over Europe, are about twice higher than in current inventories. This increase is, driven by the high HCHO levels observed in southern Europe. The weekly top-down emissions better capture large emission fluctuations than an inversion based on monthly observations.

Volatile organic compounds (VOCs) play an important role for air quality and climate. They are precursors of organic aerosols and impact tropospheric ozone levels through their interaction with nitrogen oxides. Consequently, accurate estimates of emissions of VOCs are important in air quality studies.

Natural emissions from vegetation are by far the most important source of VOCs, making up roughly 85% of global emissions. Of the biogenic VOCs, isoprene is most abundantly emitted, making up about half of the total biogenic emissions, followed by monoterpenes and methanol. 

However, our current emission estimates are very uncertain, as they rely on only a few direct field measurements of VOC emissions from plants and trees, and are highly dependent on vegetation types and other conditions. Rather than using direct measurements of the emitted VOCs in order to estimate the emissions, we make use of a different approach, called inverse modelling, which is based on satellite data and atmospheric models.

Using satellites to estimate VOC emissions

Isoprene and monoterpenes are very reactive gases in our atmosphere. Their degradation leads to the production of formaldehyde (HCHO), a molecule measured by satellites in the ultraviolet spectral range. 

We use the MAGRITTE atmospheric model at a spatial resolution of 0.5°.  This model accounts for the relevant chemical reactions in our atmosphere, as well as for the transport of chemical compounds by the wind. Based on existing knowledge on the VOC emissions, the model simulates the formaldehyde concentrations, which are then compared to high-quality formaldehyde measurements from the TROPOMI satellite sensor (Fig. 1). Improved VOC emission estimates are determined from these comparisons.

We have performed the emission inversion over Europe for four years (2018-2021) with weekly temporal resolution. Our results show that isoprene emissions in southern Europe appear to be strongly underestimated; a doubling of the emission estimates over Europe is required in order to match the TROPOMI data (Fig. 2). 

In central Europe and Benelux, moderate emissions are found, indicating a good agreement with available emission inventories. The impact of the inversion on anthropogenic VOC emissions (-17%) and biomass burning VOC emissions (+13%) is relatively small over Europe.

Reference:

  • Oomen, G.-M., Müller, J.-F., Stavrakou, T., De Smedt, I., Blumenstock, T., Kivi, R., Makarova, M., Palm, M., Röhling, A., Té, Y., Vigouroux, C., Friedrich, M. M., Frieß, U., Hendrick, F., Merlaud, A., Piters, A., Richter, A., Van Roozendael, M., and Wagner, T.: Weekly derived top-down volatile-organic-compound fluxes over Europe from TROPOMI HCHO data from 2018 to 2021. Atmos. Chem. Phys., 24, 449–474, https://doi.org/10.5194/acp-24-449-2024, 2024.
Weekly observations of formaldehyde columns over Europe
Weekly observations of formaldehyde columns over Europe
Figure 1. Weekly observations of formaldehyde columns over Europe as observed by the TROPOMI satellite sensor in July 2019. The observations display a large variability, driven by week-to-week variations in meteorological conditions. These data are used as input in the emission inversion.
optimisation of emissions using satellite data compared to the current inventory
Figure 2. The optimisation of emissions using satellite data (bottom) suggests a strong increase in isoprene fluxes over the European domain compared to the current inventory (top). The largest increase occurs over Mediterranean countries.