"Climate Stress impacts on forest emissions and aerosol pollution" (ClimStress) - Detailed description of the project

Objectives of this project

• Ecosystem-scale quantification of emissions of stressed vs. less-stressed forests

Using Zeppelin-bsaed airborne flux observations, we will map and quantify BVOC emissions from forests. These data will then be compared with emission models and inventories to test their ability to represent real-world stress-BVOC emissions.

• Quantification and chemical characterization of

1. Tree emissions under combined stressors. In the real world, trees experience several stressors at the same time - we will do the same in the plant chamber PLUS.
2. Atmospheric oxidation processes towards particles from stressed emissions in the oxidation chamber SAPHIR. Chemical ionization mass spectrometry with NH₄⁺ and H₃O⁺ ionization enables the detection of a wide range of volatilities.

Field and Chamber Experiments

Combining chamber and field work to study the impact of plant stress on atmospheric chemistry

Biogenic VOCs are rapidly oxidized in the atmosphere. Their oxidation products can condensate to form secondary organic aerosol (SOA), which impacts light scattering, cloud formation and human health. In the presence of NO_x, these processes also lead to ozone formation.

In this project, we will apply field and chamber experiments to study both the primary emissions from healthy and stressed trees/forests, and the oxidation products in the gas and particle phases. Measurement techniques used will be, amongst others, PTR-ToF-MS, NH₄⁺-CIMS, and GC-MS.

Chamber experiments

Airborne eddy covariance measurements

Airborne eddy covariance provides measurements of emission and deposition at ecosystem scale. High time resolution (usually 10 Hz) observations of vertical wind speed and trace gas concentrations are necessary for airborne eddy covariance calculations. The flux, i.e. the emission or deposition per area and time, is calculated from the covariance of vertical wind speed and trace gas concentration. In airborne eddy covariance, the method of choice is wavelet transformation.

Thismproject will for the first time use a Zeppelin NT airship for airborne flux measurements.

In contrast to concentration measurements, airborne eddy covariance has several advantages:

• Direct emission measurement
• No influence of meteorology
• Spatially resolved
• Enables source separation

Spatially resolved VOC fluxes

Examples for volatile organic compound (VOC) fluxes at landscape scale are shown in the maps below (from Pfannerstill et al. 2023, ACP). In contrast to concentration measurements, the fluxes are localized to where the emission sources are (e.g. the forest for isoprene or the highway and cities for aromatics).

Using airborne flux observations to validate emission inventories

Footprint models allow us to estimate the spatial origin of the measured emissions. These footprints can be matched with emission inventories to validate how well they represent the observed emissions (Pfannerstill et al., ES&T 2023). Matching with landcover information also helps to find out about the sources of the emissions, and multilinear regression allows to quantify the emissions of each source type (Pfannerstill et al, ACP 2023).

Planned Zeppelin payload

The planned Zeppelin payload will enable us to measure fluxes of volatile organic compounds and greenhouse gases, and to investigate particle composition. To understand the completeness of our gas phase observations, we will also observe the total OH reactivity (loss rate of the hydroxyl radical). Cameras will be used to identify plants and their stress state, as well as the surface temperature.

Timeline