Sources and sinks of long-lived greenhouse gases at Réunion Island

In the context of international atmospheric monitoring networks, BIRA-IASB is measuring time series of atmospheric concentrations of CO2, CH4 and CO at Réunion Island. In order to understand the underlying causes of the observed concentrations and their variability, we compare them to the concentrations simulated with the regional atmospheric transport model WRF-GHG. It shows that surface observations are dominated by local emissions and dynamical processes such as wind speed and direction, while column observations are influenced by larger-scale mechanisms such as biomass burning plumes from Africa or South America.

To enable climate change mitigation, it is crucial to monitor the trends and variability of greenhouse gases (GHG) and to understand their emission sources and sinks.

BIRA-IASB has a long-standing expertise in performing observations of the concentrations of these gases in the atmosphere. Its ground-based remote sensing observations provide the total amount of gas in the column from the Earth’s surface to the top of atmosphere. The in situ observations measure the local atmospheric concentrations close to the instrument.

The data are part of international monitoring networks such as:

One of the stations where BIRA-IASB is performing such measurements is located at Réunion Island, in the Southeast Indian Ocean. To gain a deeper understanding of the observed time series, we have simulated them with an atmospheric transport model, called WRF-GHG.

Atmospheric model

WRF-GHG is a specific option of the widely used dynamic Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). WRF-GHG simulates the propagation of CO2, CH4 and CO in the atmosphere and their emission sources and sinks at the Earth's surface.

In the WRF-GHG option, the regional atmospheric transport and surface-atmosphere fluxes of CO2, CH4 and CO are simulated without chemical reactions with other species. This is a valid assumption since these gases have long lifetimes.

The model output consists of hourly 3D-concentration fields. We simulated almost 2 years of observations of the target gases at Réunion Island and found a good agreement between the simulated and observed time series.

Results

Local emissions and dynamical processes

The simulations confirmed that in situ and column observations provide complementary information about the influencing processes. The variability of the surface measurements is mostly caused by local fluxes and atmospheric dynamics.

Local fluxes are those that are produced close to the instrument, primarily by anthropogenic sources in the capital (burning of fossil fuels) or biogenic sources (photosynthesis and respiration of vegetation) at the top of the Maïdo mountain.

Large-scale mechanisms

Additionally, atmospheric dynamics such as wind speed and direction have a large impact on the local concentrations. Strong winds quickly mix gases in the atmosphere, but weak breezes can allow accumulation of concentrations close to their sources.

The column observations are more sensitive to the larger scale: they are influenced by emissions much further away from the instrument and by wind patterns created by large weather systems. As such, the impact of biomass burning plumes from the African or even South-American continents can be detected at Réunion Island.

Both types of observations are required to get a better insight into the carbon cycle. Moreover, the WRF-GHG model has proven to be a very useful tool to achieve this goal.

Currently, a similar study is ongoing at the Xianghe site, near Beijing in China. In the future, this model will also be used to verify emissions of CO2, CH4, and CO in Belgium.

 

Reference

Callewaert, S., Brioude, J., Langerock, B., Duflot, V., Fonteyn, D., Müller, J.-F., Metzger, J.-M., Hermans, C., Kumps, N., Ramonet, M., Lopez, M., Mahieu, E., & De Mazière, M. (2022). Analysis of CO2, CH4, and CO surface and column concentrations observed at Réunion Island by assessing WRF-Chem simulations. Atmospheric Chemistry and Physics, 22(11), 7763–7792. https://doi.org/10.5194/acp-22-7763-2022 

Time series surface and column CO2 concentrations Réunion Island
Example time series of (a) surface and (c) column CO2 concentrations at Réunion Island, together with the simulated tracer contributions (b, d). The black line/dots in (a) and (c) are the concentrations as measured by the local instruments, while the blue line/dots are the total simulated CO2 concentrations by WRF-GHG. Figures (b) and (d) show the contribution of different components that together make up the total concentrations simulated by the model. It represents the contribution of different sources: anthropogenic (red line), biogenic (green line), biomass burning (brown) and ocean fluxes (blue line).
Map total column concentration of CO Réunion Island plot with values
Example of the WRF-GHG model output at Réunion Island. The map on top displays the total column concentration of CO (in ppb, parts per billion). The simulated and observed values at the site on Réunion Island are shown in the bottom panel of the plot below (red line and black dots respectively).