Interpretation of time series of key gases in the carbon cycle

Chemical composition and climate
BIRA-IASB is contributing to international initiatives for long-term monitoring of the atmospheric composition, by delivering measurements of CO2, CH4 and CO, gases that play an important role in the carbon cycle, at Ile de La Réunion. The collected time series show variations of the concentrations at different time scales: from daily to seasonal to interannual. In order to understand the origin of these variations, BIRA-IASB implemented the WRF-GHG model to simulate the transport and surface fluxes of CO2, CH4 and CO with a spatial resolution up to 1 km2.

Observations

Since several years, BIRA-IASB has been making ground-based remote sensing (FTIR) and in situ observations of carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO).

These measurements are often part of international networks such as:

Scientists at BIRA-IASB are responsible for these kind of measurements at, for example, Ile de la Reunion, a French island in the Southeast Indian Ocean. Despite its remote location, the island is affected by the advection of biomass burning plumes from the African or even South-American continents. Looking at the collected time series, we want to gain a better understanding of the observed variabilities at the different time scales.

WRF-GHG Model

Recently we implemented a new numerical model, WRF-GHG, to exploit these observations. WRF-GHG is a specific option of the widely used Weather Research and Forecasting model coupled with Chemistry (WRF-Chem).

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

In the framework of a PhD study at University of Liège, a young researcher at BIRA-IASB has been implementing and learning this complex model for a case study at Ile de la Reunion. It is a challenge to take into account the complex topography of the island, to have an accurate representation of the local wind flows, and to find the best emission inventories at sufficiently high temporal and spatial resolution that are needed as input fields for the model.

Soon from now, the model will be run to generate a one-year time series of simulated concentration fields of the target gases (CO2, CH4 and CO) at the observation sites at La Réunion in order to compare with the available observations, and gather more insight in the local and remote emission sources that determine the observed variations. 

In the future, WRF-GHG simulations will be exploited to analyse similar observations at the Xianghe site, near Beijing in China.

Time series of surface CO2 concentrations
Example time series of surface CO2 concentrations at the capital of La Réunion, Saint Denis. The black dots are the concentrations as measured by the local instrument, while the solid black line is the total simulated CO2 concentration by WRF-GHG. The total concentration achieved by the model adds up achieves this total concentration by adding up several components coming from anthropogenic (red line), biogenic (green line) and ocean fluxes (blue line). The purple line is the background concentration, representing all influences from outside the chosen regional model domain.