Improved assessment of OVOC sources and sinks over Reunion Island through WRF-Chem model evaluation against PTR-MS data and satellite retrievals

Oxygenated volatile organic compounds (OVOC) have a significant impact on atmospheric oxidation capacity and climate. OVOCs are directly emitted from biogenic sources and are produced from the oxidation of hydrocarbons in the atmosphere. However, their budget remains poorly understood, due to incomplete representation of photochemical OVOC production and uncertainties in terrestrial emissions and ocean/atmosphere exchanges. In addition, OVOC atmospheric measurements are scarce in remote areas, in particular in tropical regions. In this work, we exploit a 2-year high-temporal resolution dataset of mass spectrometry (PTR-MS) measurements of OVOC compounds at a remote high-altitude tropical site, the Maïdo Observatory (2155m asl) on Reunion Island. More precisely, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is used to provide an updated evaluation of the budget of OVOCs over Reunion Island, based on the PTR-MS dataset complemented with meteorological measurements and satellite (TROPOMI) retrievals of relevant compounds. The model is configured to allow three nested domains centred on Reunion Island, with spatial resolution from 12.5, 2.5 and 0.5 km. The finest resolution is needed due to the complex orography of the island and the spatially heterogeneous distribution of reactive species. For computational reasons, the focus is on two one-month simulations in January and July 2019, allowing analysis of seasonal differences and their impacts on model performance and chemical budget.

The WRF-simulated meteorology is first evaluated against meteorological measurements at a remote site (Maïdo) and two urban sites (Saint Denis and Saint Pierre). The impact of physical parameterizations (i.e. planetary boundary layer parameterizations, surface scheme, etc.) is tested through sensitivity simulations. A high-resolution (1km²) anthropogenic emission inventory for Reunion is implemented, complemented with information from global inventories. Biogenic VOC emissions (primarily isoprene) are calculated on-line using the MEGAN algorithm and high-resolution distributions of standard emission factors and plant functional types (PFTs). The MOZART chemical mechanism is adopted. The chemical simulations are evaluated against (1) NO₂ and HCHO vertical columns from TROPOMI, (2) the PTR-MS OVOC dataset at Maïdo, and (3) network air quality measurements at several sites. Those comparisons will provide new constraints on the emissions of NOx and VOCs, and will result in recommendations for further refinements. This work will lead to a better appraisal of OVOC sources and sinks over the island. The main unknowns and potential issues will be discussed.