Simulation of SOA formation in the Landes pine forest in south-western France, relative weight of initial ozone, NO3 and OH attack ?

Organic aerosol (OA) still remains one of the most difficult components of the aerosol to simulate, given the multitude of its formation precursors, the uncertainty of its formation pathways and the lack of measurements of its detailed composition. The LANDEX project (The LANDes Experiment), during its intensive field campaign in summer 2017, gives us the opportunity to compare a detailed list of measurements (VOC, NOx, radicals including NO₃, aerosol components, …) obtained within and above the Landes forest canopy, to simulations performed with CHIMERE, a regional Chemistry-Transport Model. The Landes forest is situated in the south-western part of France, and is one of the largest anthropized forest in Europe (1 million ha), composed by a majority of maritime pine trees, strong terpene emitters, providing a large potential for biogenic SOA formation.

In order to simulate organic aerosol build-up in this area, the set-up of a specific model configuration, adapted to local peculiarities, was necessary. As the forest is inhomogeneous, with interstitial agricultural fields, high-resolution 1 km simulations over the forest area were performed, imbedded into a 5 km resolved French and a 25 km resolved European domains. BVOC emissions were predicted by MEGAN, but specific land cover needed to be used, chosen from the comparison of several high-resolution land-cover databases. Also, the tree species distribution needed updated for the specific conditions of the Landes forest. In order to understand the canopy effect in the forest, sensitivity tests were also performed and the diffusivity between the first two layers were changed. The impact of each of these refinements with respect to the standard model set-up on the concentration changes of biogenic VOCs and organic aerosol was calculated and compared to observations. In addition, the sensitivity of SOA build-up with respect to the organic aerosol scheme (standard scheme within CHIMERE, VBS schemes with updated yields for OA formation from BVOCs, …) was assessed.

The ensemble of simulations allowed tracing back the origin of BSOA build-up within and above the Landes forest canopy. Above the canopy, the major simulated pathway of SOA formation is monoterpene oxidation by NO₃, while within the canopy, for sufficiently low mixing during nighttime, the NO₃ radical is suppressed and only little contributes to SOA build-up. This is in accordance to observations and reactivity considerations which show that within the canopy, ozone attack on sesquiterpenes is the major nighttime SOA source.