Simulation of organic aerosol, its precursors and related oxidants in the Landes pine forest in south-western France: Need to account for domain specific land-use and physical conditions

Abstract

Abstract. Organic aerosol (OA) still remains one of the most difficult components of the atmospheric aerosols to simulate, given the multitude of its 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 not only the opportunity to compare biogenic secondary OA (BSOA), but also its precursors and oxidants obtained within and above the Landes forest canopy, to simulations performed with CHIMERE, a state of the art regional Chemistry-Transport Model. The Landes forest is situated in the south-western part of France, and is one of the largest anthropized forests in Europe (1 million ha), composed by a majority of maritime pine trees, strong terpenoid emitters, providing a large potential for biogenic SOA formation. In order to simulate OA build-up in this area, a specific model configuration set-up, adapted to the local peculiarities was necessary. As the forest is inhomogeneous, with interstitial agricultural fields, high-resolution 1 km simulations over the forest area were performed. BVOC emissions were predicted by MEGAN, but specific land cover information needed to be used, chosen from the comparison of several high-resolution land cover databases. Moreover, the tree species distribution needed to be updated for the specific conditions of the Landes forest. In order to understand the canopy effect in the forest, canopy effects on vertical diffusivity, winds and radiation were implemented in the model in a simplified way. The refined simulations show a redistribution of BVOCs with a decrease in isoprene and an increase in terpenoid emissions with respect to the standard case, in line with observations. Corresponding changes on simulated BSOA sources are tracked. Very low night-time ozone, sometimes near zero, remains overestimated in all simulations. This has implications to the night-time oxidant budget, including NO3. Despite careful treatment of physical conditions, simulated BSOA is overestimated in the most refined simulation. Simulations are also compared to air quality sites surrounding the Landes forest, reporting a more realistic simulation in these stations in the most refined test case. Finally the importance of the see breeze system which also impacts species concentrations inside the forest is made evident.

On the formation of biogenic secondary organic aerosol in chemical transport models: an evaluation of the WRF-CHIMERE (v2020r2) model with a focus over the Finnish boreal forest

“…Few isolated spikes in monoterpenes air concentrations are likely to arise from local anthropogenic activities in the nearby sawmill facilities (Heikkinen et al, 2021;Hellén et al, 2018;Vestenius et al, 2021), a feature that is not included in the western France (Cholakian et al, 2022). Even though the model uses a different chemical scheme to perform the gas-phase and particle phase chemistry, they both indicated a large overestimation of isoprene concentrations, also at other European 275 sites.…”

“…The resulting additional particulate mass is widely referred as to secondary organic aerosol (SOA), and, if produced from biogenic volatile organic compounds (BVOCs) such as isoprene and monoterpenes, to biogenic secondary organic aerosol (BSOA). 55 Numerous regional modeling studies, have focused on the formation and characterization of the BSOA component (Aksoyoglu et al, 2011;Bergström et al, 2012;Boy et al, 2022;Cholakian et al, 2022Cholakian et al, , 2018Ciarelli et al, 2016;Hodzic et al, 2009Hodzic et al, , 2009Zhang et al, 2013). However, such a highly complex system governing the formation of BSOA remains not fully understood.…”