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

Abstract. We present an evaluation of the regional chemical transport model (CTM) WRF-CHIMERE (v2020r2) for the formation of biogenic secondary organic aerosol (BSOA) with a focus over the Finnish boreal forest. Formation processes of biogenic aerosols are still affected by different sources of uncertainties, and model’s predictions largely varies depending on the levels of details of the adopted chemical and emissions schemes. In this study, air quality simulations were conducted for the astronomical summer of the year 2019 using different organic aerosol (OA) schemes (as currently available in literature) to treat the formation of BSOA. First, we performed a set of simulations in the framework of the volatility basis set (VBS) scheme carrying different assumptions for the treatment of the aging processes of BSOA. The model results were compared against high-resolution (i.e., 1-hour) organic aerosol mass and size distribution measurements performed at the Station for Measuring Ecosystem–Atmosphere Relations (SMEAR-II) site located in Hyytiälä, in addition to other gas-phases species such as ozone (O₃), nitrogen oxides (NO_x) and BVOCs measurements of isoprene (C₅H₁₀) and monoterpenes. We show that WRF-CHIMERE could well reproduce the diurnal variation of the measured OA concentrations for all the investigated scenarios (along with standard meteorological parameters) as well as the increase in concentrations during specific heat waves episodes. However, the modeled OA concentrations largely varied between the schemes use to describe the aging processes of BSOA. Additionally, comparisons with isoprene and monoterpenes air concentrations revealed that the model captured the observed monoterpenes concentrations, but isoprene was largely overestimated, a feature that was mainly attributed to the overstated biogenic emissions of isoprene. We investigated the potential consequences of such an overestimation by inhibiting isoprene emissions from the modeling system. Results indicated that the modeled BSOA concentrations generally increased compared to the base-case simulation with enabled isoprene emissions. We attributed the latest to a shift in the reactions of monoterpenes compounds against available radicals, as further suggested by the reduction in α-pinene modeled air concentrations. Finally, we briefly analyze the differences in the modeled Cloud Liquid Water Content (clwc) among the simulations carrying different chemical scheme for the treatment of the aging processes of BSOA. Model’s results indicated an increase in clwc values at the SMEAR-II site, for simulation with higher biogenic organic aerosol loads, likely as a results of the increased numbered of biogenic aerosol particles capable of activating cloud droplets.