Analysis of chemical and transport processes of biogenic aerosols over the northern Apennines: insights from the WRF-CHIMERE model†

Sources and transport processes of aerosols over the Italian northern Apennines are investigated with a focus on the area of the WMO/GAW global station of Mt. Cimone (CMN, 2165 m a.s.l.). The site is characterized by complex orography, representing a challenge for chemical transport model (CTM) applications when simulating processes controlling advection and diffusion of air pollutants within and above the planetary boundary layer (PBL). First, we extensively evaluated the skills of the WRF-CHIMERE (v2020r3) coupled CTM in reproducing both the meteorological conditions observed at the surface level of multiple weather stations and the sub-micrometre aerosol mass concentrations from intensive in situ measurements performed at CMN during July 2017. The analysis of the meteorological fields revealed that the local thermally-driven flows occurring over the adjacent coastal and mountainous regions are very well reproduced by the model. The accuracy is less at higher altitudes in proximity of CMN and on the slopes facing the Po valley, where also fewer observational meteorological data were available. The discrepancies between the model output and observations, especially in the near-surface wind dynamics, are mainly associated with the smoothed topography of the terrain as represented in the model: at the resolution of 1 km small-scale orographic features and related meteorological phenomena cannot be adequately reproduced. Our results indicate that the modeled particle mass concentrations and its chemical composition are in good agreement with observational data, with organic aerosol contributing to about 60% of the total sub-micrometer aerosol load during the investigated time period and sulphate being the most important inorganic component. Additionally, a model-based source apportionment analysis revealed that organic aerosol, and specifically secondary organic aerosol (SOA), were mostly of biogenic origin (contributing up to 66% of the secondary organic aerosol fraction). We further analyze the transport of organic aerosol particles associated with the typical wind pattern developing at the interface between plains, valleys and ridges of the northern Apennines mountains. Despite uncertainties in source areas and formation mechanisms, the model results indicated that the upslope valley winds might sustain the funneling of biogenic aerosol particles to higher elevations up to the Apennines ridge, eventually to above the diagnosed PBL height. For biogenic organic aerosol this process is more effective on the south-western slope of the Apennines range. This may result from either more favourable meteorological conditions or larger availability of aerosol particles over the lowlands. This work represents the first high-resolution (1 km) CTM study investigating the region of Mt. Cimone and is intended to provide original insights on the vertical transport of aerosols particles into the free troposphere in regions characterized by a complex orography, such as the Alpine range, the European Alps, and the Apennines.