How much is particulate matter near the ground influenced by upper level processes within and above the PBL? A summertime case study in Milan (Italy)

Abstract

Abstract. Chemical and dynamical processes yield to the formation of aerosol layers in the upper planetary boundary layer (PBL) and above it. Through vertical mixing and entrainment into the PBL these layers may contribute to the ground-level particulate matter (PM), but a quantitative assessment of such contribution is still missing. This study investigates this aspect combining chemical and physical aerosol measurements with WRF/Chem model simulations. The observations were collected in the Milan urban area (Northern Italy) during summer of 2007. The period coincided with the passage of a meteorological perturbation that cleansed the lower atmosphere, followed by a high pressure period that favoured pollutant accumulation. Lidar observations reveal the formation of elevated aerosol layers and show evidences of their entrainment into the PBL. We analyze the budget of ground-level PM2.5 (particulate matter with aerodynamic diameter less than 2.5 μm with the help of the online meteorology-chemistry WRF/Chem model, with particular focus on the contribution of upper level processes. We find that an important player in determining the upper PBL aerosol layer is particulate nitrate, which may reach higher values in the upper PBL (up to 30% of the aerosol mass) than the lower. The nitrate formation process is predicted to be largely driven by the relative humidity vertical profile, that may trigger efficient aqueous nitrate formation when exceeding the ammonium nitrate deliquescence point. Secondary PM2.5 produced in the upper half of the PBL may contribute up to 7–8 μg m−3 (or 25%) to ground level concentrations on hourly basis. A large potential role is also found to be played by the residual aerosol layer above the PBL, which may occasionally contribute up to 10–12 μg m−3 (or 40%) to hourly ground level PM2.5 concentrations during the morning. This study highlights the importance of considering the interplay between chemical and dynamical processes occurring within and above the PBL when interpreting ground level aerosol observations.