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Chemical and dynamical processes lead to the formation of aerosol layers inthe upper planetary boundary layer (PBL) and above it. Through verticalmixing and entrainment into the PBL these layers may contribute to theground-level particulate matter (PM); however, to date a quantitativeassessment of such a contribution has not been carried out. This studyinvestigates this aspect by combining chemical and physical aerosolmeasurements with WRF/Chem (Weather Research and Forecasting withChemistry) model simulations. The observations werecollected in the Milan urban area (northern Italy) during the summer of 2007.The period coincided with the passage of a meteorological perturbation thatcleansed the lower atmosphere, followed by a high-pressure period favouringpollutant accumulation. Lidar observations revealed the formation ofelevated aerosol layers and evidence of their entrainment into the PBL. Weanalysed the budget of ground-level PM2.5 (particulate matter with anaerodynamic diameter less than 2.5 μm) with the help of the onlinemeteorology–chemistry WRF/Chem model, focusing in particular on thecontribution of upper-level processes. Our findings show that an importantplayer in determining the upper-PBL aerosol layer is particulate nitrate,which may reach higher values in the upper PBL (up to 30% of the aerosolmass) than in the lower PBL. The nitrate formation process is predicted tobe largely driven by the relative-humidity vertical profile, which maytrigger efficient aqueous nitrate formation when exceeding the ammoniumnitrate deliquescence point. Secondary PM2.5 produced in the upper halfof the PBL may contribute up to 7–8 μg m−3 (or 25%) to ground-level concentrations on an hourly basis. The residual aerosol layer above the PBL is also found to potentially play a large role, which mayoccasionally contribute up to 10–12 μg m−3 (or 40%) to hourlyground-level PM2.5 concentrations during the morning hours. Althoughthe results presented here refer to one relatively short period in onelocation, this study highlights the importance of considering the interplaybetween chemical and dynamical processes occurring within and above the PBLwhen interpreting ground-level aerosol observations

atmospheric chemistry and physics

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) evidences the distinctive role of nitrate