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

Chemical and dynamical processes lead to theformation of aerosol layers in the upper planetary boundarylayer (PBL) and above it. Through vertical mixing andentrainment into the PBL these layers may contribute tothe ground-level particulate matter (PM); however, to datea quantitative assessment of such a contribution has notbeen carried out. This study investigates this aspect by combiningchemical and physical aerosol measurements withWRF/Chem (Weather Research and Forecasting with Chemistry)model simulations. The observations were collected inthe Milan urban area (northern Italy) during the summer of2007. The period coincided with the passage of a meteorologicalperturbation that cleansed the lower atmosphere, followedby a high-pressure period favouring pollutant accumulation.Lidar observations revealed the formation of elevatedaerosol layers and evidence of their entrainment intothe PBL. We analysed the budget of ground-level PM2:5(particulate matter with an aerodynamic diameter less than2.5 μm) with the help of the online meteorology–chemistryWRF/Chem model, focusing in particular on the contributionof upper-level processes. Our findings show that an importantplayer in determining the upper-PBL aerosol layer is particulatenitrate, which may reach higher values in the upper PBL (up to 30% of the aerosol mass) than in the lower PBL. Thenitrate formation process is predicted to be largely driven bythe relative-humidity vertical profile, which may trigger efficientaqueous nitrate formation when exceeding the ammoniumnitrate deliquescence point. Secondary PM2:5 producedin the upper half of the PBL may contribute up to 7–8 μgm􀀀3(or 25 %) to ground-level concentrations on an hourly basis.The residual aerosol layer above the PBL is also found to potentiallyplay a large role, which may occasionally contributeup to 10–12 μgm􀀀3 (or 40 %) to hourly ground-level PM2:5concentrations during the morning hours. Although the resultspresented here refer to one relatively short period in onelocation, this study highlights the importance of consideringthe interplay between chemical and dynamical processes occurringwithin and above the PBL when interpreting groundlevelaerosol observations.