Aerosol source apportionment in two contrasting Italian sites: a comparison between physical and chemical PMF in Aosta and Lecce

Positive Matrix Factorization (PMF) is a powerful method for the apportionment of aerosol sources. Traditionally applied to chemical datasets, it has more recently been extended to physical datasets, focusing predominantly on size distributions of ultra-fine particles (Hopke et al., 2022). In this study, aerosol physical properties (particle size distributions, from ultrafine to coarse mode, and spectral aerosol absorption) were used as input to PMF (EPA 5.0) for identifying emission sources of two distinct sites, and results compared with those from chemical PMF. 

Located at the two extremes of the Italian territory, the sites of Aosta and Lecce represent two markedly different environments. The first lies in the mountainous area of the northwestern Alps, the second in a flat area at the southeastern edge of the peninsula, in a typical Mediterranean context. In previous studies, PMF of aerosol chemical composition was performed using chemical speciation of PM over the period 2019-2021 (Aosta, e.g. Diemoz et al., 2019) and 2017 (Lecce, Giannossa et al., 2022).  

The PMF input dataset at the urban-background site in Aosta included aerosol size distributions measured by an Optical Particle Counter (OPC) across the 0.18–18 µm range and wavelength-dependent aerosol absorption from an AE33-aethalometer. At the urban-background observatory in Lecce, an aerosol size distribution range (0.02–10 µm) was available using both a Scanning Mobility Particle Sizer (SMPS) and an OPC, complemented by single-wavelength aerosol absorption (Multi-Angle Absorption Photometer, MAAP) providing eBC concentrations. Ancillary data included meteorological parameters and trace-gases concentration. Although with differences in the physical properties used, at both sites the physical PMF allowed the identification of 6 aerosol sources. In Aosta, the aerosol sources included fossil fuel combustion, biomass burning, secondary droplet mode, secondary condensation, dust, and coarse particles. In Lecce, the sources were nucleation (thanks to the additional use of the SMPS), traffic emissions, secondary nitrate, secondary sulfate, regional transport (sea spray and dust), and local resuspension (coarse particles). 

The work will present a comparison of the two PMF approaches (Chemistry- and Physics-based PMF) and advantages and limits of the different physical input datasets used at the two sites. For example, at the Lecce site, the information on ultrafine and fine particle distribution well captured features of the nucleation and traffic factors, while the absence of wavelength-dependent absorption coefficients limited the ability to distinguish biomass burning from fossil fuel sources, which was key for Aosta. Overall, the use of physical aerosol data as input to PMF proved to be an effective method for source apportionment and could usefully complement the chemical-PMF analysis. In fact, this approach offers significant advantages, such as the capability for quasi real-time monitoring and the relative ease of instruments use and data analysis compared to the ‘traditional’ chemical analysis of PM. 

The authors acknowledge the MUR for funding the research through the CIR01_00015-PER-ACTRIS-IT. 

 

Diémoz, H.et al., https://doi.org/10.5194/acp-19-10129-2019, 2019.  

Giannossa, L. et al., http://doi.10.1016/j.jenvman.2022.115752, 2022. 

Hopke, P. K. et al., https://doi.org/10.1016/j.scitotenv.2022.153104, 2022.