Effects of COVID-19 lockdown on PM10 composition and sources in the Rome Area (Italy) by elements' chemical fractionation-based source apportionment

During the national lockdown imposed by the Government of Italy (from March 9th to May 18th 2020) to counter the Covid-19 pandemic, 24-h PM10 samples were collected at three sites in the Rome area (Central Italy), two urban (Sapienza and Via Saredo, highly impacted by vehicular traffic) and one peri-urban (Montelibretti, more impacted by biomass domestic heating). Further, at Sapienza and Montelibretti PM10 daily sampling had been carried out in the period immediately before lockdown, and at Via Saredo samples were additionally collected also after the end of lockdown. PM10 was chemically speciated for main components (major elements, inorganic ions, EC, OC, levoglucosan), and trace elements. The latter were chemically fractionated and considered for their water-soluble and insoluble fractions, which proved to be more source-selective than total element. Three datasets were thus built and analyzed by Positive Matrix Factorization (PMF), with the aim of identifying and apportioning mass contributions of sources acting in the periods before, during and after lockdown, in the Rome area. Identified emission sources were mostly from long-range advection (two different contributions of mineral dust, fresh sea spray, heavy oil combustion), while local sources (vehicular traffic and biomass burning) were strongly abated during lockdown, with respect to previous sampling period, and inorganic secondary aerosol showed a progressive increment of sulfates, driven by seasonal evolution from winter to spring. Since the lockdown interrupted all non-essential productive and work activities, thus reducing the chemical fingerprinting of local sources, this occurrence allowed to clearly describe both profiles and source contribution estimates of long-range transported PM10 components. Moreover, it allowed assessing the reduction of the impact of anthropogenic sources (such as vehicular traffic) and the efficiency of mitigation measures generally taken to control PM10 mass concentration. Acidic sulfates (bisulfate and letovicite) resulted associated to mineral dust transport events, and the role of chemically fractionated elements as source-specific tracers was further confirmed.