Analysis of elemental concentrations of particle matter samples collected in a hyperarid environment in Namibia (Metadata RADIATE TA Proposal 23003138-ST)

Proposal title "Analysis of elemental concentrations of particle matter samples collected in a hyperarid environment in Namibia". Measurements carried out at the external beam set-up for atmospheric aerosol analysis of the INFN LABEC accelerator laboratory), from 19 till 21 April 2023. Hands-off mode.

A sampling campaign was carried out in April 2022 at the Gobabeb Namib Research Institute (GNRI; https://gobabeb.org/), in the Namib Desert of Namibia (-23.55 S, 15.03 E). Aerosol samples were collected for a three-week period on polytetrafluoroethylene filters (PTFE; Whatman; 47 mm in diameter, 2.0 μm pore size) both on the PM1 and PM10 fractions using homemade filter samplers from certified Leckel sampling heads (Sven Leckel, Germany).

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The main objective is to measure the elemental composition of these samples by PIXE to evaluate for the first time (1) the source apportionment and relative contribution of the mineral dust and advected marine aerosols to the total aerosol mass; (2) the total<br>
concentrations of elements possibly soluble in water (sulfur, iron, phosphorous) forming clouds and/or contributing to ocean productivity. Complementary measurements with ion chromatography and ICP are performed at LISA.

40 PM10 and 40 PM1 samples (including blanks) were analysed by PIXE, obtaining the concentrations of all elements with Z&gt;10. In particular all main elements of interest (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Fe, Cu and Zn) were detected. A set of elemental standards were analyzed as well for quality assurance purposes and to check the accuracy of quantitative results.

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A 2.7 MeV proton beam was used. This energy was chosen to minimise the gamma ray emission from F in the collecting membranes, i.e. avoiding strong resonances in the 19F(p,p'γ)19F inelastic reactions (110 e 197 keV gamma-rays), that will increase the continuous background in the PIXE spectra in the energy region &gt; 10 keV, increase pile-up and dead-time effects. This was particularly important for this experiment as the PTFE membranes used in this project are indeed quite thick. To identify the best working energy we<br>
used literature data (Zamboni et al. NIM B 342, 2015, 266), and also measured Fluorine gamma-ray yield over a limited proton energy range of interest.<br>