Atmospheric concentrations of total and dissolved elements and water-soluble ions measured over coastal Namibia in 2017

Aerosol particles smaller than 10 µm in aerodynamic diameter (PM10) were collected at the Henties Bay Aerosol Observatory (HBAO) in Namibia by an automated sampler (model Partisol Plus 2025i, Thermo Fisher Scientific, Waltham, MA USA) on 47 mm Whatman Nuclepore polycarbonate filters (1-µm pore size). The air was drawn through a certified sampling inlet (Rupprecht and Patashnick, Albany, New York, USA) located at approximately 30 m above ground and operated at a flow rate of 1 m3 h-1. Samples were collected for 9 hours during the daytime (from 9:00 to 18:00 UTC time) and night-time (21:00 to 06:00 UTC time) for 13 non-consecutive weeks from April to December 2017 (7-14 April, 26 April-3 May, 19-26 May, 07-14 July, 2-9 1ugust, 15-22 August, 18-25 September, 02-09 October, 31 October-7 November, 13-20 November, 28 November-04 December, 12-19 December). In total, 176 samples (including 13 blanks, one per week of sampling) were collected. Analysis of those samples are provided in three datasets. The dataset “Namibia_HBAO_elemental_concentrations_2017.txt“ reports the atmospheric concentrations of 24 elements (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Sr, Pb, Nd, Cd, Ba). Concentrations were obtained at LISA by wavelength-dispersive X-ray fluorescence (WD-XRF) using a PW-2404 spectrometer (Panalytical, Almelo, Netherlands), according to the protocol previously described by Denjean et al. (2016). The relative analytical uncertainty on the measured atmospheric concentrations (expressed in ng m-3) is evaluated as 10%. This represents the upper limit uncertainty, taking into account: • The uncertainty related to the uniformity of the aerosol deposit on the filters, and the scaling error that can occur due to the fact that the area of the deposit which is analysed is smaller than the area of the aerosol deposit; • The statistical error on the photon counts, in particular for trace elements whose concentrations are close to their detection limits; • The percent error on the certified mono- and bi-elemental standard concentrations (Micromatter Inc., Surrey, Canada) used for calibration of the XRF apparatus; • For the lightest elements (Z < 20, Na to Ca), the choice of the correction factor to account for the self-attenuation of the X-ray signal, in particular for particles larger than 1 µm in diameter (Formenti et al., 2010). Constant correction factors were estimated through the sampling period assuming a mean diameter of 4.5 µm to represent the average coarse particle size. The dataset “Namibia_HBAO_water-soluble-ionic_concentrations_2017.txt“ reports the atmospheric concentrations of 16 water-soluble ions (F-, propionate, formate, acetate, methanesulfonic acid (MSA), Cl-, Br-, NO3-, PO43-, SO42-, oxalate, Na+, NH4+, K+, Ca2+ and Mg2+). The water-soluble fraction was obtained by placing filters in 20 mL of ultrapure water (MilliQ® 18.2 MΩ.cm) for 30 minutes. The solution was then filtered (Nuclepore polycarbonate filters with 0.2μm pore size) then divided into two sub-samples. First sub-samples were analysed at LISA by ion chromatography (IC) with a Metrohm IC 850 device (injection loop of 100 µl). For anionic species, the IC was equipped with MetrosepA supp 7 (250/4.0mm) column associated with a MetrosepA supp 7 guard pre-column heated at 45°C. For simultaneous separation of inorganic and short-chain organic anions, elution has been realised with the following elution gradient (eluent weak:Na2CO3/NaHCO3 (0.28/0.1mM) and eluent strong: Na2CO3/NaHCO3 (28/10mM): 100% eluent weak from 0 to 23.5 minutes; then 15% eluant strong from 23.5 to 52 minutes and 100% eluent weak to finish. The elution flow rate was 0.8 ml min-1. For cationic species, IC has been equipped with a Metrosep C4 (250/4.0mm) column associated to a Metrosep C4 guard column heated at 30°C. Elution has been realised with an eluant composed with 0.7 mM of dipicolinic acid and 1.7mM of nitric acid. The elution flow rate was 0.9 ml min-1. The uncertainty of water-soluble ionic concentrations (also expressed in ng m-3) is within 5%, the maximum uncertainty obtained during calibration by standard certified mono- and multi-ionic solutions. The dataset “Namibia_HBAO_dissolved-elemental_concentrations_2017.txt“ reports the atmospheric concentrations of dissolved mass concentrations of 25 water-soluble metals and metalloids, including Fe, Al, and Si. These were measured by acidifying the second sub-sample of the water-soluble fraction 1% with ultrapure nitric acid (HNO3) and analysed by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) using a Spectro ARCOS Ametek® ICP-AES and by High-resolution Inductively Coupled Plasma-Mass Spectrometry (HR-ICP-MS) using a Neptune Plus™ instrument by Thermo Scientific™. The calibration curve was performed using standard multi-element solutions ranging from 2 to 1000 ppb for ICP-AES and 1 to 1000 ppt for HR-ICP-MS (Desboeufs et al., 2022). For each chemical species, the detection limit (DL) was calculated as 3 times the square root of the standard deviation of the concentration of laboratory blank samples, corresponding to filter membranes prepared as actual samples but stored and analyzed without exposure to external air. All concentrations above the DL are corrected of blank interferences. Only values above DL are reported in the datasets. For elemental concentrations, DL was below 20 ng cm-2 with the exception of Sr, As, and Pb, for which the DT was 100, 140 and 80 20 ng cm-2, respectively. For water soluble concentrations, the DL was of the order of 10 µg L-1 for the major ions, and 5 µg L-1 for MSA. For ICP analysis, the DL ranged from 25 ppt (Mn, Ni, Ti, Zn) to 800 ppt for Si, and 100 ppt for Fe and 50ppt for Al, Cd, Cr, Co, Cu and V.