AEROCLO-sA PEGASUS aerosol particle water-soluble composition in the TSP fraction

The AEROCLO-sA field campaign took place from 21 August to 13 September 2017 at the Sam Nujoma Marine and Coastal Resources Research Centre (SANUMARC) of the University of Namibia at Henties Bay (Formenti et al., 2019). This sampling site, operated on the long-term as described by Formenti et al. (2018) and Klopper et al. (2020).
During the campaign, aerosol samples in the total suspended particulate (TSP). were collected both during day (approximately 07:00-17:00 UTC) and night time (approximately 17:30-06:30 UTC). The sampling duration was marginally adapted in real-time to the nature and the aerosol load of air masses using the readings of the local wind speed and direction and of the aerosol mass concentration, also measured online. Air sampling was performed from the PortablE Gas and Aerosol Sampling UnitS (PEGASUS; https://pegasus.aeris-data.fr/) facility with two high-volume aerosol inlets delivering approximately 450 L min-1 each. At wind speeds between 5 to 10 m s 1, typical for coastal Namibia, sampling is almost isokinetic for particles up to 40 µm in aerodynamic diameter (Rajot et al., 2008). The total sampled flow rate was distributed to various instruments, including a custom-made filter holder loaded with a polycarbonate membrane (Nuclepore®, 0.4-µm pore size diameter, 47-mm filter diameter) operated at an average sampling flow rate of the order of 20 L min-1. Prior the campaign, filters were acid-washed according to the protocol described in Desboeufs et al. (2024).
In total, 36 TSP filter samples (including blanks) were collected between 21 August and 12 September 2017.
The analysis of the water-soluble fraction was performed by extracting the filters with 20 mL of ultrapure water (MilliQ® 18.2 MΩ.cm) for 30 minutes. The solution was divided into two sub-samples filtered to 0.2 µm of porosity (Nuclepore). One half was analyzed by Ion chromatography (IC) using a Metrohm IC 850 device equipped with a column MetrosepA supp 7 (250/4.0 mm) for anions and with a Metrosep C4 (250/4.0 mm) for cations. The IC analysis provided the concentrations of the following water-soluble ions: F−, formate, acetate, MSA- (methanesulphonic acid), Cl−, NO3−, SO42−, oxalate, Na+, NH4+, K+, Ca2+ and Mg2+. A calibration with certified standard multi-ions solutions of concentrations ranging from 5 to 5000 ppb was performed and, the uncertainty of the analysis was estimated to be 5% (Klopper et al., 2020).
The second half of the solution was acidified to 1% with ultrapure nitric acid (HNO3) and analysed by a combination of inductively coupled plasma-atomic emission spectroscopy (ICP AES) using 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™ as described in Desboeufs et al. (2024). The calibration curve was performed using standard multi-element solutions ranging from 1 to 1000 ppt. The elemental fractional solubility (FS) for element is calculated as the ratio between the dissolved and the total concentration.