AEROCLO-sA PEGASUS aerosol particle water-soluble composition in the PM1 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 of particles smaller than 1 µm in diameter (hereafter named PM1 size fraction) 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 4-stage Dekati® PM10 impactor operated at 10 L min-1. This sampler was operated with 25-mm polycarbonate membranes on three impactor stages (> 10 µm, 10-2.5 µm and 2.5-1 μm) while the final filter stage, where the PM1 fraction is collected, was a polycarbonate membrane (Nuclepore®, 0.4-µm pore size, 47-mm filter diameter). Prior the campaign, filters were acid-washed according to the protocol described in Desboeufs et al. (2024).
In total, 31 PM1 samples (including blanks) were collected from 26 August to 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 filtered to 0.2 µm of porosity (Nuclepore) and 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).