Aerosol mass concentrations measured over coastal Namibia from 2012 to 2019

Surface observations of aerosol particles are conducted at the Henties Bay Aerosol Observatory (HBAO; http://www.hbao.cnrs.fr/), a recent regional station in the Global Atmosphere Watch (GAW) programme of the World Meteorological Organization (WMO). The research centre is located on the Sam Nujoma Marine and Coastal Resources Research Centre (SANUMARC) of the University of Namibia in Henties Bay (22∘ S, 14∘05′ E), Namibia (Fig. 1). Henties Bay is a small town in an arid environment with no vegetation, no industrial activity and very little traffic. Energy usage is predominantly a mix of electricity and gas, with some solid fuel combustion (wood) due to low availability. The monitoring site, situated on the university campus, is located on the coast approximately 100 m from the shoreline. To the east are the Namibian gravel plains, 3 km to the south of the campus is the town of Henties Bay and to the north is the Omaruru riverbed (river mouth approximately 100 m from SANUMARC). The population of Henties Bay ranges between 4600 and 6000 inhabitants, according to the Namibia Population and Housing Census Main Report (available at http://cms.my.na/assets/documents/p19dmn58guram30ttun89rdrp1.pdf.
The particle mass concentration was measured with a Tapered Element Oscillating Microbalance (TEOM, model 1400a, Rupprecht and Patashnick, Albany, New York, USA). Between 2012 and 2014, the instrument was operated from a certified inlet (also from Rupprecht and Patashnick) to select particles of diameter smaller than 2.5 µm in equivalent aerodynamic diameter (PM2.5). After 2014, and until 2019, the inlet was replaced to sample particles of diameter smaller than 10 µm in equivalent aerodynamic diameter (PM10, Rupprecht and Patashnick, Albany, New York, USA). The total flow rate at the inlet was 16.7 L min-1, and the sampling flow rate driving the aerosol-laden air to the microbalance was 3 L min-1. The temperature of the sample stream was kept constant at 50ºC. The instrument has a nominal detection limit of 5 µg m-3 and an accuracy of 1 µg m-3. Data were acquired and stored as 5-minute averages. During the acquisition, the instrument flags occurrences of invalid data based on the stability of the temperatures and flow rates. Additionally, the observation record was manually inspected to discard spurious and outlier data (peaks of short duration when concentrations exceeded 200 µg m-3).