Aerosol scattering coefficient measured over coastal Namibia from 2015 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 aerosol scattering coefficient between 7° and 170° and at 450, 550, and 700 nm was measured by a three-wavelength integrating nephelometer (TSI Inc., model 3563, 5-min resolution) sampling at 13.67 L min-1 from the certified PM10 inlet of the TEOM (sampling from the auxiliary line). The instrument also measured the particle backscattering coefficient between 90° and 170° thanks to a rotating shutter which was found to be malfunctioning and was shut down in March 2018. While the airflow temperature and relative humidity (RH) were ambient, the RH inside the sensing volume was always reduced to less than 60% due to heat from illumination, so that the measured scattering coefficients can be considered as referring to dry air conditions. The particle scattering and backscattering coefficients (units of Mm-1 = 10-6 m-1) reported in the level-2 were not corrected for angular truncation (Anderson and Ogren, 1998). The particle scattering Ångström exponent (SAE) calculated between 450 and 700 nm is also reported in the dataset and can be used to evaluate the dominant aerosol type, and so, indirectly as it provides an indication of particle size, the magnitude of the correction. The air pressure, relative humidity and temperature in the measurement volume are provided, as well as the air temperature measured at the inlet. These measurements allow calculation of standard temperature and pressure conditions for data normalisation. The nephelometer was calibrated using zero-air and CO2 as reference gases prior to being sent in the field, but not in the field due to the difficulties in bringing and conserving the gas bottles on top of the building. As a quality-control check, regular zeroing was performed to monitor the extent by which the measurement cavity could be affected by particle deposition and accumulation of dirt. Both the stability and the noise of the zero signal were monitored for this purpose. Data records for which the scattering coefficient at 550 nm was lower than 5 Mm-1 or higher than 200 Mm-1 for short durations (less than two hours) were discarded from further data analysis.