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.

本研究在亨蒂斯湾气溶胶观测站（Henties Bay Aerosol Observatory, HBAO；http://www.hbao.cnrs.fr/）开展气溶胶颗粒物地表观测，该站点是世界气象组织（World Meteorological Organization, WMO）全球大气监测计划（Global Atmosphere Watch, GAW）旗下新建的区域观测站。观测研究中心位于纳米比亚亨蒂斯湾的纳米比亚大学萨姆·努乔马海洋与海岸资源研究中心（Sam Nujoma Marine and Coastal Resources Research Centre, SANUMARC），地理坐标为南纬22°、东经14°05′（图1）。亨蒂斯湾是干旱环境中的小型城镇，无植被覆盖、无工业活动，交通流量极低。当地能源以电力与天然气混合为主，由于固体燃料（木材）可获得性较差，仅少量使用木材燃烧。监测站点设于大学校园内，地处海岸区域，距海岸线约100米。站点东侧为纳米比亚砾石平原，校园以南3公里处为亨蒂斯湾小镇，北侧为奥马鲁鲁河床（河口距SANUMARC约100米）。根据《纳米比亚人口与住房普查总报告》，亨蒂斯湾的人口规模介于4600至6000之间，报告全文可通过http://cms.my.na/assets/documents/p19dmn58guram30ttun89rdrp1.pdf获取。
研究采用三波长积分浊度计（TSI公司，型号3563，5分钟分辨率）测量7°至170°角域内、450 nm、550 nm及700 nm波长下的气溶胶散射系数，采样流量为13.67 L·min⁻¹，采样取自TEOM的认证可吸入颗粒物（PM10）进气口（经辅助管线采样）。该仪器可通过旋转快门测量90°至170°角域内的粒子后向散射系数，但该旋转快门被发现存在故障，已于2018年3月停用。尽管采样气流的温度与相对湿度（relative humidity, RH）为环境本底值，但由于照明产热，传感腔体内的相对湿度始终被控制在60%以下，因此测得的散射系数可视为对应干空气条件。二级数据中报告的粒子散射与后向散射系数（单位为Mm⁻¹=10⁻⁶ m⁻¹）未进行角度截断校正（Anderson与Ogren，1998）。数据集同时报告了450 nm至700 nm波长区间内计算得到的粒子散射安斯特朗指数（Scattering Ångström Exponent, SAE），该指数可用于评估主导气溶胶类型，间接反映粒子粒径特征，进而为校正幅度提供参考依据。数据集还提供了测量腔体内的气压、相对湿度与温度，以及进气口处的空气温度，上述数据可用于计算标准温压条件以开展数据归一化处理。
该积分浊度计在送往野外前采用零气与二氧化碳（CO₂）作为参比气体完成校准，但由于在楼顶搬运和保存气瓶存在实际困难，未在野外现场开展校准。作为质量控制措施，研究团队定期进行零点校准，以监测测量腔体受颗粒物沉积与污垢积累的影响程度，同时同步监测零点信号的稳定性与噪声水平。对于550 nm波长下散射系数低于5 Mm⁻¹或高于200 Mm⁻¹且持续时间短于2小时的数据记录，将其剔除以用于后续数据分析。