Atmospheric particulate matter smaller than 2.5 µm in diameter (PM_2.5) has a negative impact on public health, the environment, and Earth's climate. Consequently, a need exists for accurate, distributed measurements of surface-level PM_2.5 concentrations at a global scale. Existing PM_2.5 measurement infrastructure provides broad PM_2.5 sampling coverage but does not adequately characterize community-level air pollution at high temporal resolution. This motivates the development of low-cost sensors which can be more practically deployed in spatial and temporal configurations currently lacking proper characterization. Wendt et al. (2019) described the development and validation of a first-generation device for low-cost measurement of AOD and PM_2.5: the Aerosol Mass and Optical Depth (AMODv1) sampler. Ford et al. (2019) describe a citizen-science field deployment of the AMODv1 device. In this paper, we present an updated version of the AMOD, known as AMODv2, featuring design improvements and extended validation to address the limitations of the AMODv1 work. The AMODv2 measures AOD and PM_2.5 at 20 min time intervals. The sampler includes a motorized Sun tracking system alongside a set of four optically filtered photodiodes for semicontinuous, multiwavelength (current version at 440, 500, 675, and 870 nm) AOD sampling. Also included are a Plantower PMS5003 sensor for time-resolved optical PM_2.5 measurements and a pump/cyclone system for time-integrated gravimetric filter measurements of particle mass and composition. AMODv2 samples are configured using a smartphone application, and sample data are made available via data streaming to a companion website (https://csu-ceams.com/, last access: 16 July 2021). We present the results of a 9 d AOD validation campaign where AMODv2 units were co-located with an AERONET (Aerosol Robotics Network) instrument as the reference method at AOD levels ranging from 0.02 ± 0.01 to 1.59 ± 0.01. We observed close agreement between AMODv2s and the reference instrument with mean absolute errors of 0.04, 0.06, 0.03, and 0.03 AOD units at 440, 500, 675, and 870 nm, respectively. We derived empirical relationships relating the reference AOD level to AMODv2 instrument error and found that the mean absolute error in the AMODv2 deviated by less than 0.01 AOD units between clear days and elevated-AOD days and across all wavelengths. We identified bias from individual units, particularly due to calibration drift, as the primary source of error between AMODv2s and reference units. In a test of 15-month calibration stability performed on 16 AMOD units, we observed median changes to calibration constant values of −7.14 %, −9.64 %, −0.75 %, and −2.80 % at 440, 500, 675, and 870 nm, respectively. We propose annual recalibration to mitigate potential errors from calibration drift. We conducted a trial deployment to assess the reliability and mechanical robustness of AMODv2 units. We found that 75 % of attempted samples were successfully completed in rooftop laboratory testing. We identify several failure modes in the laboratory testing and describe design changes that we have since implemented to reduce failures. We demonstrate that the AMODv2 is an accurate, stable, and low-cost platform for air pollution measurement. We describe how the AMODv2 can be implemented in spatial citizen-science networks where reference-grade sensors are economically impractical and low-cost sensors lack accuracy and stability.

中文翻译：

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用于同时测量细颗粒物和气溶胶光学深度的低成本监测器——第 3 部分：自动化和设计改进

直径小于 2.5 µm 的大气颗粒物 (PM _2.5 ) 会对公众健康、环境和地球气候产生负面影响。因此，需要在全球范围内对地表 PM _2.5浓度进行准确、分布式的测量。现有的 PM _2.5测量基础设施提供广泛的 PM _2.5 采样覆盖范围，但不能以高时间分辨率充分表征社区层面的空气污染。这推动了低成本传感器的开发，这些传感器可以更实际地部署在目前缺乏适当表征的空间和时间配置中。温特等人。(2019) 描述了用于低成本测量 AOD 和 PM _2.5的第一代设备的开发和验证：气溶胶质量和光学深度 (AMODv1) 采样器。福特等人。(2019) 描述了 AMODv1 设备的公民科学领域部署。在本文中，我们展示了 AMOD 的更新版本，称为 AMODv2，具有设计改进和扩展验证功能，以解决 AMODv1 工作的局限性。AMODv2 测量 AOD 和 PM _2.5以 20 分钟的时间间隔。采样器包括一个电动太阳跟踪系统以及一组四个光学滤波光电二极管，用于半连续、多波长（当前版本为 440、500、675 和 870 nm）AOD 采样。还包括用于时间分辨光学 PM _2.5测量的 Plantower PMS5003 传感器和用于粒子质量和成分的时间积分重力过滤器测量的泵/旋风系统。AMODv2 样本使用智能手机应用程序进行配置，样本数据通过数据流传输到配套网站 ( https://csu-ceams.com/，最后访问时间：2021 年 7 月 16 日）。我们展示了 9 天 AOD 验证活动的结果，其中 AMODv2 单元与 AERONET（气溶胶机器人网络）仪器共同定位，作为 AOD 水平范围从 0.02  ±  0.01 到 1.59  ± 0.01。我们观察到 AMODv2 与参考仪器之间的一致性非常好，在 440、500、675 和 870 nm 处的平均绝对误差分别为 0.04、0.06、0.03 和 0.03 AOD 单位。我们得出了参考 AOD 水平与 AMODv2 仪器误差的经验关系，发现 AMODv2 的平均绝对误差在晴天和 AOD 升高天之间以及所有波长之间偏差小于 0.01 AOD 单位。我们发现来自单个单元的偏差，特别是由于校准漂移，是 AMODv2 和参考单元之间误差的主要来源。在对 16 个 AMOD 装置进行的 15 个月校准稳定性测试中，我们观察到的校准常数值的中值变化为− 7.14 %、− 9.64 %、− 0.75 % 和−在 440、500、675 和 870 nm 处分别为 2.80 %。我们建议每年重新校准以减轻校准漂移带来的潜在错误。我们进行了试验部署，以评估 AMODv2 装置的可靠性和机械稳健性。我们发现 75% 的尝试样品在屋顶实验室测试中成功完成。我们在实验室测试中确定了几种故障模式，并描述了我们为减少故障而实施的设计更改。我们证明 AMODv2 是一个准确、稳定且低成本的空气污染测量平台。我们描述了如何在空间公民科学网络中实施 AMODv2，其中参考级传感器在经济上不切实际，低成本传感器缺乏准确性和稳定性。