[论文解读] An analysis of degradation in low-cost particulate matter sensors

Priyanka deSouza, Karoline K. Barkjohn+5 more|arXiv (Cornell University)|Oct 26, 2022

Air Quality Monitoring and Forecasting被引用 5

一句话总结

本研究利用双传感器冗余机制与监管监测仪对比，分析低成本PurpleAir颗粒物（PM2.5）传感器随时间的性能退化情况。研究发现，4年后传感器不一致（‘标记’测量值）的比例上升至4%，偏差每年恶化-0.12 μg/m³，且在高温高湿气候区中永久性退化最为普遍，提示此类地区应缩短传感器更换周期。

ABSTRACT

Low-cost sensors (LCS) are increasingly being used to measure fine particulate matter (PM2.5) concentrations in cities around the world. One of the most commonly deployed LCS is the PurpleAir with about 15,000 sensors deployed in the United States. However, the change in sensor performance over time has not been well studied. It is important to understand the lifespan of these sensors to determine when they should be replaced, and when measurements from these devices should or should not be used for various applications. This paper fills in this gap by leveraging the fact that: 1) Each PurpleAir sensor is comprised of two identical sensors and the divergence between their measurements can be observed, and 2) There are numerous PurpleAir sensors within 50 meters of regulatory monitors allowing for the comparison of measurements between these two instruments. We propose empirically-derived degradation outcomes for the PurpleAir sensors and evaluate how these outcomes change over time. On average, we find that the number of 'flagged' measurements, where the two sensors within each PurpleAir disagree, increases in time to 4 percent after 4 years of operation. Approximately, 2 percent of all PurpleAir sensors were permanently degraded. The largest fraction of permanently degraded PurpleAir sensors appeared to be in the hot and humid climate zone, suggesting that the sensors in this zone may need to be replaced sooner. We also find that the bias of PurpleAir sensors, or the difference between corrected PM2.5 levels and the corresponding reference measurements, changed over time by -0.12 ug/m3 (95% CI: -0.13 ug/m3, -0.11 ug/m3) per year. The average bias increases dramatically after 3.5 years. Climate zone is a significant modifier of the association between degradation outcomes and time.

研究动机与目标

• 评估低成本颗粒物（PM2.5）传感器（尤其是PurpleAir设备）的长期性能退化情况。
• 确定传感器在真实部署环境中随时间推移的可靠性与准确性变化情况。
• 识别影响传感器退化的环境与时间因素，特别是不同气候区的影响。
• 为空气质量监测应用中的传感器更换与数据质量控制提供实证指导。

提出的方法

• 利用每个PurpleAir设备内两个相同传感器的差异检测机制，将分歧程度作为退化的代理指标。
• 将PurpleAir传感器读数与附近监管级PM2.5监测仪进行对比，评估偏差与漂移随时间的变化。
• 追踪4年期间‘标记’测量值（即两内建传感器不一致）的时间变化。
• 使用线性混合效应模型估算偏差的年变化率，同时调整气候区与传感器年龄的影响。
• 基于持续分歧或偏差超过阈值的情况，将传感器分类为永久性退化。
• 应用空间过滤方法，筛选出距离监管监测仪50米以内的PurpleAir传感器，以确保对比的可靠性。

实验结果

研究问题

• RQ1 PurpleAir传感器中，传感器不一致（标记测量值）的发生率如何随时间变化？
• RQ2 PurpleAir PM2.5测量值与监管监测仪数据之间的偏差如何随年份演变？
• RQ3 气候区（尤其是高温高湿条件）对传感器退化与故障率有何影响？
• RQ4 传感器性能退化到何种程度会显著影响数据可靠性？
• RQ5 在4年运行周期内，有多少比例的PurpleAir传感器会变为永久性退化？

主要发现

• 表示双传感器不一致的‘标记’测量值比例，在运行4年后上升至4%。
• PurpleAir传感器的平均偏差每年恶化-0.12 μg/m³（95%置信区间：-0.13至-0.11 μg/m³），且在3.5年之后出现显著加剧。
• 研究期间约2%的PurpleAir传感器被归类为永久性退化。
• 永久性退化发生率最高的区域为高温高湿气候区，表明环境条件会加速传感器失效。
• 气候区显著调节了时间与退化结果之间的关系，热带与亚热带地区观察到更高的退化速率。
• 本研究提供了实证证据，表明传感器性能随时间系统性退化，因此需要定期更换与重新校准。