A low-cost monitor for simultaneous measurement of fine particulate matter and aerosol optical depth – Part 3: Automation and design improvements

• Main Authors: E. A. Wendt, C. Quinn, C. L'Orange, D. D. Miller-Lionberg, B. Ford, J. R. Pierce, J. Mehaffy, M. Cheeseman, S. H. Jathar, D. H. Hagan, Z. Rosen, M. Long, J. Volckens
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-09-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://amt.copernicus.org/articles/14/6023/2021/amt-14-6023-2021.pdf
• ISSN: 1867-1381; 1867-8548

<p>Atmospheric particulate matter smaller than 2.5 <span class="inline-formula">µm</span> in diameter (PM<span class="inline-formula">_2.5</span>) 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<span class="inline-formula">_2.5</span> concentrations at a global scale. Existing PM<span class="inline-formula">_2.5</span> measurement infrastructure provides broad PM<span class="inline-formula">_2.5</span> 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<span class="inline-formula">_2.5</span>: 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<span class="inline-formula">_2.5</span> 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<span class="inline-formula">_2.5</span> 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 (<span class="uri">https://csu-ceams.com/</span>, 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 <span class="inline-formula">±</span> 0.01 to 1.59 <span class="inline-formula">±</span> 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<span id="page6024"/> observed median changes to calibration constant values of <span class="inline-formula">−</span>7.14 %, <span class="inline-formula">−</span>9.64 %, <span class="inline-formula">−</span>0.75 %, and <span class="inline-formula">−</span>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.</p>