Monitoring particulate matter with commodity hardware

Interested readers can now download the full text of my just-released Ph.D. dissertation, titled Monitoring particulate matter with commodity hardware [15 MB PDF]. Chapters 2 and 3 have previously been published as journal articles. New results obtained with the Shinyei PPD42NS sensor are presented in Chapter 4, “Observing urban plumes”.

Abstract (edited slightly from the original)

Health effects attributed to outdoor fine particulate matter (PM2.5) rank it among the risk factors with the highest health burdens in the world, annually accounting for over 3.2 million premature deaths and over 76 million lost disability-adjusted life years. Existing PM2.5 monitoring infrastructure cannot, however, be used to resolve variations in ambient PM2.5 concentrations with adequate spatial and temporal density, or with adequate coverage of human time-activity patterns, such that the needs of modern exposure science and control can be met. Small, inexpensive, and portable devices, relying on newly available off-the-shelf sensors, may facilitate the creation of PM2.5 datasets with improved resolution and coverage, especially if many such devices can be deployed concurrently with low system cost. Datasets generated with such technology could be used to overcome many important problems associated with exposure misclassification in air pollution epidemiology.

Chapter 2 presents an epidemiological study of particulate matter’s effects on human health. Contrasts in time-series data from ambient monitoring stations in the Los Angeles basin were used to observe a decrease of 6.1 g (95% CI: 3.5, 8.7) in population mean birthweight following in utero exposure to the Southern California wildfires of 2003. Further elaborations were limited by the sparsity of the empirical basis for exposure assessment.

Chapter 3 demonstrates technical potential for remedying PM2.5 monitoring deficiencies, beginning with the generation of low-cost yet useful estimates of hourly and daily PM2.5 concentrations at a regulatory monitoring site. The context (an urban neighborhood proximate to a major goods-movement corridor) and the method (an off-the-shelf sensor costing approximately USD $10, combined with other low-cost, open-source, readily available hardware) were selected to have special significance among researchers and practitioners affiliated with contemporary communities of practice in public health and citizen science. As operationalized by correlation with 1 h data from a Federal Equivalent Method (FEM) β-attenuation monitor, prototype instruments performed as well as commercially available equipment costing considerably more, and as well as another reference instrument under similar conditions at the same timescale (R2 ≈ 0.6). Correlations were stronger when 24 h integrating times were used instead (R2 = 0.72).

Chapter 4 replicates and extends the results of Chapter 3, showing that similar calibrations may be reasonably exchangeable between near-roadway and background monitoring sites. Chapter 4 also employs triplicate sensors to obtain data consistent with near-field (< 50 m) observations of plumes from a major highway (I-880). At 1 minute timescales, maximum PM2.5 concentrations on the order of 100 μg m−3 to 200 μg m−3 were observed, commensurate with the magnitude of plumes from wildfires on longer timescales, as well as the magnitude of plumes that might be expected near other major highways on the same timescale. Finally, Chapter 4 quantifies variance among calibration parameters for a large sample of the sensors, as well as the error associated with the remote transfer of calibrations between two sufficiently large sets (± 10 % for n = 12). In so doing, Chapter 4 demonstrates how a calibration against continuous reference monitoring equipment can be remotely transferred, within practical tolerances, with minimal risk of disruption to existing monitoring infrastructure (i.e., established monitoring sites). Given a collaborator with a short window of access to a reference monitoring site, this would overcome a nominally important barrier associated with non-gravimetric, in-situ calibration of continuous PM2.5 monitors.

These findings suggest that datasets generated with similar sensors could improve upstream scientific understandings of fluxes resulting from indoor and outdoor emissions, atmospheric transformations, and transport. They may also facilitate timely and empirical verification of interventions to reduce emissions and exposures in many important contexts (e.g., the provision of improved cookstoves; congestion pricing; mitigation policies attached to infill development; etc.). Progressive and disruptive prospects linked to a proliferation of comparable sensing technologies based on commodity hardware are discussed in Chapter 5.

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