Design and Testing of a Low-Cost Sensor and Sampling Platform for Indoor Air Quality

Abstract

Americans spend most of their time indoors at home, but comprehensive characterization of in-home air pollution is limited by the cost and size of reference-quality monitors. We assembled small "Home Health Boxes" (HHBs) to measure indoor PM2.5, PM10, CO2, CO, NO2, and O3 concentrations using filter samplers and low-cost sensors. Nine HHBs were collocated with reference monitors in the kitchen of an occupied home in Fort Collins, Colorado, USA for 168 h while wildfire smoke impacted local air quality. When HHB data were interpreted using gas sensor manufacturers' calibrations, HHBs and reference monitors (a) categorized the level of each gaseous pollutant similarly (as either low, elevated, or high relative to air quality standards) and (b) both indicated that gas cooking burners were the dominant source of CO and NO2 pollution; however, HHB and reference O3 data were not correlated. When HHB gas sensor data were interpreted using linear mixed calibration models derived via collocation with reference monitors, root-mean-square error decreased for CO2 (from 408 to 58 ppm), CO (645 to 572 ppb), NO2 (22 to 14 ppb), and O3 (21 to 7 ppb); additionally, correlation between HHB and reference O3 data improved (Pearson's r increased from 0.02 to 0.75). Mean 168-h PM2.5 and PM10 concentrations derived from nine filter samples were 19.4 μg m-3 (6.1% relative standard deviation [RSD]) and 40.1 μg m-3 (7.6% RSD). The 168-h PM2.5 concentration was overestimated by PMS5003 sensors (median sensor/filter ratio = 1.7) and underestimated slightly by SPS30 sensors (median sensor/filter ratio = 0.91).

Keywords: NO2; electrochemical gas sensors; household air pollution; indoor air quality; particulate matter.

Conflict of interest statement

CONFLICT OF INTEREST John Volckens is a scientific founder of Access Sensor Technologies, LLC and has an equity interest in the company. The terms of this arrangement have been reviewed and approved by Colorado State University in accordance with its conflict-of-interest policies. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Figure 1.

Left: The Home Health Box enclosure had exterior dimensions of 220 × 170 × 130 mm (excluding the feet, sample inlets, exhaust vents, hinge, and latch) and weighed approximately 2 kg. Right: A top view of the Home Health Box with the cover removed to show internal components.

Figure 2.

Placement of the Home Health Boxes, LI-820 CO2 Gas Analyzer, QTrak Indoor Air Quality Monitors (used to measure CO), and TEOM 1405 (used to measure PM2.5) relative to the natural gas cooking burners along the south interior kitchen wall. The >1-m gap between the top of the wall and the ceiling on the east side of the kitchen is visible around the TEOM. The 42C NO-NO2-NOx Analyzer and 49C O3 Analyzer were installed in the same room but are not visible from this perspective. Occupants’ personal effects have been blurred in the photo.

Figure 3.

Concentrations of CO2, CO, NO2, O3, and PM2.5 measured over the duration of the experiment. Solid lines represent 15-minute average indoor concentrations reported by reference monitors. Dashed lines and gray shaded areas represent the median and total range, respectively, of 15-minute average concentrations measured using eight (for CO, NO2, and O3) or nine (for CO2 and PM2.5) Home Health Boxes (HHBs). HHB data include uncorrected CO2 concentrations, CO and NO2 concentrations calculated using Equation 4, O3 concentrations calculated using Equation 3, as well as filter-corrected PMS5003-reported PM2.5 concentrations. Dotted lines represent 1-h average concentrations reported at Colorado Department of Public Health and Environment (CDPHE) monitoring sites located 4.3 km (CO and O3) and 3.7 km (PM2.5) from the home. Horizontal bars along the top of each panel indicate when doors and/or windows were opened, when a central air conditioning unit was on, when an indoor air cleaner was on, when scripted use of natural gas cooking burners (which typically involved boiling water) occurred, and when normal cooking activities occurred. Bars accompanied by a shaded backdrop of the same color represent activities noted in occupants’ original log data. The reference PM2.5 monitor (TEOM) stopped operating reliably approximately 48 h after the experiment began.

Figure 4.

Comparison of 1-h average concentrations calculated from low-cost sensor data in the Home Health Boxes and measured using reference monitors (CO: TSI QTrak 7575-X with model 982 probe; NO2: Thermo Environmental Instruments 42C; O3: Thermo Environmental Instruments 49C). Equations 1-4 were Alphasense algorithms. Equations 7-9 were empirical calibration models. Each HHB is represented by a different color. The dashed line is y = x.

Figure 5.

Left: Concentration-time ratios were obtained from HHB data consisting of uncorrected CO2 concentrations as well as CO, NO2, and O3 concentrations calculated using the best-performing Alphasense algorithms (Equations 4, 4, and 3, respectively). Right: Concentration-time ratios were obtained from HHB data consisting of CO2, CO, NO2, and O3 concentrations calculated using the best-performing empirical calibration models (Equations 10, 9, 8, and 9, respectively). In both panels, the dashed diagonal line is y = x and dotted lines represent concentration-time ratios = 1.