Pilot study of the effectiveness of low-cost gas-phase sensors for monitoring indoor and outdoor air quality in Beijing, China

Abstract

Ground-level ozone values have been reported to be at unhealthy levels in many populated regions of China. An efficient and effective way for people to combat its harmful effects is seriously needed. However, there are only sparse measurements of ozone being made across China and more measurement sites are needed to understand health relevant concentrations both temporally and spatially. This is due in part to the fact that current monitoring approaches are costly and bulky. Ambient sensors with low cost, small size, and fast response time could potentially fill the current need. Our purpose in this study is to evaluate low cost, portable, and real-time sensors we hope to use in future studies. This study tested the effectiveness of the sensors in monitoring indoor and outdoor air quality, in particular ozone, in Beijing, China. Seventeen sensors, which were to monitor indoor, outdoor, and personal ozone exposure in 7 homes, were collocated at Peking University (PKU) before and after running in selected residential homes. Pairwise comparisons were conducted using collocated sensor data with data from a standing reference sensor which was maintained by PKU to have best-fit regressions. Based on the best-fit regression when compiling both pre- and post- collocation periods, cleaned sensor data was calibrated and compared with reference data with R2 ranging from 0.63 to 0.97. Overall, the sensors are able to measure O3 within ±17 ppb. Average error of sensors after calibration is about -1 ppb. The geographical conditions and surroundings caused significant differences to the ambient ozone concentrations from sensors located across Beijing. Second, the sensors successfully showed that indoor ozone concentrations were likely to be lower when air filtration was on compared to the concentrations when filters were off. Moreover, the results confirmed higher levels of ozone outdoor concentrations compared to indoor. After calibration, average indoor O3 concentration ranged -2 ppb to 23 ppb, while outdoor O3 concentration was from 19 ppb to 47 ppb. The only personal sensor that worked well measured the average personal exposure as about 33 ppb. The peak of ozone outdoor concentration was usually 100 ppb higher than indoor concentration.Several factors were considered that could affect the accuracy of the sensors including temperature (degree C), relative humidity (RH%), and concentrations of other chemicals in the ambient environment such as Nitrogen Oxides (NO2, NO). However, the impacts of those factors on the performance of sensors were not significant after calibration with little or no correlation between errors and those factors. Therefore, the calibrations based on the simple linear regressions between the sensors and the reference when collocated were valid on the data during the whole sampling period.