The use of electrochemical sensors for monitoring urban air quality in low-cost, high-density networks

Abstract

Measurements at appropriate spatial and temporal scales are essential for understanding and monitoring spatially heterogeneous environments with complex and highly variable emission sources, such as in urban areas. However, the costs and complexity of conventional air quality measurement methods means that measurement networks are generally extremely sparse. In this paper we show that miniature, low-cost electrochemical gas sensors, traditionally used for sensing at parts-per-million (ppm) mixing ratios can, when suitably configured and operated, be used for parts-per-billion (ppb) level studies for gases relevant to urban air quality. Sensor nodes, in this case consisting of multiple individual electrochemical sensors, can be low-cost and highly portable, thus allowing the deployment of scalable high-density air quality sensor networks at fine spatial and temporal scales, and in both static and mobile configurations. In this paper we provide evidence for the performance of electrochemical sensors at the parts-per-billion level, and then outline results obtained from deployments of networks of sensor nodes in both an autonomous, high-density, static network in the wider Cambridge (UK) area, and as mobile networks for quantification of personal exposure. Examples are presented of measurements obtained with both highly portable devices held by pedestrians and cyclists, and static devices attached to street furniture. The widely varying mixing ratios reported by this study confirm that the urban environment cannot be fully characterised using sparse, static networks, and that measurement networks with higher resolution (both spatially and temporally) are required to quantify air quality at the scales which are present in the urban environment. We conclude that the instruments described here, and the low-cost/high-density measurement philosophy which underpins it, have the potential to provide a far more complete assessment of the high-granularity air quality structure generally observed in the urban environment, and could ultimately be used for quantification of human exposure as well as for monitoring and legislative purposes.

The rise of low-cost sensing for managing air pollution in cities

“…Some research programmes are already using sensor networks to assess their performance for both fixed-site and mobile monitoring (Mead et al 2013). There are also community-led sensing networks in operation (Air Quality Egg 2014), allowing the general public to participate in discussions on air quality.…”

“…Correction factors provided by manufacturers for temperature and relative humidity are adequate at ppm levels (Lane et al 2010;Mead et al 2013), but more sophisticated corrections are required for outdoor conditions where sensitivity is required at the ppb level and ambient temperature changes significantly on both diurnal and seasonal timescales (Mead et al 2013;Shallcross et al 2009). …”

“…These also recommend specific numbers of monitoring stations for individual pollutants, on the basis of the number of inhabitants and geographic partitioning. Demonstration studies have applied mobile sensor networks in some cities, such as Cambridge (UK), Valencia (Spain) and Lagos (Nigeria) (Mead et al 2013), but their widespread long-term application is yet to find a legislative purpose. (Kim et al 2012), but our focus here remains on the application of low-cost sensing for air pollution management in urban outdoor environments.…”

“…Some studies have found encouraging results in relation to the performance of CO, NO and NO 2 sensors that can provide parts-per-billion (µg m -3 ) level mixing ratio sensitivity with low noise and high linearity (Mead et al 2013). Furthermore, prototype sensors (Bell et al user needs, as identified through discussions with potential users of pervasive sensor array Suresh et al 2009).…”

Practical field calibration of electrochemical NO<sub>2</sub> sensors for urban air quality applications

“…Jiao et al, 2016, Duvall et al, 10 2016Mead et al, 2013;Moltchanov et al, 2015. For NO 2 monitoring, mostly metal oxide and electrochemical sensors are used (Borrego et al, 2016;Spinelle et al, 2015b;Thompson, 2016). Typical ambient concentrations of NO 2 are at part-perbillion (ppb) level.…”