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

TLDR: It is shown that miniature, low-cost electrochemical gas sensors can, when suitably configured and operated, be used for parts-per-billion level studies for gases relevant to urban air quality, and that measurement networks with higher resolution are required to quantify air quality at the scales which are present in the urban environment.

About: This article is published in Atmospheric Environment.The article was published on 2013-05-01 and is currently open access. It has received 619 citations till now. The article focuses on the topics: Wireless sensor network & Air quality index.

Figures

Table 2.1. Cross sensitivities of CO-AF, NO-AF and NO2-A1 electrochemical sensors to applied concentrations of CO, NO and NO2 at typical ambient levels. Quoted errors are ± 1σ, with the manufacturer’s average cross sensitivities, derived from ppm-level measurements, shown in brackets.

Figure 2.6. Hourly averaged NO data from a roadside reference instrument (details) are shown in red and uncorrected

Table 3.2. Slopes and correlation coefficients for linear fitting equations generated for sensor pairs colocated for an hour-long walk in central Cambridge (UK).

Figure 3.3. (Left) Time series plots of CO, NO and NO2 for sensor pairs co-located for the whole deployment period, and (right) corresponding scatter plots. Data used are a 30-second running average. NO2 data are not corrected for cross interference with O3 and the likely maximum bias   is 24 ppb (equal to the nearest background AURN O3 measurement (Wicken Fen, see section 3.3)).

Figure 3.10. Time series and histograms for a mobile sensor unit carried by a pedestrian as part of a mobile sensor deployment. NO2 data are not corrected for interference with O3 and therefore should be taken to be [O3] + [NO2], i.e. there may be an additional baseline effect related to response to [O3]. The inset NO Figure shows that the peak at 14.40 exceeded 2.5 ppm. Histograms of the data for NO and NO2 also show, in dotted lines, the average of two concurrent hourly mean values derived from each of the static sites maintained by the local authority in central Cambridge.

Figure 2.1. Schematic of an electrochemical cell of the type used in this study. The gas diffusion barrier is a gaspermeable PTFE membrane used to prevent water and dust ingress to the cell. During operation the working and counter electrodes are maintained with a fixed voltage bias, and the current between them is the output of the sensor.

Frequently asked questions

Q1. What are the future works in this paper?

The sensors are generally highly selective, although there emerged from this work some cross-sensitivities e. g. between O3 and NO2 sensors ; in future work, these will be catered for through the use of multiple sensors. Selected results also show that the sensors can be operated without significant gain attenuation over long periods ( up to 12 months thus far ), with the expectation of operable lifetimes of several years without replacement. Overall, this work has demonstrated the potential of low-cost sensor network systems for air composition measurements in the urban environment, and to be capable of doing so at an appropriate granularity ( and cost ) to quantify airborne pollution levels on local scales. 

Q2. What are the contributions in this paper?

In this paper the authors 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 the authors 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. The widely varying mixing ratios reported by this study confirm that the urban environment can not 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. The authors 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.