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

Abstract

Abstract. In many urban areas the population is exposed to elevated levels of air pollution. However, air quality is usually 10 only measured at a few locations. These measurements provide a general picture of the state of the air, but they are unable to monitor local differences. Since a few years new low-cost sensor technology is available, which has the potential to extend the official monitoring network significantly. These sensors, however, are still in an experimental stage and suffer from various technical issues which limit their applicability.This study explores the added value of alternative air quality measurements, focusing on nitrogen dioxide (NO 2 ) in 15Amsterdam, the Netherlands. 16 low-cost air quality sensor devices were built and distributed among volunteers living close to roads with high traffic volume for a two-month measurement campaign.Careful calibration of individual sensors is essential to measure ambient concentrations of NO 2 significantly. Field calibration was done next to an air monitoring station during an 8-day period, resulting in R 2 ranging from 0.3 to 0.7. The NO 2 accuracy can be improved by including temperature and humidity measurements from an additional low-cost sensor, R 2 20 ranging from 0.6 to 0.9. Recalibration is crucial, as all sensors show significant signal drift after the two-month measurement campaign. The measurement series between the calibration periods can be corrected in hindsight by taking a weighted average of the calibration coefficients.Validation against an independent air monitoring station shows good agreement. Using our approach, the standard deviation of a typical sensor device for NO 2 measurements was found to be 7 μg m -3 . This shows that, if properly treated, low-cost 25 sensors based on the current generations of electrochemical NO 2 sensors may provide useful complementary data on local air quality in an urban setting.

Review of the Performance of Low-Cost Sensors for Air Quality Monitoring

“…Fortunately, the majority of these studies also report the slope and intercept of the regression line between LCS data and reference measurements that describe the possible bias of LCS data. A few studies also report the RMSE [10,20,22,36,[41][42][43]51,52,58,60,62,63,85] which clearly indicates that the magnitude of the error in LCS data is also sensitive to extreme values and outliers. Only a few studies report the measurement uncertainty [10,22,25,30,48,52,59,61].…”

“…In addition, 19 projects evaluating OEMs, SSys, or both, and reporting quantitative comparisons of LCS data and reference measurements were identified. They include the Air Quality Egg, Air Quality Station, AirCasting [19,[35][36][37], Carnegie Mellon [36,38], CitiSense [30], Cairsense [39], Developer Kit [19], HKEPD/14-02771 [40], making-sense.eu [41], communitysensing.org [32], MacPoll.eu [20], OpenSense II [42,43], Proof of Concept AirSensEUR [22], and SNAQ Heathrow [44,45]. Out of the 1423 records collected from literature, we identified 1188 records (197 OEMs and 991 SSys) from 89 alive LCS (24 OEMs and 65 SSys) and 235 records (123 OEMs and 112 SSys) from 23 "non active" (or discontinued) LCS (7 OEMs and 16 SSys).…”

The BErkeley Atmospheric CO<sub>2</sub> Observation Network: Field Calibration and Evaluation of Low-cost Air Quality Sensors

“…In addition, many of the sensors have responses to gases other than the target analyte (Mead et al, 2013;Spinelle et al, 2015;Cross et al, 2017;25 Michael et al, 2017;Mijling et al, 2017;Spinelle et al, 2017;Zimmerman et al, 2017). One approach to addressing this challenge is to combine periodic re-calibration and co-location with regulatory reference instruments in the lab or the field (Williams et al, 2013;Moltchanov et al, 2015;Jiao et al, 2016;Mijling et al, 2017). Field calibration is preferred as in-lab performance is often a poor approximation of sensor behavior under ambient conditions (Piedrahita et al, 2014;Masson et al, 2015).…”

Citizen sensing for environmental information

“…The informal sensors were calibrated by collocating sensor devices to one formal station at Vondelpark before and after the measurement campaign and validated by comparing data from one informal sensor device to a nearby formal station at Oude Schans. More details on the followed calibration and validation procedures can be found in (Mijling et al, 2017). The case study is used as a specific example for expert validation.…”

“…If existing formal environment sensing stations are taking into account to complement informal environmental sensing network, the informal network can be designed in a better way. For instance, in the case study (Mijling et al, 2017), there were two official monitoring stations in the project area. They could be used to complement the informal sensing network.…”