Practical field calibration of electrochemical NO 2 sensors for urban air quality applications

In many urban areas the population is exposed to elevated levels of air pollution. However, air quality is usually 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 Amsterdam, 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 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 sensors based on the current generations of electrochemical NO 2 sensors may provide useful complementary data on local air quality in an urban setting.