Characterizing the Aging of Alphasense NO₂ Sensors in Long-Term Field Deployments

Low-cost NO₂ sensors have been widely deployed for atmospheric sampling. While their initial performance has been characterized, few studies have examined their long-term degradation. This study focused on the performance of Alphasense low-cost NO₂ sensors (NO2-B42F and NO2-B43F) over 4 years (2016-2020). A total of 29 NO₂ sensors from 10 batches were collocated 78 times at two sites with reference instruments. Raw signals from "functional" NO₂ sensors correlated linearly with reference NO₂ concentrations. After long-term deployment, sensor raw signals started to deviate from reference NO₂ concentrations due to sensor aging, an accumulated effect after sensor unpacking. Several sensors eventually became "non-functional" as sensor raw signals showed no correlation with reference NO₂ concentrations. Sensor aging and non-functionality may be primarily caused by expiration of the ozone (O₃) scrubber built into these sensors so that sensors responded to both ambient NO₂ and O₃. The influence of O₃ on sensor response is quantified through the permutation importance method. Most of the sensors are non-functional after approximately 200-400 days of deployment, and no sensor was functional after 400 days of deployment. This result agrees well with the estimated lifetime of the built-in ozone scrubbers considering the ambient ozone concentration in the Pittsburgh area where these sensors were deployed. To ensure reliable data quality in long-term field deployments, we recommend collocating NO₂ sensors with reference instruments regularly after 200-400 days of deployment to identify and replace non-functional sensors in a timely manner.