Figure 2.

The potential range of RMSE values for a given co-location length for three low-cost sensors (NO₂, O₃, and NO). A calibration length of 1 day indicates that a random 24-hour period was selected out of all available days between February 2019 and February 2020. The RMSE for a given test calibration period was determined by comparing the 1-hour averaged reference and the corrected sensor data associated with that calibration across the evaluation period (all days not included in the calibration period). For each calibration length, 250 randomly selected calibration periods were used to assess the potential RMSE range. All sensors were calibrated using previously identified predictors in a multiple linear regression using data from the calibration period only. The reference NO₂ concentrations ranged between 1 and 58 ppb over the full year, with a median concentration of 5 ppb. The reference O₃ concentrations ranged between 1 and 110 ppb, with a median concentration of 31 ppb. The reference NO concentrations ranged between 0.1 and 137 ppb, with a median concentration of 0.5 ppb.