Long-Term Performance Evaluation of the PurpleAir PA-II Sensor in New Delhi, India

India experiences some of the worst particulate air pollution in the world, with the highest number of deaths from ambient air pollution. Delhi is particularly polluted, experiencing annual mean PM _2.5 mass concentrations in the range of 120-160 μg/m ³. Low cost optical aerosol instruments have attracted attention as a means of augmenting reference methods, especially in environments with comparatively few official monitors. However, many assumptions are made by OPCs when deriving mass concentrations from aerosol light scattering, and therefore in-situ calibration is vital. From July 2018 through June 2019, two PurpleAir PAII units (each comprising two Plantower PMS5003 sensors) were collocated with a regulatory grade instrument (MetOne BAM-1020) located in the diplomatic quarter in Delhi. Simultaneously, at the IIT Delhi campus, about 5 miles from the site, a Scanning Mobility Particle Sizer (SMPS) and Aerosol Chemical Speciation Monitor (ACSM) were also operated to understand particle size distribution and composition. Over the course of the entire calibration period, PA attained an OLS R ² ranging between 0.65 and 0.70 for the four sensors. The RMSE ranged between 57 - 62 μg/m ³ with a NRMSE range of 0.46 to 0.50. After aggregating to 8-hr block averages, R2 improved to 0.84 - 0.85, and NRMSE improved to 0.34 - 0.37, with no significant improvement for longer averaging times. After dividing the year of data into seasons, the R2 for the Fall period was 0.75, while the monsoon season R2 was below 0.3. The most likely cause for this precipitous drop in performance is the influence of water vapor content on mass concentration, and size distribution. Average diurnal profiles showed that while absolute concentrations differed, both PA and the reference instrument captured early morning peaks and afternoon troughs.