Evaluating the PurpleAir monitor as an aerosol light scattering instrument

Abstract

The Plantower PMS5003 sensors (PMS) used in the PurpleAir monitor PA-II-SD configuration (PA-PMS) are equivalent to cell-reciprocal nephelometers using a 657ĝnm perpendicularly polarized light source that integrates light scattering from 18 to 166ĝ . Yearlong field data at the National Oceanic and Atmospheric Administration's (NOAA) Mauna Loa Observatory (MLO) and Boulder Table Mountain (BOS) sites show that the 1ĝh average of the PA-PMS first size channel, labeled ">ĝ0.3ĝμm"("CH1"), is highly correlated with submicrometer aerosol scattering coefficients at the 550 and 700ĝnm wavelengths measured by the TSI 3563 integrating nephelometer, from 0.4 to 500ĝMm-1. This corresponds to an hourly average submicrometer aerosol mass concentration of approximately 0.2 to 200ĝμgĝm-3. A physical-optical model of the PMS is developed to estimate light intensity on the photodiode, accounting for angular truncation of the volume scattering function as a function of particle size. The model predicts that the PMS response to particles >ĝ0.3ĝμm decreases relative to an ideal nephelometer by about 75ĝ% for particle diameters ≥ĝ1.0ĝμm. This is a result of using a laser that is polarized, the angular truncation of the scattered light, and particle losses (e.g., due to aspiration) before reaching the laser. It is shown that CH1 is linearly proportional to the model-predicted intensity of the light scattered by particles in the PMS laser to its photodiode over 4 orders of magnitude. This is consistent with CH1 being a measure of the scattering coefficient and not the particle number concentration or particulate matter concentration. The model predictions are consistent with data from published laboratory studies which evaluated the PMS against a variety of aerosols. Predictions are then compared with yearlong fine aerosol size distribution and scattering coefficient field data at the BOS site. Field data at BOS confirm the model prediction that the ratio of CH1 to the scattering coefficient would be highest for aerosols with median scattering diameters