Evaluating the PurpleAir monitor as an aerosol light scattering instrument
Authors: Ouimette James R. , Malm William C. , Schichtel Bret A. , Sheridan Patrick J. , Andrews Elisabeth , Ogren John A. , Arnott W. Patrick 
Published: February 2022

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 657 nm perpendicularly polarized light source that integrates light
scattering from 18 to 166∘. 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 1 h average of the
PA-PMS first size channel, labeled “> 0.3 µm” (“CH1”), is
highly correlated with submicrometer aerosol scattering coefficients at the
550 and 700 nm wavelengths measured by the TSI 3563 integrating
nephelometer, from 0.4 to 500 Mm−1. This corresponds to an
hourly average submicrometer aerosol mass concentration of approximately 0.2 to 200 µg m−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 > 0.3 µm decreases relative to an ideal nephelometer by about 75 % for particle diameters ≥ 1.0 µm. 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
< 0.3 µm. The PMS detects aerosols smaller than 0.3 µm
diameter in proportion to their contribution to the scattering coefficient.
The results of this study indicate that the PMS is not an optical
particle counter and that its six size fractions are not a meaningful
representation of particle size distribution. The relationship between the
PMS 1 h average CH1 and bsp1, the scattering coefficient in
Mm−1 due to particles below 1 µm aerodynamic diameter, at
wavelength 550 nm, is found to be
bsp1 = 0.015 ± 2.07 × 10−5 × CH1, for relative humidity below 40 %. The
coefficient of determination r2 is 0.97. This suggests that the
low-cost and widely used PA monitors can be used to measure and predict the
submicron aerosol light scattering coefficient in the mid-visible nearly as
well as integrating nephelometers. The effectiveness of the PA-PMS to serve
as a PM2.5 mass concentration monitor is due to both the sensor
behaving like an imperfect integrating nephelometer and the mass
scattering efficiency of ambient PM2.5 aerosols being roughly constant.