Over the past decade, a range of sensor technologies became available on the market, enabling a revolutionary shift in air pollution monitoring and assessment. With their cost of up to three orders of magnitude lower than standard/reference instruments, many avenues for applications have opened up. In particular, broader participation in air quality discussion and utilisation of information on air pollution by communities has become possible. However, many questions have been also asked about the actual benefits of these technologies. To address this issue, we conducted a comprehensive literature search including both the scientific and grey literature. We focused upon two questions: (1) Are these technologies fit for the various purposes envisaged? and (2) How far have these technologies and their applications progressed to provide answers and solutions? Regarding the former, we concluded that there is no clear answer to the question, due to a lack of: sensor/monitor manufacturers' quantitative specifications of performance, consensus regarding recommended end-use and associated minimal performance targets of these technologies, and the ability of the prospective users to formulate the requirements for their applications, or conditions of the intended use. Numerous studies have assessed and reported sensor/monitor performance under a range of specific conditions, and in many cases the performance was concluded to be satisfactory. The specific use cases for sensors/monitors included outdoor in a stationary mode, outdoor in a mobile mode, indoor environments and personal monitoring. Under certain conditions of application, project goals, and monitoring environments, some sensors/monitors were fit for a specific purpose. Based on analysis of 17 large projects, which reached applied outcome stage, and typically conducted by consortia of organizations, we observed that a sizable fraction of them (~ 30%) were commercial and/or crowd-funded. This fact by itself signals a paradigm change in air quality monitoring, which previously had been primarily implemented by government organizations. An additional paradigm-shift indicator is the growing use of machine learning or other advanced data processing approaches to improve sensor/monitor agreement with reference monitors. There is still some way to go in enhancing application of the technologies for source apportionment, which is of particular necessity and urgency in developing countries. Also, there has been somewhat less progress in wide-scale monitoring of personal exposures. However, it can be argued that with a significant future expansion of monitoring networks, including indoor environments, there may be less need for wearable or portable sensors/monitors to assess personal exposure. Traditional personal monitoring would still be valuable where spatial variability of pollutants of interest is at a finer resolution than the monitoring network can resolve.

Applications of low-cost sensing technologies for air quality monitoring and exposure assessment: How far have they gone?

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Aerosol Measurement Principles Techniques And Applications

Abstract. This paper presents the light-scattering properties of atmospheric
aerosol particles measured over the past decade at 28 ACTRIS observatories,
which are located mainly in Europe. The data include particle light
scattering (σsp) and hemispheric backscattering
(σbsp) coefficients, scattering Ångström exponent
(SAE), backscatter fraction (BF) and asymmetry parameter (g). An increasing
gradient of σsp is observed when moving from remote
environments (arctic/mountain) to regional and to urban environments. At a
regional level in Europe, σsp also increases when moving
from Nordic and Baltic countries and from western Europe to central/eastern
Europe, whereas no clear spatial gradient is observed for other station environments.
The SAE does not show a clear gradient as a function of the placement of the
station. However, a west-to-east-increasing gradient is observed for both
regional and mountain placements, suggesting a lower fraction of fine-mode
particle in western/south-western Europe compared to central and eastern
Europe, where the fine-mode particles dominate the scattering. The g does
not show any clear gradient by station placement or geographical location
reflecting the complex relationship of this parameter with the physical properties of the aerosol
particles. Both the station placement and the
geographical location are important factors affecting the intra-annual
variability. At mountain sites, higher σsp and SAE values
are measured in the summer due to the enhanced boundary layer influence
and/or new particle-formation episodes. Conversely, the lower horizontal and
vertical dispersion during winter leads to higher σsp values
at all low-altitude sites in central and eastern Europe compared to summer.
These sites also show SAE maxima in the summer (with corresponding g
minima). At all sites, both SAE and g show a strong variation with aerosol
particle loading. The lowest values of g are always observed together with
low σsp values, indicating a larger contribution from
particles in the smaller accumulation mode. During periods of high
σsp values, the variation of g is less pronounced, whereas
the SAE increases or decreases, suggesting changes mostly in the coarse
aerosol particle mode rather than in the fine mode. Statistically significant
decreasing trends of σsp are observed at 5 out of the
13 stations included in the trend analyses. The total reductions of
σsp are consistent with those reported for PM2.5
and PM10 mass concentrations over similar periods across Europe.


/pdf/a-european-aerosol-phenomenology-6-scattering-properties-of-1x8qczzdc3.pdf

A European aerosol phenomenology-6: Scattering properties of atmospheric aerosol particles from 28 ACTRIS sites [Discussion paper]

Abstract. We report on an inter-comparison of black-carbon- and
aerosol-absorption-measuring instruments with laboratory-generated soot
particles coated with controlled amounts of secondary organic matter (SOM).
The aerosol generation setup consisted of a miniCAST 5201 Type BC burner for
the generation of soot particles and a new automated oxidation flow reactor
based on the micro smog chamber (MSC) for the generation of SOM from the
ozonolysis of α-pinene. A series of test aerosols was generated with elemental to total carbon (EC / TC) mass fraction ranging from about 90 % down to 10 % and single-scattering albedo (SSA at 637 nm) from almost 0 to about 0.7. A dual-spot Aethalometer AE33, a photoacoustic extinctiometer
(PAX, 870 nm), a multi-angle absorption photometer (MAAP), a prototype
photoacoustic instrument, and two prototype photo-thermal interferometers
(PTAAM-2λ and MSPTI) were exposed to the test aerosols in parallel.
Significant deviations in the response of the instruments were observed
depending on the amount of secondary organic coating. We believe that the
setup and methodology described in this study can easily be standardised and provide a straightforward and reproducible procedure for the
inter-comparison and characterisation of both filter-based and in situ black-carbon-measuring
(BC-measuring) instruments based on realistic test aerosols.


/pdf/comparing-black-carbon-and-aerosol-absorption-measuring-3tfvsuww.pdf

Comparing black-carbon- and aerosol-absorption-measuring instruments – a new system using lab-generated soot coated with controlled amounts of secondary organic matter

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 “&gt; 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 &gt; 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 &lt; 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. 

Evaluating the PurpleAir monitor as an aerosol light scattering instrument

. The Plantower PMS5003 sensors (PA-PMS) used in the PurpleAir (PA) monitor PA-II-SD configuration are equivalent to cell-reciprocal nephelometers using a 657 nm perpendicularly polarized light source that integrates light scattering from 18 to 166 degrees. 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 nm and 700 nm wavelengths measured by the TSI 3563 integrating nephelometer, from 0.4 Mm−1 to 500 Mm−1. This corresponds to an hourly average submicrometer aerosol mass concentration of approximately 0.2 to 200 ug m−3. A physical-optical model of the PA-PMS is developed to estimate light intensity on the photodiode, accounting for angular truncation as a function of particle size. Predictions are then compared with yearlong fine aerosol size distribution and scattering coefficient field data at the BOS site. It is shown that CH1 is linearly proportional to the model-predicted intensity of the light scattered by particles in the PA-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. 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 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 loss in the instrument before reaching the laser. The results of this study indicate that the PA-PMS is not an optical particle counter and that its six size fractions are not an accurate representation of particle size distribution. The relationship between the PA-PMS 1 h average CH1 and bsp1, the scattering coefficient in Mm−1 due to particles below 1 μm aerodynamic diameter, at wavelength 550 nanometers, 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 aerosol light scattering coefficient in the mid-visible nearly as well as integrating nephelometers.

/pdf/evaluating-the-purpleair-monitor-as-an-aerosol-light-4gnwccqm6y.pdf

James R. Ouimette

Papers

Evaluating the PurpleAir monitor as an aerosol light scattering instrument