Low-cost Alphasense OPCs shine in US trials

A third-party evaluation of Alphasense’s low-cost particulate matter (PM) air quality sensors has demonstrated good correlation with reference instruments at a fraction of their cost. Welcoming the results of the trial, Will Parrett, European Sales Manager at Alphasense, Alphasense, said: “With an ever-increasing number of low-cost sensors appearing on the market, it is becoming difficult for customers to pick the technologies they can trust. Third-party assessments such as this are therefore extremely helpful.

The publication of these results is well-timed, as there has been a greater focus on fine particulates in recent months as the health effects become widely understood. In September 2021, the World Health Organization slashed the yearly average PM guideline values to 5 µg/m3, and in March 2022 the UK government announced a consultation on new targets to be included in the Environment Act, including a reduction in the PM2.5 target to 10 µg/m3 by 2040.

The PM sensor evaluation involved three Alphasense OPC-R2 sensors, which were deployed at the South Coast AQMD stationary ambient monitoring site in Rubidoux, California, over three months from October to December 2021. The sensors were run alongside Federal Equivalent Method (FEM) instruments measuring the same parameters: particulates PM1, PM2.5 and PM10, relative humidity and temperature.

OPC trial background

The South Coast Air Quality Management District (South Coast AQMD) is the agency responsible for attaining state and federal clean air standards in the South Coast Air Basin in southern California. With a population of 14.6 million, the South Coast Air Basin covers an area of 6,745 square miles, which includes Los Angeles.

In order to inform the public about the actual performance of commercially available ‘low-cost’ air quality sensors, the South Coast AQMD has established the Air Quality Sensor Performance Evaluation Center (AQ-SPEC) program, which characterises sensors under both field and laboratory conditions. The AQ-SPEC program was established in 2014 and during the past 8 years, the group has worked closely with more than 100 sensor manufacturers/developers/vendors to evaluate more than 170 sensors measuring National Ambient Air Quality Standards (NAAQS) criteria pollutants. To be eligible for evaluation, sensors must measure one of a list of target parameters in real- or near-real time and cost less than $2,000. Priced in the sub $500 range, Alphasense’s PM sensors qualified for this study.

Recognising the growing popularity of low-cost air quality sensors, the AQ-SPEC program was created to evaluate the performance of devices, amid concerns that sensor data should not be accepted at face value without an evaluation of precision and overall quality. With new devices entering the market, there were concerns that poor quality data from unreliable sensors may not only lead to confusion but also jeopardise the successful evolution of low-cost sensor technology. In addition to the need to characterise the performance of air monitoring sensors South Coast AQMD also seeks to educate the public about the advantages of such devices and their potential limitations.

AQ-SPEC evaluations are being undertaken for both gaseous and particulate pollutants. The health risks associated with particulate matter equal or smaller than 10 and 2.5 microns (µm) in diameter (PM10 and PM2.5, respectively) are of particular public health relevance. Both PM2.5 and PM10 are capable of penetrating deep into the lungs but PM2.5 can even enter the bloodstream, primarily resulting in cardiovascular and respiratory impacts, and also affecting other organs.

The Alphasense OPC-R2 particulate sensor

Weighing less than 30g and with 72mm as its longest dimension, the OPC-R2 (optical particle counter) offers impressive performance for such a small sensor. A laser is utilised to detect particles from 0.30 to 12.4 µm in diameter with 16 software ‘bins’. Count measurements are converted into mass concentrations of PM1.0, PM2.5 and PM10 using embedded algorithms. The sensor also measures temperature and relative humidity.

All Alphasense sensors are factory calibrated prior to dispatch, and data acquisition software can be downloaded free of charge from www.alphasense.com. South Coast AQMD did not perform any further calibrations prior to the commencement of the trial.

OPC test evaluation results

Data recovery for the Alphasense sensors was close to 100% for all PM measurements. The performance of the OPC-R2 sensors was evaluated against two FEM reference instruments, which South Coast AQMD estimated to cost between $21,000 and $25,000 each. There was strong correlation between the two FEM instruments across the PM range.

The Alphasense OPC-R2 sensors 1-hr mean results showed:

  • PM1: strong correlations with the FEM instruments (0.77 < R2 < 0.88).
  • PM2.5: moderate to strong correlations with the FEM instruments (0.67 < R2 < 0.80).
  • PM10: strong to very strong correlations with the FEM instruments (0.70 < R2 < 0.91).
  • Very strong correlations with the corresponding South Coast AQMD Met Station data for both temperature and humidity (R2 ~0.95 & R2 ~0.97 respectively)

R2 is the linear correlation coefficient which is a measure of the linear relationship between the average measurements from the three sensors under test and the corresponding reference instrument values. An R2 approaching the value of 1 reflects a near perfect agreement between the sensors and the reference readings, whereas a value of 0 indicates a complete lack of correlation.

The successful evaluation of the OPC-R2 sensors builds on the high levels of performance demonstrated by Alphasense’s other PM sensor, the OPC-N3, during a 2018 evaluation by South Coast AQMD. A little larger than the OPC-R2, the OPC-N3 detects particles from 0.35 to 40 µm in diameter, sorting them into 24 software ‘bins’. The OPC-N3 measures 65mm (wide) x 75mm (long) and weighs less than 105g. Like the OPC-R2, the OPC-N3 registers PM1.0, PM2.5 and PM10 (with PM4.25 as an option) and also measures temperature and relative humidity. The trial results for the Alphasense OPC-N3 sensors (1-hr mean) evaluation showed:

  • PM1: correlated well with GRIMM instruments (R2 ~ 0.80).
  • PM2.5: moderate to good correlations with FEM GRIMM, FEM BAM and FEM T640 instruments respectively (R2 ~ 0.64, 0.44 & 0.57).
  • PM10: low to moderate correlations with GRIMM, FEM BAM and T640 instruments (R2 ~ 0.51, 0.28 & 0.47).
  • Very strong correlations with the corresponding South Coast AQMD Met Station data for both temperature and humidity (R2 ~0.97 & R2 ~0.98 respectively)

OPC test evaluation summary

The Alphasense OPC-R2 sensors performed very well across all three PM ranges. “We were delighted with the South Coast AQMD evaluation; the results provide our customers with assurance that good quality data can be obtained from these low-cost sensors,” said Will Parrett.

“We applaud the efforts of the South Coast AQMD because third-party evaluations such as these help customers to choose reliable technologies. This builds confidence and trust in low-cost monitors, which in turn can dramatically improve air quality monitoring and encourage more widespread adoption of air quality monitoring programmes and initiatives.”

The improved time and spatial resolution provided by low-cost monitors delivers the granularity of data that is necessary to help governments, local authorities and educational institutions better understand the impacts of air quality, whilst also enabling the implementation and evaluation of effective air quality mitigation measures.

By reducing the cost per monitoring point without compromising the accuracy and quality of data, these sensors facilitate the development of monitoring networks which provide much greater insight into the factors affecting air quality.

To find out more about Alphasense OPC products and gas sensors, visit www.alphasense.com.

The full evaluation report is available on the South Coast AQMD website http://www.aqmd.gov/aq-spec/evaluations/field