Are we overpredicting air pollution?

Pakistan was ranked second in the list of countries with the worst air quality in 2019 by a Swiss air quality technology company. Lahore and Karachi often compete for the first position in the list of the world’s most polluted cities on the company’s website. These rankings are based on the Air Quality Index (AQI), which is based on the concentration of breathable particulate matter, PM2.5, and its values range from 0-500. The higher the AQI, the higher the air pollution, and the greater the health concern.

This company uses low-cost air monitors (LCAM) to obtain the concentration of respirable particles (PM2.5) and the corresponding AQI. Several not-for-profit organisations, individuals, and companies have acquired these monitors and started reporting hourly values of PM2.5 and AQI online. People use this information to know their locality’s air quality and make informed decisions such as staying indoor, shortening outdoor activities and wearing masks, etc. However, there are many questions regarding the reliability, accuracy, and suitability of LCAMs for air quality monitoring.

Elevated levels of PM2.5 in ambient outdoor and indoor air are the primary cause of mortality worldwide. Such particles are among the major constituents of smog that blankets northern India and Pakistan mid-October every year. These are responsible for respiratory and cardiovascular diseases and irritate the eyes, nose, and throat. Low visibility and haze caused by particulate air pollution have resulted in many car accidents in areas heavily blanketed by smog.

The EPA recommended method for measuring PM2.5 is based on the weight of particles collected on a filter paper after a 24-hour sampling of air (Federal Reference Method, FRM). Another method (BAM) employs Beta-rays’ absorption by the particles extracted from the air and continuously monitors PM2.5 and PM10 (Federal Equivalent Method, FEM). BAM is the choice method in many countries for regulatory air monitoring. Instruments based on FRM and FEM have heaters at the inlet to dry air and avoid any possible effects of high humidity on the measurement. However, these monitors are expensive and a big hurdle in expanding air quality monitoring capacity.

LCAMs measure the amount of light scattered by the particles and provide air quality data in real-time. They are affordable, easy to operate, small-sized, and consume low energy. A network of such monitors would be economical and provide data with high spatial and temporal resolution. However, the composition, concentration, and refractive index of particles and environmental conditions such as humidity, can affect the monitors’ response.

Research has shown that low-cost air monitors overestimate particle number and mass concentrations at relative humidity above 75%. For instance, during a fog period, the total particle number count may increase to 28%. In Lahore, humidity stays very high from October to February when smog blankets the city. Therefore, it is likely that unless calibrated, LCAMs might over-predict PM2.5 levels during times of high humidity. The Environmental Protection Department Punjab (EPD) has recently raised questions regarding the accuracy of the LCAMs

The effect of humidity depends on the composition of PM particles. Therefore it would be highly relevant to analyse the particles for their chemical composition. Performance evaluation of LCAMs against a reference instrument is also essential as our environmental conditions are different from the countries that developed the technology. It would also allow the development of algorithms to calibrate these low-cost monitors and produce high-quality data.

If properly calibrated against a reference station, LCAMs can be used to construct a “hyper-local” network that could deliver street-level air quality data in real-time. Air pollution maps based on this could be made available publicly for both public awareness and the assessment of air quality interventions. Corrected air quality data from low-cost monitors would help in policymaking, legislation, and making informed decisions to manage air pollution.

Published in The Express Tribune, January 31^st, 2021.

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