Visibility is not the best indicator of air quality: NEA

NEA's project director and chief scientific officer Indrani Rajaram explaining how the agency measures PM2.5 concentrations in the air. ST VIDEO: AUDREY TAN

SINGAPORE - The haze blanketing countries in the region has resulted in flight cancellations, obscured panoramas and affected visibility.

However, poor visibility does not always mean the air is more polluted. In fact, during the current haze episode, there were a number of days when visibility remained poor even though there were fewer pollutants in the air, the National Environment Agency (NEA) said at a media briefing on Thursday.

The lower visibility could be due to the presence of water vapour in the air. Water droplets themselves scatter light, resulting in poorer visibility. In addition, when some pollutant particles attract water molecules to themselves, they could scatter more light than when dry, said Dr Felicia Shaw, director of the risk and resource department of the NEA's Meteorological Service Singapore.

The agency found, through an analysis of meteorological and air quality data between Sept 9 and Oct 6, that Singapore experienced the same visibility during periods of high humidity and low pollution, as periods of low humidity and high pollution.

 

A PM2.5 concentration of 60 micrograms (mcg) metre cube under high humidity, for instance, gave the same visibility of about 4km as drier conditions with higher amounts of PM2.5 at 250 mcg a metre cube.

On a regular, non-hazy day, the maximum concentration of PM2.5 is usually between 20 and 35 mcg a cubic metre. Visibility on these days can also go beyond 10km if there is no rain.

During periods of transboundary haze, the dominant pollutant in the air is PM2.5 - small, toxic particles that measure less than 2.5 microns in diameter. PM2.5 is one of six pollutants, which also include ozone, carbon monoxide and sulphur dioxide, that the Pollutant Standards Index (PSI) measures.

The 24-hour PSI is the average of readings taken from the previous 24 hours, and is what the authorities here base their health advisories on.

PSI readings are measured at 22 air quality monitoring stations across Singapore. Each station is equipped with at least six different systems that measure concentrations of each pollutant. Such systems cost at least $300,000, excluding the cost of other data mining and calibration equipment.

To measure PM2.5, authorities here use a measurement method known as the beta-attenuation method.

It works by sucking in 16.67 litres of air a minute and then filtering out the harmful PM2.5 particles from it. The air sample containing PM2.5 particles then pass through a glass fibre filter tape that retains these particles.

A low radioactive beam, a beta-ray, is then exposed to the filter tape. PM2.5 weakens the intensity of the beta ray and the change in the intensity of the ray is measured by a detector and this is a measure of PM2.5 concentration.

The NEA also said that it is looking at ways to improve the user interface on its MyENV app and website. More details will be released later.

audreyt@sph.com.sg