New chemical pathway found to worsen air quality in harsh winters Premium

In a study published in the journal Science Advances, researchers found that lower sulphate concentrations in air combined with low temperatures to make PM particles less acidic. This in turn increased the production of PM2.5 in the air.

In winter, the temperature in Dras in Ladakh drops to -20º C, making it the coldest place in India. On the other side of the world, Fairbanks, the capital city of Alaska, holds a similar record in the U.S., its temperature hovering around -22.4º C in winter. But the two cities have drastically different air quality. Unlike Dras, where the air is remarkably healthy, Fairbanks is among the U.S.’s worst performing cities. One estimate ranked it tenth in a list of the country’s most air-polluted cities. Another ranked it first for particle pollution.

Particle pollution, also called ‘particulate matter’ (PM), is a mix of solid particles and liquid droplets suspended in the air. PM can be classified into two broad categories: PM10-2.5 and PM2.5. PM10-2.5 refers to particles whose diameter ranges between 2.5 and 10 micrometres (µm, equal to one millionth of a metre) and PM2.5 refers to particles that are less than 2.5 µm in diameter.

PM2.5 particles are also called ultrafine particles. They are considered to be particularly dangerous: they enter the lungs through the nose and throat; once in, they reduce lung function, aggravate asthma, and — for people with lung or heart disease — pave the way for premature death.

In 2009, authorities from the Division of Air Quality in Alaska declared Fairbanks to be a “PM2.5 nonattainment area”: that is, the amount of PM2.5 in the city exceeded the limit of 35 µg per cubic metre of air. The main sources of these pollutants were identified to be emissions from wood stoves, the burning of distillate fuel oil, industrial sources, and automobiles, all of which also emit a large amount of sulphur dioxide.

To bring PM2.5 levels below the permissible limits, the Division in a 2022 directive banned the use of fuel with sulphur concentrations exceeding 1,000 parts per million in Fairbanks. Now, a study led by researchers from the University of Alaska Fairbanks and the Georgia Institute of Technology, both in the U.S., has found that the ban may not be entirely effective because the chemistry of PM2.5 particles changes in cold weather.

In their study, published in the journal Science Advanceson September 4, the researchers found that lower sulphate concentrations in air combined with low temperatures (around -35℃) made the PM particles less acidic. This in turn increased the production of hydroxymethanesulphonate — another component of PM2.5 — in the air.

Rodney J. Weber, a professor at the School of Earth and Atmospheric Sciences, Georgia Institute of Technology, and one of the corresponding authors of the study, told this reporter that the study’s findings have implications for the “effectiveness of emission controls to reduce pollution levels”.