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A study shows that cold temperatures in Fairbanks (around -35º C) make PM2.5 particles less acidic, promoting the formation of a compound called hydroxymethanesulphonate, which contributes to air pollution.
Cold Cities and Air Quality
Dras, Ladakh is the coldest place in India, with temperatures dropping to -20º C in winter.
Fairbanks, Alaska also experiences extreme cold, with temperatures around -22.4º C during winter.
However, Fairbanks suffers from poor air quality, ranking among the U.S.'s most polluted cities, while Dras has notably healthy air quality.
Particle Pollution and Health Risks
Particulate matter (PM) includes tiny solid particles and liquid droplets suspended in the air, with two major types:
PM10-2.5: Particles between 2.5 and 10 micrometers in diameter.
PM2.5: Extremely small particles (less than 2.5 micrometers), also called ultrafine particles, which can enter the lungs and worsen conditions like asthma, heart disease, and even cause premature death.
Pollution Sources in Fairbanks
Fairbanks struggles with high PM2.5 levels due to wood stoves, distillate fuel oil, industrial emissions, and automobile exhaust.
In 2009, Fairbanks was designated a "PM2.5 nonattainment area" due to pollution levels exceeding the allowable limits.
In response, in 2022, the use of high-sulphur fuel was banned to reduce sulphur dioxide emissions, but its effectiveness remains uncertain.
Aerosol particles can form hydroxymethanesulphonate during winter in Fairbanks, particularly when formaldehyde and sulphur dioxide react in the presence of water.
This process happens even in the cold, dark conditions of winter, where liquid water droplets exist in supercooled aerosols, making it easier for hydroxymethanesulphonate to form.
Sulphate (SO₄²⁻) and ammonium (NH₄⁺) ions play a key role in the acidity of aerosol particles.
When sulphate and ammonium ions are present in aerosol particles, they determine whether the particles will be acidic or less acidic.
In Fairbanks, after the sulphur fuel ban, the concentration of ammonium ions in aerosol particles increased, lowering the acidity.
This created conditions where hydroxymethanesulphonate could form more easily, which is unexpected because it typically requires acidic conditions to form.
Implications for Global Understanding
The study changes the understanding of secondary aerosol formation, showing it can occur in cold, dark conditions, like those found in cold urban and industrial regions.
The findings are particularly relevant to regions like the Andes and Himalayas, but scientists are keen to expand this understanding to other cold regions globally.
The study highlights the need to understand how temperature changes could impact air quality and climate, especially in a world experiencing global warming.
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