In 2020, Taiwan's Taichung began tracking haze for 3 years; by 2022, industrial solvents drove 57% of the chemistry behind organic haze, showing why cleaner air did not always mean clearer skies

A cleaner atmosphere does not always look like one. In Taiwan, air quality has improved over the past decade, yet the hazy, milky-white skies that blur mountains and city skylines have remained a familiar sight in many places.

That contrast led a research team at National Taiwan University to take a closer look at what was actually floating in the air over Taichung, a mixed urban-industrial region in central Taiwan.

Led by Prof. Ta-Chih Hsiao from the Graduate Institute of Environmental Engineering, the team monitored the city's air continuously from 2020 through 2022. They tracked PM2.5, the fine particles that scatter and absorb sunlight, but they also went further by examining the size and chemical makeup of individual particles, as per a Phys.org report.


Their findings, published in the Journal of Hazardous Materials, show that the ingredients behind Taichung's haze changed during those three years.

In 2020, ammonium sulfate and ammonium nitrate, salts associated with vehicle and industrial emissions, were the main contributors to hazy conditions.

By 2022, organic particles had become the largest contributor. During summer, they were responsible for as much as 52% of the light-blocking effect, as per the Phys.org report.

Industrial solvents emerged as a major source

The researchers then looked at where the organic particles were coming from.

They focused on volatile organic compounds, or VOCs, reactive vapors released by vehicles, factories and industrial solvents. When these compounds react in sunlight, they can contribute to the formation of particles that create haze.

By using the chemical fingerprints of dozens of airborne compounds, the researchers were able to trace the VOCs back to their sources.

Industrial solvents used in coatings, paints and other manufacturing processes accounted for 57% of the potential to form the organic particles linked to haze.

That was a larger contribution than vehicle exhaust.

Cutting NOx did not always clear the air

The research also revealed why reducing one familiar pollutant does not necessarily produce the expected result.

Nitrogen oxides, or NOx, are mainly released by vehicles and factories and are a common target of clean-air policies. Yet the researchers found that reducing NOx could sometimes make nitrate haze worse.

In autumn, cutting NOx by half was calculated to increase nitrate haze levels by nearly 30%.

The explanation lies in the chemistry of the atmosphere. Ammonia is abundant in the region, so nitrate formation depends less on the amount of NOx present and more on the atmosphere's oxidizing capacity.

Sunlight and reactive chemicals can make the atmosphere more capable of converting pollutants into particles.

The researchers found that daytime nitrate formation was roughly three times more responsive to this oxidizing capacity than to NOx levels, as per the Phys.org report.

The haze changed even as air quality improved

That helps explain the gap between cleaner air and skies that still look hazy.

As emissions changed, so did the mix of particles responsible for blocking light. The main contributors in 2020 were different from those that dominated by 2022.

Organic particles eventually became the biggest contributor during summer, making the sources of VOCs increasingly important to visibility.

The findings suggest that focusing only on vehicle emissions may not be enough to address the haze.

Factories could be part of the solution

The researchers say regulators should also look more closely at solvent vapors released by factories.

One approach they point to is encouraging the use of water-based coatings, which could reduce solvent emissions and the atmospheric chemistry that produces organic haze.

The study's message is not simply that one pollutant replaced another. Instead, it shows how complicated the chemistry behind haze can become when different emissions interact in the atmosphere.

Hsiao said, "Our data show that visibility in this region is no longer just a story about NOx and vehicle exhaust—industrial solvent vapors are now writing a big part of that story, and controlling them is essential to seeing clearer skies again," as quoted by Phys.org report.

After three years of watching Taichung's air, the researchers found that cleaner air does not automatically mean clearer views. By 2022, industrial solvents had become a major part of the chemistry behind the organic haze, pointing to a different target for efforts to bring clearer skies back.