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Researchers at The University of Alabama in Huntsville (UAH) have published a in Communications Earth & Environment that demonstrates for the first time that using data gathered on atmospheric particles from Chinese megacities to characterize air quality for U.S. urban centers leads to significant inaccuracies.

Dr. Lee Tiszenkel, a UAH alumnus at the UAH Earth System Science Center and Dr. Shanhu Lee, a professor in the Department of Atmospheric and Earth Sciences at UAH, a part of The University of Alabama System, found the key to understanding the critical differences from one city to another came through painstaking field research in Houston, Texas.

"Much of our understanding of particle formation comes from studies conducted in Chinese megacities, so predictions and models of air quality in the urban United States will not be accurate," Tiszenkel says. "Research in Chinese megacities found that ultrafine particles could be traced back almost entirely to human activity like traffic, cooking, industry or heating.

"In the United States, however, the existence of green spaces in and surrounding urban areas means that emissions from trees and other emerging air pollutants emitted from urban human activities form particles by a different mechanism than was found in urban China. We found that the causes of urban air pollution cannot be generalized, and it highly depends on the specific cocktail of human and natural emissions."

In fact, according to the new research, urban new particle formation in Houston may even vary from season to season.

The work focuses on atmospheric ultrafine particles with sizes smaller than 100 nanometers, where 1 nanometer equals one billionth of a meter. Particles this small can have negative health effects, because they can be diffused into the lungs even more easily than larger particles.

"The knowledge that particles less than 100 nm penetrate deeply into the lungs has been known for a few decades now," Tiszenkel notes. "Similarly, the knowledge that these ultrafine particles form via gas-to-particle conversion in significantly polluted areas has been known since the 80s or 90s. However, the technology in particle detection and chemical composition is more recently catching up with this knowledge."

The study was made possible by a special instrument recently acquired by UAH's Earth System Science Center. The device, known by the tongue-twisting name of Filter Inlet for Gases and Aerosols High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer, has UAH as its home base.

Taking science to the streets

"Chinese megacities tend to have a lot of concrete, higher populations and more relaxed vehicle emissions standards," Tiszenkel explains, "which results in an atmospheric chemistry that is heavily weighted towards anthropogenic compounds," substances that originate from human activity, rather than naturally occurring processes.

"In Houston, we found more influence from surrounding agriculture and rural areas dominated by biogenic emissions like monoterpenes that can become extremely low-volatility compounds upon oxidation."

Monoterpenes are volatile organic compounds (VOCs) emitted from plants and other sources, such as household cleaning products and deodorants, that can affect air quality and human health. They are a significant component of biogenic VOCs—organic gases released into the atmosphere by living organisms—and play a significant role in atmospheric chemistry, influencing ozone and secondary organic aerosol formation, which in turn affects air quality.

"My previous research was mostly in the lab, where we can very precisely control the kind of environment we want to observe," the researcher says. "Results from those experiments are extremely useful for making conclusions about very specific chemical mechanisms, but, due to the constraints of our experimental setup, they are somewhat difficult to generalize to the real atmosphere. The ability to bring all of our instrumentation out into ambient environments links scientific theory and experimental observations to the real world."

The ability to detect freshly formed clusters, for example—particles under 3 nanometers in diameter—is a relatively recent innovation, with instruments capable of detecting these particles only coming out in the last 10 or 15 years, Tiszenkel reports.

"In addition, the mass spectrometry technology that enables the measurement of aerosol chemical composition and organic precursors at the kind of resolution required for comprehensive surveys of urban new particle formation is a more recent innovation," the researcher says. "Especially in urban areas with significant anthropogenic and biogenic emissions, these studies are essential to understand and improve urban air quality."

The study grew out of Tiszenkel's doctoral work with Lee as his advisor.

"I came to UAH with an atmospheric chemistry background, and I knew that atmospheric aerosols represented the biggest unknowns in our understanding of the chemical composition and climatic effects of Earth's atmosphere," Tiszenkel says. "It was easy to pursue that in Dr. Lee's lab at UAH where we have one of the most comprehensive instrument suites for understanding atmospheric particle formation and evolution in the country.

"We are the first group in the U.S., to our knowledge, to simultaneously measure size-resolved aerosol concentrations from newly formed particles up to micron sizes, gas-phase precursors and particle-phase chemical composition in-situ," the researcher says. "This means we are uniquely poised to make definitive conclusions of the causes of urban new particle formation at a molecular level, while minimizing unknown factors."