Renyi Zhang

Principal Investigator
Texas A&M University

From ozone and particulate matter to acid rain, air pollution profoundly impacts the ecosystem, human health and climate. Renyi Zhang, a Welch principal investigator and professor at Texas A&M University, is working to understand the basic chemical reactions that create air pollution. By tracking the processes at the molecular level, he hopes to provide policy makers with more effective approaches to protect the environment.

Recently, his group and a team of researchers from China, Florida, California, Israel and the UK published findings that shed light on the current haze afflicting many cities in China and other parts of the world and the similarity to the historic London fog that killed thousands in 1952. The work combined laboratory experiments with real-world measurements using instruments Dr. Zhang’s group developed.

Dr. Zhang says that people have long known that sulfate was a big contributor to the deadly effects of London fog, but how the sulfur dioxide (SO2) from coal-burning was turned into sulfuric acid in the atmosphere was unclear. Their research showed that nitrogen dioxide (NO2) from coal-burning and automobiles is instrumental in facilitating the conversion. Usually, however, the chemical conversion of SO2 to sulfate produces acid, which then inhibits further conversion. Why were these situations different?

It turns out that difference mechanisms were at play although delivering the same health-hazardous results.

In London, the interactions started with the natural fog that contained large particles, tens of micrometers in size, and so the acid formed was diluted enough for the conversion to sulfate to continue. When the sun rose, water evaporation from the droplets led to highly acidic particles, contrary to complete dissipation for unpolluted fog. Lack of wind to disperse the resulting stew then ended in tragedy.

In China, on the other hand, the haze starts from much smaller nanoparticles. Here, they discovered, ammonia in the atmosphere from fertilizers and automobile exhaust serves to neutralize the acid, allowing the conversion to sulfate to continue.

“A better understanding of the air chemistry holds the key for development of effective pollution reduction actions in China and elsewhere,” he says. “Our work shows that reducing the emissions of nitrogen oxides and ammonia will likely be effective in disrupting this sulfate formation process.”

A Welch principal investigator since 1998, Dr. Zhang is the University Distinguished Professor and the Harold J. Haynes Chair of Atmospheric Sciences and Professor of Chemistry. He coordinated the international effort with finding published in the Proceedings on the National Academy of Sciences