And so, from Taiyuan to Beijing, Syracuse to Providence, and all over the United States, he has been working toward figuring that out for himself.

Chai is a postdoctoral research associate in the lab of Meredith Hastings, Associate Professor of Earth, Environmental and Planetary Sciences and pioneering biogeochemist. In 2014, the Hastings Lab developed a novel technique for separating different isotopes of nitrogen from nitrogen oxides, or NOx: a component of the atmosphere that is responsible for smog, acid rain, and more.

By analyzing the types of nitrogen present in air samples from different environments across the country, Hastings, Chai, and the rest of the lab are piecing together a method of identifying what types of reactive nitrogen come from what sources.

Chai's current research focuses on a molecule known as nitrous acid, or HONO.

HONO is important because it is a major precursor to OH, the so-called "atmospheric vacuum cleaner" that reacts with, and thereby starts to clean up, many of the primary pollutants in the lower atmosphere, and potentially generate a large number of secondary pollutants.

But scientists don't fully understand where HONO comes from. Its primary sources—wildfires, soil, and vehicle emissions—have been somewhat identified; however, these sources don't account for the amount of HONO that researchers like Chai observe in the atmosphere.

"Air quality models always underpredict and underestimate the HONO we observe in the field," he says. "That means there are significant, missing HONO sources in the model. We want to find those sources, constrain them, and quantify them so as to improve the air quality model predictions."

Chai's work requires traveling all over the country, using a customized instrument to collect air samples from different places, and transporting them safely back to the lab. He then uses the state-of-the-art technology at Hastings Lab to make two crucial measurements: how much HONO is in the air, and what composition of isotopes makes up the HONO.

He explains that the instrument, which he designed and created himself, enables him to collect this data in a novel way.

"The whole idea is not brand new, but the combination is new—especially the application for isotopic signature determination," says Chai. "This is the first system with the capability to determine the HONO isotopic signatures of oxygen and nitrogen."

"It's 'ho-new'," he laughs. "And it has very high detection limit, because the HONO concentration in the air is very low."

So far, his analysis has revealed that different sources of HONO in the air do, indeed, have different isotopic profiles. Last summer, Chai deployed his instrument at a biomass burning site in Montana and a manure fertilized farm in Pennsylvania.

"The isotopic signature from soil is quite different from that of the biomass burning," he says. "That can potentially be distinguished."

In order to pick up the isotopic signature of vehicle emissions, Chai has also driven his system across highways up and down the east coast, and has stationed it at an air quality research station right here in Providence. This year, he plans to deploy the instrument at another biomass burning site in the western United States.

Once he has collected enough data to tease apart the isotopic signatures of fire, soil, and vehicle emissions, he plans to begin collecting air samples and determining how much of the HONO is the air has been contributed by each of the sources.

"The next step is that we can sample the air from anywhere," says Chai. "Then, we can put all the source signatures into a mixing model, which can tell us the relative contribution from different sources."

Chai's research will have implications for other atmospheric scientists; however, he also believes that policymakers can and should use this information to inform laws and best practices.

"Understanding the chemistry of soil emissions can serve as a benchmark for determining which sort of fertilizer we want to use and which type of soil management we want to use," he says. "And many of the biomass burnings in the US are prescribed fires, which are used for soil management. This can all provide useful information for air quality control."

Moreover, his research is important for its significance to human health. With any luck, future atmospheric scientists will not need to get share Chai's own experience enduring the effects of poor air quality in order to follow in his footsteps.