An Unexpected Spike in Nighttime Pollution

When Southwestern PA residents wake in the middle of the night to the stench of rotten eggs, many open SmellPGH, a mobile device app that crowdsources pollution odor reports in the Pittsburgh region. Researchers at Carnegie Mellon University’s Center for Atmospheric Particle Studies investigated the nighttime odor-causing plumes, and for the first time, have determined that the smell correlates with sulfur compounds, including sulfuric acid which typically is only observed during the day.

Sulfuric acid plays a widespread role in industry from petroleum refining to battery, fertilizer, and steel production. When this “king of chemicals” escapes to the atmosphere, it mixes with other pollutants to form acid rain and smog. It is no surprise that the Southwestern PA region, home to heavy industry including a steel mill, coke plant, and steel processing plant, has concentrations of sulfuric acid in its air.  

Sulfuric acid is thought to form primarily during the day through chemical reactions triggered by sunlight, or photochemistry. However, data from the researchers suggests a different mechanism is at play during the night.

The Carnegie Mellon team conducted field campaigns in the fall of 2023 and 2024, using air mass tracing to identify the source of the high concentrations of sulfuric acid as originating from the southeast of Pittsburgh. The team identified sulfuric acid concentrations reaching levels typically only seen during daylight hours. Notably, the nocturnal concentrations increased significantly on more than half of the nights monitored. 

The formation of sulfuric acid increased even in conditions where we would typically expect levels to remain low.

Coty Jen, Associate Professor, Chemical Engineering, Carnegie Mellon University

In some instances, concentrations peaked at 10⁸ molecules per cubic centimeter. This figure exceeds previously reported nighttime levels and rivals typical daytime measurements. It also surpasses concentrations observed in Beijing, China, a megacity that is addressing its historic air pollution.

"These measurements indicate a previously unrecognized nighttime formation pathway for sulfuric acid," said Coty Jen, an associate professor of chemical engineering. “The formation of sulfuric acid increased even in conditions where we would typically expect levels to remain low."

The researchers hypothesize that the acid is being formed within a plume containing metal particles and black carbon emitted by the steel mill, coke plant, and steel processing plant industrial sites. These particles could serve as heterogeneous catalysts, stimulating a process where chemical reactions occur on surfaces. This allows sulfur dioxide to convert into sulfuric acid without the need for sunlight.

The research findings, published in the journal Environmental Science & Technology, observed a positive correlation between sulfuric acid spikes and an increase in particulate mass, specifically metals commonly associated with coal combustion and steel production.

While the findings provide a clearer picture of Pittsburgh’s nocturnal air chemistry, the researchers emphasize that more work is needed. Future studies will focus on identifying the specific chemical compounds and precise catalytic processes that drive these nighttime increases.

For the residents of Southwestern PA, this could mean cleaner air and a better night’s sleep.

 

Researchers in Center for Atmospheric Studies at Carnegie Mellon University focus on the intersection of industrial emissions and regional air quality. The team specializes in high-precision measurements of trace gases and particulate matter to better understand their chemical evolution and impact on Earth's atmosphere.  

For media inquiries, please contact Lisa Kulick at lkulick@andrew.cmu.edu.