Understanding how urban smog particles grow quickly

Microscopic particles hanging in the air above some of the world’s most populated cities often form a hazy smog that can block the midday sun and worsen certain health conditions. A combination of environmental factors unique to big-city winters plays key roles in enabling the smallest of those particles to survive long enough to grow into the larger particles that cause smog, according to a new study (Nature 2020, DOI: 10.1038/s41586-020-2270-4).

Airborne particulate matter and other forms of air pollution are closely tied to respiratory disease and cardiovascular problems. In animal studies, low-quality air has been shown to cause preterm birth and pregnancy complications. The tiny particles also affect climate through their interactions with sunlight and clouds.

To understand and help improve air quality, especially in urban centers, atmospheric researchers have developed models that describe the complex interplay between three types of airborne species—molecules, clusters, and particles. The theories describe the way gas-phase molecules—for example, nitrogen and sulfur species from power plant and automobile emissions—can stick to other molecules and grow into molecular clusters. Clusters can grow by trapping nearby molecules and by glomming onto other clusters, eventually growing to form particles measuring hundreds of nanometers in diameter and larger.

To reach the low end of the nanometer size range, newly formed clusters need to bulk up quickly, gobbling hapless molecules and joining forces with other clusters before the clusters are scavenged by larger particles. Researchers, though, haven’t figured out how that can happen in urban centers.

In densely populated cities, concentrations of relatively large airborne particles, which are voracious scavengers of smaller species, can be more than 100 times as high as those in rural locations. At the same time, there isn’t much difference in the measured rates at which the tiniest particles grow in both types of regions. If tiny urban particles don’t grow much faster than their rural counterparts, they should have next to no chance of surviving to particle adulthood because of the concentrated presence of the larger, hungry particles. Yet they do survive, growing up to form the constituents of city smog.

To sort out this atmospheric puzzle, a team of roughly 80 researchers led by Mingyi Wang and Neil M. Donahue of Carnegie Mellon University, Ruby Marten of the Paul Scherrer Institute, and Weimeng Kong of California Institute of Technology conducted a series of experiments in a controlled-atmosphere chamber. The team tested a mixture of gas-phase nitric acid, ammonia, and other components of big-city air pollution, and used a number of mass spectrometry methods to probe the gases’ behavior over a range of temperatures.

The researchers found that under various conditions, nitric acid and ammonia react to form ammonium nitrate. That compound can rapidly condense onto newly formed clusters, causing them to grow 10 to 100 times as fast as previously observed, quickly reaching sizes large enough to avoid being consumed by other particles. Growth rates depend strongly on temperature. For example, the rate at -10 °C is 200 times as great as the rate at 5 °C.

“This is a very impressive study that demonstrates the potential contribution of nitric acid and ammonia to the early growth of newly formed particles under urban atmospheric conditions,” says Lin Wang, an environmental scientist at Fudan University.

The University of Manchester’s Hugh Coe, another environmental scientist who was not involved in the study, also finds the work impressive. Ammonium nitrate is an important component of urban winter and springtime particulate matter, Coe says, but has not been thought to play a major role in particle formation. “These observations were made in a laboratory chamber, but the authors convincingly argue that similar conditions can occur transiently in megacities.”

Transient conditions may be the key, the study’s authors say. Earlier studies, which reported much lower particle growth rates, typically used numbers for environmental factors that were averaged over space and time. Yet real-world conditions can cause values to vary widely. Rush-hour traffic, for example, can cause emissions from cars and trucks to vary greatly over short periods of time and small distances. And closely spaced tall buildings, roads, and traffic can cause large variations in temperature over short distances because of shadows, heat absorption and reflection, and variations in air flow.

Now that the researchers have come up with a piece of the puzzle describing particle growth in big cities, they’re turning their attention to studying how this mechanism plays out in Earth’s upper atmosphere.