Finding Out Why a Seemingly Low-Emissions City Has So Much Smog

Looking to control perplexing seasonal smog, Norwegian city enlists help from high-resolution modeling

 

by Nancy D. Lamontagne

Desipite low air pollution emissions in the region, the city of Bergen in Norway experiences seasonal periods of smog.

Despite low air pollution emissions in the region, the city of Bergen in Norway experiences seasonal periods of smog.

On a sunny summer day, the streams of tourists who flow through the coastal city of Bergen, Norway enjoy crisp blue skies and crystal-clear views of the region’s scenic valleys. But if those same tourists were to come during a cloudless winter day, many would be shocked to find the city shrouded in hazy gray smog. These seasonal air pollution spikes are worrisome to residents and have brought the city unwanted media attention.

 

The pollution is perplexing in the isolated city with seemingly low emissions, no heavy industry, and a population of just 275,000. Searching for ways to reduce pollution without causing a heavy economic burden to Bergen’s businesses and residents, city officials have struggled to pinpoint the source of the smog. Some point to emissions from traffic or log fires commonly used for domestic heating, while others blame the heavy ship traffic in the city’s harbor.

 

Seeking to settle the matter, the Bergen Harbor Authority put out a call for research proposals, which led to a partnership with Tobias Wolf, a doctoral student at Nansen Environmental and Remote Sensing Center in Bergen. For the past few years, Wolf has combined sophisticated simulations with meteorological and air pollution measurements to investigate the complex conditions that lead to smog events in the long, narrow valley in which Bergen is situated. Now, in partnership with the Harbor Authority, Wolf and his colleagues are adapting their high-resolution simulations to help illuminate the harbor’s role in the city’s air pollution.

 

“The Bergen Harbor Authority has been severely criticized because people think that the ships may be contributing to the high air pollution events,” explained Wolf. “They need information on the actual severity of the impact from the ships in order to plan appropriate measures to lessen that impact.”

 

Modeling an air-pollution trap

On otherwise clear days in the winter, temperature inversions in the Bergen Valley trap air pollution at ground level, creating smog in the city center.

On otherwise clear days in the winter, temperature inversions in the Bergen Valley trap air pollution at ground level, creating smog in the city center.

As anyone who has ascended a mountain can tell you, air temperature usually drops as elevation increases. But in Bergen, a collection of factors combine to turn this conventional wisdom on its head. During Bergen’s long winter nights, clear skies and low, winds that could sweep through the valley exert a strong cooling effect on the ground. This periodically causes an atmospheric temperature inversion in which air at lower altitudes is cooler than air higher up. Temperature inversions trap air pollutants at ground level, and in Bergen this creates smog despite overall low emissions in the region.

 

During his doctoral work, Wolf discovered several important characteristics of high-pollution events and their associated temperature inversions. For example, measurements showed that although temperature inversions in the Bergen valley were more common than previously assumed, high air-pollution events only occurred when temperature inversions remained consistent for a relatively long time.

 

Small-scale influences

The Bergen valley is only 1 kilometer wide at the base, making it important to examine how small-scale air circulation influences air pollution events. For this, Wolf and his colleagues turned to the PALM LES model, a computer-intensive research tool that can simulate circulations at a 10-meter resolution. For comparison, a typical weather forecast uses simulations that run on a 1-kilometer or larger scale.

 

Combining the PALM simulation with their other findings, the researchers showed that during a high pollution event the local valley topography causes a reversing of normally southeasterly winds a few hundred meters above the valley. These high-level winds are balanced by lower level southeasterly, or down-valley, winds that arise from convection created by the contrasting temperatures of the cold city center and warm ocean currents.

 

Although the down-valley winds imply that air pollution from ships would likely be carried away from the city center, this information is not easily obtained by examining wind speeds and direction in the city center alone. With funding from the Bergen Harbor Authority, the researchers will use PALM simulations to study exactly how ship pollutants are transported.

 

This plot shows air pollution emissions (red) originating from the largest street in Bergen traveling through the valley. The blue arrows show the wind direction and speed at 35 meters above sea level. The length of the arrow in the box denotes a wind-speed of 3 meters/second.

This plot shows air pollution emissions (red) originating from the largest street in Bergen traveling through the valley. The blue arrows show the wind direction and speed at 35 meters above sea level. The length of the arrow in the box denotes a wind-speed of 3 meters/second.

Testing mitigation strategies

Wolf and his colleagues at the Nansen Environmental and Remote Sensing Center could use the same simulation approach to help decision makers understand how various mitigation strategies would actually affect air pollution. Wolf emphasizes the importance of “virtually testing” mitigation strategies ahead of implementing policies, especially since cutting back on ships in the harbor or reducing vehicle driving can have large economic impacts.

 

“We can set up a certain meteorological scenario and then simulate the emissions and the pollution transport,” he explained. “By changing the emissions according to the planned air pollution mitigation plan, we can see what happens to the distribution of the pollutants.”

 

The same high-resolution simulation approach could be used to study the impact of ships on air quality in other harbor towns in Norway and elsewhere. It is also applicable for exploring how small-scale air circulation affects air pollution in cities where mountains influence wind currents or skyscrapers create street canyons that might trap pollution.

 

Wolf’s doctoral work was funded by GC Rieber Funds.

 

Nancy D. Lamontagne is a freelance science communicator and a contributing writer for Creative Science Writing and the Thriving Earth Exchange.