Soundscapes of Smog: Researchers Let You Hear the Pollution of Cities (Literally)

*Listen* to some of America's most pristine and polluted airways.

Oakland's Caldecott Tunnel (allaboutgeorge/Flickr)

In the flat lands of California's Central Valley, oil pumps obscured by waving lines of fuel-richened air dip and rise on the horizon. Two hundred miles to the north and west, aging eighteen-wheelers pound through an urban bypass tunnel, staining the walls black with diesel fumes. Farther to the north, High Sierra pines scent the mountain air with notes of cinnamon and nutmeg, sending blue wisps of haze trailing gently upward.

Air is not the same everywhere. Across the extremes of the human environment, in both urban areas and wild, powerful natural and human forces combine to create intricate mixtures of chemicals that compose the air we breathe, seek for pleasure, or avoid. And now that air is made audible.

We created sounds from air samples (atmospheric particulate matter collected on filters) by first using gas chromatography to separate the thousands of compounds in the air (try it with markers at home) and then using mass spectrometry, which gives us a unique "spectrum" for chemicals based on their structure, to identify the compounds and assign them tones. Some compounds end up sounding clear and distinct, while others blur together into unresolvable chords. The result is a qualitative, sensory experience of hard, digital data. You can actually hear the difference between the toxic air of a truck tunnel (clogged with diesel hydrocarbons and carcinogenic particulate matter) and the fragrant air of the High Sierras.

In the following soundscapes you can listen to the air quality at study sites established across California by air pollution scientists at the University of California-Berkley's Department of Environmental Science, Policy, and Management, where new efforts are underway to better understand the air we breathe and to devise new efforts to improve our polluted areas.

Take a listen.

The Caldecott Tunnel, Oakland, CA

The Caldecott Tunnel cuts east from Oakland through the Berkley Hills, linking greater Contra Costa County with the Bay area. To capture the direct emissions of cars and trucks (which often vary greatly from projected emissions) we dangled an air sampler from a ventilation passageway above the busy road. What you hear in the soundscape is an eerie mixture of highly unsaturated compounds called "polycyclic aromatic hydrocarbons" (those distinct chirps at the beginning) and complex, saturated heavy hydrocarbons (the long, low droning chords at the end). Both of these result from burning fossil fuels. And many are dangerous carcinogens, mutagens, and teratogens -- linked to cancers, gene mutations, and birth and developmental defects.

Bakersfield, California

The town of Bakersfield sits in the middle of California's Central Valley on swampland reclaimed from the nearby Kern River. It hosts, supposedly, the world's largest ice cream plant (Dreyer's Grand!) and sits in one of our country's most productive oil counties (Kern County). It is also, according to the American Lung Association, America's most air-polluted city. You'll notice it sounds a lot like a contained highway tunnel -- the result of fresh hydrocarbons from a main trucking highway and oil and gas fields surrounding the sampling site.

Pasadena, California

Several years ago almost 100 air and climate scientists joined forces to resolve a lingering question in atmospheric science: Why do existing air quality models under-predict urban particulate (unhealthy airborne particle) concentrations by a factor of between two and 10? Their efforts met on the campus of Caltech in Pasadena, which sits just downwind of greater Los Angeles.

Because it is so close to LA and major shipping ports, you would expect Pasadena to sound the same as Bakersfield, or perhaps the Oakland tunnels. But in Southern California winds blow in from the ocean and trap the smog of LA at the foot of the surrounding San Gabriel Mountains before carrying it down to Pasadena. The usual hydrocarbon slurry then has a chance to "cook" in the oxidizing atmosphere of the hot mountain foothills. The resulting soundscape is more bubbly than the pure hydrocarbon samples above, as you can hear the new presence of complex oxygenated compounds.

The High Sierras

In a remote pine forest deep in the Sierra Mountains we gathered particulate data from the top of a swaying tower, which was installed to take measurements from above and inside the tree canopy. This soundscape starts with bubbly, diverse tones -- the result of small compounds (smaller compounds show up earlier in the data and hence earlier in the soundscape) that plants release to attract or repel insects. When you smell the sharp tang of pine pitch or fragrance of mountain laurel, you are smelling the volatile chemicals produced by the plant, which have evaporated and taken to the air, where hungry herbivores and pollinators can detect them.

Despite the remoteness of the study site, you can hear the influence of vehicle emissions later in the soundscape, as low, Bakersfield-type drones fill in. Though this site is far from any major cities or highways, winds bring emissions up from Sacramento during the day and back down the mountain at night -- a common pattern in the Central Valley of California -- making the influence of humanity nearly impossible to escape.

Lingering Pollution

The negative influence of human emissions on forests and remote areas is a problem that is increasing throughout the country and the world, and it is leading to one of the major issues currently being tackled in the scientific community. When human and natural emissions interact, as they do outside our cities, more particulate matter (smog and haze) is formed than we expect. This leads to inaccuracies in pollution models and, unsurprisingly, makes improving air quality more difficult. In some places, like the American Southeast, these poorly understood interactions have resulted in unpredicted, large-scale trends in regional warming and cooling.

These interactions are also, incidentally, one of the reasons President Reagan once said that trees pollute more than people do.

In short, to better understand and regulate our changing climate, the consequences of mixing those bubbly, natural tones with that droning, fossil-fuel chord need to be better studied.

Data analysis contributing to this project was performed by the research group of Dr. Goldstein at UC Berkeley. We'd like to thank the research groups of Dr. Harley (UC Berkeley) and Dr. Surratt (UNC Chapel Hill) for collecting samples.