by Gordy Slack
Cities are major players in the phenomenon of global warming: transportation, air conditioning and industry all convert fossil fuels into carbon dioxide (CO2). But cities can also absorb a significant amount of CO2; trees, greenways, gardens, parks, and agricultural zones can use it as the raw material for photosynthesis. While measuring a city’s net output of CO2 is generally straightforward, it has proven more difficult to calculate how much CO2 people are actually generating before some of it gets absorbed by plant life. That gross number will be important to track, though, says Elliott Campbell, assistant professor of engineering at UC Merced, if urban planners and policy makers are going to help California’s cities minimize their contribution to climate change.
Planners and politicians must be able to sort out where urban CO2 is coming from, and ultimately where it is going, says Campbell, so they can measure, predict, and influence the impact of different policies and practices on a city’s overall emissions.
For example, what would be the effect on Santa Cruz’s or Merced’s CO2 contributions if a light rail were built in those cities? It might reduce transportation-related emissions that could be detected through observations of traffic and fuel sales. However, the ultimate validation of light rail and related climate strategies would be to measure the levels of atmospheric CO2 above the cities themselves. Unless they know how much CO2 green corridors and trees are absorbing, though, policy makers cannot make that calculation, says Campbell.
To fill in those variables, Campbell, in partnership with UC Santa Cruz engineer Claire Gu, is investigating a new approach that employs sensor networks and mathematical models to calculate and predict both the gross and net production of CO2 throughout urban environments.
In a 2008 Science paper, Campbell describes a way of calculating gross human contributions of CO2 by tracking another carbon-based molecule. Carbonyl sulfide (COS) is produced by plants in a biological process parallel to photosynthesis. The method, which determines how much carbon-absorbing photosynthetic activity is occurring, has already proven effective on a broader, regional scale. Campbell’s and Gu’s new project, supported by a $50,000 CITRIS seed grant, aims to sharpen the resolution of the method for more focused, urban-scale readings.
Their plan is straightforward. A dome of CO2 forms around each city. The gas is most concentrated at the center of urban action; moving toward the city’s edge, CO2 tapers off as it diffuses and mixes with other atmospheric gasses. Campbell and Gu simply plan to measure and compare atmospheric levels of CO2 and COS at numerous strategically determined points around that urban CO2 dome.
“If you measure CO2 and COS at the same time,” says Campbell, “and then compare them, you get a picture of what the contributions are of the biosphere—the trees, the lawns, parks—and what the contributions are of the anthrosphere—the vehicles, the power plants, the HVAC systems.”
In practice, however, it is not so easy. While CO2 is abundant and relatively easy to detect and measure with existing sensors, COS is not.
“Carbon dioxide can be measured in ambient urban air in parts per million,” says Gu. “Carbonyl sulfide can only be measured in parts per trillion.”
Gu has approached Santa Clara instrument maker Picarro about co-developing a COS-measuring sensor. Picarro already makes a robust, cavity ring-down spectrometer for sensing CO2. “You can drop the briefcase-size device—something you do not usually do to optical instruments—and it still works,” says Gu. That kind of durability would make deploying the instrument on building tops, antennae, and on other exposed and perilous urban environments feasible, she says.
An alternative sensor employs something called photonic crystal fiber Raman spectroscopy, in which an air sample is injected into empty spaces that honeycomb a small crystal fiber and then a light is shone through the fiber’s core. The signature interactions between the molecules and the light reveal which kinds of molecules are in the air.
While Gu and her colleagues develop the network of sensors, Campbell is studying how many sensors will be needed as well as where and how densely they need to be distributed throughout a city. He is also developing the data assimilation models that they will apply to the sensor results in order to calculate how much CO2 is being emitted, how much is being absorbed, and where those emissions and absorptions are occurring. “The challenge is starting with a limited number of point observations and then inferring what emissions are in different locations all across the city,” Campbell says.
“We have robust methods for saying what the carbon cycle is doing at the global scale. That is well understood. But at smaller scales, the problem becomes much more challenging,” says Campbell.
Cities are not only where most of our greenhouse gasses come from, they are also where it is now easiest to promote the urgently needed policies that restrict them, he says.
“Studying climate change at the global scale is very important,” says Campbell. “But when I work on studies of global climate change, I am often frustrated by the lack of political will for nations to take advantage of these scientific findings. Currently, a better connection between policy makers and scientists may be at the city and state levels where climate policy is actually being enacted.”
“There is deadlock at the national and international levels,” he says. And there’s little indication that will change any time soon. “But there are a number of cities, such as Santa Cruz and Merced, that are actually moving forward with policies that will help significantly reduce climate change. So it makes sense to focus our science and engineering efforts on supporting those cities with useful data.”