Description:
About carbon monoxide in Spokane:
Carbon monoxide (CO) is odorless and colorless gas that is produced naturally by both biological and abiotic sources (such as wildfires and volcanoes); natural air contains about 0.02 to 1 ppm of CO. CO is also a common byproduct of incomplete combustion of materials such as tobacco, wood, coal and petroleum derivatives (oil and gas). Automobile exhaust is rich in carbon monoxide, along with its more oxidized cousin, carbon dioxide (CO2). About 60% of anthropogenic CO comes from automobiles.
Carbon monoxide interferes with proper respiration in animals, by its very efficient substitution for molecular oxygen (O2) in red blood cells. Breathing air laden with carbon monoxide can quickly lead to an oxygen deficit in the bloodstream, and can impact health, especially among those with pre-existing respiratory problems and heart conditions. Carbon monoxide can build up in an automobile-rich urban environment, especially during poor atmospheric conditions such as no wind (stagnant air is typical of skyscraper-bounded streets) and a density inversion (often in winter, when automobile combustion is also inefficient). The Environmental Protection Agency has set CO standards of 9 ppm averaged over 8 hours and 35 ppm averaged over 1 hour.
The Washington State Department of Ecology has established real-time air quality monitoring stations in selected localities, to measure various pollutants such as CO, fine particulate matter, and ozone. These data are posted on the web (see below). The data shown here were collected in downtown Spokane, a city of 250,000 in easternmost Washington. The data have already been "smoothed" through calculation of a running 8-hour average, to better compare with the EPA standard.
We show the smoothed data, rather than the original values, to emphasize the cyclical nature of street-level carbon monoxide concentrations. As one might expect, CO peaks around the evening rush hour, and then declines steadily until the morning traffic surge. Superimposed on this cyclical behavior is a linear trend of declining carbon monoxide concentrations over the three days shown. What might be the cause of this decline?
Students can model the CO concentrations by first subtracting the linear decrease, to discover a damped oscillation. The damped oscillation can then be modeled with a sine function. Monday's late morning data will be underestimated by the model; why?
