NetLogo User Community Models

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Download If clicking does not initiate a download, try right clicking or control clicking and choosing "Save" or "Download".(The run link is disabled for this model because it was made in a version prior to NetLogo 6.0, which NetLogo Web requires.)

## WHAT IS IT?

This model simulates the formation of photochemical smog over time. Nitrogen dioxide (NO2) and volatile organic compounds (VOC) are primary pollutants that are released into the atmosphere as a result of fossil fuel combustion. In the presence of oxygen (O2) and sunlight, they undergo chemical reactions that form ozone (O3) and photochemical oxidants (Ox), which are the principal components of smog.

## HOW IT WORKS

The primary pollutants, NO2 and VOC, are emitted at rates designated by the user. When nitrogen oxide (NO2) is the only primary pollutant present, the following reactions occur:

(1) NO2 + sunlight = NO + O (nitrogen dioxide breaks down)
(2) O + O2 + sulight = O3 (ozone is created)
(3) NO + O3 = O2 + NO2 (ozone is destroyed)

When VOC is present in addition to NO2, the following reaction replaces reaction (3) above:

(4) NO + VOC = Ox (photochemical oxidants are created)

So, the presence of VOC causes a build-up of ozone in the atmosphere and leads to the formation of photochemical oxidants, which together are what we call smog. The chemical reaction rates of (1) and (2) are dependent on sunlight, so higher levels of sunlight cause this reaction to occur faster. When there is no sunlight, these reactions do not occur.

## HOW TO USE IT

You can adjust several variables that will affect the behavior of the model:

"nitrogen-dioxide" - the number of NO2 molecules created during each tick
"volatile-organic-compounds" - the number of VOC molecules created during each tick
"sunlight-intensity" - the level of sunlight (0 - 100)

You can also choose the conditions under which the model will run:

When the "day-night-cycle" switch is OFF, the sunlight level will remain the same over time, at the level that the user selected. When the switch is ON, the level of sunlight will change over time, going from 0 at "night" to the user's selected setting during the "day."

When the "pollutants-cycle" switch is OFF, the emissions rates for NO2 and VOC will remain the same over time, at the level that the user selected. When the switch is ON, the emissions rate will change over time going from 0 at "night" to the user's selected setting during the "day." This reflects the fact that most emission-creating activities occur during daylight hours.

When the "atmosphere-inversion" switch is OFF, molecules that reach the edge of the screen disappear. This simulates a situation in which normal air flow causes dispersion of pollutants away from their source. When the switch is ON, molecules that reach the edge of the screen bounce back into the screen area. This reflects a situation in which a temperature inversion in the atmosphere prevents pollutants from dispersing.

When you are ready to run the model, click the "Setup" button, then click "Go."

## THINGS TO NOTICE

The model assumes that the supply of oxygen (O2) is infinite. This is a realistic assumption for the reactions occuring in this model. Oxygen makes up about 21% of the atmosphere--that's 210,000 parts per million (ppm). Dangerous levels of smog are on the order of 0.4 ppm. In order to make the number of oxygen molecules in the model be in realistic proportion to the pollutant molecules, there would have to be millions of them, which is not feasible. So instead of using O2 turtles, the model assumes that O2 is always available.

## THINGS TO TRY

Try each of the switches separately, then try them together. Compare the results.