Peroxyacyl Nitrates (PAN's)

Peroxyacyl Nitrates (PAN's) are one of the main components in photochemical smog. First discovered in the 1950's, their general structure is RC(O)OONO2, and they are often formed from the reaction between aldehydes, NOx and the OH radical. PAN's are considered to be secondary pollutants as they are not emitted directly, but come about from the combination of other pollutants.

A general formula for the formation of PAN's is:

NO2 --> NO + 1/2 O2
1/2 O2 + O2 --> O3
NO + Hydrocarbons + O3 --> Peroxyacyl nitrates + O2

Peroxyacetyl nitrate, the first compound in the series of PAN's is formed from the reaction of acetaldehyde with OH:

                          .
CH3CHO + OH --> CH3CO + H2O
CH3CO + O2 --> CH3C(O)O2
CHC(O)O2 + NO2 + M <==> CH3C(O)OO2NO2 + M

Here's a moveable model of Peroxyacetyl Nitrate!

Image from "http://www.ci.fort-collins.co.us/ENVIRONMENTAL/INDOOR_AIR/webcam.html"

An important thing to note about PAN's are that they are not easy to get rid of. They do not photodissociate in the atmosphere, and are not very water soluble. The main sink of PAN's is thermal decomposition, which converts PAN's back to a peroxyacyl radical and NO2. The lifetime of PAN's can range from about 30 minutes to 8 hours, and are strongly dependent on temperature. In the upper levels of the troposphere, where temperatures are low, PAN tends to have lower levels and is important in that it acts as a reservoir species for NOx.

Quick Quiz: What is the main sink of PAN's?
Thermal Decomposition
Photodissociation
Wet Deposition
Dry Deposition

Effects of PAN's

PAN's are considered to be photochemical oxidants. Basically, that means they are very reactive and will cause thing to oxidize that normally wouldn't (like other chemicals or your lungs). One noticeable effect of PAN's is that they cause eyes to water and burn. PAN's are one of the key elements in photochemical smog, and along with other components such as NOx and ozone compose a type of smog known as "brown air". Smog is believed to be responsible for a number of health problems including respiratory problems and decrease immune function.

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