Ozone Photochemistry - Part 2

The purpose of this series of posts is to discuss the photochemistry leading to formation of tropospheric ozone and smog.

In Part 1 Eli discussed how tropospheric ozone forms and how photolysis of ozone leads to formation of HO radicals.  The story starts with the photodissociation of NO2 below 420 nm to form O atoms and NO.  The O atoms react with O2 molecules (plenty of them) to form ozone, O3.  Ozone it self is not the greatest thing in the world to breathe, and photolysis of ozone produces excited O(¹D) atoms, which either react with water vapor to form HO radicals or are collisionally quenched back to O(³P), which, in turn reacts with O2 to reform ozone. 

Wither HO (or OH, depends on your age and field).  Let's start by not worrying about hydrocarbons.  In that case, in a really clean atmosphere the OH will react with carbon monoxide, CO  to form hydrogen atoms and CO2.  The carbon in CO2 is fully oxidized and that is the end of that.  The hydrogen atoms react with O2 to form hydroperoxyl radicals, HO2.  HO2 is a lot less reactive than HO, so as a general rule the atmosphere has a lot more HO2 than HO, but HO2 does react with NO and that reforms NO2

Eli is quite happy with the figure above, moving NO2 to the center emphasizes the intermingled NOx and HOx cycles.

There are a few things left out here.  The major one, of course, is reactions with volatile organic molecules including methane, CH4.  Eli has discussed that previously.  For another HO2 can react with ozone to form two molecules of O2 and HO, but that is roughly three orders of magnitude slower than the reaction with NO.  There are also some termination steps.  For example, the reaction of NO2 with HO yields nitric acid HNO3, which can rain out.  For another, HO + HO2 --> H2O + O2.  And then, of course, there is deposition.  Ozone hitting the ground will never rise.  Same for most of these other molecules.

Next we will discuss the implications of this chemistry for ozone in the unpolluted troposphere.