1. Tropospheric Ozone System
Just Ox Ox XO O3
The key to producing ozone is a free oxygen
We can think of ozone as an O2 molecule
“carrying” the O X O2
Reactions:
(1a) O2 + O  O3
(1b) O2 + hn  O2 + O

2. Tropospheric Ozone System
Adding NOx Ox
NOx XO O3 2a
NO2
The main source of that free O in the troposphere is NO2
We can think of NO as a “carrier” molecule for the free oxygen 2b hn M 2b 2a X O2 NO
Reactions:
(2a) NO2 + hn  NO + O
(2b) O3 + NO  NO2 +O2 M
We keep track of nitrogen with NOx = NO + NO2
The free O bound up in NO2 can be thought of as a “potential Ox”
XO = Ox + NO2

3. Tropospheric Ozone System
Adding NOx Ox
NOx XO O3 2a
NO2
In equilibrium, we have 2b hn M 2b 2a X O2 NO
This is not unique – depends on initial conditions How much NOx and XO do you start with?
Reactions:
(2a) NO2 + hn  NO + O
(2b) O3 + NO  NO2 +O2 M
All NOx and no XO means all NO – no ozone producing potential
All XO and no NOx means all O3 – fixed amount of O3
In the end, this underpredicts observed O3 (i.e. assuming we start with pure NO2, not enough free oxygen there to explain that which ends up in Ox.
Need additional source of XO to explain observations.

4. Tropospheric Ozone System
Adding HOx Ox
NOx
HOx XO O3 2a
NO2 3
HO2
NO can get “recharged” when HO2 passes along its free oxygen in (3)
We can think of OH as a “carrier” molecule for the free oxygen in HO2. 2b hn M 2b 2a 3 3 X O2 NO OH
Reactions:
(3) HO2 + NO  NO2 + OH
We keep track of hydrogen with HOx = OH + HO2
The free O bound up in HO2 can be thought of as another “potential Ox”
XO = Ox + NO2+ HO2

5. Tropospheric Ozone System
Hydrocarbons: CO Ox
NOx
HOx CO
OH can get “recharged” when it reacts with carbon monoxide CO
CO is lost (CO2 is pretty inert) by this reaction, so it does not participate in a COx cycle here. XO O3 2a
NO2 3
HO2 4 CO 2b hn M 2b 3 3 4 X O2 NO OH
Reactions:
(4) CO + OH +O2  CO2 + HO2
The potential for CO to steal a free O from atmospheric O2 can be thought of as another “potential Ox”
XO = Ox + NO2+ HO2 + CO

6. Tropospheric Ozone System
Hydrocarbons: CH4 Ox
NOx
HOx CO
CH4 XO O3 2a
NO2 3
HO2 4 CO 5
CH4 2b hn x3 hn M 2b 3 5 3 4 5 X O2 NO OH
Reactions:
(5a) CH4 + OH + O2  H2O+ CH3O2
CH4 has many pathways for its degradation.
Low NO  sink for HOx and minor source of CO
High NO  source of 2HOx, 4 XO, one CO
(5b) CH3O2 +NO + 3O2
CO + NO2 + 3HO2
(5c) CH3O2 + HO2  CH3OOH + O2
The potential for methane to steal free O from atmospheric O2 is potentially strong, if there’s lots of NO around, otherwise, it’s not as strong.
XO = Ox + NO2+ HO2 + CO + CH4f(NOx)

7. Summary of Trop Ozone
NOx needed, but O3 determined only by a ratio between NO2 and NO, not the abundance
Need sources of free oxygen
Emissions of NO2
Emissions of CO
Oxidation of hydrocarbons in the presence of NO
Actual O3 amount is not the total amount of XO – how the XO gets trapped in it’s non-Ox forms and how these are destroyed are important to ultimate O3 concentrations. Think of stratospheric examples.

8. Budgets
Now it’s important to understand the sources and sinks of the “carrier” species NO OH CO
hydrocarbons

9. NOx Sources Sinks NO2 + OH  HNO3(g)  HNO3(aq)
Emissions by fossil fuel burning
Emissions by biomass burning
Soil emission (biological and fertilizer)
Lightning
Sinks
Dry deposition
Wet deposition of nitrate
NO2 + OH  HNO3(g)  HNO3(aq)

10. HOx Sources Sinks O3 + hn  O2 + O(1D) H2O + O(1D)  2OH
Oxidation of hydrocarbons (see Ozone chain)
Sinks
Wet deposition of nitrate and sulfate
NO2 + OH  HNO3(g)  HNO3(aq)
Wet deposition of hydrogen peroxide
HO2 + HO2  O2 + H2O2

11. Carbonaceous species CO Methane (CH4) Non-methane hydrocarbons NMHC
Main source is fossil fuels and photooxidation of hydrocarbons
Main sink is OH – short CO lifetime led to discovery of photochemistry in troposphere
Methane (CH4)
Main sources are anoxic decomposition of organic material (natural, landfills, mines, etc)
Main sink is photooxidation in atmosphere via formaldehyde HCHO
Non-methane hydrocarbons NMHC
Alkanes, alkenes, aromatics, and oxygenated hydrocarbons
Many natural and industrial sources (“trees pollute”)
Sinks are photooxidation by OH and directly

12. Nitrogen Cycle

13. Sulfur Cycle