1. OZONE PRODUCTION IN TROPOSPHERE
Photochemical oxidation of CO and volatile organic compounds (VOCs)
catalyzed by hydrogen oxide radicals (HOx) and nitrogen oxide radicals (NOx)
HOx = H + OH + HO2 + RO + RO2
NOx = NO + NO2
OH can also add to double bonds of unsaturated VOCs
Oxidation of VOC:
Oxidation of CO:
RO can also decompose or isomerize; range of carbonyl products
Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HOx radicals (branching reaction)

2. Energy states of the O atom (1s22s22p4)
determined by the arrangement of the four electrons in the 2p orbitals
total electronic
orbital angular
momentum number
multiplicity
Multiplicity = 2S+1, where S is the spin. The spin of an electron is (+/‐) 1/2.
Hund’s Rule: lowest-lying energy state is when the maximum number of orbitals
is filled, with electrons of the same spin
Find the energies (and related wavelengths of transitions) for O(1D, 1S) and O2 (3Sigma, 1Delta, 1Sigmal); H-O-H bond energy 493 kJ/mole
O(1 S)
O(1D)
O(3P)
: : O
: : O
Energy
O(3P) is a diradical
O(1D) is not a radical
(but still more reactive than O(3P)

3. NOx tropospheric columns observed from space
OMI NO2
2013
1015 molecules cm-2

4. High- and low-NOx oxidant regimes
Ozone loss, OH production,
initiation of HOx-catalyzed reaction chains
Low-NOx conditions
High-NOx conditions
HOx-catalyzed
ozone loss
Three branches
HOx- and NOx-catalyzed
ozone production
HOx loss,
chain termination
HOx loss, chain termination
Null (other than CO oxidation)

5. Methane oxidation cascade
(Carbon oxidation number)
CH4
(-IV)
CH3O2
(+I)
(0)
(0)
CH3OOH CH2O
CH3OH
(+II) CO
(+II)
CO2
(+IV)

6. also isomerization,
decomposition
multifunctional organics,
organic aerosol…
multifunctional organics,
epoxides, organic aerosol…
loss of organics to deposition
Brasseur and Jacob, chap 3

7. GLOBAL BUDGET OF TROPOSPHERIC OZONE (Tg O3 yr-1)
IPCC (2007) average of 12 models
Chem prod in troposphere
4700±700
Chem loss in troposphere
4200
±500
Transport from stratosphere
500 ±100
Deposition
1000
±200 O2 hn O3
Ozone lifetime: 24 ± 4 days
STRATOSPHERE
8-18 km
TROPOSPHERE hn
NO2 NO O3
hn, H2O OH
HO2
H2O2
Deposition
CO, VOC

8. Hu et al. [2017]

9. Dependences of ozone production and OH on NOx and CO
(where CO also serves as simple proxy for VOCs)
Initiation: HOx production
HOx production rate: P(HOx) ~ [O3][H2O]
Ozone production: P(O3) = k5[HO2][NO]
Efficient propagation: [HO2]/[OH] ~ [CO]/[NO]
HOx steady state:
Propagation: HOx cycling
First limiting case: rate(7) >> rate (8)
(“NOx-limited regime”)
[HO2] ~ P(HOx)1/2
[OH] ~ [O3]1/2[H2O]1/2[NO]/[CO]
P(O3) ~ [O3]1/2[H2O]1/2[NO]
Second limiting case: rate(8) >> rate (7)
(“NOx-saturated regime”)
Termination: HOx loss
[OH] ~ [O3]1/2[H2O]1/2 /[NO2]
P(O3) ~ [O3]1/2[H2O]1/2 [CO]/[NO2]

10. Factors controlling tropospheric ozone and OHhn NO
NO2 O3 hn
O(1D) M OH
HO2
H2O2
H2O CO
HNO3
P(O3) [OH]
NOx-limited regime: ~ [O3]1/2[H2O]1/2[NO] ~ [O3]1/2[H2O]1/2[NO]/[CO]
NOx-saturated regime: ~ [O3][H2O][CO]/[NO2] ~ [O3][H2O]/[NO2]

11. OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONS Box model calculation
NOx-limited regime
Ridge
NOx-
saturated
regime

12. Questions
Maximum photon flux during summer results in a seasonal maximum of ozone in polluted regions but a seasonal minimum of ozone in very clean regions. Why is that?
We found that ozone production is self-amplifying: P(O3) ~ [O3]1/2. Why don’t we get explosive behavior?
If the CO source to the atmosphere were to double, would the CO concentration (a) double, (b) less than double, (c) more than double?
If the NOx source to the atmosphere were to double, would the NOx concentration (1) double, (2) less than double, or (3) more than double?
Methane has an atmospheric lifetime of about 10 years. However, estimates of the global warming potential from methane emissions assume a lifetime of 17 years for decay of this added methane. Why is that? [Hint: think about the effect of increasing methane on OH]