1. TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY
The many faces of atmospheric ozone:
In stratosphere: UV shield
Stratosphere:
90% of total
In middle/upper troposphere: greenhouse gas
Troposphere
In lower/middle troposphere: precursor of OH,
main atmospheric oxidant
In surface air: toxic to humans and vegetation

2. Direct effect
Indirect effect
“official chart”
IPCC [2014]

3. Radiative forcing from tropospheric ozone: absorption in atmospheric window
Question: How many watts/m2 radiated to space on a clear night in the Niger with the surface still at 320K? (200 cm-1 x 150x10-3 W/m2) x 2pi= 6 x (30+20) = 300 Wm [ + 200x100x10-3 ?]
Question: What would be the emission rate if T=280 instead of 320 (a cool clear winter night)?
Estimate that it goes at T4: (7/8)4 = => 100 Wm-2.
How much does that cool the air? Assume snow cover (perfect insulator), effect through 100 m depth of air.
Heat capacity of air = 1005 J/kg/K. In 1 hour each sq. meter loses 36 KJ m of air = 10,000 kg/m2, so 200m is 300 kg.
DelT= 360KJ/300/1.005 = 1.2 K/hr. In a night, 12 hr, 14K; 16 hr, 19K T decline!

4. The atmosphere as oxidizing medium in global biogeochemical cycling
Atmospheric oxidation is critical for removal of many pollutants, e.g.
methane (major greenhouse gas)
Toxic gases such as CO, benzene, mercury…
Gases affecting the stratosphere
Oxidation
Oxidized gas/
aerosol
Reduced gas
EARTH
SURFACE
Emission
Uptake
Reduction

5. CO and methane account for most of reduced gas flux to atmosphere
CO observed from space: ppb
Methane observed from space: ppb

6. OH is the principal tropospheric oxidant
Almost all non-radical reduced species are oxidized by OH in troposphere
OH production in troposphere follows same pathway as in stratosphere:

7. fsI Global mean [OH] ≈ 1.0×106 molecules cm-3 τCO = 2 months
Production of O(1D) in troposphere takes place in narrow band [ nm]
solar flux I
at sea level
ozone absorption
cross-section s
O(1D)
quantum
yield f
fsI
Global mean [OH]
≈ 1.0×106 molecules cm-3
τCO = 2 months
τCH4 = 10 years

8. Estimating global tropospheric OH using methylchloroform
Methylchloroform (CH3CCl3):
Uniquely anthropogenic (industrial solvent), banned by Montreal Protocol
Removed from troposphere by oxidation by OH, transport to troposphere
Mass balance equation for troposphere (T):
Observed decay rate
loss rate from
CH3CCl3 + OH
exchange with stratosphere
(small)
exponential decay
Lifetime =1/k[OH]T
= 5 years

9. TRENDS IN GLOBALTROPOSPHERIC OH inferred from methylchloroform and computed from models
Holmes et al. [2013]

10. Questions
How will recovery of the stratospheric ozone layer affect tropospheric OH concentrations?
How might global warming affect tropospheric OH concentrations?