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

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

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-2. [ + 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 = .58 => 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. 7000m 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!

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

CO and methane account for most of reduced gas flux to atmosphere CO observed from space: 50-200 ppb Methane observed from space: 1700-1900 ppb

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:

fsI Global mean [OH] ≈ 1.0×106 molecules cm-3 τCO = 2 months Production of O(1D) in troposphere takes place in narrow band [290-320 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

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

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

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