METO 637 Lesson 14

Photochemical chain initiation In the troposphere several species are present that absorb solar ultraviolet radiation and can initiate radical- chain reactions. Ozone is phololyzed at wavelengths less than 310 nm and the following reactions occur: O 3 + hν → O 1 (D) + O 2 ( 1 Δ g ) O 1 (D) + H 2 O → OH + OH Since H 2 O is a minor constituent in the atmosphere, and O 1 (D) is quenched by collisions with O 2 and N 2 the OH production rate is not high. Note that the O 3 P that results from the quenching will quickly be converted back to ozone.

Photochemical chain initiation Note that the rate of production of OH depends uniquely on the quantum yield of O 1 (D) as a function of wavelength. Another source of atomic oxygen in the troposphere comes from the dissociation of nitrogen dioxide at wavelengths less than 400 nm: NO 2 + hν → NO + O The NO molecule can be oxidized back to NO 2, and so the NO2/NO cycle can be catalytic. However, in contrast to the stratosphere, the oxidant is not O.

Oxidation steps The OH radical reacts mainly with CO and CH 4 : OH + CO → H + CO 2 OH + CH 4 → CH 3 + H 2 O In the unpolluted atmosphere about 70% of the OH reacts with CO, and 30% with CH 4. These reactions are followed by: H + O 2 + M → HO 2 + M CH 3 + O 2 + M → CH 3 O 2 + M Both products are peroxy radicals (they oxidize) Where the NO concentrations are low we get HO 2 + HO 2 → H 2 O 2 + O 2 CH 3 O 2 + HO 2 → CH 3 OOH + O 2

Oxidation steps – high NO If NO is high then the hyperoxides react rapidly: HO 2 + NO → OH + NO 2 CH 3 O 2 + NO → CH 3 O + NO 2 The methoxy radical, CH 3 O, can then react with O 2 : CH 3 O + O 2 → HCHO + HO 2 HCHO, formaldehyde, is dissociated ijn the atmosphere to produce H and CHO Followed by HCO + O 2 → CO + HO 2 Hence CH 4 has been oxidized to CO and HO X

Chemistry of the troposphere

Higher hydrocarbons Higher hydrocarbons have similar oxidation steps to methane. In this case we represent the hydrocarbon as RH, (for methane R = CH 3 ). For ethane and propane R is formed by subtraction of the hydrogen atom. Next RO 2 is formed which oxidizes NO to NO 2 RO 2 + NO → RO + NO 2 RO → R’ + R”CHO RO + O 2 → R’R”CO + HO 2 Followed by NO 2 + hν → NO + O O + O 2 + M→ O 3 + M

Tropospheric ozone production Note that what the complete cycle has done is to dissociate molecular oxygen using two photons. One at about 310 nm (4 eV) and one at 400 nm (3 eV). NO 2 + hν → NO + O O + O 2 + M→ O 3 + M OH + CO → H + CO 2 H + O 2 + M → HO 2 + M HO 2 + NO → OH + NO 2 CO + 2O 2 + hν → CO 2 +O 3 Similar chain reactions can be written for RO 2

Tropospheric ozone production In the free troposphere, and with a small amount of NO, the loss mechanism for ozone is: HO 2 + O 3 → OH + 2O 2 OH + CO → H + CO 2 H + O 2 + M → HO 2 + M CO + O 3 → CO 2 + O 2 This loss mechanism is large for low NO concentrations – provides a photochemical sink for ozone.

Tropospheric ozone oxidation

Importance of NO X As we noted before NO is crucial to the production of ozone, especially in polluted atmospheres. Two reservoirs for NOX have been identified – nitric acid and peroxyacetylnitrate (PAN). Both of these gases dissociate in the daytime, but are quite significant at night. There are, in fact, a whole range of peroxynitrates that exist in the atmosphere.

Oxidation of a VOC - daytime

The nitrate radical The nitrate radical was first observed in It is formed by the reaction: NO 2 + O 3 → NO 3 + O 2 It can be stored as Nitrogen Pentoxide N 2 O 5 During the day the NO 3 radical is rapidly photolyzed, the product being either NO or NO 2. Although the OH radical is usually the main agent of attack on the hydrocarbons in daylight, NO 3 can be the most important agent at night: NO 3 + RH → HNO 3 + R

Concentrations of NO 3 and peroxy radicals

Oxidation of VOC’s at night