OZONE PRODUCTION IN TROPOSPHERE

1. Photochemical oxidation of CO and volatile organic compounds (VOCs) catalyzed by hydrogen oxide radicals (HOx) and nitrogen oxide radicals (NOx) OZONE PRODUCTION IN TROPOSPHERE 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 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 http://disc.sci.gsfc.nasa.gov/giovanni

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. (Carbon oxidation number) Methane oxidation cascade CH4 (-IV) (+I) CH3O2 (0) (0) (+II) CH3OOH CH2O CH3OH 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 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. (where CO also serves as simple proxy for VOCs) Dependences of ozone production and OH on NOx and CO 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 OH hn NO NO2 hn O3 O(1D) M HO2 H2O2 OH 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 EMISSIONSBox 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]