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)

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)

NOx tropospheric columns observed from space OMI NO2 2013 1015 molecules cm-2 http://disc.sci.gsfc.nasa.gov/giovanni

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)

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

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

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

Hu et al. [2017]

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]

Factors controlling tropospheric ozone and OH hn 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]

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

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]