Tropospheric Ozone System Just Ox Ox XO O3 The key to producing ozone is a free oxygen We can think of ozone as an O2 molecule “carrying” the O X O2 Reactions: (1a) O2 + O  O3 (1b) O2 + hn  O2 + O

Tropospheric Ozone System Adding NOx Ox NOx XO O3 2a NO2 The main source of that free O in the troposphere is NO2 We can think of NO as a “carrier” molecule for the free oxygen 2b hn M 2b 2a X O2 NO Reactions: (2a) NO2 + hn  NO + O (2b) O3 + NO  NO2 +O2 M We keep track of nitrogen with NOx = NO + NO2 The free O bound up in NO2 can be thought of as a “potential Ox” XO = Ox + NO2

Tropospheric Ozone System Adding NOx Ox NOx XO O3 2a NO2 In equilibrium, we have 2b hn M 2b 2a X O2 NO This is not unique – depends on initial conditions How much NOx and XO do you start with? Reactions: (2a) NO2 + hn  NO + O (2b) O3 + NO  NO2 +O2 M All NOx and no XO means all NO – no ozone producing potential All XO and no NOx means all O3 – fixed amount of O3 In the end, this underpredicts observed O3 (i.e. assuming we start with pure NO2, not enough free oxygen there to explain that which ends up in Ox. Need additional source of XO to explain observations.

Tropospheric Ozone System Adding HOx Ox NOx HOx XO O3 2a NO2 3 HO2 NO can get “recharged” when HO2 passes along its free oxygen in (3) We can think of OH as a “carrier” molecule for the free oxygen in HO2. 2b hn M 2b 2a 3 3 X O2 NO OH Reactions: (3) HO2 + NO  NO2 + OH We keep track of hydrogen with HOx = OH + HO2 The free O bound up in HO2 can be thought of as another “potential Ox” XO = Ox + NO2+ HO2

Tropospheric Ozone System Hydrocarbons: CO Ox NOx HOx CO OH can get “recharged” when it reacts with carbon monoxide CO CO is lost (CO2 is pretty inert) by this reaction, so it does not participate in a COx cycle here. XO O3 2a NO2 3 HO2 4 CO 2b hn M 2b 3 3 4 X O2 NO OH Reactions: (4) CO + OH +O2  CO2 + HO2 The potential for CO to steal a free O from atmospheric O2 can be thought of as another “potential Ox” XO = Ox + NO2+ HO2 + CO

Tropospheric Ozone System Hydrocarbons: CH4 Ox NOx HOx CO CH4 XO O3 2a NO2 3 HO2 4 CO 5 CH4 2b hn x3 hn M 2b 3 5 3 4 5 X O2 NO OH Reactions: (5a) CH4 + OH + O2  H2O+ CH3O2 CH4 has many pathways for its degradation. Low NO  sink for HOx and minor source of CO High NO  source of 2HOx, 4 XO, one CO (5b) CH3O2 +NO + 3O2 CO + NO2 + 3HO2 (5c) CH3O2 + HO2  CH3OOH + O2 The potential for methane to steal free O from atmospheric O2 is potentially strong, if there’s lots of NO around, otherwise, it’s not as strong. XO = Ox + NO2+ HO2 + CO + CH4f(NOx)

Summary of Trop Ozone NOx needed, but O3 determined only by a ratio between NO2 and NO, not the abundance Need sources of free oxygen Emissions of NO2 Emissions of CO Oxidation of hydrocarbons in the presence of NO Actual O3 amount is not the total amount of XO – how the XO gets trapped in it’s non-Ox forms and how these are destroyed are important to ultimate O3 concentrations. Think of stratospheric examples.

Budgets Now it’s important to understand the sources and sinks of the “carrier” species NO OH CO hydrocarbons

NOx Sources Sinks NO2 + OH  HNO3(g)  HNO3(aq) Emissions by fossil fuel burning Emissions by biomass burning Soil emission (biological and fertilizer) Lightning Sinks Dry deposition Wet deposition of nitrate NO2 + OH  HNO3(g)  HNO3(aq)

HOx Sources Sinks O3 + hn  O2 + O(1D) H2O + O(1D)  2OH Oxidation of hydrocarbons (see Ozone chain) Sinks Wet deposition of nitrate and sulfate NO2 + OH  HNO3(g)  HNO3(aq) Wet deposition of hydrogen peroxide HO2 + HO2  O2 + H2O2

Carbonaceous species CO Methane (CH4) Non-methane hydrocarbons NMHC Main source is fossil fuels and photooxidation of hydrocarbons Main sink is OH – short CO lifetime led to discovery of photochemistry in troposphere Methane (CH4) Main sources are anoxic decomposition of organic material (natural, landfills, mines, etc) Main sink is photooxidation in atmosphere via formaldehyde HCHO Non-methane hydrocarbons NMHC Alkanes, alkenes, aromatics, and oxygenated hydrocarbons Many natural and industrial sources (“trees pollute”) Sinks are photooxidation by OH and directly

Nitrogen Cycle

Sulfur Cycle