Workshop on Air Quality Data Analysis and Interpretation Chemistry of Ozone Formation

Chemistry of Ozone Formation The formation of ozone in the troposphere is by one and only one process. The photolysis of NO2. NO2 + hν (λ<420nm) → NO + O(3P) O(3P) is a ground state oxygen atom and will simply be indicated by O. O + O2 + M → O3 + M

NO2 Photolysis NO2 Absorption Spectrum NO2 Quantum Yield Data

Why is this the only route to O3 formation? Earlier we talked about stratospheric ozone being formed by the photolysis of O2. Why can’t this occur in the troposphere? One of the important functions of the stratosphere is to protect living organisms from harmful, short wavelength UV radiation.

Atmospheric Absorption of Light, λ<300 nm

Solar Radiation available for Tropospheric Photolysis Processes

Where does NO2 come from? NO is the primary pollutant emitted by combustion sources. NO2 emissions are only about 10% of the NO emissions. NO2 is produced by the atmospheric oxidation of NO. The thermal oxidation of NO by O2 is much to slow to be of much significance under atmospheric conditions. 2 NO + O2 → 2 NO2 slow

So how can NO be oxidized to NO2? NO + O3 → NO2 + O2 O3 can oxidize NO to NO2, but this process does not allow net O3 production. NO2 + hν → NO + O O + O2 + M → O3 + M The net effect of these three reactions (the sum or the reactions) is no net reaction. O3 is destroyed in the process that initiates the oxidation of NO to NO2.

The importance of radical reactions! Peroxy types of radicals can react with NO to form NO2. The hydroperoxy radical (HO2 or H-O-O•) can react with NO to form NO2 and the hydroxyl radical (OH) HO2 + NO → OH + NO2 By analogous processes, the alkylperoxy radical (RO2) can react with NO to form NO2 and the alkoxy radical (RO) RO2 + NO → RO + NO2 These reactions are fast, but limited by the available peroxy radicals.

Where do peroxy radicals come from? Peroxy radicals can be formed as a result of some photolysis processes. Formaldehyde (HCHO) HCHO + hν (λ<340 nm) → H + CHO The H atom reacts with O2 in the air to form HO2 H + O2 + M → HO2 + M The formyl radical can also react with O2 in the air to form HO2 and CO CHO + O2 → HO2 + CO

Acetaldehyde Photolysis Acetaldehyde (CH3CHO) CH3CHO + hν (λ<330 nm) → CH3 + CHO The methyl radical (CH3) can react with O2 in the air to form CH3O2 CH3 + O2 + M → CH3O2 + M This is one possible RO2 radical. As before, the formyl radical can react with O2 in the air to form HO2 and CO CHO + O2 → HO2 + CO

Hydroperoxy and Hydroxy Radicals We have seen the peroxy radicals, such as the hydroperoxy radical is important because it can oxidize NO to NO2. HO2 + NO → OH + NO2 This forms the hydroxy radical. What is the role of the hydroxy radical in the atmosphere? The hydroxy radicals primary role in the atmosphere is initiating the oxidation of hydrocarbons. OH + RH → H2O + R R + O2 → RO2

Role of Hydrocarbon Oxidation – Ethane (an example) OH + CH3CH3 → H2O + CH3CH2 CH3CH2 + O2 → CH3CH2O2 CH3CH2O2 + NO → CH3CH2O + NO2 CH3CH2O + O2 → CH3CHO + HO2 HO2 + NO → OH + NO2 OH + CH3CHO → CH3CO + H2O CH3CO + O2 → CH3C(O)O2 CH3C(O)O2 + NO → CH3C(O)O + NO2 CH3C(O)O → CH3 + CO2

Role of Hydrocarbon Oxidation – Ethane (continued) Peroxyacetynitrate (PAN) can be formed from the reaction of the radical with NO2. CH3C(O)O2 + NO2 ↔ CH3C(O)O2NO2 (PAN) CH3 + O2 → CH3O2 CH3O2 + NO → CH3O + NO2 CH3O + O2 → HCHO + HO2 HO2 + NO → OH + NO2

Role of Hydrocarbon Oxidation – Ethane (continued) Formaldehyde can either photolyze, as described earlier, or react with OH radicals. HCHO + OH → HCO + H2O CHO + O2 → HO2 + CO HO2 + NO → OH + NO2 OH + CO → H + CO2 H + O2 + M → HO2 + M In this sequence of reactions, 4 OH radicals were consumed and 4 OH radicals were produced. But as this sequence was written, 7 NO molecules were oxidized to NO2.

What if formaldehyde had been photolyzed? HCHO + hν (λ<340 nm) → H + CHO H + O2 + M → HO2 + M HO2 + NO → OH + NO2 CHO + O2 → HO2 + CO OH + CO → H + CO2 In this sequence of reactions, 3 OH radicals were consumed and 5 OH radicals were produced. And as this sequence was written, 8 NO molecules were oxidized to NO2.

Role of Hydrocarbon Oxidation – Ethane (concluded) OH radicals initiate a chain reaction oxidation of atmospheric hydrocarbons. Along the way a number of NO molecules are converted to NO2. It is likely fairly obvious that the rate of oxidation of NO to NO2 depends on the reactivity (with OH) of the hydrocarbon. Hence, the rate of ozone production is related to the reactivity of the hydrocarbon mixture that is present in the atmosphere.

What limits these hydrocarbon oxidation processes? As long as one has OH radicals, the hydrocarbon oxidation processes will continue. How are the hydrocarbon oxidation chain reactions terminated? OH + NO2 + M → HNO3 + M OH radicals can react with NO2 to produce nitric acid. This is a relatively stable product that results in net radical loss (OH and NO2 – both radicals for a stable molecule HNO3 – not a radical)

Workshop on Air Quality Data Analysis and Interpretation Seasonality of ozone concentrations

Monthly average O3 for each site

Normalized Monthly Average O3 for each site

Monthly average NOx for each site

Normalized Monthly Averaged NOx for each site

Observations The ambient data in Bangkok tends to suggest that ozone concentrations at the higher ozone sites tend to be higher after the first of the year. For many sites, the primary pollutants are a bit higher before and at the end of the year. Pollutant dilution and dispersion factors do not seem to fully explain the seasonality of O3.

Why does ozone peak early in the year? Since primary pollutants are highest at the end of the year, atmospheric dilution and dispersion favor highest concentrations at the end of the year. The peak solar flux is expected to be in the June – July period, when the sun is most directly overhead.

Average of Daily Max Global Radiation – National Housing 10T

Possibly Photolysis Rate Driven! It is possible that photolysis is highest during the early part of the year, with clear skies, that are less common during the rainy season. The seasonal effects on solar radiation are relatively small in Thailand, in comparison to higher latitude areas. The early part of the year may be favored by a combination of poorer pollutant dispersion and less cloud cover, relatively higher solar radiation?

NO2 Photolysis rate in Los Angeles

NO2 Photolysis rate in Bangkok

NO2 Photolysis Much less seasonal dependence on the photolysis rate in Bangkok than in Los Angeles. There is very little seasonal dependence on the length of the solar day in Bangkok. But even so, the NO2 photolysis rate in Los Angeles during August (peak ozone season) is greater than the photolysis rate in Bangkok during January.

Comparison of NO2 Photolysis rates in Peak O3 periods

Comparison of Photolysis Rates The peak photolysis rates for NO2, formaldehyde and acetaldehyde in Bangkok during Jan. is about 80% of the peak photolysis rates in L.A. during Aug. The integrated photolysis in Bangkok during Jan. is about 65% of that in L.A. during Aug.

Workshop on Air Quality Data Analysis and Interpretation Nitrogen Oxides Species in the Atmosphere

Nitrogen Species in Urban Air Urban concentrations NO 1 - 1000 ppb NO2 1 - 800 ppb HNO3 - 50 ppb PAN - 50 ppb HNO2 - 8 ppb NO3 0.005 - 0.4 ppb N2O5 -  15 ppb NH3 - 50 ppb

Nomenclature of Nitrogen Oxides NOx = NO + NO2 Measured NOx includes NO + NO2 + fraction of others, that depends on details of sampling and analysis system. NOy = NOx + HNO3 + 2 N2O5 + NO3 + organic nitrates + particulate nitrate + ... Total Odd Nitrogen Species – made through atmospheric chemistry of nitrogen oxides NOz = NOy - NOx

Atmospheric reactions of NO 2 NO + O2  2 NO2 k = 2.0 x 10-38 cm6 molecule-2 s-1 At a high NO = 1 ppm lifetime of NO = 4.6days RO2 + NO  RO + NO2  7.6 x 10-12 cm3 molecule-1 s-1  RONO2 At RO2 = 120 ppt τNO  44s HO2 + NO  OH + NO2 8.3 x 10-12 cm3 molecule-1 s-1 At HO2 = 80 ppt τNO  60s O3 + NO  NO2 + O2 1.8 x 10-14 cm3 molecule-1 s-1 At O3 = 200 ppb τNO  11s OH + NO  HONO 6.8 x 10-12 cm3 molecule-1 s-1 At OH = 0.4 ppt τNO  4h NO3 + NO  2 NO2 2.6 x 10-11 cm3 molecule-1 s-1 At NO3 = 0.36 ppb τNO  4s

Atmospheric reactions of NO2 NO2 + hv  NO + O τNO2 > 2m OH + NO2 (+M)  HNO3 (+ M) 1.1 x 10-11 cm3 molecule-1 s-1 At OH = 0.4 ppt τNO2  3h O3 + NO2  NO3 + O2 3.2 x 10-17 cm3 molecule-1 s-1  NO + 2 O2 At O3 = 200 ppb τNO2  2h NO3 + NO2 (+ M) <=> N2O5 (+ M) 1.3 x 10-12 cm3 molecule-1 s-1  NO + NO2 + O2 At NO3 = 0.36 ppb τNO2  1m HO2 + NO2 (+ M) <=> HO2NO2 (+ M) 1.4 x 10-12 cm3 molecule-1 s-1 At HO2 = 80 ppt τNO2  6m RO2 + NO2 <=> RO2NO2 7 x 10-12 cm3 molecule-1 s-1 At RO2 = 120 ppt τNO2  1m The last three processes achieve equilibrium quickly, hence they do not act as significant net sinks for NO2.

Photostationary State NO2 + hv (λ < 430 nm)  NO + O (1) O + O2 (+ M)  O3 (+ M) (2) O3 + NO  NO2 + O2 (3) Rxn 2 is sufficiently fast that as soon as O is formed in (1) it reacts and forms O3. This leads to a dynamic balance NO, NO2 and O3 during the day, when the photolysis (1) can occur.