Air Pollution– Inorganic Gaseous Pollutants

1. Air Pollution– Inorganic Gaseous Pollutants

2. Major inorganic gaseous pollutants • Carbon monoxide (CO) • Sulfur dioxide (SO2) • Nitrogen Oxide (NO, NO2) NOx=NO+NO2 • Ozone (O3)

3. HK 1-hour Air Quality Objectives for various air pollutants a: 3-hr standard, b: annual arithmetic mean

4. O-O O2 Fe Fe Fe C-O CO Carbon monoxide: Health effect CO enters the blood stream and binds preferentially to hemoglobin, thereby replacing oxygen. 320 times stronger than hemoglobin-O2 binding

5. Carbon monoxide: sources and sinks Such as automobiles • Sources • Incomplete combustion (internal engine) • Biomass burning • Methane oxidation • Oxidation of non-methane hydrocarbon • Decay of plant matter • Sink • Reaction with OH radical .OH + CO  CO2 + H. H. + O2 + M  HO2. + M • Removal by soil microorganism

6. CO formation from methane oxidation

7. CO emission sources in Hong Kong

8. Carbon monoxide: Atmospheric chemistry CO + OH + O2 CO2 + HO2. HO2. + NO  NO2 + OH NO2 + hv  NO + O O + O2 + M  O3 + M Net: CO + 2 O2 + hv  CO2 + O3 The net reaction can be viewed as a catalytic oxidation of CO to CO2. Net formation of O3 occurs.

9. Carbon monoxide: control strategies on the automobile source • Employ a leaner air/fuel mixture (higher air/fuel ratio) • Employ catalytic exhaust reactors • Excess air is pumped into the exhaust pipe. • Air-exhaust mixture pass through a catalytic converter to oxidize CO to CO2. • Addition of oxygenates to gasoline • Examples of oxygenates: methanol, ethanol, MTBE

10. Sulfur dioxide: Health effect • Produce irritation and increasing resistance in the respiratory tract. • Mucus secretion • In sensitive individuals, the lung function changes may be accompanied by perceptible symptoms such as wheezing, shortness of breath, and coughing. • may also lead to increased mortality, especially if elevated levels of suspended particles are also present.

11. Sulfur dioxide: Sources and sinks Sources • Combustion of S-containing fuel in electric power plants, vehicles. • S (organic S + FeS2 pyrite) + O2 --> SO2 • Oxidation of H2S: 2H2S + 3 O2 --> 2 SO2 + 2 H2O • H2S is produced as an end product of the anaerobic decomposition of S-containing compounds by micro organisms. • Oxidation of DMS Sink • Converted into sulphuric acid in either gas or liquid phase

12. SO2 emission sources in Hong Kong

13. Formation of sulfuric acid and sulfate from SO2 • In gas-phase SO2 + .OH + M  HOSO2. + M HOSO2. + O2  HO2. + SO3 SO3 + H2O + M  H2SO4 + M • In aqueous phase, dissolved SO2 is oxidized to sulfate by • O3 (dominant pathway when pH>5) • H2O2 (dominant pathway when pH<5) • organic peroxides • O2 catalyzed by iron and manganese • Sulfate formation: 2 NH3 + H2SO4  (NH4)2SO4

14. Sulfur dioxide: Control strategies • Removal of S before DURING burning. Fludized bed combustion: Coal is burned with limestone (CaCO3) (finely pulverized) or dolomite (Ca-Mg carbonate) or both. CaCO3 --> CaO + CO2, CaO + SO2 --> CaSO3. CaSO3 is removed from the stack by an electrostatic precipitator. • removal of S from smokestacks before entering the atmosphere. Flue-gas desulfurization: SO2 is washed from the chimney (flue) gases by absorption in an alkaline solution.

15. Sulfur dioxide: Control strategies (Continued) 3. Dilution Installation of tall stacks reduces SO2 levels in the immediate neighborhood by dispersing them more widely

16. Nitrogen oxides: Health Effects NO • Cellular inflammation at very high concentrations. • May be incorporated into hemoglobin in the blood to interfere with the transport of oxygen around the body. NO2 • irritate the lungs • lower resistance to respiratory infection such as influenza.

17. Nitrogen oxides: Sources and sinks Sources • Fuel combustion in power plants and automobiles. N2 + O2 --> NO 2 NO + O2 --> 2 NO2 • Natural sources: electrical storms; bacterial decomposition of nitrogen-containing organic matter

18. NOx emission sources in Hong Kong

19. Nitrogen oxides: Atmospheric chemistry Interconversion of NO and NO2 NO2 + hv  NO + O (1) O + O2 + M  O3 + M (2) NO + O3 NO2 + O2 (3) No net O3 formation NO2 + hv  NO + O (1) O + O2 + M  O3 + M (2) HO2. + NO NO2 + OH (4) RO2. + NO  NO2 + RO. (5) O3 is formed

20. Nitrogen oxides: Atmospheric chemistry Formation of nitric acid Gas-phase reaction NO2 + OH  HNO3daytime (dominate pathway) Heterogeneous reaction NO2 + O3  NO3 + O2 NO3 + NO2 Û N2O5 N2O5 + H2O (aq) 2 HNO3 (aq) Minor pathway Only operative during nighttime

21. Nitrogen oxides: Atmospheric chemistry Formation of nitrate HNO3 + NH3  NH4NO3 HNO3 + NaCl(s)  NaNO3 + HCl

22. Nitrogen oxides: Control strategies 1. Lower the combustion temperature of the furnace in electric power plants 2. Install catalytic converters: catalytic converters in automobiles can remove 76%of NOx from tailpipes.

23. Two-stage combustion to reduce both NOx and VOCs • First stage: combustion condition—rich in fuel • Second stage: combustion condition—rich in air

24. Three-way catalytic converter for automobile exhaust (Remove CO, NO and HC) HC + H2O = H2 + CO 2NO + 2H2 = N2 + 2 H2O 2CO + O2 = 2CO2 HC + 2O2 = CO2 + 2H2O Catalyst: Rhodium Catalyst: Rhodium Catalyst: Platium/palladium

25. NOx control in power plants • Ammonium reduction of NO • 4NH3 + 6NO = 5 N2 + 6 H2O • Urea reduction of NO • 2CO(NH2)2 + 6NO = 5 N2 + 2 CO2 + 4 H2O