Air Pollution– Inorganic Gaseous Pollutants

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

HK 1-hour Air Quality Objectives for various air pollutants AQ Objectives (mg/m3) AQ Objectives (ppm) US AQ Standards (ppm) Carbon monoxide 30000 26.2 35 Sulfur dioxide 0.306 0.50a Nitrogen dioxide 0.159 0.05 b Ozone 800 0.122 0.12 a: 3-hr standard, b: annual arithmetic mean

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

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

CO formation from methane oxidation

CO emission sources in Hong Kong

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.

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

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.

Sulfur dioxide: Sources and sinks 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

SO2 emission sources in Hong Kong

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

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.

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

Nitrogen oxides: Health Effects 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.

Nitrogen oxides: Sources and sinks 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

NOx emission sources in Hong Kong

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

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

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

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.

Two-stage combustion to reduce both NOx and VOCs First stage: combustion condition—rich in fuel Second stage: combustion condition—rich in air This approach is being incorporated into new power plants; It has been tried in cars via the “stratified-charge” engine, but with less success.

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

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