HK 1-hour Air Quality Objectives for various air pollutants Air Pollutants AQ Objectives ( g/m3) AQ Objectives (ppm) US AQ Standards (ppm) Carbon monoxide Sulfur dioxide a Nitrogen dioxide b Ozone 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 Fe C-O O-O 320 times stronger than hemoglobin-O2 binding
Carbon monoxide: sources and sinks 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 Such as automobiles
CO formation from methane oxidation
CO emission sources in Hong Kong
Carbon monoxide: Atmospheric chemistry CO + OH + O 2 CO 2 + HO 2. HO 2. + NO NO 2 + OH NO 2 + hv NO + O O + O 2 + M O 3 + M Net: CO + 2 O 2 + hv CO 2 + O 3 The net reaction can be viewed as a catalytic oxidation of CO to CO 2. Net formation of O 3 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 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
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 1.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. 2.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 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.
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
NOx emission sources in Hong Kong
Nitrogen oxides: Atmospheric chemistry Interconversion of NO and NO2 NO 2 + hv NO + O(1) O + O 2 + M O 3 + M(2) NO + O 3 NO 2 + O 2 (3) NO 2 + hv NO + O(1) O + O 2 + M O 3 + M(2) HO 2. + NO NO 2 + OH(4) RO 2. + NO NO 2 + RO. (5) No net O3 formation O3 is formed
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
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: 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