Department of Chemistry CHEM1020 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

CHEM1020/General Chemistry _________________________________________ Chapter 12. (L22)-Chemistry of Earth Today’s Outline …The troposphere structure and characteristics basic chemistry …The “bad” ozone and photochemical smog basic troposphere ozone chemistry photochemical smog

Chapter 12. (L22)-Chemistry of Earth The troposphere …Characteristics of the troposphere dimension: 0~10-15 km; -temperature profile: 15– -53˚C -solar radiation profile: > 290 nm (UVB, visible, and above) -mixing: rapid, strong, continuous, day-night difference -exchange between troposphere and ground surface, exchange between troposphere and stratosphere -major trace compounds in troposphere: N-compounds, C- compounds, plus water vapor, besides oxygen

Chapter 12. (L22)-Chemistry of Earth The troposphere...Two types of troposphere background (unpolluted) troposphere urban (polluted) troposphere …Two types of air pollutants Primary pollutants: Emitted pollutants Secondary pollutants: Those formed physically and chemically in the air …Major pollutants (see Table 12.4 on pp. 353)

Chapter 12. (L22)-Chemistry of Earth Basic troposphere chemistry …Basic reactions: Ozone chemistry and oxidation …Basic oxidation chemistry -organic C compounds: hydrocarbon → aldehydes → acids → CO 2 (H 2 CO 3 ) -N compounds (NO x = NO + NO 2 ): NO → NO 2 → HNO 3 (acid rain) -S compounds: H 2 S or CH 3 SCH 3 → SO 2 → H 2 SO 4 (acid rain)

Chapter 12. (L22)-Chemistry of Earth Basic troposphere ozone chemistry A simplified system containing only NO, NO 2, and O 3, plus M = O 2 and N 2 NO 2 + hν (λ < 424 nm) → NO + O(source)(1) O + O 2 + M → O 3 + M(2) O 3 + NO → NO 2 + O 2 (sink)(3) Note: The reaction: O 3 + O → O 2 + O 2 is slow, cannot compete with reaction 3 The above mechanism (reactions 1-3) is sort of like the Chapman mechanism for stratosphere ozone chemistry

Chapter 12. (L22)-Chemistry of Earth Atmospheric smog and atmospheric pollution...Two types of smog Smog = Smoke + fog Smog is a general term referring to various forms of air pollution in which atmospheric visibility is partially obscured by a haze consisting of solid particles and/or liquid aerosols as well as gaseous phase pollutants such as O 3, aldehydes (aerosols), PAN (peroxyacetyl nitrate)

Chapter 12. (L22)-Chemistry of Earth Atmospheric smog and atmospheric pollution...Two types of smog London-type smog and photochemical smog (Classic) London-type smog: It results from the burning of coal and other raw materials in the presence of a fog or strong inversion (more associated with S compounds and aerosol particles)

Chapter 12. (L22)-Chemistry of Earth Atmospheric smog and atmospheric pollution...Two types of smog Photochemical smog: It results from the emissions of hydrocarbons and oxides of nitrogen from automobiles and other sources in the presence of sunlight, and it does not require smoke or fog for its production. Both types of smog are enhanced by a strong temperature inversion (air could not rise if it is warmer at higher altitude and thus trapped in the location, like LA valley). In many places, urban air pollution involves both types.

Chapter 12. (L22)-Chemistry of Earth Atmospheric smog and atmospheric pollution Consequences of photochemical smog …O 3 health effects -irritating -chest pain ( > 0.25 ppm v ) -lung problem -headache ( > 0.15 ppm v ) -sore throat and cough (> 0.30 ppm v ) -also affects plants and animals -asthma

Chapter 12. (L22)-Chemistry of Earth Photochemical smog and related troposphere chemistry Photochemical smog forms by reactions of NO x with reactive organic gases (ROGs, i.e., hydrocarbons) in the presence of sunlight …Chemical sequence of photochemical smog formation via proxy radicals (RO 2 ): ROGs + OH → RO 2 + products NO + RO 2 → NO 2 + RO NO 2 + hν (λ < 424 nm) → NO + O(1) O + O 2 + M → O 3 + M(2) O 3 + NO → NO 2 + O 2 (slow)(3)

Chapter 12. (L22)-Chemistry of Earth Photochemical smog and related troposphere chemistry …Sources of OH free radical via O 3 photolysis O 3 + hν → O 2 ( 1 Δ g ) + O( 1 D)at 310 nm (λ < 336 nm) O 2 ( 1 Δ g ): excited O 2 molecule O( 1 D): excited O atom In troposphere, O ( 1 D) is the source of OH radical: O( 1 D) + H 2 O(g) → 2OH The above reaction is the major source of OH in the entire atmosphere and it competes with: O( 1 D) + M → O( 3 P) + M O( 3 P): ground state O atom

Chapter 12. (L22)-Chemistry of Earth Photochemical smog and related troposphere chemistry …Sources of OH free radical via O 3 photolysis at 1 atm, 50% RH, 298 K, ~10% of the O( 1 D) produced reacts with water vapor to from OH, O( 3 P) would not react with water vapor (because the reaction would be endothermic) OH is unreactive towards O 2, so it can survive to react with other atmospheric trace species

Chapter 12. (L22)-Chemistry of Earth Two types of photochemical smog …Two types of photochemical smog: Los Angeles-type and Smoky Mountains-type -Los Angeles type: Automobile emitted NO x and hydrocarbons induced production of O 3 and other gaseous pollutants as well as aerosol particles involving N (HNO 3 ) and S (H 2 SO 4 ) compounds -Smoky Mountain type: Blue haze, organic aerosols produced by reactions of biogenic volatile organic carbon (VOC) compounds from trees (e.g., pine trees) with O 3 (plus O 3 smog caused by biogenic and automobile sources of VOC and automobile emitted NO x )

Chapter 12. (L22)-Chemistry of Earth Quiz Time TBA