AtmosphericAtmospheric ChemistryChemistry LectureLecture 2:2: TroposphericTropospheric ChemistryChemistry andand AerosolsAerosols Jim Smith Atmospheric Chemistry Division / NCAR [email protected] Outline for Second Lecture Tropospheric Chemistry and Aerosols Primary and secondary pollutants Sulfur chemistry Nitrogen chemistry Acid rain Chemistry of the background troposphere: • CO • Methane • Formaldehyde Urban Air Pollution • Hydrocarbon oxidation • Ozone Isopleth plots • Smog meteorology Tropospheric Chemistry Primary and Secondary Pollutants and their impacts Primary pollutants are those emitted directly from sources • CO: affects oxygen-carrying capacity of blood (but not at urban concentrations) • NO: secondary reactions • NO 2: 5% in urban, much higher for biomass burning • SO 2: respiratory constriction (asthma, bronchitis) • NH 3: aerosol formation • soot: visibility reduction, PAH is carcinogen • isoprene: emitted from plants, leads to ozone and makes a certain President think the ozone problem is caused by trees. • vanillan: yummy smell from Ponderosa Pine Secondary pollutants are those formed in the atmosphere by chemical interactions among primary pollutants and normal atmospheric constituents. • NO 2 (95%): lung damage •H2SO 4: Aerosol formation, visibility reduction and acid rain • PAN: hurts your eyes, damage to plants •O3: damage to plants & plastics, lung damage (only short-term?) Tropospheric Chemistry Sulfur Cycle Image courtesy of Hugh Powell, Univ. of Durham, UK Tropospheric Chemistry Sulfur Cycle SO 2 can be converted to H 2SO 4 in the gas phase or in aqueous solution (in clouds or fogs) Gas phase oxidation involves … OH attack! SO 2 + •OH + M → HSO 3• + M HSO 3• + O 2 + M → SO 3 + HOO • + M SO 3 is highly soluble in water, producing sulfuric acid: SO 3 + H 2O → H2SO 4(aq) Gaseous SO 2 dissolves in water giving sulfurous acid: - + SO 2 + H2O → HSO 3 (aq) + H (aq) Once in solution, there are several oxidation pathways: • by dissolved ozone or hydrogen peroxide: - - e.g., HSO 3 + H 2O2 → SO 2OOH + H 2O - + SO 2OOH + H → H2SO 4 (pH dependent) • by dissolved oxygen, catalyzed by iron and/or manganese - - HSO 3 + ½ O2 → HSO 4 Tropospheric Chemistry Nitrogen Chemistry Image courtesy of Hugh Powell, Univ. of Durham, UK Tropospheric Chemistry Nitrogen Chemistry Nitrogen chemistry is actually quite complicated! NO and NO 2 can be absorbed in water droplets: + - - 2NO 2 + H 2O → 2H + NO 3 + NO 2 NO 2 can be oxidized by OH in the gas phase: NO 2 + •OH + M → HNO 3 + M In the stratosphere, NO plays a starring role in the catalytic cycle for ozone destruction (Crutzen’s contribution!) NH 3 is absorbed in water droplets, and is the most important aqueous base in the atmosphere At nighttime, NO 2 can be oxidized by O 3 to form NO 3, a strong atmospheric oxidant which can go on to form N 2O5: NO 3 + NO 2 + M → N2O5 + M In the morning, both compounds rapidly photolyze to give the troposphere a morning boost of NOx: N2O5 + hν → NO 3• + NO 2 NO 3• + hν → NO + O 2 NO 2 + O Tropospheric Chemistry Acid Rain Normal, unpolluted rainwater has a typical pH of ~5-5.6: CO 2 + H 2O → H2CO 3(aq) + - H2CO 3(aq) → H (aq) + HCO 3 (aq) - + - HCO 3 (aq) → H (aq) + CO 3 (aq) Rain with pH < 4.5 is termed “acid rain” Term used for both dry acid deposition and acid precipitation Acids found in rainwater: H2SO 4, HNO 3, HCl, H 2CO 3, H 2SO 3, organic acids H2SO 4 and HNO 3 are main anthropogenic pollutants responsible for acidification, resulting from fossil fuel combustion. Emissions of these compounds are decreasing in most industrialized countries, however with residence times of several days, acid rain can affect areas 1000s of km from the emission source. Problems caused by acidity: • damage to vegetation • acidification of surface waters, esp. lakes. Tropospheric Chemistry The primary photochemical cycle of NOx and ozone NO is a primary pollutant formed by combustion, in the atmosphere it oxidizes to form NO 2: 2NO + O 2 → 2 NO 2 Ozone then occurs as a result of photolysis of NO 2: NO 2 + hν → NO + O (1) O + O 2 + M → O3 + M (2) Note that there are no other significant sources of ozone in the atmosphere other than reaction 2. Once formed, ozone can regenerate NO 2: NO + O 3 → NO 2 + O 2 (3) Let’s consider the system of reactions 1-3. The rate of change of O 3 is: d[O ] 3 = −k [O ][ NO ]+ k [O][ O ][ M] dt 3 3 2 2 Similarly, the rate of change of O is: d[O] = k [NO ]− k [O][ O ][ M] dt 1 2 2 2 Tropospheric Chemistry The primary photochemical cycle of NOx and ozone NO 2 + hν → NO + O (1) d[O] O + O + M O + M (2) = k [NO ]− k [O][ O ][ M] 2 → 3 dt 1 2 2 2 NO + O 3 → NO 2 + O 2 (3) In reality, reaction 2 is so fast that the oxygen atom disappears as soon as it’s formed by reaction 1. The result is that d[O]/dt is always very nearly zero …this is called the “pseudo-steady state approximation” (PSSA): k k 1[NO 2 ] = 2[O][ O2 ][ M] The steady-state ozone concentration was given by: d[O ] 3 = −k [O ][ NO ]+ k [O][ O ][ M] = 0 dt 3 3 2 2 Substituting the result of the PSSA into the above equation gives: k [NO ] [O ] = 1 2 3 k 3[NO ] The steady-state ozone concentration is proportional to [NO2] / [NO] Tropospheric Chemistry Chemistry of the background troposphere The key species involved are: • methane (CH 4) • carbon monoxide (CO) • formaldehyde (HCHO) • ozone (O 3) • NOx (NO and NO 2) • nitric acid (HNO 3) • hydrogen peroxide (H 2O2) The initiation step that starts it all is the photolysis of ozone to generate two OH radicals (“the atmosphere’s detergent”): O3 + hν → O2 + O* O* + H 2O → 2 •OH Chemistry of the Background Troposphere CO chemistry Chemical lifetime of 30-90 days Primary natural source is oxidation of methane (20-50%), with the remainder from oxidation of other hydrocarbons and from oceans. Primary sink is OH radical attack: •OH + CO → CO 2 + •H (1) •H + O 2 + M → HOO• + M (2) the hydroperoxyl radical HOO• + NO → NO 2 + •OH (3) If NOx concentrations are low, the following reaction competes with (3): HOO• + O 3 → •OH + 2O 2 (4) So the photolytic oxidation of CO to CO 2, catalyzed by OH, leads to a net production of ozone in the presence of NOx since: k [NO ] [O ] = 1 2 3 k 3[NO ] or to the net destruction of O 3 if NOx concentrations are low via (4) Chemistry of the Background Troposphere CH 4 and HCHO chemistry Chemical lifetime of 8-10 years Primary natural source is biogenic (swamps, tropical rainforests, livestock) In urban air, methane oxidation is slower than other hydrocarbons Primary sink is OH radical attack: •OH + CH 4 → •CH 3 + H 2O •CH3 + O 2 + M → CH 3OO• + M methylperoxy radical CH 3OO• + NO → NO 2 + CH 3O• methoxy radical CH 3O• + O 2 → HCHO + HOO• formaldehyde (HOO• + NO → NO 2 + •OH) HCHO is further attacked by OH to form CO: HCHO + •OH → HCO• + H 2O HCO• + O 2 → HOO• + CO (HOO• + NO → NO 2 + •OH) As we saw before, CO is oxidized to CO 2: CO + 2O 2 + hν → CO 2 + O 2 So methane is oxidized photolytically to CO 2 via OH attack, with formaldehyde and CO as intermediates and net production of ozone. Chemistry of Urban Air Pollution Key Steps in Atmospheric Hydrocarbon Oxidation Alkanes Alkenes Aromatics RH R2C=CR 2 RH OH attack R• R2C•-COHR 2 •ROH O2 addition RO2 • RO 2•C-COHR 2 •ROHO2 Endoperoxides NO/NO 2 conversion Epoxides RO • RO •C-COHR 2 ROHO • multifunctional isomerization •ROH •ROH CC βββ-cleavage •R’ + R”CHO •R’ + R”CHO •R’ + R”CHO reaction with O 2 RCHO RCHO RCHO Dicarbonyls reaction with NO x RONO x RONO x RONO x Chemistry of Urban Air Pollution Butane Oxidation (even simple molecules have complicated chemist ry!) O• 1-1 +O 2 +NO βββ-cleavage 1-C1 +NO 2 1-1.11 1-1.2 hν • H2CO + C 3H7 [1,4]H-shift [1,5] H-shift ONO ONO 2 1-1.10 H-abstraction 1-1NO OOO• 1-1NO 2 -HO 2 1-9 • 1-2 • OH 1-11 OH [1,4] H-shift -HO 2 1-9.10 H H H OH • +O +O 2 H 2 OH 1-2b - HO 2 1-10 1-3 O •OO [1,6] H-shift OO OH 1-2bO 2 H OH HOO • +NO +O 2 OH 1-3NO [1,5] H-shift 1-3b ONOO HOO OH OO - HO 2 1-3bO H HOO 2 ONO -NO 1-4NO OH 2 • OH +O 2 HOO 1-4 1-3c O O HOO 1-12 OH OH H2CO + OH 1-4C 1-4.5 OO 1-3cO 2 +O 2 [1,5]H-shift ? H OH HO • 1-5 •OOO 1-7 O2 OH OH 1-3NO.8 1-4.8 H-abstraction HO 1-6 -HONO 1-7.8 OO - HO 2 - HO 2 observed 1-6.8 - HO 2 experimentally H OH 1-8 O Chemistry of Urban Air Pollution Formation of peroxyacetyl nitrates (PAN) from oxidation of acetaldehyde Ozone has received much attention as a urban pollutant, yet there are many other compounds that, though present at trace levels, can have a profound effect on human health and welfare. Carcinogenic compounds such as polycyclic aromatic hydrocarbons (PAHs), a common constituent in diesel exhaust is one such example.