Introduction to Photochemistry Marin Robinson Department of Chemistry Northern Arizona University

Outline Layers of the atmosphere Photochemistry – definitions Energy for photochemistry Consequences of photochemistry in stratosphere (ozone depletion) Consequences of photochemistry in troposphere (smog, haze, and acid rain)

Altitude (kilometers) Temperature (°C) Thermosphere Mesosphere (0.5%) Stratosphere (9.5%) Troposphere (90%) tropopause stratopause mesopause Ozone maximum Layers of the atmosphere Troposphere Stratosphere Ozone depletion Haze, smog, global warming

Photochemistry Photochemistry – Chemistry of the atmosphere driven by sunlight Photodissociation – Cleavage of a molecule into two or more (smaller) molecules (or atoms) by absorption of light

Photolysis Processes XY + h  XY * When XY * is unstable, it may decompose into its constituent atoms XY *  X + Y (ground state) XY *  X * + Y (excited state)

Photodissociation Rate R l = J l n l J l = wavelength dependent coefficient that depends on absorption properties of molecule (s -1 ) n l = number density (molecules/cm 3 )

Photodissociation J l is large (> /s), molecule unstable Atmospheric lifetime: sec to days (example: O 3 and NO 2 ) J l is small (< /sec), molecule stable Long lifetimes (~yrs) Most atmospheric gases (N 2, O 2, CH 4 ) Other loss mechanisms important (chemical reaction, physical removal)

Solar Energy Needed for Photochemistry Stratosphere: high energy UV Troposphere: low energy UV and VIS Also IR in troposphere, but not strong enough to break bonds (excites vibrations and rotations, but not dissociation)

Earth’s Energy Balance Solar in  Infrared out IR UV-VIS-IR

Solar In – IR Out

Energy In (from Sun) Long (Troposphere) Short  Strat)

How much UV-VIS energy makes it to troposphere? VIS - all UV-A: nm (tanning salons) – all UV-B: nm (sunburn) - most absorbed by O 3 layer UV-C: nm (biocidal) – completely absorbed by O 2 and O 3 in stratosphere

UV (strat); UV-VIS (trop)

Photochemistry in Stratosphere (High-Energy UV) Natural formation of ozone O 2 + h   O + O O + O 2 + M  O 3 Natural destruction of ozone O 3 + h   O + O 2 O + O 3  2O 2

Photochemistry in Stratosphere (High energy UV) [cont.] Destruction of ozone (human-caused) CF 2 Cl 2 + light  CF 2 Cl + Cl Cl + O 3  ClO + O 2 ClO + O 3  Cl + O 2 + O 2

Photochemistry in Troposphere (UV-VIS) UV-B and UV-A (and some VIS) N 2 + light (trop)  no reaction O 2 + light (trop)  no reaction CO 2 + light (trop)  no reaction H 2 O + light (trop)  no reaction CFCs + light (trop)  no reaction “Stable” molecules, bonds too strong

So is there PC in the trop? Yes –the two most important reactions O 3 + h  O 2 + O * NO 2 + h  NO + O * Both provide source of O atom (free radical, highly reactive) – this in turn, drives much of troposphere chemistry

Free Radicals Atoms or molecules with unpaired electron in outer shell (neutral) Two important free radicals in troposphere O (from photodissociation of O 3 & NO 2 ) OH (hydroxyl radical, made from O) O + H 2 O  OH + OH

The Hydroxyl Radical OH Minor (trace) constituent, but very important! [OH] ~ 1 ppm “Ajax” of atmosphere – OH reacts with almost everything with H CH 4 (methane) + OH  CH 3 + H 2 O H 2 S + OH  HS + H 2 O CF 2 ClH (H-CFC) + OH  CF 2 Cl + H 2 O

Atmospheric Lifetimes “Short” lifetimes (< 10 yr) Photochemically unstable (NO 2 and O 3 ) React with OH (CH 4, reactive hydrocarbons, H-CFCs, SO 2 ) Wash out (soluble in water) “Long” lifetimes ( yr) CO 2 (greenhouse gas) N 2, O 2 (major gases in atmosphere)

Consequences of Tropospheric Photochemistry Direct photochemistry (reactions with h  Photolysis of NO 2 Photolysis of O 3 Indirect photochemistry (rxns with OH) HCFCs (as shown before) Photochemical SMOG Haze/acid rain (sulfate and nitrate aerosols)

Photochemical (LA) Smog London smog (1952) – 4000 deaths Coal (sulfur) + heat  H 2 SO 4 Smoke (coal) + fog = smog Los Angeles smog is different Primary pollutants (from cars) [e.g. NO, CO] Light (sun) and warmth (above 15 o C) Topography (inversion)

EPA Criterion Pollutants: Smog COcarbon monoxide (1 o ) NO x NO + NO 2 (1 o and 2 o ) O 3 ozone (2 o ) RH (VOC)reactive hydrocarbons or volatile organic carbon (1 o )

Chemistry of Smog RH + OH + NO  O 3 + NO 2 + HC O 2, light RH (VOC) = reactive hydrocarbon or volatile organic carbon (e.g. CH 4, octane, terpenes) HC = unreactive hydrocarbon (CO 2 ) OH = hydroxyl radical (requires light) NO(NO 2 ) = nitrogen oxide (nitrogen dioxide) O 3 = ozone

The Chemical Reactions (simplified) Early Morning (sun and cars) CO(cars) + OH  H + CO 2 H + O 2 + M  HO 2 Late Morning (interconversion of NO x ) HO 2 + NO  OH + NO 2 NO 2 + light  NO + O

The Chemical Reactions Early afternoon (formation of ozone) O + O 2 + M  O 3 Same as formation of stratospheric O 3, but source of O atom is NO 2, not O 2 Stratosphere: O 2 + UV light  2O Troposphere: NO 2 + UV light  NO + O

Photochemical Smog CO+ OH  H + CO 2 H + O 2 + M  HO 2 HO 2 + NO  OH + NO 2 NO 2 + light  NO + O O + O 2 + M  O 3 + M Net: CO + 2O 2 + light  CO 2 + O 3

Smog Scenario Early AM rush hour Temperature inversion NO, CO, RH (from cars) Mid-morning (photochemistry, sun) NO 2, CO 2, HC Mid-afternoon (~3-5 PM) O 3 peaks Land warms, sea breeze pushes smog to mountains

Smog Scenario (cont.) Evening Rush hour traffic (more RH, CO, NO) No sunlight, little O 3 formation Sea breeze pushes O 3 inland Late evening Land cools, sea breeze dies down Temperature inversion

Evolution of Smog over Time NO, CO, RH NO 2, CO 2, HC O 3, aerosols GC 6-9AM9 PM3-5 PM10 AM

31 Haze and Acid Rain Conversion of SO 2 to H 2 SO 4 Conversion of NO to HNO 3

Formation of H 2 SO 4 SO 2   H 2 SO 4 1. Gas-phase (homogeneous) (SLOWER) SO 2 + OH  HSO 3 HSO 3 + O 2  HO 2 + SO 3 SO 3 + H 2 O  H 2 SO 4

Formation of H 2 SO 4 (cont.) SO 2   H 2 SO 4 2. In water (heterogeneous) FASTER SO 2 + H 2 O (l)  H 2 O SO 2 (l) H 2 O SO 2 (l) + H 2 O 2  H 2 SO 4 + H 2 O

Formation of HNO 3 NO   HNO 3 1. Gas-phase (homogeneous) (FAST) NO + O 3  NO 2 + O 2 NO 2 + OH  HNO 3

Fate of H 2 SO 4 and HNO 3 Gases (H 2 SO 4 and HNO 3 ) dissolve in water to form acid rain Gases (H 2 SO 4 and HNO 3 ) nucleate to form particles Particles (~ 0.1  m) scatter light and cause haze

A Clear Day (Raleigh Day) (just molecular scattering)

Aerosol Scattering (Backward)

Aerosol Scattering (Forward)