Introduction to Photochemistry
Marin RobinsonDepartment 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)
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Alt
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Temperature (°C)
Thermosphere
Mesosphere (0.5%)
Stratosphere (9.5%)
Troposphere (90%)
tropopause
stratopause
mesopause
Ozonemaximum
Layers of the atmosphere• Troposphere• Stratosphere
Ozone depletion
Haze, smog, globalwarming
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
Rl = Jlnl Jl = wavelength dependent coefficient that
depends on absorption properties of molecule (s-1)
nl = number density (molecules/cm3)
Photodissociation
Jl is large (> 10-6/s), molecule unstable Atmospheric lifetime: sec to days
(example: O3 and NO2) Jl is small (< 10-7/sec), molecule stable
Long lifetimes (~yrs) Most atmospheric gases (N2, O2, CH4) 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 BalanceSolar in Infrared out
IRUV-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: 320-380 nm (tanning salons) – all UV-B: 290-320 nm (sunburn) - most
absorbed by O3 layer
UV-C: 250-290 nm (biocidal) – completely absorbed by O2 and O3 in stratosphere
UV (strat); UV-VIS (trop)
Photochemistry in Stratosphere(High-Energy UV)
Natural formation of ozone O2 + h O + O
O + O2 + M O3
Natural destruction of ozone O3 + h O + O2
O + O3 2O2
Photochemistry in Stratosphere(High energy UV) [cont.]
Destruction of ozone (human-caused) CF2Cl2 + light CF2Cl + Cl
Cl + O3 ClO + O2
ClO + O3 Cl + O2 + O2
Photochemistry in Troposphere(UV-VIS)
UV-B and UV-A (and some VIS) N2 + light (trop) no reaction
O2 + light (trop) no reaction
CO2 + light (trop) no reaction
H2O + 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 O3 + h O2 + O*
NO2 + 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 O3 & NO2)
OH (hydroxyl radical, made from O)
O + H2O 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 CH4 (methane) + OH CH3 + H2O
H2S + OH HS + H2O
CF2ClH (H-CFC) + OH CF2Cl + H2O
Atmospheric Lifetimes
“Short” lifetimes (< 10 yr) Photochemically unstable (NO2 and O3) React with OH (CH4, reactive hydrocarbons,
H-CFCs, SO2) Wash out (soluble in water)
“Long” lifetimes (10-200 yr) CO2 (greenhouse gas) N2, O2 (major gases in atmosphere)
Consequences of Tropospheric Photochemistry
Direct photochemistry (reactions with h Photolysis of NO2
Photolysis of O3
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 H2SO4
Smoke (coal) + fog = smog Los Angeles smog is different
Primary pollutants (from cars) [e.g. NO, CO] Light (sun) and warmth (above 15 oC) Topography (inversion)
EPA Criterion Pollutants: Smog
CO carbon monoxide (1o) NOx NO + NO2 (1o and 2o)
O3 ozone (2o)
RH (VOC)reactive hydrocarbons or
volatile organic carbon (1o)
Chemistry of Smog
RH + OH + NO O3 + NO2 + HC
O2, light
RH (VOC) = reactive hydrocarbon or volatile organic carbon (e.g. CH4, octane, terpenes)
HC = unreactive hydrocarbon (CO2)OH = hydroxyl radical (requires light)
NO(NO2) = nitrogen oxide (nitrogen dioxide)
O3 = ozone
The Chemical Reactions (simplified)
Early Morning (sun and cars)
CO(cars) + OH H + CO2
H + O2 + M HO2
Late Morning (interconversion of NOx)
HO2 + NO OH + NO2
NO2 + light NO + O
The Chemical Reactions
Early afternoon (formation of ozone)
O + O2 + M O3
Same as formation of stratospheric O3, but source of O atom is NO2, not O2
Stratosphere: O2 + UV light 2O
Troposphere: NO2 + UV light NO + O
Photochemical Smog
CO+ OH H + CO2
H + O2 + M HO2
HO2 + NO OH + NO2
NO2 + light NO + O
O + O2 + M O3 + M
Net: CO + 2O2 + light CO2 + O3
Smog Scenario
Early AM rush hour Temperature inversion NO, CO, RH (from cars)
Mid-morning (photochemistry, sun) NO2, CO2, HC
Mid-afternoon (~3-5 PM) O3 peaks Land warms, sea breeze pushes smog to
mountains
Smog Scenario (cont.)
Evening Rush hour traffic (more RH, CO, NO) No sunlight, little O3 formation
Sea breeze pushes O3 inland
Late evening Land cools, sea breeze dies down Temperature inversion
Evolution of Smog over Time
NO,CO, RH
NO2,CO2, HC
O3,aerosols
GC
6-9AM 9 PM3-5 PM10 AM
31
Haze and Acid Rain
Conversion of SO2 to H2SO4
Conversion of NO to HNO3
Formation of H2SO4
SO2 H2SO4
1. Gas-phase (homogeneous) (SLOWER)
SO2 + OH HSO3
HSO3 + O2 HO2 + SO3
SO3 + H2O H2SO4
Formation of H2SO4 (cont.)
SO2 H2SO4
2. In water (heterogeneous) FASTER
SO2 + H2O (l) H2 O SO2(l)
H2 O SO2(l) + H2O2 H2SO4 + H2O
Formation of HNO3
NO HNO3
1. Gas-phase (homogeneous) (FAST)
NO + O3 NO2 + O2
NO2 + OH HNO3
Fate of H2SO4 and HNO3
Gases (H2SO4 and HNO3) dissolve in water to form acid rain
Gases (H2SO4 and HNO3) 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)