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TRANSCRIPT
Introduction to Photochemistry
DEPARTMENT OF CHEMISTRY MOOLJI JAITHA COLLEGE
JALGAON.
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)
120-
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-100 -60-80 -20 0-40 20 40 60
Altitude
(kilo
mete
rs)
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, 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 + hn 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 Balance
Solar in Infrared out
IRUV-VIS-IR
Solar In – IR Out
Energy In (from Sun)
Long l
(Troposphere)
Short l
(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 + hn O + O
O + O2 + M O3
Natural destruction of ozone
O3 + hn 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 + hn O2 + O*
NO2 + hn 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 hn)
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 mm) scatter light
and cause haze
A Clear Day (Raleigh Day)(just molecular scattering)
Aerosol Scattering (Backward)
Aerosol Scattering (Forward)