Introduction to Photochemistry Marin Robinson Department of Chemistry Northern Arizona University.

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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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

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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)