Spectroscopy & Photochemistry I
Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7
Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2013 Jose-Luis Jimenez
Importance of Spectroscopy and Photochemistry I
• Most chemical processes in the atmosphere are initiated by photons
– Photolysis of O3 generates OH – the most important atmospheric oxidizer: 1 O3 + hv O2 + O( D) 1 O( D) + H2O 2 OH – Solar photodissociation of many atmospheric molecules is often much faster than any other chemical reactions involving them:
NO2 + hv O + NO (source of O3 in the troposphere)
CF2Cl2 + hv CF2Cl + Cl (photolysis of CFCs in the stratosphere) HONO + hv OH + NO (source of OH in the troposphere)
NO3 + hv O2 + NO or O + NO2 (removal of NO3 generated at night)
Cl2 + hv Cl + Cl (source of Cl radicals)
H2CO + hv H2 + CO or H + HCO (important in hydrocarbon oxidation) etc.
1 Importance of Spectroscopy and Photochemistry II
• Absorption of solar and earth radiation by atmospheric molecules directly influences the energy balance of the planet
– Greenhouse effect (CO2, H2O, N2O, CFCs)
– Stratospheric temperature inversion (O3 photochemistry) • Spectroscopy of atmospheric molecules is used to detect them in situ – OH is detected via its electronic transition at 310 nm
–NH3 is detected via its fundamental vibrational transition at 1065 cm-1, etc.
Blackbody Radiation
Linear Scale P/A=T4 Log Scale T= b
Interactive demo: http://phet.colorado.edu/en/simulation/blackbody-spectrum From R.P. Turco, Earth Under Siege: From Air Pollution to Global Change, Oxford UP, 2002.
2 Solar & Earth Radiation Spectra • Sun is a radiation source with an effective blackbody temperature of about 5800 K • Earth receives circa 1368 W/m2 of energy from solar radiation From Turco
• Clicker Question: are relative vertical scales correct in right plot? A. Yes B. Somewhat off C. Completely wrong
D. I don’t know Adapted from S. Nidkorodov
Solar Radiation Spectrum
UV Photon Energy C B A
IR
From Turco
3 Solar Radiation: Initiator of Atmos. Reactions Average thermal energy of collisions: Each degree of freedom ~ ½ kT (per molecule) Collision energy ~ RT = 8.3 J mol-1 K-1 x T (per mole) RT = 2.5 kJ mol-1 @ 300 K
Energy of photons (E = hv): 300 nm photon = 380 kJ mol-1 600 nm photon = 190 kJ mol-1 Typical bond strengths: -1 D0(O2) = 495 kJ mol -1 D0(Cl2) = 243 kJ mol C-H, O-H, C-O ~ 400 kJ mol-1 Atmospheric chemistry on Earth is driven by photolysis, not by thermal excitation!!!
Adapted from S. Nidkorodov
What is light? • Dual nature – Photon: as particle • Energy but no mass – As wave: electric and magnetic fields oscillating in space and time • Wavelength, From F-P&P frequency • c ~ 3 x 109 m/s Discuss in class: at a fundamental physical level, why are molecules capable of absorbing light?
4 The Electromagnetic Spectrum
• Units used for photon energies and wavelengths: -1 c – 1 eV = 8065.54 cm = 96.4853 kJ/mol = 23.0605 kcal/mol – 1 Å = 0.1 nm = 10-10 m; – 1 micron (m) = 10-6 m = 1000 nm E h -34 2 – Planck's constant = 6.626068 × 10 m kg / s 1 • Clicker Q: the energy of a green photon ( = 530 nm) is: v A. 1 kJ/mol B. 230 kJ/mol C. 230 kcal/mol (wavenumber) D. 6 x 10-7 kJ/mol E. I don’t know
Reminder of EM Spectrum
5 Types of radiation important in lower atmosphere • Ultraviolet and visible radiation ( = 100-800 nm) – Excites bonding electrons in molecules – Capable of breaking bonds in molecules photodissociation) – Ultraviolet photons ( = 100-300 nm) have most energy, can break more and stronger bonds. We will pay special attention to them. • Infrared radiation ( = 0.8 - 300 m) – Excites vibrational motions in molecules – With very few exceptions, infrared radiation is not energetic enough to break molecules or initiate photochemical processes • Microwave radiation ( = 0.5 - 300 mm) – Excites rotational motions in molecules
http://phet.colorado.edu/en/simulation/molecules-and-light
Spectroscopy & Photochemistry II
Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7
Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2013 Jose-Luis Jimenez
6 Fundamentals of Spectroscopy • Molecules have energy in translation, vibration, rotation, and electronic state – Translation (= T) cannot be changed directly with light – We will focus on the other 3 energy types v', J', … • Molecule can absorb radiation efficiently if: photon
– The photon energy matches the energy E spacing between molecule’s quantum levels v", J", … – Optical transition between these quantum levels is allowed by “selection rules” – “Forbidden” transitions can occur but are weaker
Vibrational Energy & Transitions • Bonds can be viewed as “springs” • Energy levels are quantized,
– Ev = hvvib(v+1/2)
– vvib is constant dependent on molecule – v = 0, 1, 2… is vibrational quantum number
From F-P&P
7 Vibrational Energy Levels • Ideally: Harmonic Oscillator – Restoration force of “spring” follows Hooke’s law: F= k x
– Ev = hvvib(v+1/2), v = 0, 1, 2… – Energy levels are equally spaced • Really: Anharmonic oscillator – Restauration force rises sharply at small r, bond breaks at large r
2 3 E hν v 1 hx v 1 hy v 1 ... vib 2 e 2 e 2 – Vibrational quantum levels are more closely spaced as v increases
From F-P&P
http://phet.colorado.edu/en/simulation /energy-skate-park
Example: Ground Electronic State of HF
Etotal Erot Evib
Erot BJ(J 1)
Evib hνv
Rotational level manifolds for Possible different vibrational quanta rovibrational overlap with each other transition: v=0 v=1 J=14 J=15 HF molecular constants -1 Bv=0 = 20.557 cm (rotational constant) = 4138.32 cm-1 (harmonic frequency)
xe = 89.88 cm-1 (anharmonicity) From S. Nidkorodov
8 Vibration-rotation of HCl • Molecules vibrate and rotate simultaneously
From F-P&P
Electronic Transitions (ETs) • Molecules can undergo an ET upon absorption of an appropriate photon – Simultaneous vibrational and rotational transitions – No restriction on v, many vib. trans. can occur – J = -1, 0, +1 • P, Q, and R branches • Frank-Condon principle – Time for ET so short (10-15 s) that internuclear distance cannot change – “vertical” transitions
From F-P&P
9 Pathways for Loss of e- Excitation
From Wayne • Photophysical processes – Lead to emission of radiation – Energy converted to heat • Photochemical processes – Dissociation, ionization, reaction, isomerization
Business Items • Piazza details – Student’s answer, instructor answer, comments – Email alerts digest
10 Repulsive States • No minima in PE vs r curves • Dissociation occurs immediately after absorption of light
From F-P&P
Molecules & Light Simulation
http://phet.colorado.edu/en/simulation/molecules-and-light
11 Quantum Yields () • Relative efficiency of various photophysical and photochemical processes: Number of excited molecules proceeding by process i i Total number of photons absorbed
* • E.g.: NO3 + hv NO3 (3) * NO3 NO2 + O (4a)
NO + O2 (4b)
NO3 + hv (4c) Number of NO molecules formed 2 4a Total number of photons absorbed and so on
• i Are wavelength dependent, above all important at different
Quantum Yields II
From F-P&P
12 Spectroscopy and Photochemistry III
Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7
Atmospheric Chemistry ATOC-5151 / CHEM-5151 Spring 2013 Prof. Jose-Luis Jimenez
On Beer’s Law “The taller the glass, the darker the brew, Clicker prediction: The less the amount of light that comes through” If I set up (1) 200 M concentration & 1 cm path (2) 100 M concentration and 2 cm path The transmittance will be: A. Same B. More C. Less D. A lot less E. I don’t know http://phet.colorado.edu/en/simulation/beers-law-lab
13 Gas Absorption: Beer-Lambert Law I
I I0 exp( L N) • Allows the calculation of the decay in intensity of a light beam due to absorption by the molecules in a medium
Definitions:
• A = ln(I0/I) = Absorbance = LN (also “optical depth”) • absorption cross section Solve in class: Show that in the small [cm2/molec] absorption limit the relative change in light intensity is approximately equal to • L absorption path length [cm] absorbance. • n density of the absorber [molec/cm3]
From F-P&P & S. Nidkorodov
Strength of the bands
Quantify w/ absorption cross-section for one molecule: cm2
17 2 What is a large a. 10 cm 2 (but still realistic) b. 10 cm -12 2 value of for c. 10 cm -17 2 molecules? d. 10 cm e. 10-20 cm2
From Tolbert
14 Physical interpretation of • , absorption cross section (cm2 / molecule) – Effective area of the molecule that photon needs to traverse in order to be absorbed. – The larger the absorption cross section, the easier it is to photoexcite the molecule.
– E.g., pernitric acid HNO4
Collisions Light absorption 10-15 cm2/molec 10-18 cm2/molec From S. Nidkorodov
Measurement of Absorption Cross Sections
Measurement of absorption cross sections is, in principle, trivial. We need a light source, such as a lamp (UV), a cell to contain the molecule of interest, a spectral filter (such as a monochromator) and a detector that is sensitive and responds linearly to the frequency of radiation of interest:
Gas cell Filter I0 I Detector n [#/cm3] L
Measurements are repeated for a number of concentrations at each wavelength of interest.
From S. Nidkorodov
15 Clicker Q A sample contains 1 mbar of molecules of interest (A) with =2x10-21 cm2/molec and 1 bar of impurity (I) with =2x10-18 cm2/molec, both at =530 nm. Which contributes the most to the total absorbance in a 50 cm cell with 1012 photons cm-2 s-1 at the wavelength of interest? A) Molecules A contribute most B) Impurity I contributes most C) They contribute equally D) They are both negligible E) I don’t know
Oxygen Spectrum
•O2 photolysis in the 200- 240 nm range is the
major source of O3 in the stratosphere
•O2 can absorb nearly all radiation with = 10- 200 nm high up in the atmosphere Hole in the Clicker Q: Estimate the spectrum coincident with length of air column at P Lyman line of = 0.01 mbar and T= 200 H-atom K (characteristic of 80 km altitude) required to reduce the radiation flux at 150 nm by 10 orders of magnitude. Neglect the fact that a A. 2.3 cm B. 23 m substantial portion of C. 23 km D. 230 m oxygen is atomized at E. I don’t know this altitude. From Brasseur and Solomon
16 Calculation of Photolysis Rates I Generic reaction: A + hν B + C d[A] J [A] dt A • “First-order process”
• What does JA for a given molecule depend on? A. Light intensity from above B. Light intensity from all directions C. Absorption cross section () & Path length (L)
D. Quantum yield for fluorescence (f) E. I don’t know
Calculation of Photolysis Rates II Generic reaction: A + hν B + C d[A] J [A] () ()F()d [A] dt A A A -1 JA – first order photolysis rate of A (s ) 2 σA() – wavelength dependent cross section of A (cm /#)
A() – wavelength dependent quantum yield for photolysis F() – spectral actinic flux density (#/cm2/s)
17 Group Problem
The graphs below show the approximate solar flux at the Earth surface, absorption cross
section of NO2 molecule, and photodissociation quantum yield
of NO2. What is the photodissociation lifetime of NO2 (NO2)?
A. 16 s B. 6 s-1 C. 6000 s D. 116 s-1 E. I don’t know Part of 2005 Final exam Final Part of 2005
Corollary: What are the smallest cross sections that matter in the atmosphere?
Group Problem
The graphs below show the approximate solar flux at the Earth surface, absorption cross
section of NO2 molecule, and photodissociation quantum yield
of NO2. What is the photodissociation lifetime of NO2 (NO2)?
A. 16 s B. 6 s-1 C. 6000 s D. 116 s-1 E. I don’t know Part of 2005 Final exam Final Part of 2005
18 TUV Model from NCAR (as run for previous slides)
http://cprm.acd.ucar.edu/Models/TUV/Interactive_TUV/
Actinic Flux @ surface & 20 km Output of TUV model of TUV for Output clean atmosphere
19 Nitrogen Dioxide (NO2)
•NO2 is one of a very few atmospheric molecules that absorb & photolyze in the visible range • Photolysis of NO2 generates ozone in the troposphere: 3 NO2 + hv NO + O( P) 3 O( P) + O2 + M NO2 • Absorption cross sections are structured, and have a non-trivial dependence on T,P.
•NO2 contributes to the brown color of air in very polluted cities (but most due to aerosols!).
-19 8x10
6 , base e)
-1 4 molec Cross Section 2 2 (cm
300 350 400 450 500 550 600 From S. Nidkorodov Wavelength, nm
Photochemistry of NO2
• Photolysis occurs with nearly 100% yield below 397.8 nm. O-atom immediately makes O3 3 NO2 + hv NO + O( P) 3 O( P) + O2 (+ M) O3 • Between 398 nm and 415 nm, room temperature NO2 still partially photodissociates because of contributions of internal energy to the process, but the quantum yield declines rapidly with wavelength
• Above 410 nm, electronic excitation of NO2 can result in the following processes: * NO2 NO2 + hv Fluorescence * 1 NO2 + O2 NO2 + O2(a g) Electronic energy transfer * NO2 + N2 NO2 + N2(v) Electronic-to-vibrational energy transfer * NO2 + NO2 NO + NO3 Disproportionation (lab conditions only) From F-P&P
20 Clicker Q • What is the order of magnitude of the shortest possible species lifetime due to photolysis at the 210 nm radiation peak @ 20 km altitude? A. 50 years 20 km B. 50 months C. 50 days D. 50 hrs E. 50 min
Corollary: what about the surface? Surface
Examples of Photolysis Rates
From F-P&P
21 General Remarks Photodissociation is the most important class of photochemical process in the atmosphere: AB + hv A + B
• In order to photodissociate a molecule it must
be excited above its dissociation energy (D0). • In the lower troposphere, only molecules with
D0 corresponding to > 290 nm are photochemically active. Most common
atmospheric molecules, including N2, CO, O2, CO2, CH4, NO, etc. are stable against photodissociation in the troposphere. • In addition, the molecule should have bright
electronic transitions above D0. For example, HNO3 has a low dissociation energy (D0 = 2.15 eV) but it needs UV for its photolysis because it does not have appropriate electronic transitions in the visible. • In general, both the absorption cross sections and photodissociation quantum yields are wavelength dependent. • Photoionization processes are generally not important in the lower atmosphere (ionization potentials for most regular molecules > 9 eV). From S. Nidkorodov
O3 Absorption Spectrum Infrared Hartley absorption bands
Huggins bands Chappuis bands
From Yung & DeMore
22 O2 Photochemistry • Schumann continuum very efficient screening radiation below 200 nm • Solar radiation more intense towards longer From Brasseur and Solomon • Dissociation of O2 in Herzberg continuum (200- 240 nm) is very important for O3 in the stratosphere 3 3 O2 + hv O( P) + O( P) 3 O( P) + O2 (+ M) O3 • Troposphere > 290 nm – Not enough energy
for O2 dissociation –O3 from NO2 + hv From F-P&P
Importance of O3 • Central role in atmospheric chemistry • Highly reactive • Highly toxic => health effects in humans • Crop degradation => billions of $ in losses • Absorbs UV – Shield surface from hard UV – Its photolysis produces O(1D), which yields OH • OH is most important tropospheric oxidant 3 – Photolysis to O( P) regenerates O3, not important! • Absorbs IR – Greenhouse gas
23 O3 Photochemistry • Most important aspect is production of O(1D) (and thus OH) 1 1 O3 + hv O2 + O( D) ( D) 90% below 305 nm 3 3 O3 + hv O2 + O( P) ( P) 10% below 305 nm 1 O( D) + H2O 2 OH OH yield 10% (at the surface) O(1D) + M O(3P) the rest of O(1D) atoms are quenched
Energy Threshold
From F-P&P
UV absorption by O2 and O3
http://www.ccpo.odu.edu/SEES/ozone/oz_class.htm
24 Solar Radiation Spectrum IV • Solar spectrum is strongly
O3 modulated by atmospheric absorptions O2 • Remember that UV photons have most energy
–O2 absorbs extreme UV in mesosphere; O3 absorbs most UV in stratosphere – Chemistry of those regions partially driven by those absorptions – Only light with >290 nm penetrates into the lower troposphere – Biomolecules have bonds that can break with UV absorption => damage to life • Importance of protection From F-P&P provided by O3 layer
Solar Radiation Spectrum vs. altitude
From F-P&P
• Very high energy photons are depleted high up in the atmosphere • Some photochemistry is possible in stratosphere but not in troposphere • Only > 290 nm in trop.
25 Photoionization
Clicker Q: is photoionization important in the troposphere? A. Yes for many species B. Yes for Na C. Yes for Na & Mg D. No E. I don’t know
Chlorofluorocarbons (CFCs)
Photolysis Rates
Absorption Spectra
• No other CFC sinks than photolysis
• Known that Cl would destroy O3 • 1995 Nobel Prize (M&R) is the idea in this slide: CFCs will provide large source of Cl in stratosphere and lead to O3 destruction From Brasseur and Solomon
26 Photolysis Rates for O2 , NO2, and O3 Typical values for photodissociation
From Yung & DeMore coefficients, J, for O3, O2, & NO2 as a function of altitude. Photolysis rate for O2 is strongly altitude dependent because the lower you go the less UV radiation capable of breaking O2 is available (self-shielding). Photolysis rate for O3 becomes altitude dependent below 40 km for similar self-shielding reasons. On the contrary, visible NO2 photolysis occurs with about the same rate throughout the atmosphere because there is not enough of it for self-shielding.
From Warneck
Solve in class: Based on the figures shown here,
estimate the lifetime of NO2, O2 and O3 against photodissociation at 20 km and 50 km.
Solar Radiation Spectrum II
From F-P&P
•Solar spectrum is strongly modulated by atmospheric scattering and absorption From Turco
27 Scattering by Gases • Purely physical process, not absorption • Approximation: 5 2 4 tsg 1.04410 (n0 1) / • Strongly increases as decreases • Reason why “sky is blue” during the day From Turco
Spectroscopy and Photochemistry IV
Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7
Atmospheric Chemistry ATOC-5151 / CHEM-5151 Spring 2013 Prof. Jose-Luis Jimenez
28 Solar Radiation Intensity
To calculate solar From F-P&P spectral distribution in any given volume of air at any given time and location one must know the following: – Solar spectral distribution outside the atmosphere – Path length through the atmosphere – Wavelength dependent attenuation by atmospheric molecules – Amount of radiation indirectly scattered by the earth surface, clouds, aerosols, and other volumes of air
Solar Radiation: Further Details Output of TUV model of TUV for Output clean atmosphere
29 Solar Zenith Angle • Aside form the altitude, the path length through the atmosphere critically depends on the time of day and geographical location. • Path length can be calculated using the flat atmosphere approximation for zenith angles under 80º. Beyond that, Earth curvature and atmospheric refraction start to matter. Solar Flux 1015
Actual pathlength L "Air Mass" m sec ]
-1 Vertical pathlength h nm
-1 14 s 10 -2 SZA = 0º SZA = 86º
1013 At large SZA very little UV-B radiation
Solar Flux [Photons cm reaches the troposphere
1012 300 400 500 700 1000 Wavelength [nm]
Radiation vs. time of day, location, season
From F-P&P Q: summer/winter solstices intensity at 445 nm: @ 8 am? @ noon?
30 Surface Albedo Reflected Radiation() Albedo() Incident Radiation() • Wavelength dependent! • Question: for the same incident solar flux, will you tan faster over snow or over a desert?
From F-P&P
Direct Attenuation of Radiation
-tm From F-P&P & S. Nidkorodov I I0 e
t tsg tag tsp tap I radiation intensity (e.g., F)
I0 radiation intensity above atmosphere m air mass t attenuation coefficient due to – absorption by gases (ag) – scattering by gases (sg) – scattering by particles (sp) – absorption by particles (ap)
Rayleigh scattering t -n -4 sp tsg much
more Deep UV – O, N2, O2 complex Mid UV & visible – O3 Near IR – H2O Infrared – CO2, H2O, others tag
31 Scattering & Absorption by Particles • Particles can scatter From Jacob and absorb radiation • Scattering efficiency is very strong function of particle size – For a given wavelength • Visible: ~ 0.5 m – Particles 0.5-2 m are most efficient scatterers! • Will discuss in more detail in aerosol lectures
Spectroscopy and Photochemistry V (and last!)
Required Reading: FP Chapter 3B, 3C, 4 Required Reading: Jacob Chapter 7
Atmospheric Chemistry ATOC-5151 / CHEM-5151 Spring 2013 Prof. Jose-Luis Jimenez
32 Review Clicker Q
• Jose said during last lecture O3 Infrared that: absorption – ozone is a greenhouse gas – the absorption spectrum of ozone to the right is centered around 1000 cm-1 = 1 m • Is that correct? A. Yes B. Only approximately C. No D. I don’t know
Database of Absorption Spectra http://www.atmosphere.mpg.de/enid/2295
33 Example of Spectra from Database
Limitations of Available Information
Spectra in Quantum database: yields in database:
Much less data for quantum yields, much harder to measure than absorption spectra. (Why?)
34 Structure of Important N-Species
• From Jacobson (1999) Table B – http://cires.colorado.edu/jimenez/AtmChem/Jacobson_Table_B.pdf – Many other species there, useful when you don’t know detailed structure
From Warneck Absorption Spectra and Photolysis Rates of Selected N-Species
Note sensitivity of photolysis rates to shape of absorption spectra
Solve in class: Estimate the photodissociation
lifetimes of N2O and HONO at 20 km. Compare those with characteristic times for vertical transport in the stratosphere ( 2 years).
From S. Nidkorodov
35 Nitrous Acid (HONO) • HONO + hv OH + NO 1 below 400 nm
• HONO, like NO2, has strong absorption in visible and a highly structured spectrum. Its photochemical lifetime in the atmosphere is ~ a few minutes. From F-P&P • Very important as source of OH radicals in the morning
Formaldehyde (HCHO)
• Two competing photodissociation channels: HCHO can be a dominant HCHO + hν H + HCO (a) Hox source (e.g. Mexico City)
HCHO + hν H2 + CO (b)
• Channel (a) leads to HO2 radical production via: All aldehydes have relatively weak transitions at around 300 nm. H + O2 + M HO2 (19) Photoexcitation of aldehydes normally results in a release of HCO, which is quickly HCO + O2 HO2 + CO (20) converted into HO2 and CO in the atmosphere: RCHO + hν R + HCO (slow) • Sources of HO are 2 R + O RO (very fast) effectively sources 2 2 HCO + O HO + CO (very fast) of OH because: 2 2
HO2 + NO OH + NO2 (17)
36 Sources of HOx = OH + HO2 in Mexico City
•HOx drive smog and secondary aerosol chemistry • HONO photolyzes at long , very important in early morning • HCHO (formaldehyde) is dominant source
•O3 source needs to wait for O3 to be produced! (depends on others)
From R. Volkamer & W. Brune (MIT & PSU)
Nitric Acid (HNO3)
HNO3 + hv OH + NO2 1 between 200 and 315 nm
HNO3 + hv O + HONO requires vacuum UV; is only 0.03 at 266 nm
HNO3 + hv H + NO3 requires vacuum UV; is only 0.002 at 266 nm
• Overall relatively long lifetime against photolysis
From F-P&P
37 From F-P&P Nitrate Radical (NO3)
•NO3 is important in nighttime chemistry. It has unusually large cross sections in the red photodissociates in seconds in the morning. 3 NO3+ hv NO2 + O( P) is dependent; important towards the blue
NO3+ hv NO + O2 is dependent; important in the red; competes with fluorescence; this process is nearly thermoneutral but it is inhibited by a tall energy barrier.
Nitrous Oxide (N2O) From •N2O is extremely long-lived because it is Calvert unreactive, and it does not absorb much & Pitts above 200 nm. Below 200 nm: 1 N2O + hv N2 + O( D) - Note: this is a popular laboratory method of generating O(1D); 1 1 • Subsequent reactions of O( D) with N2O lead to production of other nitrogen oxides in the stratosphere: 1 N2O + O( D) NO + NO major source of NO in the stratosphere
N2 + O2 competing step
• Because of its stability, N2O is used a "tracer". Concentrations of other molecules Solve in class: Find a conversion between absorption coefficient in cm-1atm-1 an are often compared to that of N O to see is 2 cross sections in cm2/# at room they are well mixed. temperature. Evaluate N2O cross sections at 193 nm using the data shown here.
38 Atm. Profiles of Important N-Species Important N- containing molecules in lower atmosphere: – extremely photo-stable N2O – easily-degradable NO2, NO3, and N2O5, HO2NO2
Total Density Total – multitude of poorly quantified organic nitrates, RONO2 (most Altitude (km) nitrates are relatively stable towards UV) Generally, concentrations are inversely proportional to photodissociation / reaction rates in the From NASA atmosphere
HW 5 discussion • How to load data from databases into Igor • Interpolation in HW5.2 • Diagram for HW5.3
–NO2 + hv → NO + O (1) O + O2 + M → O3 (2) NO + O3 → NO2 + O2 (3) 1 –O3 + hv → O( D) + O2 (4) 1 –O(D) + H2O → 2 OH (5) 1 3 –O(D) + O2 → O( P) (6) 1 3 –O(D) + N2 → O( P) (7)
– OH + NO2 → HNO3 (8)
–H + O2 → HO2 (9)
–HO2 + NO → OH + NO2 (10)
–HO2 + HO2 → H2O2 + O2 (11)
39 Space for Diagram for HW5.3
40