23 april 2002 creative research enterprises’ presentation at egs 2002 meeting slide 1 a greenhouse...

23 April 2002 Creative Research Enterprises’ Presentationat EGS 2002 Meeting

Slide 1

A GREENHOUSE GAS (N2O) IS A POSSIBLY SIGNIFICANT BYPRODUCTOF THE

UV-SHIELDING OF THE BIOSPHERE BY O3

PAPER # EGS02-A-02049; ST4.01-1TU3A-005

Sheo S. PrasadCreative Research Enterprises

(e-mail: [email protected]: +1 925 426 9341)


Slide 2

TABLE OF CONTENTS

• OVERVIEW

• BACKGROUND

• RECENT DEVELOPMENTS

• SUMMARY & CONCLUSIONS

• URGENT TASKS AHED


Slide 3

OVERVIEW OF THE UV-SHIELDING PROCESS AND N2O PROSUCTION

• The shielding of the biosphere from harmful UV-A (240-320 nm) and UV-B results from the excitation of ground state O3(X1A1) to its highly dissociated 1B2 and 2 1A1 electronic states associated with the Hartley & Huggins bands.

• Intriguingly, a potentially significant amount of a greenhouse gas (N2O) may be generated as a byproduct, when atmospheric O3 is optically pumped by 310 340 nm photons to the state responsible for the Huggins bands. This may occur via the reaction O3(2 1A1) + N2 N2O + O2


Slide 4

EVOLUTION OF N2O-O3 CONNECTIONHas twists & turns very familiar in path to progress!

• Prasad,1981; Electronically excited metastable triplet O3 was thought to be a possible source of N2O based on experiments that appeared to support formation of triplet O3 in O, O2 recombination

• Prasad,1994; 10 experiments further support N2O formation via some type of excited O3. But, by this time the prospect of metastable excited triplet was lost. So, excited O3(X1A1, very high v) from O, O2 recombination was proposed as N2O source

• Zipf & Prasad, 1998; O3(X1A1, very high v) appeared to explain the high yield of N2O in Zipf-Prasad experiment

• 2001; Role of O3(X1A1) eliminated (Estupinan). But, electronically excited O3 may be forming N2O after all ! (Prasad)


Slide 5

ELECTRONICALLY EXCITED O3 MAY BE FORMING N2O AFTER ALL !

• Formation of N2O from electronically excited O3 follows from analysis of the following experiments

– Estupinan on N2/O2/O3 photolysis at 266 nm

– Gaedtke et al on on N2/O2/O3 photolysis at 254 nm

– Kajimoto & Cvetanovic on N2/O2/O3 photolysis at 254 nm and at N2 pressures from 27 to 113 atmospheres

– DeMore & Raper in liquid phase and 200 to 350 nm

• using a model of N2O quantum yield that explains the yield observed in all these experiments encompassing a very wide range of pressures and radiation wavelength and relative N2, O2 & O3 amount


Slide 6

GAEDTKE ET AL EXPERIMENT SUGGEST N2O FORMATION FROM PROCESS OTHER THAN

O(1D), N2 ASSOCIATION

• Gaedtke et al (1973) found N2O formation in photolysis of N2/O2/O3 mixture at 254 nm and 1 atm pressure and determined 2.7x10-36 as the the rate coefficient for the O(1D) + N2 + M -> N2O + M, if the observed N2O is attributed to that reaction.

• Later work by Kajimoto & Cvetanovic (1975) suggested a much smaller (by factor of 7.7) rate coefficient (or, 3.5x10-37) and this smaller coefficient is currently recommended by NASA Panel and by IUPAC.

• In retrospect, Gaedtke et al experiment imply that at lower pressures N2O may form more efficiently by process (es) other than O(1D)+N2


Slide 7

EFFICIENT FORMATION OF N2O BY PROCESSES OTHER THAN O(1D), N2 ASSOCIATION IS

CONFIRMED BY ESTUPINAN EXPERIMENT

• 28 year later, Estupinan et al found a linear variation of the quantum yield N2O in O3/O2/N2 photolysis at 266nm and at 200 P 800 torr, i.e.,

N2O/PN2 = 2.1x10-6 when PN2 is in atm

• If the observed N2O is attributed to O(1D), N2 association (as Estu-pinan et

al did), the rate coefficient for the association reaction at 1 atm again turns out to be too large by a factor of almost 8 compared to current NASA Panel recommendation based on Kajimoto & Cvetanovic experiment.

• Thus, it is urgent to search for the process(es) that could produce N2O

more efficiently than the O(1D), N2 association at 1 atm.


Slide 8

PRODUCTION FROM ELECTRONICALLY EXCITED O3 IS A LOGICAL CHOICE

• None of the other species expected in N2/O2/O3 photolysis at 266 nm

(like O2(1g) , O2(b 1)) has enough energy to produce N2O

• In principle photolysis of O3 with Hartley band photons can produce

vibrationally excited O2 with vibrational energy needed to possibly

generate N2O. However, at 266 nm this too is not possible.

• O3(X,1B1) with high v attainable in O, O2 recombination has already

been eliminated by Estupinans experiments with 532 nm

• Thus, O3(1B2) + N2 -> N2O# + O2*# (where superscripts * and #

represent, respectively, electronic and combined vibrational and translational excitations) may be the logical process.


Slide 9

THE FACT THAT ELECTRONICALLY EXCITED O3 HAS LIFETIME OF MOSTLY FEMTOSECONDS

SHOULD NOT CAUSE MUCH CONCERN

• Possibly, when a N2 comes so close to a O3(1B2) that it might react

then the close proximity perturbs the dissociation dynamics to an extent that there is time to form the transition state.

• The "net" reaction may involve a hitherto unrecognized, electronically excited, O3 with shallow minimum in its potential

energy surface into which a fraction of O3(1B2) may change by

curve crossing

• Also, after all short lived O2(B3 ) with lifetime of ps or less is

known to react


Slide 10

RATE CONSTANT FOR O3(1B2) + N2 -> N2O# + O2*#

DERIVED FROM ESTUPINAN’S N2O AT 266 nm IS VERY REASONABLE

• At any point in the irradiated region the very small but finite number density of O3(1B2) that have not as yet lost their identity is

n(O3(1B2)) = J n(O3) / kdiss ; kdiss = (lifetime)-1

• The corresponding quantum yield is : (k/kdiss )n(N2) where k is the rate constant of the title reaction

• With Estupinan’s N2O & lifetime = 10 fs, k = 8x10-12cm3s-1


Slide 11

IF O3(1B2) FORMS N2O, THENTHE FOLLOWING SHOULD ALSO HOLD

• There should be considerable N2O formation when O3 is excited to the

secondary minima of the 2 1A1 or to the quasi-bound portion of the 1B2

potential energy surface that are responsible for the Huggins bands

(despite the negligible yield of O(1D) in this region).

• The N2O derived in from Estupinan’s data should be consistent with that

from the high pressure data of Kajimoto and Cvetanovic.

From a reinterpretation of Kajimoto & Cvetanovic and DeMore & Raper

experiments, using a more complete model of N2O, both constrains are

found to be fulfilled (as will be now explained).


Slide 12

THE MODEL OF N2O USED TO REINTERPRET KAJIMOTO & CVETANOVIC AND DEMORE & RAPER DATA HAS FOLLOWING FEATURES

• Quantum yield from O3(1B2) and O3(2 1A1) [Linear in pressure p]

• All elements of Kajimoto & Cvetanovic’s model of N2O from O(1D), N2 association [p2 variation]

• Contribution of O3N2 + hv N2O+ O2 that represents the photolysis of O3 component of the O3N2 and the O(1D), N2 association inside the dimer. [Also, p2 variation]

• Details are in a preprint available for distribution to those interested.


Slide 13

THE NEW MODEL OF N2O HAS THE ELEGANT POTENTIAL FOR EXPLAINING OBSERVED N2O

OVER A VAST RANGE OF PRESSURES (OVER THREE ORDERS OF MAGNITUDE)

N2 Pressure (atmospheres)

0.1 1 10 100

Qua

ntum

yie

lds

1e-7

1e-6

1e-5

1e-4

1e-3press vs High pressure (>27 atm) of KCpress vs Total of the three contributions

press vs O3 (1B2) contribution

press vs O3-N2 dimer contribution

press vs O(1D)+N2+M contribution

press vs Low pressure data of Estupinan


Slide 14

THE ELECTRONICALLY EXCITED O3(1B2) STANDS OUT AS THE N2O PRODUCTION MECHANISM AT THE LOW PRESSURE

Note that the role of the O(1D), N2 association and the O3N2 decrease rapidly at the lower pressures. Thus, the electronically excited O3(1B2) mechanism becomes the dominant process in Estupinan's experiment at low P. The crossover between the O3(1B2) and the association mechanisms occurs at about 10 atm. Beyond that, the association mechanism rapidly becomes predominant. The fit of the observational data with the new model is excellent for both Estupinan's and KC's data. Observed and modeled N2O overlap.


Slide 15

IF O3(1B2) CAN FORM N2O THEN O3(2 1A1) MUST ALSO DO THE SAME, POSSIBLY EVEN MORE

EFFICIENTLY

Due to interference by O(1D), we must look at the almost O(1D) free Huggins band region to appreciate a unique role of electronically excited O3. This effort may also reveal possible variation of (k3/kdiss) with the variation in the lifetime of excited O3 from very low values (i.e., 10 - 20 fs for the Hartley bands) to significantly higher values (i.e., 0.3 - 1.1 ps for the Huggins bands). The only one experiment where the production of N2O was studied in the Huggins band region was done in the liquid nitrogen at 77K by DeMore and Raper.


Slide 16

IF O3(1B2) CAN FORM N2O THEN O3(2 1A1) MUST ALSO DO THE SAME, POSSIBLY EVEN MORE

EFFICIENTLY -- (Continued from slide 15)

• Done 40 years ago, in 1962, DeMore-Raper experiment has been almost forgotten. Specifically, for no really valid reason, the N2O observed in the 320-340 nm region has been held in grave suspicion.

• The situation now should changes in view of the present interpretation of Estupinan’s et al experiment


Slide 17

THE N2O FROM O3 EXCITED BY HUGGINS BAND AND DIMER EFFECT ARE VERIFIED BY

DEMORE-RAPER EXPERIMENT

Wavelength

240 260 280 300 320 340 360

N2O yield

0.000

0.005

0.010

0.015

0.020

0.025Observed N2O Yield with error bars

O3(1B2) part O3-N2 dimer part

O(1D) + N2 part Sum of all partspotential yield from O3(

1B2)over

the entire Hartley-Huggins band

Figure shows N2O observed by DeMore &Raper, and the its various modeled components.

Note that the production from O3(2 1A1) dominates at 310330 nm


Slide 18

THE ATMOSPHERIC PRODUCTION OF N2O FROM O3 EXCITED BY HUGGINS BANDS MAY BE

SIGNIFICANT

N2O Production rate (cm-3 s-1)

0 50 100 150 200 250

Altitude (km

)

0

10

20

30

AB

O3 Concentration (cm-3)

1e+9 1e+10 1e+11 1e+12 1e+13

n(O3)A & B representtwo different ways of diurnal averaging.

Significance

•Direct way of producing mass-independent heavy O-atom enrichment in N2O

•Total atmospheric production (2-3)x108 N2O cm-2 s-1 is substantial


Slide 19

SUMMARY & CONCLUDING REMARKS

(#1) N2O is a quite possible and atmospherically significant product when O3 is excited by Huggins band (310-340 nm) photons in air

(#2) This production occurs in both the troposphere and the stratosphere

(#3) Since this production occurs in the stratosphere also, missing sinks of N2O are implied, if the possibility in (1) is upheld by experiments


Slide 20

EXPERIMENTAL RESEARCH TASKS THAT NEED URGENT ATTENTION

• Since DeMore & Raper experiment was done in condensed phase, it is most important to study the production of N2O with high spectral resolution when gas phase mixtures of air and O3 are irradiated by Huggins band photons at various atmospherically significant temperatures and pressures, simultaneously examining the isotopic composition of the product N2O


Slide 21

EXPERIMENTAL RESEARCH TASKS (Cont.)

• The fact that Estupinan et al experiments done with the marvels of modern laboratory techniques gave the same answer as was obtained 28 years ago with much simpler techniques available at that time shows that

• the needed set of experiments can be done with relatively simple techniques available at even moderately equipped laboratories.

• It is therefore hoped that this presentation will enthuse many others to experimentally check the interpretations presented here.


Slide 22

APOLOGY, THANKS & REQUEST FOR FEEDBACK

• The author apologizes for not being present here

(due to budgetary constrains)

• Sincere thanks are due to Peter Fabian for hanging

the poster version of the paper originally planned for

oral presentation

• Please send feedback (comments and/or questions)

or preprint request to the author using Internet e-mail

address [email protected]

23 april 2002 creative research enterprises’ presentation at egs 2002 meeting slide 1 a greenhouse...

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