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[Music]

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hi there so my name is Chloe Stanton I

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just finished my first year of grad

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school at Penn State University but

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today I'm going to talk about a project

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that I did here at Georgia Tech with Dan

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glass there's an undergrad I actually

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just finished this project and we

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literally just got the reviews back from

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the authors like yesterday and I'm not

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even that upset so I'm kind of like in a

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good mood right now I'm sorry I know a

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lot of you guys have heard the spiel

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already many times before but you're

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right here again so I'd like to set the

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stage first by talking about luminosity

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and early Earth and how we perceive

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climate to have changed throughout

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Earth's history so early Earth

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experience what we called the faint

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young Sun paradox and that is because

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luminosity was much dimmer in early

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Earth history so if I have the timeline

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here on the x-axis well that begs axis

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is the timeline of Earth's history and

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on these Y axes I have temperature and

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solar luminosity

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this changes throughout time in a way

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that we can proceed with these

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relationships here so the bold line here

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is just luminosity increasing through

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time and this translates to temperature

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also increasing through time the bottom

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line here is the effective temperature

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of the earth and how it would increase

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if there was no greenhouse effect on

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earth CS represents the line with a

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modern greenhouse atmosphere what

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temperature we would expect throughout

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Earth's history now we have a problem

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because early life evolved during the

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Archean at some point I'm not gonna say

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a number and the problem that we see

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here is even with a modern greenhouse we

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weren't reaching the point of liquid

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water due to loop low luminosity on

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earth so we have a paradox here and

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there's been some proposed solutions to

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this paradox namely greenhouse

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atmospheres first proposed was carbon

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dioxide and then methane but today I'd

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like to spend the time

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I'm talking about nitrous oxide as a

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potential mediator for this effect so in

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order to talk about nitrous oxide I'd

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like to focus specifically on the

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Proterozoic because it has a very

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interesting redox environment again the

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timeline of Earth history but now these

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while keys are related to the oxygen

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content of Earth's atmosphere the

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Proterozoic is boxed off here and oxygen

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is shown to be increased at least to

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some degree during this period in Earth

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history

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however the oceans were hypothesized to

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remain relatively anoxic sand and thus

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were very iron rich prior to this

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neoproterozoic an option that we heard

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about earlier today so this is important

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for one particular reason

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nitrous oxide photo dissociates a very

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low oxygen content because there is no

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ozone shield to protect nitrous oxide

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from catalysis so when we're talking

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about nitrous oxide and early Earth it's

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important that we have some kind of

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oxygen protection to protect some

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nitrous oxide from breaking apart so we

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proposed a new model for the Proterozoic

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nitrogen cycle and we have a background

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here of 0.12 0.001 to 0.1 p al oxygen

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and atmosphere we have micromolar to low

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millimolar iron in the oceans and these

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are just drawn from the literature in

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this regime i have also an axis for the

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nitrogen oxygen state oxidation state

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for these species again this is just the

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Potala sis here in the atmosphere that

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you can have at low oxygen first nitrous

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oxide will be broken down to nitrogen

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gas however

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abus koukin during the time at

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University of Washington hypothesized

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that NIST had evolved by this time in

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Earth's history so nitrogen fixation

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could have been important during this

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time also others have proposed that the

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Arabic nitrogen cycle had also been

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kicking to some degree during this time

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nurses 3 as well producing a bunch of

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different bioavailable intermediates

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for the nitrogen cycle these species all

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could have translated

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through circulation into anoxic waters

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that covered most of Earth's ocean and

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these could have been removed back to

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nitrogen gas through an amok or through

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a more familiar denitrification

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involving several different ear media's

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now an interesting feature of

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denitrification during the Proterozoic

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is that we had low copper concentrations

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in the Proterozoic ocean

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which may have limited this last step in

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denitrification inhibiting conversion of

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nitrous oxide to nitrogen gas I'd like

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to focus on this top though specifically

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for an abiotic form of denitrification

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called femur denitrification this is a

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subsequent translation of these nitrogen

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intermediates back to nitrous oxide gas

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using different metals today I'd like to

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focus just on iron though and just on

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this last step so for this project I set

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out to document the rate of this

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reaction shown here shown to be

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thermodynamically favorable and modern

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conditions and I set out to determine

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the exact variables involved in the rate

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law of documenting nitrous oxide

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production as a function of each

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variable in this equation here the way I

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did this was I set out with an anoxic

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chamber incubated sterile anoxic

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seawater with variable amounts of iron -

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and synthetic nitric oxide

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I removed the headspace from these

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bottles regularly and measured the

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nitrous oxide produced over time using

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gas chromatography some of the data from

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these experiments is shown here so a

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single substrate you only see very

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little very minimal nitrous oxide on the

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y-axis produced over time on the x-axis

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however if we start increasing the

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reactant concentration holding nitric

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oxide constant here you see increased

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nitrous oxide production as we increase

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iron concentrations as well then holding

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iron constant you see the same effect if

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you increase nitric oxide concentrations

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as well this translate back to the rate

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law by plotting in a special way so here

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I've gotten two plots just pulled from

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this same data color coordinated if I

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plot

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the log of the initial reactant

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concentration on the x axis and on the y

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axis the log of the initial rate I see a

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linear relationship shown here and the

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slopes of these relationships

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corresponds to the order of reaction

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with respect to each of these reactants

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and then I can plug in to this rate law

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that I derived here the pH was found to

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be constant throughout these reactions

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so I dropped the proton and then I can

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solve for this reactant constant K here

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and we have a complete rate law for this

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reaction but this is important for a

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couple reasons

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because we can determine essentially a

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theoretical nitrous oxide production

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rate in any environment but we have to

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understand things about the reactant

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concentrations first so we understand a

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bit about iron in modern and ancient

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react environment so in modern OMGs or

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oxygen minimum zone you see very minimal

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iron concentration but there have been

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modeling studies shown that Proterozoic

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iron concentrations could have been up

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to 10 millimolar or even more based on

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certain studies nitric oxide is a little

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bit harder to constrain it's actually

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not commonly measured even in the modern

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but we do see picomolar 2 nano molar

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levels in modern environment so we

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translate that to equivalent load nano

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molar levels in the clitoral so what

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does this mean for the nitrous oxide

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flux currently our flux is around 17

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tera grams per year and this translates

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to a mixing ratio of about 300 ppb and

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that appears and I set out using these

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reactant concentrations to think about

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what this meant for the Proterozoic in

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different environments so I used this

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rate law in tandem with a couple

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assumptions first of all we assumed that

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there was a 10 meter thick smoke line in

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pretty rizzo ik oceans in which ecology

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nitrification occurred and that this

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ocean covered 70% of Earth's surface and

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then we cap things with the nitrogen

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fixation rate so the nitrogen fixation

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rate in the modern environment is

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hundreds of 200 tare grams of nitrogen

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per year and we assumed that all of this

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fixed nitrogen in the Proterozoic would

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convert to n 2o through Kimura

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denitrification so using our rate law

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we've made this thing you're here so we

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have the nitric oxide concentration the

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first reactant on the y axis and then

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iron the second reactant on the x axis

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and then the contours represent the flux

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of n 2o you can get out using our rate

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law the the dashed lines represent this

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nitrogen fixation rate of 100 to 200

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teragrams and given these dashed lines

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we would assume that this could happen

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in environments with lone animal or NO

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and low millimolar iron which we think

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could have been possible in the

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Proterozoic based on our previous study

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so this is cool now we know how fast the

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intro is being made but how much and

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what did that mean for the climate and

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the car was doing so we set out to do

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some photochemical modeling so this is a

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complex figure but all we have here on

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the x-axis is that flux out from our

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kinetic model and then on the y-axis we

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have the atmospheric concentration of

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n2o how much actually is retained in the

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atmosphere and this changes at the

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function of oxygen due to that fatah

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lysis and that point there represents

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our current modern state but we targeted

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depending on the oxygen concentration

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that you think was present during the

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Proterozoic this regime for how much

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into oh and how much flux of into oh we

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could have during a fritter is oh it

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pumping this into a climate model that

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was previously completed by Roberson at

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all we wanted to see how much this would

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translate in terms of temperature and

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warming during the clitoris oeq so here

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this figure before I put any lines on it

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I'd like to just talk about it a little

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bit on the x-axis we have the mixing

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ratio of n2o

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which translates to the y-axis on this

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first figure on the x-axis or on the

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y-axis on the second figure we have

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surface temperature in Kelvin as a

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result from this climate model and all

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lines on this

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are gonna correspond to a background

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near monitoring greenhouse of carbon

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dioxide and methane and here I'm just

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kind of bookended a couple important

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values that we think could have been

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possible as a minimum and a maximum for

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potential Proterozoic nitrous oxide

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concentration exam this year and what

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this means in terms of temperature

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depends on when in the Proterozoic we're

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speaking about because the Proterozoic

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is a really long time so in the early

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Proterozoic moon Asti was much lower so

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we experienced lower surface

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temperatures in the late Proterozoic it

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was higher so this relationship between

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the into a mixing ratio and temperature

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change throughout this time period but

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during this time period with a

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background into 0.125 ppm which we could

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expect given our assumptions and our

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modeling calculation we do see a

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significant level of warming just from

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nitrous oxide so just to conclude and

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wrap-up this the climate modeling that

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we've done assumes that we can get up to

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five degrees of warming from nitrous

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oxide just from chemo denitrification

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but we have built in a couple

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assumptions into these calculations I

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think have a little bit of wiggle room

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and I think it's really exciting to

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start thinking about how this affects

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the rest of the ancient nitrogen cycle

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and in turn how further ancient nitrogen

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cycle studies could affect you know

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denitrification for example we don't

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have very good constraints on ancient

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nitric oxide concentrations could have

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been much higher considering the ocean

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was primarily anoxic also if ancient

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nitrogen fixation was faster than it is

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today this is the possibility this would

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also change our calculations in the same

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sense if it was slower this would change

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our calculations as well also it is

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likely that not all of this chemo or all

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of this fix nitrogen engages in chemo

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denitrification so we've presented one

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end member here conversely there's

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another end member in which no nitrogen

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is smutty nitrified also there are

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interesting caveats that we've only

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recently kind of thought about in terms

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of feedbacks with ancient oxygen

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in high oxygen environments nitrous

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oxide is broken down into nitric oxide

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which engages in a zone breakdown so

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there would be a feedback in which ozone

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protection would be scavenged even in

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high oxygen environments so I'm really

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excited about this project

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I'm excited some first submitted paper

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so I'm glad it's done if you guys have

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any questions just let me know and I'd

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like to thank my lab group it's good to

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see everybody and some of our funding

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sources as well alright questions for

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Chloe I'm just curious what is the sink

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of and cool in your model and how much

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how long was the residence time of

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entering your model yeah so that would

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be the Fatah Lass's just as soon as you

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release into oh and that McPhee R you're

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breaking it down into nitrogen gas here

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oh there we go

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so you've considered it in your model

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right yes that how long was the lifetime

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like it's very rapid but it depends on

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the oxygen concentration yeah that's why

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I must use yeah so I mean if you look

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like back all the way yeah so he looked

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here it depends greatly on how much

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oxygen you have so your residence time

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in changing with the concentration of

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oxygen

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hi thanks for the great talk um I was

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curious you had mentioned I forget

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exactly which reaction it was but metal

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limitation in a protozoa koushin

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potentially changing the rates of

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writing these biological processes and

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so I had a question specifically for a

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nitrogen fixation because yeah I guess

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presumably the Proterozoic ocean was

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also molybdenum limited and so maybe

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that might have affected the rates right

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and that student fixation so I was

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wondering what your thoughts were so I

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think well I think all of these I kind

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of

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but I put them in really small font on

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purpose kind of get some of these

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studies they're a little bit skeptical

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in my eyes but if you look here Ava

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Dukan did this paper studying nitrogen

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fixation enzyme and she is particularly

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interested in alternative cofactors

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besides molybdenum and how that could

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have pertained to ancient enzymes in a

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nitrogen cycle

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so this one paper in particular hinges

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the entire nitrogen fixation crux on

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something else besides molybdenum is

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engaging with reaction so whether or not

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that's true I don't know I think there

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is definitely some isotope evidence that

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they're kind of hinging their argument

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upon there but I think again today we

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see mostly a molybdenum cofactor I think

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that's something that I'm going to think

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about when we talk before your somatic

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you'd like this thank you other

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could you go back to rape Grafton's rate

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which yes and so I know this is just a

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general chemistry question now so this

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00:17:17,830 --> 00:17:15,830
is assuming first-order kinetics for

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each of these reactants so there's a

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00:17:23,710 --> 00:17:20,270
little bit of there's a little bit of

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00:17:26,590 --> 00:17:23,720
wiggle room here but this is essentially

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just kinetic models that you can run for

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different orders of reaction for each

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reactant and this is the one in which

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gives us a linear relationship so this

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00:17:39,010 --> 00:17:35,420
is the one that we use so depending on

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00:17:42,070 --> 00:17:39,020
how linear your data is is determining

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00:17:47,260 --> 00:17:42,080
how you plot your data you've got the

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different order for these are called

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pseudo first order because they're not

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00:17:54,610 --> 00:17:51,890
zero with order and they're not second

402
00:17:57,330 --> 00:17:54,620
order but they don't fall along the

403
00:18:02,350 --> 00:17:57,340
theoretical first order kinetics which

404
00:18:04,440 --> 00:18:02,360
almost nothing does but this is as close

405
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as you're gonna get in the real world I

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00:18:06,960 --> 00:18:06,380
don't know does that answer your

407
00:18:15,940 --> 00:18:06,970
question

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Chloe all right if not let's thank her