Parts per million for a gas in atmosphere

[alpha tech]

if a gas has 400 PPM in an atmosphere that is 4.2 billion km^3 what is the actual volume of that gas

[p1t1o]

PPM is just like percentage, just in millions instead of hundreds:

( 4.2billion / 1million ) * 400 = km^3 of said gas

 

[alpha tech]

I did that before I looked and the exponent was negitave so its millionths

0.04

Edited March 2, 2017 by alpha tech

[YNM]

PPM : Parts Per Million

So whatever amount of mixture you have, that part of the mixture will only be (PPM/(1,000,000))*(Amount of mixture). It could be mass, volume, number of particles... (but converting from one to the other is another thing.)

[alpha tech]

ideal gas equation of co2 in earths atmosphere

[p1t1o]

35 minutes ago, alpha tech said:

I did that before I looked and the exponent was negitave so its millionths

0.04

Heh?

[alpha tech]

on Wikipedia

[Shpaget]

4 hours ago, alpha tech said:

I did that before I looked and the exponent was negitave so its millionths

0.04

400 millionths, as in 400/1 000 000

0,0004.

[Snark]

On 3/2/2017 at 7:51 AM, alpha tech said:

if a gas has 400 PPM in an atmosphere that is 4.2 billion km^3 what is the actual volume of that gas

 

So, a couple of things to consider.

 

One is, when you say "PPM", is that by volume, or by mass?  Reason that matters:  if you're talking about CO₂ in Earth's atmosphere, it has a molecular mass roughly 50% higher than that of the rest of the atmosphere.  So the mass PPM will be roughly 50% higher than the volume PPM.

 

Next... it's potentially tricky (or misleading) to talk about our atmosphere's "volume" because that's not a well-defined number.  The atmosphere isn't uniformly distributed (density changes rapidly with altitude), and has no specific "top" to it.  So, be careful to understand what the meaning of that number you're using for "volume" actually is.  One number you could use would be to just say, "if the atmosphere were all the same uniform density that it is at sea level (which it isn't), how much volume would it occupy."  From plugging a few numbers in, I think that's where you're getting your number-- that looks about right.  That's fine, as long as that's the number that you actually want; just making sure that's the assumption here.  The actual atmosphere occupies a lot more space, since it rapidly gets less dense with altitude.

 

[mikegarrison]

On 3/3/2017 at 2:36 PM, Snark said:

 

So, a couple of things to consider.

 

One is, when you say "PPM", is that by volume, or by mass?  Reason that matters:  if you're talking about CO₂ in Earth's atmosphere, it has a molecular mass roughly 50% higher than that of the rest of the atmosphere.  So the mass PPM will be roughly 50% higher than the volume PPM.

 

Next... it's potentially tricky (or misleading) to talk about our atmosphere's "volume" because that's not a well-defined number.  The atmosphere isn't uniformly distributed (density changes rapidly with altitude), and has no specific "top" to it.  So, be careful to understand what the meaning of that number you're using for "volume" actually is.  One number you could use would be to just say, "if the atmosphere were all the same uniform density that it is at sea level (which it isn't), how much volume would it occupy."  From plugging a few numbers in, I think that's where you're getting your number-- that looks about right.  That's fine, as long as that's the number that you actually want; just making sure that's the assumption here.  The actual atmosphere occupies a lot more space, since it rapidly gets less dense with altitude.

 

Since it's obviously a reference to CO₂ on the Earth, let me explain. The answer is "neither". It's mole fraction.

When they report that CO₂ is at 400 ppm, that means that for every million molecules in the atmosphere (including CO₂ itself but not including water vapor), 400 of them are CO₂.

Edited March 22, 2017 by mikegarrison

[Snark]

3 hours ago, mikegarrison said:

The answer is "neither". It's mole fraction.

If what he intended was mole fraction, then fine-- of the two possibilities I suggested, it's not "neither", it's "volume."

Since "mole fraction" and "volume fraction" are, in fact, the same thing for a gas. 

[mikegarrison]

14 hours ago, Snark said:

If what he intended was mole fraction, then fine-- of the two possibilities I suggested, it's not "neither", it's "volume."

Since "mole fraction" and "volume fraction" are, in fact, the same thing for a gas. 

Hey, you could be right. I'm just repeating what the experts on the subject say.

https://www.esrl.noaa.gov/gmd/ccgg/trends/

Quote

Data are reported as a dry air mole fraction defined as the number of molecules of carbon dioxide divided by the number of all molecules in air, including CO₂ itself, after water vapor has been removed. The mole fraction is expressed as parts per million (ppm). Example: 0.000400 is expressed as 400 ppm.

But I kind of vaguely think there is a difference. pv=nRT, right? But that's for ideal gases. I think there may be some slight differences for non-ideal gases.

Edited March 23, 2017 by mikegarrison

[SuperFastJellyfish]

Why do they discount water vapor?  Because it cycles from ocean/freshwater to water vapor and back again?

[mikegarrison]

1 minute ago, SuperFastJellyfish said:

Why do they discount water vapor?  Because it cycles from ocean/freshwater to water vapor and back again?

Yeah, I think that's why.

[Snark]

52 minutes ago, mikegarrison said:

Hey, you could be right. I'm just repeating what the experts on the subject say.

https://www.esrl.noaa.gov/gmd/ccgg/trends/

Yes, that's experts say, who mention molar fraction and make no comment whatsoever about how that relates to volume fraction.  So that particular link isn't relevant to the question "is volume fraction the same as molar fraction."

52 minutes ago, mikegarrison said:

pv=nRT, right?

Yes, that's right.  That's why they're the same thing.

52 minutes ago, mikegarrison said:

But that's for ideal gases. I think there may be some slight differences for non-ideal gases.

"Slight" as in "very tiny and generally not worth bothering with."  Earth atmosphere at surface atmospheric pressure and temperature is a pretty darn good approximation of an ideal gas, which is why everyone does all their math assuming it to be one.

In extreme conditions that bring the gas molecules very close together-- e.g. at cryogenic temperatures, or very high pressures-- there may be more of a discernable difference.  But that's not the situation we're talking about here.

50 minutes ago, SuperFastJellyfish said:

Why do they discount water vapor?  Because it cycles from ocean/freshwater to water vapor and back again?

I would guess that it's simply because it's so highly variable from place to place and from day to day.  Unlike all the other significant components of the atmosphere, which are pretty much uniformly mixed and don't vary significantly by location or over short time periods.

[mikegarrison]

25 minutes ago, Snark said:

I would guess that it's simply because it's so highly variable from place to place and from day to day.  Unlike all the other significant components of the atmosphere, which are pretty much uniformly mixed and don't vary significantly by location or over short time periods.

Um, no. Sorry. But the concentration of CO2 and many other gasses does vary significantly by location or over short time periods. If you turn on a gas burner and light it up, the concentration of CO2 (and water) will spike in a plume near that burner. If you don't light it up, the concentration of methane will spike in a plume near that burner.

The concentration of CO2 (and other gases) is both seasonal and depends on circulation patterns. There is more CO2 in the northern hemisphere, for example, because the circulation patterns between hemispheres are quite a bit slower than those that spread the air around to different longitudes. So because more CO2 is made in the northern hemisphere, there is more up it up here.

If all CO2 production stopped, then yes, it would become much more evenly distributed. But that's not the case.

[Snark]

1 hour ago, mikegarrison said:

Um, no. Sorry. But the concentration of CO2 and many other gasses does vary significantly by location or over short time periods. If you turn on a gas burner and light it up, the concentration of CO2 (and water) will spike in a plume near that burner. If you don't light it up, the concentration of methane will spike in a plume near that burner.

Um, yes.  Sorry. 

First, apologies if I wasn't sufficiently clear, but I assumed it was obvious that I wasn't talking about CO₂  itself, since that is in fact what the graph you linked was talking about, and it was in that context that @SuperFastJellyfish was asking why don't they include water.  The graph itself shows the variation of CO₂, including the seasonal component.

But more importantly than that:  You'll note that I said "significant components".  As in, "numbers big enough to make any visible difference on the graph of CO₂ fraction."  Which excludes CO₂ and methane, since those are pretty much insignificant as far as things like mass fraction or molar fraction are concerned.  (Significant for global warming, yes, but that's not what we're talking about here.)  Those are measured in parts per million, i.e. far tinier than 1%, and a variation in them isn't going to make much difference to the overall molar fraction of whatever you're measuring.  And besides, they're not even all that variable; the summer concentration of CO₂ is only barely (about 2%) higher than the winter concentration.

As opposed to water vapor, which is not only extremely variable (a lot more than 2%), but is abundant enough to make a significant difference to the fractions involved.  At room temperature (20 C), the molar fraction of water vapor in the air can be anywhere from nearly zero to over 2%.  On a warm day (30 C), over 4%.  On a really hot day (40 C), it can get over 7% at maximum humidity.  It averages around 1% at sea level, i.e. 10,000 ppm.  That's a lot.  It's enough, for example, to affect the bulk density of air:  humid air is noticeably lighter than dry air, with concomitant effects on weather patterns.

The other components of the air that are big enough to matter-- oxygen, nitrogen, and argon-- don't change around much and are pretty much constant over time and geography.

Basically, what I was trying to say is:  If you leave water out, the bulk composition of air is pretty darn constant and uniform, which makes for a nice, "controlled" data set for making graphs.  The variable components (such as CO₂) are tiny in comparison, and don't need to be controlled for on a graph such as the one shown on the page you linked.  Water vapor, however, is "special," in that it's basically the only highly-variable component of air that is abundant enough to skew graphs.

And that is why I assume they excluded it on that graph, in answer to @SuperFastJellyfish's question.

[mikegarrison]

1 hour ago, Snark said:

Um, yes.  Sorry. 

First, apologies if I wasn't sufficiently clear, but I assumed it was obvious that I wasn't talking about CO₂  itself, since that is in fact what the graph you linked was talking about, and it was in that context that @SuperFastJellyfish was asking why don't they include water.  The graph itself shows the variation of CO₂, including the seasonal component.

But more importantly than that:  You'll note that I said "significant components".  As in, "numbers big enough to make any visible difference on the graph of CO₂ fraction."  Which excludes CO₂ and methane, since those are pretty much insignificant as far as things like mass fraction or molar fraction are concerned.  (Significant for global warming, yes, but that's not what we're talking about here.)  Those are measured in parts per million, i.e. far tinier than 1%, and a variation in them isn't going to make much difference to the overall molar fraction of whatever you're measuring.  And besides, they're not even all that variable; the summer concentration of CO₂ is only barely (about 2%) higher than the winter concentration.

As opposed to water vapor, which is not only extremely variable (a lot more than 2%), but is abundant enough to make a significant difference to the fractions involved.  At room temperature (20 C), the molar fraction of water vapor in the air can be anywhere from nearly zero to over 2%.  On a warm day (30 C), over 4%.  On a really hot day (40 C), it can get over 7% at maximum humidity.  It averages around 1% at sea level, i.e. 10,000 ppm.  That's a lot.  It's enough, for example, to affect the bulk density of air:  humid air is noticeably lighter than dry air, with concomitant effects on weather patterns.

The other components of the air that are big enough to matter-- oxygen, nitrogen, and argon-- don't change around much and are pretty much constant over time and geography.

Basically, what I was trying to say is:  If you leave water out, the bulk composition of air is pretty darn constant and uniform, which makes for a nice, "controlled" data set for making graphs.  The variable components (such as CO₂) are tiny in comparison, and don't need to be controlled for on a graph such as the one shown on the page you linked.  Water vapor, however, is "special," in that it's basically the only highly-variable component of air that is abundant enough to skew graphs.

And that is why I assume they excluded it on that graph, in answer to @SuperFastJellyfish's question.

I admit, I was going outside the bounds of the original question.

But plumes are of course real. They are often used to calculate emissions index from combustion sources when you can't just stick your instruments right up the tailpipe. For instance, you want to measure emissions from a train. So you set up downwind of the tracks. As the train comes by, the plume from its engine exhaust washes over your gas analyzers. You can measure the concentration of NOx, hydrocarbons, particulates, etc., but how do you know how diluted they are? Well you also measure the CO2. You measure the readings above ambient and convert that back to the mass of fuel that your sample represents. Then you compare the amount of other emissions to that mass of fuel and you get the emissions index (ie. 30g of hydrocarbons per kg of fuel).

On a global scale, this interesting graph shows that CO2 levels fluctuate much more in the northern latitudes than they do in the southern latitudes.

https://www.esrl.noaa.gov/gmd/dv/iadv/graph.php?code=MLO&program=ccgg&type=lg