Thursday, July 28, 2016

What's Going On Here

Lately Eli's thoughts have wandered to building a good, excellent, wonderful lab experiment illustrating the greenhouse effect.  Well there are a fair number of demonstrations on the web that purport to do this, but, they really don't. So Eli is going to put a few of them up for the bunnies to look at, and ask what is going on here

Eli did screw the pooch, answering the question a couple of weeks ago on Twitter, so those of you who saw it sssh now.  There is some didactic purpose in going through a few of these. 

For the old timers there is always Eli's light bulb, a pretty good illustration of a single layer model of the atmosphere.  Chris Colose has a more complex multilayer model.


E. Swanson said...

Eli, Have you tried that experiment using a heat lamp instead of one which emits SW and LW? Ideally, it would be best to use a source which is near the IR absorption bands for CO2. One might even use a low wattage CO2 laser, splitting the beam to illuminate each bottle equally. Wrap the bottles with a layer of reflective foil, except for the spot where the laser beam enters the bottle. Offset the entrance slightly, thus allowing multiple reflections between the reflectors.

Also, what are the effects of water vapor in the bottle with the fizzy froth? Don't use water in the two bottles, generate the CO2 in a third bottle and connect to one target bottle with tubing. Without the water, the time to achieve equilibrium would be much less.

Tom Curtis said...

Although the alka-saltzer reaction is endothermic, and should cool the second bottle, all else being equal:
1) The bottles are sealed so the increased pressure from the CO2 given of by the reaction should warm the second bottle; and
2) The alka-saltzer reaction itself should absorb more visible light, as also will the bubbles, again distorting the experiment.

You can see the experiment done properly here. In that experiment, the CO2 is produced in a separate container and fed into the CO2 enhanced bottle be a tube, thus reducing impacts of the endothermic reaction, and ensuring the reaction itself does not cause an increase in temperature. The lid is left open, ensuring no change in gas pressure, and relying on the fact that CO2 is heavier than air to enrich the CO2 in the bottle. There is still some potential contamination from the tube potentially absorbing light; and issues about the slapdash alignment of lights with the bottles such that it is not certain each receives the same light.

Finally, both demonstrate the increased absorption of energy by CO2, but that is not by itself the greenhouse effect.

Russell Seitz said...

After a century and a half, Tyndall's original steampunk CO2 demo is still hard to beat

EliRabett said...

As pointed out all of these experiments show absorption of energy. The greenhouse effect depends on decreasing optical density with altitude resulting in a decreasing amount of absorption and emission of thermal energy. In a real sense they are irrelevant.

The fun in analyzing them is to figure out first what wavelengths are emitted by the lamps being used and what wavelengths penetrate into the containers and what wavelengths are being absorbed by CO2. Tom is most of the way there. Anybunny who cares to look at it again should ask themselves why the bulb with the alka seltzer lights up. OTOH, you can't hold much of an overpressure with a stopper, and, in fact the experiment he points to (it was going to be next) fails through the analysis at the top of the paragraph.

CO2 does not absorb photons emitted from a CO2 laser. Otherwise you could not have a CW CO2 laser.

Kenneth Hayes said...

You may find this article interesting:'Climate_change_in_a_shoebox'_A_critical_review


Russell Seitz said...

Eli correctly observes:"CO2 does not absorb photons emitted from a CO2 laser. Otherwise you could not have a CW CO2 laser."

OTOH a tub of water makes a dandy beam dump for one, illustrating the synergy of CO2 and H20 in radiative equilibrium in the IR

EliRabett said...

Except if you try that you spray water all over the place, which, if you are using salt optics tends to not be a good thing. Eli always preferred an old fashioned brick

Bernard J. said...

Some would say (not Bernard J. to be sure) that a few of the resident commenters here are opaque to the wavelengths of understanding of the lay lurkers.

I love a private scientific joke as much as the next squint, but...


EliRabett said...

Unpacking this a bit: In this example, if you look at the alka seltzer bottle you see a lot of light being scattered (it lights up) That has to be from bubbles, aka aerosols and in addition to the scattering there is absorption so that's is why the bottle lights up.

If you get rid of the bubbles and just put CO2 gas into the bottle, there still will be absorption, but it is in the near IR, if for no other reason that the lamps are made out of glass and the bottles too (plastic, specific plastics can pass IR though). That means that the only wavelength of the light that gets into the bottle is somewhere shorter than 2 u (u stands for microns).

Vibrational transitions in CO2 that absorb light are at ~ 3.9 u and 15 u. The 15 u one is the one that drives the greenhouse effect). So what is happening, well there are weak IR absorptions that involve simultaneous excitation of more than one vibrational mode at the same time. These are called combination bands and/or overtones. These extend down into the near IR and actually are easily observed in the solar spectrum at the surface. These weak absorption can also be easily seen if you have high concentrations of CO2 as in this case.

It ain't the greenhouse effect.

Russell Seitz said...

As it was a 10 kw beast with diamond optics, spray wasn't an issue. The water drum was brought in when folks on the other side began to worry about the slag patches on our lab's cinderblock walls.

A graphite brick was tried as an output port beam dump on another gas dynamic CO2 laser, but it tended to fall off when the duct tape got hot , posing a haircut threat to bystanders and pedestrians.

Susan Anderson said...

I don't mind a bit, but delighted with the locution: "opaque to the wavelength of the lay lurkers" is me to a T. Carry on folks, though ...

And Russell, I still get pleasure from your peculiar sense of humor, so carry on there as well. Thanks.