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Comment by contemporary343 | Hacker News Reader

There’s been a ton of development in super reflective materials since the first demonstration of daytime radiative cooling at Stanford in 2014. Ultimately though, reflectivity does drop with dirt and pollution exposure over time, even for these kinds of materials. So most super reflective materials plateau at a lower number long term outdoors.

Also, just for reference, Spectralon, a sintered PTFE reflectance standard has had this level of solar reflectance for decades. So, in a sense, not that much new here. a sintered ceramic is gonna be expensive and will have a hard time competing with the simplicity of a paint based approach. Super white paints using various pigments have been well studied the last 5 years.

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IMO simply being reflective is a dead end. Future materials will have to absorb wideband energy and actively emit it in an atmospheric bandgap to be effective enough to matter. I would think this would be an excellent application for quantum dots and have been sort of low-key waiting for an announcement or paper to drop for the last year or two.

We also need to reduce aircraft contrails. Contrails increase high-altitude clouds (cirrus and related), which blocks the heat that would have been lost by radiative cooling. It isn't hard to reduce contrails. We have the atmospheric data telling us the height of and where clouds are likely to form. Aircraft need to change altitude by 2000 ft from what was planned to avoid the cloud-forming regions.

research paper https://acp.copernicus.org/articles/19/8163/2019/

AI is helping reduce the problem https://blog.google/technology/ai/ai-airlines-contrails-clim...

Reducing contrails would cause more warming unless we have a ton of these coatings out there

That's not how clouds in the atmosphere work. Lower clouds reflect infrared back up and away from Earth. When large oceangoing vessels reduce their soot emissions, the formation of low altitude clouds declined letting more infrared in to the surface and increase the amount of heating in the lower atmosphere and surface of the earth. On the other hand, contrails hold heat in and prevent it from radiating out into space.

The "ideal" situation would be lots of low altitude clouds and no high-altitude clouds like Cirrus and contrails. This conjecture comes from the known property of low altitude clouds reflect the heat up and eliminated heat retention properties of high-altitude clouds.

For more detail:https://earthobservatory.nasa.gov/features/Clouds#:~:text=Lo....

We already pay to clean lots of buildings, and rain should help with that too, no? Seems like this would be a very solvable problem to me, but maybe I'm misunderstanding the problem fundamentally.

The less rain you get (i.e. during a drought) the more you tend to get dust in the air. We went through this last summer; my dog would kick up clouds of dirt dust running through our yard.

Worse than this, though, is the durability of the material. Lots of these super reflective paints don't hold up very well to rainwater (which itself is not especially clean or PH neutral) or seasonal extremes.

Typically, you wouldn't want to paint the sides of buildings, as that'll just reflect the light mostly down. You want it on rooftops, which most people don't pay to clean frequently or at all.

This makes sense; I guess it just becomes a new added expense with no major benefit aside from maybe saving on some cooling costs? Seems like a thing you'd be able to convince a company to buy into completely, but only once it's cheap and durable.

Maybe it's just dirty coal plants and the nearby streets, but rain seems to be the main reason why our windows get dirty.

You could add a lotus-effect coating, but given that window cleaners are still in business I assume that is quite expensive at scale.

>Ultimately though, reflectivity does drop with dirt and pollution exposure over time, even for these kinds of materials.

I guess this will be the new white washing?, like people used to do on buildings 100 years ago.

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