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Author Topic: The Science of Aerosols  (Read 76430 times)

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Re: The Science of Aerosols
« Reply #250 on: April 16, 2020, 10:15:24 PM »
<snip>
This team analyzed dozens of dust observations made by aircraft and compared them to how much dust current climate models predict should be in the atmosphere. And, while climate models predict only about 4 million metric tons, the team found that there is closer to 17 metric tons of coarse dust in our atmosphere.
<snip>
I assume that is 17 million metric tons

kassy

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Re: The Science of Aerosols
« Reply #251 on: April 16, 2020, 11:39:18 PM »
I just autocompleted that.  ;)

Quote
Abstract
Coarse mineral dust (diameter, ≥5 μm) is an important component of the Earth system that affects clouds, ocean ecosystems, and climate. Despite their significance, climate models consistently underestimate the amount of coarse dust in the atmosphere when compared to measurements. Here, we estimate the global load of coarse dust using a framework that leverages dozens of measurements of atmospheric dust size distributions. We find that the atmosphere contains 17 Tg of coarse dust, which is four times more than current climate models simulate. Our findings indicate that models deposit coarse dust out of the atmosphere too quickly. Accounting for this missing coarse dust adds a warming effect of 0.15 W·m−2 and increases the likelihood that dust net warms the climate system. We conclude that to properly represent the impact of dust on the Earth system, climate models must include an accurate treatment of coarse dust in the atmosphere.
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kassy

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Re: The Science of Aerosols
« Reply #252 on: June 22, 2020, 10:44:12 AM »
The study into Southern Ocean clouds is in.

Scientists just sampled the most pristine air on Earth. Here's what they found.

...

But due to how remote the Southern Ocean is, there have been very few actual studies of the clouds there. Because of this lack of data, computer models that simulate present and future climates overpredict how much sunlight reaches the ocean surface compared to what satellites actually observe. The main reason for this inaccuracy is due to how the models simulate clouds, but nobody knew exactly why the clouds were off. For the models to run correctly, researchers needed to understand how the clouds were being formed.

To discover what is actually happening in clouds over the Southern Ocean, a small army of atmospheric scientists, including us, went to find out how and when clouds form in this remote part of the world. What we found was surprising — unlike the Northern Hemisphere oceans, the air we sampled over the Southern Ocean contained almost no particles from land. This means the clouds might be different from those above other oceans, and we can use this knowledge to help improve the climate models.

Ice clouds and liquid clouds
Clouds are made of tiny water droplets or ice crystals, or often a mixture of the two. These form on small particles in the air. The type of particle plays a big role in determining whether a liquid droplet or ice crystal forms. These particles can be natural — like sea spray, pollen, dust or even bacteria — or from human sources like cars, stoves, power plants and so on.

To the untrained eye, an ice cloud and a liquid cloud look much the same, but they have very different properties. Ice clouds reflect less sunlight, precipitate more and don't last as long as liquid clouds. It matters to the weather — and to climate models — what kinds of clouds are around.

...

This was the mystery: Why are there more liquid clouds than the models think there are? To solve it, we needed to know what kinds of particles are floating around in the atmosphere around Antarctica.

Before we went down there, we had a few clues.


Previous modeling studies have suggested that the ice–forming particles found over the Southern Ocean may be very different from those found in the Northern Hemisphere. Dust is a great ice cloud seeder, but due to the lack of dusty land sources in the Southern Hemisphere, some scientists have hypothesized that other types of particles might be driving ice cloud formation over the Southern Ocean.

...

Bacterial maps

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The atmosphere is full of microorganisms that are carried hundreds to thousands of kilometers on air currents before returning to Earth. These bacteria are like airborne license plates, they are unique and tell you where the car — or air — came from. Since scientists know where most bacteria live, it's possible to look at the microbes in an air sample and determine where that air came from. And once you know that, you can predict where the particles in the air came from as well - the same place the bacteria usually live.

...

Ocean bacteria alone
In most ocean regions around the world, especially in the Northern Hemisphere where there is a lot of land, the air contains both marine and terrestrial particles. That's what we expected to find down south.

With the frozen filters safely back at our lab in Colorado, we extracted DNA from the bacteria and sequenced it to determine what species we had caught. Much to our surprise, the bacteria were essentially all marine species that live in the Southern Ocean. We found almost no land-based bacteria.

If the bacteria were from the ocean, then so were the cloud-forming particles. This was the answer we were looking for.

https://www.livescience.com/most-pristine-air-on-earth-bacteria.html

Airborne bacteria confirm the pristine nature of the Southern Ocean boundary layer

We found that the summer airborne bacterial community in the marine boundary layer over the Southern Ocean directly south of Australia is dominated by marine bacteria emitted in sea spray, originating primarily from the west in a zonal band at the latitude of collection. We found that airborne communities were more diverse to the north, and much less so toward Antarctica. These results imply that sea spray sources largely control the number concentrations of nuclei for liquid cloud droplets and limit ice nucleating particle concentrations to the low values expected in nascent sea spray. In the sampled region, the sources of summer cloud-active particles therefore are unlikely to have changed in direct response to perturbations in continental anthropogenic emissions.

https://www.pnas.org/content/117/24/13275
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kassy

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Re: The Science of Aerosols
« Reply #253 on: November 05, 2020, 04:33:18 PM »
Soot particles influence global warming more than assumed

...

Burning wood, petroleum products or other organic materials releases soot particles into the atmosphere that consist mainly of carbon. This soot is considered the second most important anthropogenic climate forcing agent after carbon dioxide. In the atmosphere or as deposits on snow and ice surfaces, soot particles absorb the short-​wave radiation of the sun and thus contribute to global warming.

In the atmosphere, soot particles also have an indirect effect on the climate by altering the formation, development and properties of clouds. A research team led by Ulrike Lohmann, professor at the Institute for Atmosphere and Climate at ETH Zurich, has now for the first time investigated how two specific types of soot particles influence clouds and, in turn, the climate: on the one hand, soot aerosols that age due to ozone and, on the other, those that age due to sulfuric acid.

Soot chemistry changes cloud formation

"Until now, it was assumed that these two types of soot ageing had little effect on cloud formation and climate," says David Neubauer, scientific programmer in Lohmann's research group. However, the results of the simulations now carried out on the CSCS supercomputer "Piz Daint" paint a different picture.

...

Simulations of ozone-​aged soot show that when the carbon dioxide content of the atmosphere doubles compared to the pre-​industrial era, fewer low clouds form. Considerably more cloud droplets are initially formed by ozone ageing of soot. However, their high concentration leads to more cloud top cooling causing more dry air being mixed in from above.  "These clouds then evaporate more quickly, especially in a warmer climate," explains Lohmann. "In a warmer climate, the air mixed in also has a lower relative humidity". Due to the faster evaporation, less low-​lying clouds remain, and more short-​wave radiation reaches the earth and warms it.

The soot particles aged by sulfuric acid, on the other hand, cause more ice crystals to form and make cirrus clouds optically thicker, i.e. they are less permeable to radiation. They extend as far as the tropopause, which is located at an altitude of 10-​18 kilometres, and also linger longer in higher regions of the atmosphere. As a result, cirrus clouds absorb more of the long-​wave thermal radiation emitted by the Earth and allow less of it to escape into space. The warming effect of cirrus clouds increases and exacerbates global warming: When the carbon dioxide content of the atmosphere doubles compared to pre-​industrial times, both types of soot ageing together lead to a 0.4 to 0.5 ºC increase in global warming. As a result, the water cycle will further accelerate and global precipitation will further increase, the researchers write.

..

https://ethz.ch/en/news-and-events/eth-news/news/2020/11/soot-particles-influence-global-warming.html
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