Paper: Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation

Mick West

Staff member
Some interesting and unexpected results:

Formation of cirrus clouds depends upon the availability of ice nuclei to begin condensation of atmospheric water vapor. While it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective has not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, while sulfate/organic particles are underrepresented and elemental carbon and biological material are essentially absent. Further, composition analysis combined with relative humidity measurements suggest heterogeneous freezing was the dominant formation mechanism of these clouds.
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Heterogeneous freezing means that the water freezes on something, a seed aerosol, rather than homogenous freezing which is where pure water freezes. It's a little confusing terminology

homogeneous = uniform, kind of like homogenized milk (or think two of the same, like homosexual)
heterogeneous = the opposite, not homogenized, varied in makeup, a mixture

Host – Kerry Klein
There’s a substantial research base examining aerosols and their effect on the atmosphere
and climate. What’s less known is their effect on cloud formation. Clouds that contain ice
require tiny particles to nucleate, and it has been thought that organic carbon and other
biological particles make up the lion’s share of cloud seeds. Now, using an innovative
sampling methodology, Dan Cziczo and colleagues have examined some cloud seeds—
and they found something quite unexpected. Cziczo spoke with me about the formation of
cirrus clouds.

Interviewee – Daniel Cziczo
Cirrus clouds are the clouds that you often see on a clear day. They’re very high altitude.
They look very wispy. And what you’re actually seeing is snow taking place high in the
atmosphere. These are ice crystals forming. They become so big that they start to
sediment out. They fall, and then upper level winds are moving those ice crystals off to
the side. And one reason that they’re important, one reason that we’re interested in
investigating them, is that they cover a lot of the Earth’s surface. About 30% of the globe
is covered by cirrus clouds, and we don’t often see them because there’s lower level
clouds that are in the way. They’re so high in altitude that we just simply don’t see them,
for example, when it’s raining out or when it’s cloudy. Those clouds are lower in the
atmosphere, and they obscure the cirrus clouds.

Interviewer – Kerry Klein
So you were interested in figuring out how these cirrus clouds form. I mean, I think for
most of us clouds are either there or they’re not. We don’t stop to really think about why
or how they form. Why was this important to you?

Interviewee – Daniel Cziczo
Yeah, that’s a great question. So we were very interested in how these clouds formed,
and one of the things that’s maybe not obvious if you don’t start looking at these clouds
in depth is that clouds are forming on pre-existing particles in the atmosphere. So there’s
always some amount of particles in the atmosphere – and these are things like dust
particles that are getting kicked up; they’re the smoke that’s coming out of the back of
cars or out of smokestacks. So those particles are always there. And the cirrus clouds are
forming on some of those particles that are present in the upper atmosphere. So they’re
acting as condensation sites or something that we call ice nuclei onto which that ice
forms the cloud. So what we were really trying to do in this study was figure out what
those seeds were – what those nuclei were that were forming the ice crystals.

Interviewer – Kerry Klein
So how on Earth does one measure the seeds of clouds?

Interviewee – Daniel Cziczo
That’s a great question, too. Well first off, as I mentioned at the very beginning, you
have to get to these very high altitude ice clouds, and to do that we needed to use high
altitude research aircraft from NASA. And we also used one from the National Science
Foundation. So these are specialized aircraft that are able to take measurements in the
upper part of this atmosphere. So that’s the first step is to find a platform to get to them.
The second thing that you have to do is that you have to separate out the ice crystals, and
this is maybe a little harder than it sounds like. Very often what we’re talking about are a
few ice crystals per liter, whereas there might be hundreds, thousands, or tens of
thousands of particles per liter. So they’re very tenuous clouds. There’s only a few ice
crystals there. So we have to very efficiently separate out the ice crystals from the
particles. And to do that, we’ve made some special inlets to bring that material in. And
the analogy that I like to use is it’s a bit like flying around with a hair dryer. And what I
mean by that is that you can imagine that if you have a hair dryer, and you sort of point it
up, and you drop something like a ping pong ball on it, the flow from that hair dryer sort
of stops the ping pong ball, and it shuffles it off the side. And so we built an inlet to do
that, and we force a flow of gas out of the front of the aircraft. And what that does is it
stops all of the small particles that we don’t want to sample, and it sort of shuffles them
off to the side. If you think about that same hair dryer analogy, and you drop a bowling
ball on the hair dryer, it’s not going to care that the hair dryer is on, it’s just going to
come crashing down. And that’s the same idea behind these ice crystals that are in the
cirrus. So we allow those particles to make it into the inlet. And you can imagine that
the flow of that gas, just like a hair dryer, is it’s going to be warm, and it’s going to be
dry. We melt off that ice, and what happens is we release the original aerosol particle
that that ice crystal formed on. And that’s what we’re interested in investigating. So we
look at those particles both on the aircraft – we fly instruments, mass spectrometers, on
the aircraft – to look at the composition. We collect some of that material to bring back
to the laboratory where we can use techniques like electron microscopy. And we also
compare it to other things. So we compare it to, for example, relative humidity
measurements made on the aircraft to understand formation conditions. And we also
compare it to models afterwards. We try to understand these formation mechanisms and
sort of back up the data that we collect in the atmosphere by understanding it from sort of
a theoretical sense.

Interviewer – Kerry Klein
Wow, that is awesome! So how big are these particles?

Interviewee – Daniel Cziczo
So it depends on what we’re talking about. The ice crystals can be quite large. They can
be some hundreds of microns in diameter. The seed particle that we start with is actually
quite small in comparison. It’s usually only about a micron in diameter, so you’ve got
many orders of magnitude difference between the size of the ice crystal and the little
particle that seeded it.

Interviewer – Kerry Klein
And so what sorts of cloud-forming seeds did you find?

Interviewee – Daniel Cziczo
Well, the results were pretty interesting. One of the things that we were very interested in
from the outset was if the particles that these cirrus clouds formed on were just the
background aerosol, were they just the background particles that we very often find in the
atmosphere? And they weren’t, so the first result was that these ice nuclei were very
special particles in the atmosphere not representative of the whole. We found a lot of
mineral dust in these clouds, so the sort of seed particle, the ice nucleus, was very often a
small bit of mineral. Very often it was also a metal, so something that came from human
activities. One of the reasons that this is interesting is that if it was just sort of the
background aerosol, the background particles, there isn’t sort of a lever to change cirrus
cloud properties. It would way that you would have to change all of the aerosol in the
atmosphere very radically to get a big effect on the clouds. But because mineral dust and
metallic particles are such a small amount of the particulate matter – just a percent or two
– it means that you only have change something about a percent or two of the particles to
get a big effect on these clouds. We also had an interesting sort of negative result, which
is what the clouds don’t form on. So it had been theorized for a good bit of time that
maybe they form on something from human activities like black carbon, and this is the
sort of sooty material that comes out of the back of a dirty truck or out of a smokestack.
And we actually didn’t find this material in the ice crystals. So it sort of implied that we
don’t have to worry about this as a source of cirrus cloud formation. We also didn’t find
a lot of biological material, so we know that very close the Earth’s surface, there’s some
bacteria that are very good at nucleating ice. It doesn’t appear that these make it in
sufficient number into the upper atmosphere to have an effect on cirrus clouds. So that
was sort of another interesting negative effect, something that we also don’t have to
worry about.

Interviewer – Kerry Klein
So you could rule out black soot, but many of the other particles forming clouds are still
man-made, right?

Interviewee – Daniel Cziczo
That’s absolutely right. In fact, it’s an interesting point that you make about minerals
because we sort of think of mineral dust, or we want to think if it, as a natural particle –
you know, something that comes from a dust storm, which is just naturally occurring.
But it turns out that human activities actually have a big effect on the amount of mineral
dust in the atmosphere, and this maybe isn’t surprising when you think of land usage, you
know, farmland being created out of forested land, or maybe plowing under fields instead
of leaving the root systems intact, so when the wind comes by, it can blow mineral dust
into the atmosphere. And there’s some estimates that there’s as much maybe 50% more
mineral dust in the present atmosphere than there was before human activity took place,
so this isn’t really a natural material anymore, it’s now sort of a man-made material. And
then, as you mentioned, the other thing that we found was this metallic aerosol, so these
are little bits of metals from things like smelting activities, from industrial activities, that
type of industry puts metal particles into the atmosphere. And we’re seeing that these
maybe are a quarter of the particles that are in the cirrus clouds.

Interviewer – Kerry Klein
And which metals and minerals in particular did you find?

Interviewee – Daniel Cziczo
The mineral dust is actually going to take more work. So we haven’t sort of specifically
found one source region for this, and probably that implies that it comes from a number
of different areas. It seems to be aluminous silicate material, which is quite common at
the Earth’s surface. The metallic particles are also very variable. The big one that we’ve
found is lead. There’s still a bit of lead in the environment. It comes from things like
tetraethtyl lead in fuels; most of that has been eliminated from cars, you know, for about
25 years now, but it’s still used in some light aviation. Metals like lead are also emitted
by things like fossil fuel burning. Coal burning puts a bit lead into the atmosphere. So
that’s probably the biggest metal that we find, or the most frequent metal that we find.
But we find a whole host of different metals, actually.

Interviewer – Kerry Klein
So if human activity is influencing the amount of these particles in the air, does this mean
that the amount of cirrus clouds being formed is changing as the globe industrializes and

Interviewee – Daniel Cziczo
That’s what we’re trying to understand, and I think that this is the first attempt to do this
– the first cut that we’ve taken at it – is this type of research. So, you know, what we’re
finding is that a significant amount of the material that seeds these clouds is from human
activities, and it does imply that there could a human effect. It’s something that’s going
to take follow-on studies – things like modeling studies, better understanding of the
emission of, for example, mineral dust and metal aerosols to the atmosphere – to really be
able to pin down sort of a global effect of what these clouds could be doing to the Earth’s
climate. It’s very uncertain right now. So if you look at things like the recent
Intergovernmental Panel on Climate Change report, cirrus clouds and clouds in general
are one of the things that lead to a high error bar in our understanding of climate.
They’re sort of the most uncertain thing in the climate system right now, and what we’re
hoping is that this work can really help us to shrink that error bar, that it’s going to give
us an increased ability to understand cloud formation, and specifically these high altitude

Interviewer – Kerry Klein
Great! Well, Dan Cziczo, thank you so much.

Interviewee – Daniel Cziczo
Thank you!

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Fodder for conspiracy theories:

After each flight, Cziczo and his colleagues analyzed the collected particles in the lab using high-resolutionelectron microscopy. They compared their results with analyses from the onboard mass spectrometer and
found the two datasets revealed very similar cloud profiles: More than 60 percent of cloud particles
consisted of mineral dust blown into the atmosphere, as well as metallic aerosols.

Cziczo notes that while mineral dust is generally regarded as a natural substance originating from dry or
barren regions of the Earth, agriculture, transportation and industrial processes also release dust into the

"Mineral dust is changing because of human activities," Cziczo says. "You may think of dust as a natural
particle, but some percentage of it is manmade, and it really points to a human ability to change these

He adds that some global-modeling studies predict higher dust concentrations in the future due to
desertification, land-use change and changing rainfall patterns due to human-induced climate effects.

Cziczo's team also identified a "menagerie of metal compounds," including lead, zinc and copper, that may
point to a further human effect on cloud formation. "These things are very strange metal particles that are
almost certainly from industrial activities, such as smelting and open-pit burning of electronics," Cziczo
adds. Lead is also emitted in the exhaust of small planes.

Contrary to what many lab experiments have found, the team observed very little evidence of biological
particles, such as bacteria or fungi, or black carbon emitted from automobiles and smokestacks. Froyd says
knowing what particles are absent in clouds is just as important as knowing what's present: Such
information, he says, can be crucial in developing accurate models for climate change.
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I predict the term "very strange metal particles" will get interpreted as anything from advanced nanotech to evidence of covert geoengineering with barium and aluminum.
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Fascinating post. Cloud microphysics will continue to yield fascinating findings.

Here is a paper that discusses the global electric circuit and the role that human altered clouds may play in weather.
The global electric circuit may be involved in climate change via non-linear electrical effects on cloud microphysical processes (Aplin et al. 2008; Carslaw 2009; Harrison and Ambaum 2008, 2010; Nicoll and Harrison 2010), as discussed later in this paper.
d) Global circuit cloud coupling
This modulation of the fair weather current density by solar activity and associated cosmic ray changes provides a potential mechanism whereby the properties of clouds at low heights in fair weather regions could be changed by the currents passing through them, with implications, currently not quantified, for changes in weather and climate as a result. Theory indicates that, at the edge of a horizontal layer cloud, the transition from low conductivity air within a cloud to air of greater conductivity outside the cloud will be accompanied by a region of enhanced space charge, when the current flows vertically through the cloud boundary. Nicoll and Harrison (2009) have confirmed empirically that current flow passes through layer clouds.

Cirrus clouds are especially fascinating in regards to electic and chemical dynamics.
Stratospheric electrical processes will be better understood when more is known about dusty plasmas.



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