Slowly evaporating particles refute assumption used in air quality and climate models

February 12, 2015
Slowly evaporating particles refute assumption used in air quality and climate models
Under all atmospherically relevant conditions SOA particles evaporate significantly slower than models assume, providing a simple explanation for the discrepancy between models and field data.

Ubiquitous carbon-rich aerosol particles created by emissions from cars, trees, and other sources alter our climate and affect air quality. Until recently, the properties of these aerosols were hard to experimentally characterize, forcing computational models to rely on unsupported assumptions. For several years, scientists at Pacific Northwest National Laboratory have chipped away at these notions. They have provided hard data about viscosity, shape, morphology, volatility, and other fundamental particle properties. Recently, the team tackled how the particles, called secondary organic aerosols (SOAs), evaporate when the relative humidity is high. They found that these aerosols actually evaporate very slowly, sticking around for days.

"Together, our studies of evaporation rates and viscosity provide incontrovertible experimental data that the assumptions invoked to model SOA formation and evolution are fundamentally flawed," said Dr. Alla Zelenyuk, the PNNL chemist who led the study.

Models have persistently and significantly underestimated atmospheric loadings of SOA. Similarly, simulations could not explain how carry pollutants thousands of miles away from the sources to pristine environments. The assumption that SOAs quickly evaporated got in the way. The SOAs were assumed to be at constant equilibrium with the through rapid evaporation or condensation and internal mixing. Now, scientists can add aerosols' fundamental properties into the models, opening the door to discover how the most abundant particulates in the atmosphere affect our climate and .

The research group has previously shown that at low relative humidity (RH), SOA particles evaporate much slower than assumed. This led some scientists to assume that, under atmospherically relevant conditions of high RH, these particles would become less viscous and evaporate nearly instantaneously, therefore retaining equilibrium with gas phase.

To test the assumption, the team created SOAs from alpha-pinene and limonene, natural aerosol progenitors responsible for the smell of pine trees and lemons. At DOE's EMSL, a national scientific user facility, the team studied the evaporation of these aerosols under RHs ranging from nearly zero to more than 90 percent. Using tools built at EMSL, including SPLAT II, the team showed that in all cases, evaporation rates are orders of magnitude slower than assumed, and way too slow to retain equilibrium with the rapidly changing gas phase. The team showed that the evaporation slowed even more for aged aerosols, which were stored for a day before testing.

"To everyone's surprise, our data clearly showed that while relative humidity affects SOA evaporation kinetics somewhat, the differences are rather small," said Zelenyuk. "At low and high , SOAs evaporate too slowly to reach equilibrium with the gas phase." These particles evaporate so slowly that they are best treated as semisolid and non-volatile.

The team is studying how hydrophobic organic molecules, commonly present in the atmosphere, change the aerosols' formation, properties, and behavior. They are focusing on conditions, in which biogenic SOA precursors from trees and other natural sources meet emissions from industrial or other anthropogenic sources.

Explore further: A new method for measuring the viscosity of nanoparticles

More information: Wilson J, D Imre, J Beránek, M Shrivastava, and A Zelenyuk. 2015. "Evaporation Kinetics of Laboratory-Generated Secondary Organic Aerosols at Elevated Relative Humidity." Environmental Science & Technology 49(1):243-249. DOI: 10.1021/es505331d

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syndicate_51
1.5 / 5 (8) Feb 12, 2015
From the Article:
"Until recently, the properties of these aerosols were hard to experimentally characterize, forcing computational models to rely on unsupported assumptions."

What is aggravating is not that it has anything to do with climate, more so what it says about how science was conducted.

So we have been told for decades that climate change models predict z result. So this lets us assert a rock solid prediction of what will happen to Earth's climate given current rate of pollution x for y time allowing this percentage for error. But still very accurate.

This assertion based on climate models with a good deal of assumption in their predictive processes.

As regards the scientific process it is great to speculate and test those hypothesis' but not to make an assertion about a "conclusion" that is still relying upon assumptions. This is a misleading use of the scientific process.

Especially when you tell the public it is scientific fact.
RWT
4 / 5 (6) Feb 12, 2015
I wonder, do trees perceive the RH and release aerosols at certain times in an attempt to seed fog and clouds? I wouldn't be surprised from what has been learned about plants over the past 20 years.
syndicate_51
1 / 5 (7) Feb 12, 2015
I just want real science done. How foolish does one look if they cite this or that study and then later it is proven as false because of factors other than scientific discovery. It just casts an ugly shadow on science and undermines its credibility.
alan_goldman
5 / 5 (8) Feb 12, 2015
Seriously, Syndicate? Someone in the are of climate science makes some incremental advance (otherwise known as "doing science") which helps us refine the climate models and in your mind that undermines the credibility of climate science? Think about it....
nikkiswims32
1.1 / 5 (7) Feb 12, 2015
This is just one of the many important natural phenomena that is not accurately modeled within the climate models. Good for these guys to look into it.

Aerosols generally have a cooling effect, so to understate them is to overstate predictive temperature rise. This is certainly one of the reasons why current models are so very wrong, in that they are predicting a much greater temperature rise than is currently observed. In answer to the "where did all of the heat go?" over the past 17 years, the climate researchers excuse has been "the ocean ate it". This research shows that at least part of the problem is the cooling effect of aerosols were not properly accounted for. Since aerosols are not new, this also means the modeling in the past that did coincide with temperature rise necessarily overstated the impact of CO2, meaning there is still something important missing from the models.
nevermark
5 / 5 (7) Feb 12, 2015
@syndicate,
You seem to be wildly unfamiliar with how discovery always unfolds in any complex regime. The fact that simulations of global climate will now be updated for micro effects of how aerosols evaporate is incredible.

And nobody is concerned that their previous results were not perfectly correct. That is expected! Scientists (and anyone who has any idea what they are talking about) expect even quantum mechanics and general relativity to be "corrected" in the future.

But imperfect large scale models can still capture important behavior, which is why they are tested against real data and keep improving. Now there is an opportunity to match real data even more closely.

Why are the scientific illiterate so knee-jerk negative about scientific progress? People who will never shed any light on anything criticize people who have actually moved knowledge forward just because the process isn't magically instant.
FrisbeeChemist
5 / 5 (6) Feb 12, 2015
You can't predict the future without making some assumptions (at the very least, you must assume that the world will continue to operate as it has in the past). If the use of assumptions is surprising/disappointing/ugly to you, then you have NEVER read a scientific paper or its methods section. Every study spells out exactly how their model/conclusions were reached.
Maggnus
5 / 5 (6) Feb 12, 2015
This is certainly one of the reasons why current models are so very wrong, in that they are predicting a much greater temperature rise than is currently observed. In answer to the "where did all of the heat go?" over the past 17 years, the climate researchers excuse has been "the ocean ate it".
This is inaccurate and simplistic. They have not been projecting a "much greater temperature rise than is currently observed"; within the error bars and accounting for non-predictable short-term phenomena, they are very accurate in their projections. The heat has gone to exactly where the science said it would go - mostly heating the oceans and melting glaciers and ice sheets. The denialist cry "the ocean ate it" is misleading, at the least, and represents the worst of anti-scientific jargon.

The effect of aerosols is small. Certainly not enough to disprove the underlying science behind models of the climatic heating arising from increased CO2 in the atmosphere.
nevermark
5 / 5 (6) Feb 12, 2015
@nikkiswims32,
You are way jumping the gun to conclude this will radically change where heat is being exchanged. Nowhere in the article have they run a new simulation that supports that and it isn't going to overturn the fact that oceans have taken up a large amount of heat energy as that is both inevitable and measurable.

As a recent meta-paper showed, the current (they covered 146) models are very good at predicting the heat accumulation and dynamics over decades, but not yet accurate within a decade. Perhaps this will help them become more accurate on shorter time frames. But we won't know until the models incorporate this new info.
nevermark
5 / 5 (6) Feb 12, 2015
@syndicate,
Especially when you tell the public it is scientific fact.

The fact that 9 out of 10 of the warmest years have happened since 2000, and the warmest was 2014, doesn't require any model assumptions or simulations.

Models are just part of the evidence, and despite being incomplete, models are validated with real data.

You appear to be completely uninformed about the topic you are blathering about.
jbbbbf
5 / 5 (5) Feb 12, 2015
The duration of these particles is exactly what it was before these tests. It is what it always was. Before we started burning fossil fuels there was an equilibrium. the amount of free carbon stayed the same it circulated from air, soil,and ocean to plant and animal, then back around again, but the total remained the same. Now we are steadily adding to the total amount in a new way that has never happened before. The amount in the air and oceans has gone up about 40% and it keeps going up faster. The extra in the air is causing AGW, and the extra in the ocean is causing acidification that is damaging all sorts or sea life. These findings refine our knowledge of the details of how these things happen. they do nothing to change the fact that they are happening.
mscheue1
3.2 / 5 (5) Feb 12, 2015
Before we started burning fossil fuels there was an equilibrium..


The problem is not the burning of fossil fuels. Mankind has been burning organic molecules for heat, cooking, etc. for eons. Burning wood creates as much CO2 as coal and oil. The equilibrium existed not because of the type of fuel but because the number of people was small. With a few hundred thousound or even a million people, the planetary systems had no problem maintaining a balance.

The 'we' you mention has now become 'WE'. The world's population now exceeds 8 billion, more than twice what it was in 1900, and yet the CO2 concentration in the atmosphere is up only 40%. The switch from wood to coal and oil is only a coincidence. The real problem is the number of people. Going back to burning wood is definitely not the solution.

So, unless you are advocating genocide, the only two solutions to GW are (1) adapt or (2) develop technologies that enable us all to live.
Maggnus
5 / 5 (4) Feb 12, 2015
[qThe switch from wood to coal and oil is only a coincidence. The real problem is the number of people. Going back to burning wood is definitely not the solution.
No, this is wrong.

Burning wood did not add to the global carbon cycle, because wood takes carbon from the air and converts it to cellulose. Burning it simply returns it to the air. The carbon budget remains the essentially the same.

Once we began burning the sequestered CO2 reserves of fossil fuels however, that changed. We are adding carbon to the equation faster than the natural sinks for carbon can remove it. That is why CO2 is increasing.

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