Unexpectedly large sunlight reflecting impact of big particles in a clean sky

Nov 07, 2012
Whether looking through dirty (left) or clean (right) sky, the clarity of the view can depend on the amount of particles in the air. New research by scientists at PNNL shows that even in clean sky conditions, there are large-size particles in the air that have an unexpectedly large effect on the amount of sunlight reflected back into space. Credit: U.S. National Park Service

(Phys.org)—A few large particles in a crowd of tiny ones have often been ignored when calculating the amount of sunlight bounced back into space in clean-sky conditions. Scientists at Pacific Northwest National Laboratory found that these "giant" particles have a larger-than-expected impact on the amount of sunlight reflected away from Earth, by as much as 45 percent. They also showed that particles larger than one micron (0.000039 inch) occur much more frequently than expected, up to 85 percent of the time.

"Many routine measurements are unable to sample large , thus they may overlook the residence of many 'Gullivers in the country of Lilliput,' said Dr. Evgueni Kassianov, PNNL scientist and lead author of the study. "Our results show these 'invisible giants' play a much greater role in reflecting back into space than previously thought."

Tiny called get in the way of beaming down to Earth. They either absorb or scatter sunlight. The human eye catches these effects when particles get in the way of viewing distant . Scientists can measure particle presence using ground-based and airborne instruments. Because there are so many small particles and so few large ones, measurements have concentrated on capturing the characteristics of the small ones. This new study shows that in clean air, just a few, larger particles have a much greater influence on reflected sunlight than previously thought. Revealing the true nature of these "giants" suspended in the atmosphere will improve models that predict how the climate will react in all conditions.

Many routine measurements have failed to capture the amount of large aerosols, particles greater than one micron, in the atmosphere in clean sky conditions. Without a complete picture of the effect of aerosols on the budget, these routine measurements did not provide the important observational constraints needed to assess and improve climate models.

The PNNL researchers performed a two-step study using daily observations from an integrated dataset collected during the Carbonaceous Aerosol Radiative Effects Study (CARES) field campaign held in 2010 near Sacramento, California. The data analysis revealed surprisingly large and frequent contributions (up to 85 percent of the time) of large particles to the total volume of aerosol particles.  Co-incident ground-based and aircraft data collected at and above two sites verified the existence of large particles.

In the second step, the team used state-of-the-art radiative transfer calculations on the CARES dataset to show the large aerosol particles' effect on the amount of sunlight reflected back into space. They found the large particles had an unexpectedly significant influence on the amount of reflected sunlight.

Researchers plan to perform similar two-step studies on datasets collected around the world, including data from the 2012 Intensive Observation Period of the Two-Column Aerosol Project (TCAP) near Cape Cod, Mass. These studies will provide additional improved observational constraints for regional and global climate models.

Explore further: NASA sees Hurricane Edouard far from US, but creating rough surf

More information: Kassianov E, M Pekour, and J Barnard. 2012. "Aerosols in Central California: Unexpectedly Large Contribution of Coarse Mode to Aerosol Radiative Forcing." Geophysical Research Letters 39:L20806. DOI: 10.1029/2012GL05346.

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User comments : 10

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1 / 5 (3) Nov 07, 2012
Of course this will mean that climate models will show less warming going forward since the models will also show more reflected sunlight. But they don't want to talk about those ramifications.
5 / 5 (1) Nov 07, 2012
You forget reflected sunlight that doesn't reach the earth surface, also means heat trying to escape is reflected back to the earth surface too by the same action.

So it could be in balance, it could not be - don't know till we do the model.
5 / 5 (2) Nov 07, 2012
But they don't want to talk about those ramifications.

Actually they do. Climate scientists are well aware that aerosols are one of the areas that needs to be better understood so that climate models can be improved. And here is a study taking a look at exactly that.

Funny how scientists -- even those crazy climate scientists-- are always trying to improve their understanding of the natural world around us.
not rated yet Nov 07, 2012

The earth isn't warming significantly as a result of increases in solar output.

Hence the current concentration of particulates, if held constant, will continue to reflect the same amount of sunlight.

In fact, as long as the particulate level is static, there is no effect on the model results at all, since the existing particulate level is calibrated into the calculations.

"Of course this will mean that climate models will show less warming going forward since the models will also show more reflected sunlight." - Lino235
1 / 5 (4) Nov 07, 2012
Perhaps the warming trend observered in the 80's and 90's was simply the result of scrubbers applied to coal fired power stations and the resultant cleaner air , and not rising co2 after all?
5 / 5 (2) Nov 07, 2012
You would then need to explain how CO2 magically absorbs IR in the lab but not IR in the environment.

Good luck.
2.3 / 5 (3) Nov 08, 2012
Apologies VD , I should have used the word 'predominantly' rather than 'simply'. Here's a paper they may affirm, in part, my assertion. Though predominantly about cloudyness the paper cover aerosols too.
"A new paper published in Atmospheric Chemistry and Physics finds from direct measurements that there was a significant increase in solar radiation at the surface of the Northern Hemisphere from 1982 to 2008. According to the authors, "the average increase of [surface solar radiation] from 1982 to 2008 is estimated to be 0.87 W m−2 per decade," which equates to 2.26 W m-2 over the 26 year period. By way of comparison, this forcing was 12.5 times greater than the surface forcing alleged by the IPCC from increased CO2 over the same period:"
5 / 5 (1) Nov 10, 2012

The effects of global dimming due to man-made pollution as well as natural aerosols (like forest fires, or big volcanic eruptions that inject sulfides into the stratosphere), is a well-known phenomenon and is a standard input for global circulation models of climate.

The paper you cite may help better constrain the inputs fed into those GCMs (thus helping reduce uncertainty ranges.)

However, as to the conclusions you (and this 'hockeyshtick' site you link) draw from the paper are deeply flawed. Firstly, the paper found this trend only in the Northern hemisphere (and even there, only over certain regions thereof where there is dense population and also coincidentally lots of SunDu data available), while seeing no trend at all in the Southern hemisphere. Secondly, it measured Rs only over land, whereas we all know (don't we?) that Earth's surface is 70% oceans.

Thus, it has little to say about GLOBAL warming, though it provides useful inputs for regional modelling.
1 / 5 (2) Nov 11, 2012
I didn't mean to draw any conclusions from this paper. I merely suggested that the cleaning up of smokestacks may account for some of the observed warming. The figures for the lower troposphere show that warming since 1980 has been far from Globally uniform. The southern hemisphere has warmed at .079K / decade while the Northern Hemisphere has warmed at .179 K /decade. Perhaps this is due to the greater number of coal fired power stations in the and the
subsequent cleaning up of their out put. This is just a suggestion, not a conclusion.http://wattsupwit...-update/
1 / 5 (2) Nov 11, 2012
Here is the conclusion the Hockey Schtick site makes which is not really conclusive either.
"The paper adds to several others showing that a decrease in cloudiness was largely responsible for warming in the latter 20th century, rather than man-made greenhouse gases."