One Paper and Another - Clouds and Methane

A short while ago, Eli posted about Finnish research on the formation of small atmospheric aerosols.  To repeat it was found that

First, the number of small clusters (< 1.2 nm) was essentially constant over time with loss from evaporation and reaction balancing growth by accretion and reaction. 

Second, growth up to about 1.9 nm occurs through reactions with sulfuric acid.  Significant growth only occurred on days when sulfuric acid concentrations increases and was synchronous with it.  On the other hand, theory shows that sulfuric acid/water aerosols are not stable by themselves requiring amines to stabilize and measurements with an atmospheric pressure inlet time of flight mass spectrometer showed that the intermediate aerosols did incorporate amines.  This means that sulfuric acid from SOx oxidation can be rate limiting

Third, above this limit, organic addition dominates and growth requires (photo)chemical activation by oxidation

Fourth, neutral clusters dominate.  This was surprising and casts a pall on claims that cosmic ray ionization controls aerosol production.

That study only looked at aerosols that were no bigger than 2 or 3 nm, not at the mechanism of how clouds are formed.  A recent Science paper (open link here), doesn't quite answer that question, but does provide a strong indicator by measuring the residues from the ice nuclei from which cirrus clouds form. 

The paper, Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation, D,J. Cziczo, et al. find that most of the ice nuclei form around mineral dust.  Cirrus clouds are relatively high thin clouds formed from ice crystals.  The hand wave goes that the ice crystals form by homogeneous freezing of water vapor, but, as pointed out by Cziczo, et al, this would require a much higher relative humidity than is generally found.

Somewhat complicated, but the point is that most aerosols, especially soot, aka elemental carbon, but also to a great extent mixtures of sulfates, volatile organics and/or nitrates do not much nucleate ice crystals, but mineral dust does so effectively.  The figure shows tracks for  five aircraft campaigns during which 0.2 - 3 micron residues from the ice crystals in the cirrus clouds were measured.  Homogeneous freezing was only found in two clouds.  In all of the rest the mineral dust residue dominated, accounting for 61% of freezing.  Sulfate and/or organic coatings were missing on the crystals.  Sea salt was important over the oceans.  In all, heterogeneous nucleation accounted for 94% of the cloud ice particles.

So where does the mineral dust come from?  Well a lot blows off the surface, but the bunnies blow up a few mountains of the stuff.  Eli would be curious to see how much comes from brake linings, but what is clear is that cosmic rays and adaptive irises will have a hard time with this paper.

However, you bunnies out there knew there was an however, this morning, Eli was talking to a buddy who brought up the idea that atmospheric methane lifetime might have something to do with cloud properties.  The friend (Eli has a small number and Willard Tony knows why), mentioned things like the cloud radiation field, photochemistry, etc.  Eli, demurred.  Why you ask, well, the Rabett had just read another paper in Science, Enhanced Role of Transition Metal Ion Catalysis During In Cloud Oxidation of SO2 by Harris, et al (open link here maybe)

Global sulfate production plays a key role in aerosol radiative forcing; more than half of this production occurs in clouds. We found that sulfur dioxide oxidation catalyzed by natural transition metal ions is the dominant in-cloud oxidation pathway. The pathway was observed to occur primarily on coarse mineral dust, so the sulfate produced will have a short lifetime and little direct or indirect climatic effect. Taking this into account will lead to large changes in estimates of the magnitude and spatial distribution of aerosol forcing. Therefore, this oxidation pathway—which is currently included in only one of the 12 major global climate models—will have a significant impact on assessments of current and future climate.

In other words, the SO2 will be oxidized to sulfuric acid by such reactions and fall out .  This might also play merry hell with various ideas about injecting SO2 high up in the atmosphere to counteract climate changes produced by increased carbon loading of the atmosphere,  Still, this is not where Eli is going.

Remember how the Bunny was talking with his buddy about what happens to methane (and other organics) in the atmosphere?  It is well known that transition metal ions catalyze oxidation and other reactions of organics.  There is a name for that.  It is called an oil refinery.  This also works in aqueous media.  Fun:)