clouds from aerosols from trees

[Colin Forrest]

There was a question a while back about using trees which emit aerosols (isoprene and terpenes) to promote low level clouds.

In principle it is a great idea; trees have albedos nearly as low as water, for example the boreal conifer, picea mariana, black spruce, has an albedo of 0.09. Gymnosperms generally have albedos betwen that and 0.15. Angiosperms' albedos range from 0.13 to 0.18, with the albedo largely determined by the nitrogen content in the foliage. Fertilised grass and cropland can have albedos between 0.2 and 0.25. Tropical trees have an albedo of around 0.12 to 0.14, and the strongest cooling would be obtained if clouds could be brightened over tropical trees.

Trees cover 8% of the planet surface and 45% of them are in the tropics.

Some species within some families of trees produce isoprene (C5H8) in respones to heat and water stress, emissions amounting to about 1 Mt per year. Conifers emit between 0.1 and 0.2 Mt of terpenes, which are also reduced carbon species, and are oxidised to CO2 and H2O in the atmosphere by the same OH radicals which also oxidise methane. This means that if we increased volatile emissions from trees, less methane would get broken down.

However, if we can sucessfully develop the technology to deliver aerosol spray into marine clouds, I think it could be adapted to provide cloud brightening in clean air over forests, wetlands, tundra and inland waters, if meteorological conditions are favourable.

Best Wishes,  Colin Forrest

 

[Andrew Lockley]

I looked at this around a decade ago, but never managed to get a research project going.

My understanding is that the monoturpenes are released by the trees for sky brightening, but I'm not aware they're effective cloud condensation nuclei (non polar, hence high surface tension angle).

It should, in theory, be possible to use genetic engineering or selective breeding to optimise for aerosol production. However, these have a significant impact in human (and likely animal) health. 

Andrew Lockley 

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