Sunday, July 04, 2010

How efficient are plants? (part II)

Insolation at Earth's surface (the total solar irradiance, in units of W/m2):

On sunny days, about 1 kilowatt of solar radiation bathes every square meter of the earth's surface every hour. Because of the seasons and weather, the annual average for much of the united states is between 1/2 and 1/3 of this ideal "clear sky" condition.

Photosynthetically Active Radiation, or PAR, is only ~40% of that total, depending on which chlorophyll molecules are present in the plant. Because of physiological requirements, plants actually use an order of magnitude less, usually about 2% of total solar insolation. Of that, orders of magnitude less are available for conversion into biomass. Contrast that with commercial solar arrays that can capture about 20% of the energy in solar radiation!

Either way, the rest of that kW is either reflected or converted to heat...and heat makes the wind blow. A 20mph wind contains enough energy to generate 1 kilowatt per square meter for every hour it blows.

Ship Pollution

Large ships continue to use old, polluting diesel engines, with disasterous health effects for people living close to the ocean. The EPA's proposed plan would greatly alleviate this largely unregulated source of pollution, which is currently equal to about 120 coal power plants running continuously in our nation's harbors and waterways. (source: President's Cancer Commission report, 2010)

EPA Website on large ship pollution.

Interestingly, ships also affect cloud formation: the soot in their exhaust creates nuclei for water droplets to condense and form clouds:

Friday, July 02, 2010

Do Trees Pollute?

Plants produce VOCs as part of normal respiration. These, along with anthropogenic VOCs can produce ozone smog in the lower atmosphere.

In the South, pine plantations used for their fast-growing supplies of timber have proven to be havens for sweetgum trees, which are major producers of VOCs. Indeed, virtually every tree that grows fast -- a desirable quality for forestry production -- is a heavy emitter of VOCs.

"It's just one of those biological correlations," said Purves. "What you want is a fast-growing tree that doesn't produce a lot of VOCs, but that doesn't seem to exist."

The debate continues: "... trees were reported to contribute to ozone formation. This misleading fact contains only part of the truth. Most trees do emit biogenic VOCs such as isoprene and monoterpenes which can contribute to the formation of ozone and carbon monoxide. The other side of this story is that in areas with low nitrogen oxide concentrations, such as more rural areas, VOCs are believed to remove ozone. Additionally, since trees lower air temperature, the net effect of increased trees in urban areas is an overall lowering of VOC emissions and therefore ozone formation.

Trees in urban areas require energy inputs for planting, maintaining and removing. Because we burn fossil fuels (which emit CO2, SO2, N, CO and VOCs) in all these activities, we also need to factor that into the Trees + Air equation. In this case, it tips the scale a bit to the net loss side… but not for long!"