Geoscience Reference
In-Depth Information
Figure 12.18 The effect of tropical rainforest stratification on temperature.* (A) Daily march of temperature (10 to 11 May 1936) in
the tree-tops (24 m) and in the undergrowth (0.7 m) during the wet season in primary rainforest at Shasha Reserve, Nigeria. (B) Average
weekly maximum and minimum temperatures in three layers of primary (Dipterocarp) forest, Mount Maquiling, Philippine Islands.
Sources : *After Richards (1952); (A) After Evans; (B) After Brown.
combustion producing carbon monoxide (CO),
hydrocarbons (Hc), nitrogen oxides (NO x ), ozone
(O3) and the like. A three-year survey of thirty-nine
urban areas in the United States identified forty-eight
hydrocarbon compounds: twenty-five paraffins (60
per cent of the total with a median concentration of
266 ppb carbon), fifteen aromatics (26 per cent of
the total, 116 ppb C) and seven biogenic olefins (11
per cent, 47 ppb C). Biogenic hydrocarbons (olefins)
emitted by vegetation are highly reactive. They
destroy ozone and form aerosols in rural conditions,
but cause ozone to form under urban conditions. Pine
forests emit monoterpenes, C 10 H 18 , and deciduous
woodlands isoprene, C 3 H 8 ; rural concentrations of
these hydrocarbons are in the range 0.1 to 1.5 ppb
and 0.6 to 2.3 ppb, respectively.
(especially the presence of inversions) and of the hori-
zontal air motion. In addition, it is generally greater on
weekdays than at weekends or on holidays. Second,
aerosols are removed from the atmosphere by settling
out and by washing out. Third, certain gases are suscep-
tible to complex chains of photochemical changes,
which may destroy some gases but produce others.
a Aerosols
As discussed in Chapter 3A.2 and A.4, the global energy
budget is affected significantly by the natural production
of aerosols that are deflated from deserts, erupted from
volcanoes, produced by fires and so on (see Chapter
13D.3). Over the twentieth century the average dust
concentration increased, particularly in Eurasia, due
only in part to volcanic eruptions. The proportion of
atmospheric dust directly or indirectly attributable to
human activity has been estimated at 30 per cent (see
Chapter 2A.4). As an example of the latter, the North
In dealing with atmospheric pollution it must be
remembered, first, that the diffusion or concentration of
pollutants is a function both of atmospheric stability
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