Modification of atmospheric composition

Urban pollution modifies the thermal properties of the atmosphere, cuts down the passage of sunlight, and provides abundant condensation nuclei. The modern urban atmosphere comprises a complex mixture of gases including ozone, sulphur dioxide, nitrogen oxides, and particulates such as mineral dust, carbon and complex hydrocarbons. First, we examine its sources under two main headings:

1 Aerosols. Suspended particulate matter (measured in mg m-3 or |g m-3) consists chiefly of carbon, lead and aluminium compounds, and silica.

2 Gases. The production of gases (expressed in parts per million (ppm) or billion (ppb), respectively) may be viewed in terms of industrial and domestic coal burning releasing such gases as sulphur dioxide (SO2), or from the standpoint of gasoline and oil

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.

LOCAL TIME (hours) MONTHS

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 (NOx), 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, C10 H18, and deciduous woodlands isoprene, C3 H8; rural concentrations of these hydrocarbons are in the range 0.1 to 1.5 ppb and 0.6 to 2.3 ppb, respectively.

In dealing with atmospheric pollution it must be remembered, first, that the diffusion or concentration of pollutants is a function both of atmospheric stability

(especially the presence of inversions) and of the horizontal 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 susceptible 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

Figure 12.19 Annual and daily pollution cycles. (A) Annual cycle of smoke pollution in and around Leicester, England, during the period 1937 to 1939, before smoke abatement legislation was introduced. (B) Diurnal cycle of smoke pollution in Leicester during summer and winter, 1937 to 1939. (C) Annual cycle of mean daily maximum one-hour average oxidant concentrations for Los Angeles (1964 to 1965) and Denver (1965) (dashed). (D) Diurnal cycles of nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3) concentrations in Los Angeles on 19 July 1965.

Sources: (A), (B) After Meethan et al. (1980), (C), (D) After US DHEW (1970) and Oke (1978).

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment