The main source of urban heat is not chemical emissions but the buildings made of concrete and asphalt which tend to store heat better than soil or vegetation. Another source of urban heat is the warm air arising from combustion processes and use of air-conditioners. Aerosols such as sulphuric acid mists and ammonium sulphate mists and vapours, etc., can influence the vertical temperature status of the atmosphere and the resulting thermal alterations affect mixing, dispersion and the buildup of aerosols.
A kind of particles called ice nuclei which occur in our atmosphere in low concentrations are responsible for modifying the cloud structure at subzero temperatures and the effects of ice nuclei are some time so severe that the precipitation potential of such cold clouds may also become altered. Two main sources of ice nuclei are steel mills and automobile emissions. Forest fires and pulp and paper mills are the sources of certain other particles called cloud condensation nuclei (CCN).
These CCN particles are capable of modifying precipitation from clouds that have temperatures above O°0. Other air pollutants such as acid fumes, aerosols, ammonia, and S02 are known to affect the pH of rainwater. Certain chemicals in the discharge can interact in the atmosphere resulting in an amplification of their effects; e.g., although SO2 and NH3 by themselves react slowly, their reaction rate is considerably increased in the presence of cloud droplets. Surface active pollutant molecules can coat cloud particles and influence cloud and aerosol coagulation and evaporation. Some air pollutants affect aerosols involved in cloud processes.
The structure and distribution of clouds can be affected by such pollutants, and this causes alterations in precipitation and optical scattering properties. In addition to the well-documented known effects of pollutants on the local and regional scale on some aspects of weather, these pollutants are also now believed to exert significant effects on global climate (see Hobbs et al., 1975). Combustion of coal and other fossil fuels generates two important problems, viz., acid rain, and CO2 build-up.
Nitrates are now known to contribute significantly to the acidity of rainfall (earlier it used to be attributed chiefly to SO2 from power plants.) In Europe the local impacts of SOx and NOx have been attempted to be avoided by diffusing such pollutants by discharging them through tall stacks. But in the atmosphere these oxides combine with moisture and fall back to earth as dilute acids. Forest streams and lakes are some systems most strongly affected by acid rain and the aquatic life in lakes and streams can be adversely affected by acidity. Other adverse effects of acid rain include decreases in agricultural and forest yields, depletion of nutrients from soils and aquatic systems, inactivation of useful microbes, and deterioration or corrosion of materials. Acid precipitation can damage plant leaves, accelerate cuticular erosion, change responses to pathogens, symbionts and saprophytes, affect seed germination and seedling establishment, affect soil nitrogen availability, decrease soil respiration and increase nutrient leaching from the soil.
The second energy -related problem is CO2 build-up possibly leading to “the greenhouse effect”. Atmospheric CO2 levels have increased by about 5 per cent during the last three decades. It shows the observed increases in atmospheric CO, and methane, resulting in part from human activities, CH4 is another important greenhouse gas. This CO2 helps control the earth’s heat balance by preventing part of the sun’s reflected energy from returning back to space.
With increased levels of CO2, therefore, more energy is retained and the atmospheric temperature rises. Rapid use of the world’s fossil fuels has been estimated to produce a 7-fold increase in the CO2 level by 2200 A.D., and the level may well be doubled in another 50 years. This doubling could cause an average temperature increase by at least 2 degrees Celsius. This 2°C increase can produce some negative and some positive effects; the melting of the ice caps could raise the sea level, and the agricultural belts could shift towards the poles.
On the other hand, increased C02 content could increase the rate of photosynthesis. An international symposium on sulphates in the atmosphere was organized in Dubrovnik (Yugoslavia) in September 1977. The main emphasis was on transport, transformation and removal processes as well as on the properties and measurements of sulphur compounds in the atmosphere. A few important points emerging from this symposium are summarized below. The natural volatile sulphur emissions are currently estimated at about 35 million tons/year.
Man-made emissions, which are chiefly SO2, account for about 65 million t/yr as SO2. The major effects of sulphur oxides are associated more with the reaction products than with SO2 itself. Three mechanisms are important in atmospheric SO2 conversion (mostly to sulphate): the first, i.e., indirect photo-oxidation in homogeneous and occurs in the gas phase; the other two are heterogeneous, i.
e., the reactions occur in liquid particles or on particle surfaces. These three involve catalytic SO2 oxidation using Fe, Mn, etc. Surface catalyzed oxidation uses scot (elemental carbon).
The average oxidation rate over the life time of SO2 is about half per cent per hour. The residence time and the transport distance of atmospheric sulphur depend on the overall removal rate of sulphur compounds from the atmosphere. Overall removal has four major components, viz., dry removal of SO2, wet removal of SO2, dry removal of SO4, and wet removal of SO4. Of these, the dry removal of SO2 and wet removal of SO4 are the important and principal components. It is a flow diagram of sulphur transmission through the atmosphere. About one-half of the SO2 is removed or changed to SO4 during the First day of its atmospheric residence.
SO2 oxidation leads to the formation of particulate sulphur (as sulphate ion;). In temperate climates in mid-latitudes, up to 5O per cent of the SO2 may get converted into sulphate before removal.