THE PHYSIOCHEMICAL PROPERTIES OF RAIN WATER

CHAPTER ONE
1.0    INTRODUCTION
Acid rain has become a phenomenon that draws a lot of attention. Even recently, there was a panic in the country and in the world at large as forcast was made of deadly dangers associated with acid rain. The prediction was that the initial or first would be acidic.
The U.S department of energy estimates that since 1960, there have been more than four thousand (>4000) oil spills, discharging several millions barrels of crude oil into the ponds, ditches, creeks, beaches, streams and rivers in Nigeria especially in the Niger Delta[1]. These have impacted negatively on the environment, economic life and health of the people[2].
Onitsha as a case study is a commercial urban city with a very dense population, a lot of industries (including small scale, medium and large scale industries) and also a high use of gasoline generators in homes, offices, churches, schools and industries. Because of the unstable supply and the demanding nature of power (energy) in all establishments, the use of gasoline generators becomes inevitable which has led to extensive gas flare. The smoke that comes out of the gas flare does not just contain sooty grey particles but also many invisible gases that can be very harmful to our environment. These gases (especially nitrogen oxide and sulfur dioxide) react with the tiny droplets of water in clouds to form sulfuric and nitric acids. The rain from these clouds then falls as acid rain[3]. 
Natural process such as bacterial action on soils, forest fires and lightening can contribute significantly to the high value of this acid forming gas in the atmosphere e.g HCO3-. Actually, all rain tends to be acidic because of the equilibration of water with atmospheric carbon dioxide, yielding a pH of 5.6 – 6.5, then precipitated with a pH below 5.0 as a result of significant anthropogenic contribution of pollutants mainly sulfur dioxide and nitrogen oxide[4].

1.1    OBJECTIVES OF THE RESEARCH
In carrying out this research, we have in mind;

1.2    LITERATURE REVIEW
1.2.1 ORIGIN
Since the industrial revolution, emissions of sulfur dioxide and nitrogen oxides to the atmosphere have increased[5][6].
In 1852, Robert Angus Smith was the first to show the relationship between acid rain and atmospheric pollution in Manchester, England [7]. Though acid rain was discovered in 1852, it was not until the late 1960s that scientists began widely observing and studying the phenomenon. The term “acid rain” was generated in 1972[8]. Canadian Harold Harvey was among the first to research a dead lake public awareness of the acid rain in the U.S which increased in the 1970s after the New York times promulgated reports from the Hubbard Brook Experimental forest in New Hampshire due to myriad deleterious environmental efforts demonstrated to result from it[9].
Occasionally, pH reading in rain and fog water of well below 2.4 have been reported in industrialized areas [5]. Industrial acid rain and fog water of well below 2.4 have been reported in industrialized areas [5]. Industrial acid rain is a substantial problem in Europe, China [10], Russia and areas down-wind from them.
These areas all burn sulfur containing coal to generate heat and electricity. The problem of acid rain not only has increased with population and industrial growth, but has become more widespread. The local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulations[12][13].
Often depositions occurs in a considerable distance downwind of the emissions with mountainous regions tending to receive the greater deposition (simply because of their higher rainfall). An example of this is the low pH of rain compared to the local emission which falls in scandinavia[14].

1.2.2  RAIN FORMATION
Because the energy necessary for evaporation is supplied by sunlight, the largest sources of water vapour are tropical and semi 
tropical oceans. When warm, moist air rises in updrafts, it expands, cools and its relative humidity  increases. When the humidity is slightly in excess of 100%, the moisture condenses on slightly small aerosol particles called condensation nuclei. This forms fog and cloud not all clouds form rain but when it does, it is by one of these processes. The first one by Bergeron, Norwegian meteorologist, takes place by only in the part of the cloud that is below the freezing point of water. In such cloud, when a minute ice crystal forms, the crystal grows very rapidly. Because ice is the stable form of water below 00C (320F), it has a lower vapour pressure than the super-cooled droplets, moisture therefore evaporates from the super-cooled droplets and condenses on the fewer ice crystals. These crystal gradually increases in size, eventually becoming large enough to fall from clouds as snow. Much rain is produced when snow melts as it cascades from clouds into warmer air[15]. 
In the second process, collision and coalescence of many fine clouds droplets form raindrop. Collision of cloud droplets occurs as a result of their relative motion, which may rise because of Brownian movement or because some droplets are larger and fall faster than others. As they overtake and collide with small droplets, they coalescence with them, becoming even small droplets, they coalesce with them, becoming even larger and fall even yet faster by this process, they eventually grow to rain drop size. Chemists believe that some of the drops by this process get so large that they break up under aerodynamic forces into two or more somewhat smaller drops. These smaller drops in turn grow until they become unstable and break up, thus producing raindrops growth by chain reaction. There is evidence that in thunderstorm electrical forces may aid the rain forming process by accelerating collision and coalescence and droplets.
Raindrops ranges in size from less than 0.008 inch to about 0.23 inch in diameter. Raindrops do not exceed 0.23inch in diameter because of surface tension which keeps them intact is insufficient to withstand the aerodynamic forces tending to pull them apart. The rate of fall of a raindrop is determined by the balance between its weight and its aerodynamic drag[15].

1.2.3 CAUSES OF ACID RAIN
The most important gas which leads to acidification is sulfur dioxide. Emission of nitrogen oxides which are oxidized to form nitric acids is of increasing importance due to controls on emissions of sulfur containing compounds. About 70tg(s) per year in the form of SO2 comes from fossil fuel combustion and intensity, 2.8Tg(s) from wild fires and 7-8Tg(s) per year from volcanoes [16].

1.2.3.1       NATURAL PHENOMENA
The principal natural phenomena that contribute acid producing gases to the atmosphere are emissions from volcanoes and those from biological processes that occurs on the land, in wetlands and in the oceans, The major biological source of sulfur containing compounds is dimethyl sulfide. Nitric acid in rainwater is an important source of fixed nitrogen from plant life and is produced by electrical activity in the atmosphere such as lightening. Acidity deposits have been detected in glacial ice thousands of years ago in remote part of the globe[12].

1.2.3.2       ANTHROPOGENIC ACTIVITIES
The principal cause of acid rain is sulfur and nitrogen compounds from human sources, electricity generation, factories and motor vehicles, coal power plants are other sources of the polluting components that constitute acid rain. The gases can be carried hundreds of kilometers in the atmosphere before they are converted to acids and deposited. In the past, factories had short funnels to let out smoke but this caused many problems locally. Thus factories now have taller smoke funnels. However, dispersal from these taller stacks causes pollutants to be carried farther causing widespread ecological damage. However, livestock production also plays a major role. It is responsible for ammonia produced through human activities which significantly contributes to acid rain.