Less acidic rain
in the 1970s, western Europe, Sweden in particular, was in the grip of a severe environmental crisis: acid rain. Eastern usa faced the same phenomenon in the 1980s. Ever since, many developed nations have undertaken efforts to avert the threat of acid rain. However, a number of polluted areas around the world are still subjected to the phenomenon. Belo Horizonte Metropolitan Region (rmbh) in Brazil's Minas Gerias state is one such area.
But, besides pollutants from vehicular and industrial sources, rmbh is also subjected to prevailing winds from an area where intensive calcareous exploitation occurs. Large emissions of sulphur dioxide (so 2) and nitrogen oxide (no x) gases react with water vapour in the air to form sulphuric and nitric acids, which come down with the rainwater.
But, on the other hand, these winds contain alkaline dust generated by wind erosion, calcareous dust from cement and lime processing units and ammonia gas emissions neutralise the acid rain.
Debora Vallory Figueredo, a chemical engineer and a senior researcher at Fundacao Centro Technologico de Minas Gerias in rmbh , Brazil, has reported on how calcareous soil particulates influence acid rain in the region. Rainwater samples were collected from three sites of rmbh - Belo Horizonte, Contagem and Betim cities. Belo Horizonte, capital of rmbh , is engulfed by vehicular emissions, while Betim and Contagem suffer from industrial pollution. The large calcareous area is located near the sampling stations and, therefore, all of them are influenced by the prevailing winds ( Ambio , Vol 26, No 6).
Ninety-one rainwater samples (32 from Belo Horizonte, 25 from Contagem and 34 from Betim) were collected during the wet seasons of October to February. Almost half the samples (48 per cent) were found to be acidic.
Sixty-five per cent of the samples from Bentim and 30 per cent of the samples from Belo Horizonte and Contagem stations were found to be acidic.
The primary conclusion arrived at was that the most polluted and industrialised area of Minas Gerias state, rmbh , is subjected to acid rain. However, the rainfall in rmbh is a dilute solution of minerals and acids, primarily sulphuric and nitric acids. It contains five major ions - calcium, sulphate, ammonium, nitrate and hydrogen.
Since the calcium ion is the major component of rmbh precipitation, the acid-neutralising capacity by calcium particulates becomes enormous. In the dry periods, these alkaline particles (calcium, for instance) are continuously emitted into the atmosphere and neutralise the acid gases from the industries and vehicles. Therefore, the acidity of the first rain is considerably low. However, the removal of these alkaline particles from the atmosphere by frequent rains reflects directly in the increase in rain acidity.