Towards the end of the first half of this century, major technological advancement and changes were made in major industrial countries like the United States and Great Britain. A good example was in Pittsburgh, Pennsylvania whereby the use of coal and oil as home heating fuel was substituted with the use of natural gas. In addition, was the displacement of steam locomotives and replaced with diesel locomotives. Despite the growing problem of air pollution, no country in the world adopted any air pollution laws to try and mitigate harmful emissions in this period, nevertheless, the state of California adopted the first state air pollution law in the United States.
The period between 1950-1980 was marked with a major pollution disaster in Great Britain in 1952 which was later known as the 'Great London Smog'. During this period coal was used as the main source for generating power and heating homes in London and it was unfortunate that on this particular day, 5th of December 1952, a high-pressure weather system had stalled over southern England and caused a temperature inversion. For five days, a heavy fog combined with smoke formed from the fumes of vehicle exhaust and power plants created a public health disaster which resulted in an estimated 8,000 to 12,000 deaths\cite{klein2012}. The impact of the 'The Great London Smog' provided the impetus and will for effective air quality regulations which was enacted in the Clean Air Act of 1956. This legislation ensured that the market provided cleaner fuels for homeowners and phasing out coal in most urban areas. In addition, power stations located close to residential areas were closed and height of chimneys was regulated so that pollutants can be blown away by the wind \cite{populationmatters}.
Large cities in almost all countries in Europe including Australia, New Zealand and Japan were experiencing high air pollution levels during this period and as a result, these countries were the first to create national air pollution control legislation. Cities in the United States continued to experience the smog problem; as a result, the first federal air pollution laws came in to effect in 1955 which supported research in air pollution, training and technical assistance \cite{dl1984}. Later in 1970, the United States Environmental Protection Agency (USEPA)came into existence and with the mandate of having federal authority and responsibility in the control of air and water pollution and other hazards that may be detrimental to the environment and health.
During this 30 year period, a lot of interest was on air pollution created by automobiles and how to control it, sulfur oxide pollution from flue gases and fuel desulfurization as well as nitrogen oxides produced from combustion \cite{vallero2008} . Towards the end of this period, air quality monitoring systems came of age, monitoring systems were available to conduct modelling of atmospheric processes which highlighted the fact that air pollution had become a global concern. In addition, air pollution modelling program also projected that anthropogenic heat and toxic chemicals will become a serious global pollution problem in the future.
Build up of the greenhouse' gases like carbon dioxide in the atmosphere and chlorofluorocarbons (CFCs) was a concern in the 1970s as it contributed to the depletion of the stratosphere ozone layer and in 1985 CFCs became a major problem after the discovery of the Antarctic 'ozone hole' \cite{elsom1992}. The 1990s saw the emergence of two global environmental crises, the stratospheric ozone depletion and the uncontrolled changes in climate. Central of concern to this crises was the building up of greenhouse gases in the atmosphere and the cooling trends caused by particulate matter and sulphates in the same atmosphere. Subsequently, in the early years of the 21st century, global air pollution has taken on a greater urgency amongst the scientific community and the general public.
Recent Global Ambient Air pollution
Air pollution in the industrialized and developing world has changed drastically since the last century as a consequence of the rapid global population growth \cite{fenger2009}, moreover, economic development, energy consumption, urbanization, transportation and motorization are other major driving forces of air pollution in major urban cities \cite{Chen_2008}. Nevertheless, the urban environment in the industrialized countries has improved with respect to pollution from sulphur dioxide and soot that is often produced from power and heat production\cite{Fenger_2009}. In its 2005 Air Quality Guidelines Global update , the World Health Organization summarized that the PM10 annual average concentrations in European and North American cities were generally lower than 50\(\mu\)g/m3 as shown in Table 2 \cite{who2005}. Asia, Africa and Latin America recorded the highest levels of PM10 . SO2 has substantially declined in the United States and in particular Europe whilst in some Asian cities like Bangkok, Jakarta and New Delhi the low sulfur content in the fuel used there has resulted in the decline in ambient SO2 levels. Moreover, there has also been a moderate decrease in SO2 levels in LAtin America and Africa \cite{Chen_2008}.