Natural Factors

Adiabatic Cooling

 

       Throughout the summer months of May to October, temperatures in Cairo will often reach highs of 105° F but can drop all the way down to only 50° F at night (1)! Under these circumstances, the air closer to the ground cools much quicker than the air above (2).  Due to the ideal gas law and adiabatic cooling, the rising surface air becomes denser than the surrounding environment and eventually stops its ascent, resulting in a very stable atmosphere.  Atmospheric stability actually slows down the dispersion and dilution of pollutants, keeping them closer to the lower layers of the troposphere where they remain most harmful to the environment.

 

 Temperature Inversions

 

       During periods of extreme atmospheric stability, when wind levels are low and the diurnal and nocturnal temperature variance is high, radiation temperature inversions can occur.  When this happens, polluted air at the surface becomes trapped under a    “virtual”(3) lid of warmer air.  Although temperature inversions generally do not last for more than two or three hours, the elevated concentration levels of air pollutants become acutely dangerous for many of Cairo’s residents.

 

 

 

 

 

 

 

 

 

 Photochemical Smog

 

 High levels of photochemical smog are yet another byproduct of Cairo’s hot and sunny climate.  Photochemical smog develops when sunlight interacts with primary and secondary pollutants in the atmosphere such as Nitrogen oxides (NOX) and Volatile Organic Compounds (VOCs).  When sunlight chemically breaks down these compounds into their constituent parts, free oxygen molecules (O) unite with the oxygen in the air (O2) to create the tropospheric pollutant Ozone (O3).

 

DESERT DUST

 

 Longer Lasting Clouds and Transboundary Pollution

      

The second natural factor affecting Cairo’s air pollution is desert dust, a naturally occurring atmospheric aerosol, consisting of mineral particles and dirt blown from the desert surface. The small size of these particles allows for them to be transported several thousand miles away, leading to issues of transboundary air pollution.  For example, wind currents carrying desert dust have been traced from their source in Asia to as far east as the United States. Similarly, dust from the Saharan desert is frequently transported to parts of the Caribbean and South America (4).  Furthermore, clouds containing desert dust are thought to last longer due to their increased reflective properties (5).  Scientists are still researching this issue, but if it their assertions prove to be accurate, it will compound the problems of transboundary air pollution as Cairo’s dangerously high level of airborne pollutants travel greater distances and affect larger populations.

 

 Reduction of Rainfall

 

 In addition to reflecting sunlight, atmospheric desert dust also absorbs sunlight.  In fact, it is through the absorption of sunlight that desert dust becomes most harmful to the environment. As sunlight is absorbed, the atmospheric layer surrounding the desert dust becomes warmer, a process which suppresses cloud formation (6).  It is further believed that the characteristics of clouds are changed by the addition of atmospheric aerosols.  Daniel Rosenfeld, a scientist at The Hebrew University of Jerusalem, reported in Science that the presence of aerosols in clouds can cause abnormally small water droplets to form (7).   Since it takes much longer for these smaller drops to converge with each other in order to amass into water drops large enough to fall to the ground, it results in the overall reduction of rainfall.

Notwithstanding the environmental damage incurred by Cairo as a result of reduced rainfall, which we will discuss in a later section titled Environmental Impacts, the importance of rain as a cleanser for air pollution should not be overlooked. Rain and snow remove pollutants from the atmosphere through a process call wet acid deposition, also known as acid rain. While this process merely transfers the “problem” from one sector to another, it does help to alleviate the many health risks associated with excessive concentrations of suspended air pollution.

 

In addition to the anthropogenic causes of pollution, there are two natural factors which both contribute to and compound the air pollution in Cairo: climate and desert dust. A city’s climate can greatly impact the way and rate in   which pollutants are dispersed.  The mostly dry and hot desert environment of Cairo exacerbates the harmful effects of     emitted carbon, sulfur and nitrogen oxides through the processes of adiabatic cooling, temperature inversions, and photochemical smog.

 

CLIMATE