Dust and Haze
Hazy Atmosphere
Urban Air Pollution
Smog at Crete
Dust Devil
Sahara Dust
Seasalt Aerosol



Urban Pollution - Brown Clouds and Smog

Region around Frankfurt/ Wiesbaden/ Mainz (Germany)


Quito, Equador


Sao Paulo, Brazil

Teheran, Iran

Esfahan, Iran

The phenomenon of "Brown clouds" - also called smog dome or smog - is a consequence of urban air pollution. Nitrogen oxides, organic gases, and aerosol particles are released together with several other gases within the emissions from traffic, industry and the combustion of fossil fuels. Under conditions of strong solar radiation photochemical processes will occur in the air. They lead to the formation of aerosol particles directly from the gaseous state - the so-called homogenous nucleation. The released and freshly generated aerosols blur the air and reduce the visibility in the atmosphere. Nitrogen dioxide (NO2) is built from nitrogen monoxide (NO) via oxidation. The brown colour results from the absorption spectrum of the nitrogen dioxide. It absorbs the blue and the short green wavelengths of the visible light, but permits the longer wavelengths of the green and the red light to pass. This results in the brown colouring.

BrownCloud1.jpg: The "Brown cloud" is formed at the dent in front of the mountain. Between 7 a.m. and 10 a.m. it has grown so much that the mountain was not visible anymore. The image was taken from the 20th floor of the "Hilton Colon" Hotel in Quito.
BrownCloud2.jpg: This image was taken one year later from the same place at around 10 o´clock in the morning. The brown layer in front of the mountains can be again clearly noticed.
BrownCloud9-27.jpg: Dark layers of mist with low thickness arose already fifteen minutes before landing in Sao Paulo at the upper limit of the boundary layer. It became denser and denser until it covered the whole boundary layer at the landing.

The summer smog in Teheran, Iran: Teheran is located 1300 m above sea level. It has approximately 14 million residents and 2 million cars. The cars do not have catalytic converters and neither cars nor trucks are equipped with diesel soot filters. The sulphur content is not limited in the fuel. (Since January 2005 the sulphur content in petrol and diesel has to be lower than 10 ppm(m) in the EU). In combination with the strong solar radiation it is not surprising that at each day intense photochemical smog arises in Teheran. It ranks among one of the worst in the world. The traffic is incredibly crowded and resembles the kinetic gas theory from the mathematical point of view. The north of Teheran is an amazing "green" city because of several avenues and parks. The observer can clearly see this from a viewpoint in the mountains. The topography plays an important role for the boundary layer of the atmosphere, because Teheran is enclosed in the mountain chain of the Alborz Rocks. The first mountain chain already has an altitude of 3000 m. The tourist place Touchal with view to Teheran is located at an elevation of 3800 m and the Damavand reaches up to 5000 m. In chapter "Cumulus" - paragraph "Status nasc. Smog BL top" - the consequences of the dispersion of the smog are described in detail.

Smog1-4.jpg: These images show the smog in the early morning. They were taken from the 11th floor of a tower block in the north of Teheran towards north-east. The aerosol particles scatter light most efficiently in forward direction, here from the sun in direction of the camera. Therefore the view in direction to the mountains is especially hazy.

BrownCloud28+29.jpg were taken from the building site of the Milad Tower which is located in the north-east of Teheran. The view is directed to the south. In the morning the brown smog layer can be especially clearly noticed.

Tower1-3.jpg: These pictures show the tower under construction. Finally it will reach an altitude of 350 m and resist earthquakes of magnitude 7 on the Richter scale. The building has already an altitude of 280 m at the picture. The tower is used for telecommunications as well as for measurements of the atmospheric boundary layer over Teheran.

Smog5-11.jpg were taken from the construction site at an elevation of 258 m. TeheranSmog5-9.jpg were recorded from the Milad Tower towards the south-east. They show the smog in Teheran at the forenoon. The view to the west of the Milad Tower is pictured in Smog11.jpg.

BrownCloud30.jpg: This picture was taken from the Geophysical Institute in the north of Teheran with a view to the south in the early morning.

TouchalPanorama.jpg: At the northern end of Teheran one can take the cable car to go from 1300 m to 3800 m a.m.s.l. The cable car is divided into two parts and has a total length of 8 km. The town Touchal is located at the end of the first part. The panorama picture was recorded next to a SINOP meteorological station at an altitude of 3800 m. At the left hand side is south-east, at the right side south-west and the view is directed over Teheran. The inversion above the smog layer is here at least at a level of 3000 m. Cloud images from the upper limit of the inversion as well as another panorama picture could be found under "Status nascendi, Smog BL Top" in chapter "Cumulus".

Smog1-3.jpg (Esfahan): Esfahan is located 330 km south of Teheran in an arid region at around 1500 m a.m.s.l. Also here one has to be concerned about anthropogenic air pollution, but it is less severe compared to Teheran. This pollution arises from traffic and domestic emissions in the winter as well as from industrial emissions (steel production and one refinery) predominantly in the east of the city.
Meteorological conditions: The town is embedded in mountains with an altitude of 3000 m. Since a large-scale downdraft of air masses dominates, it has a stable atmospheric layering 256 days per year. These conditions result in inversions which occur more than 180 days per year over the city. In winter there is no rainfall at all and hardly any snow (e.g. three days). The annual mean amount of precipitation is 120 mm. The annual mean temperature is 25 °C (minimum value - 8 degrees and maximum value 40 degree Celsius). The relative humidity is between 15 and 59 % in the summer, and between 35 and 95% in the winter. The wind mostly weak with 2 to 3 m/s and rarely occurs gust up to 10 m/s.

Although the city is embedded in a desert landscape, it is quite "green" as shown by the images. The abundant fine mineral dust stemming from the desert nearby is a special component of the air pollution. It is still unknown how the dust from the desert interacts with the components of the anthropogenic air pollution. See also the chapter Dust Devil.


Smog1-4.jpg: FACE II Science Team, Kleiner Feldberg, Taunus near Frankfurt, Germany, July 2005

Smog5-7.jpg: S. Borrmann, Mainz-Drais, view in direction to Kleiner Feldberg, Taunus, 14 July 2005, 8:40 a.m.

BrownCloud1.jpg: S. Borrmann, Quito, Ecuador, 1 September 2000, between 7 a.m. and 10 a.m.

BrownCloud2.jpg: S. Borrmann, Quito, Ecuador, 16 September 2001, around 10 a.m.

BrownCloud(i).jpg: S. Borrmann, arrival of flight AA163 from Miami to Sao Paulo, Brasilia, 8 April 2005, 9 a.m. to 9:03 a.m.

Iran (Teheran)
Exhaust(1).jpg: S. Borrmann, Teheran-Firooschkooh, Iran, 14 August 2005, 6:03 p.m. local time

Smog1-4.jpg: S. Borrmann, North Teheran, Iran, 17 August 2005, 6:55 a.m. local time

BrownCloud28+29.jpg: S. Borrmann, Teheran, at the building site of the Milad Tower, Iran, 17 August 2005, 9:16 to 9:17 a.m. local time

Tower1.jpg: S. Borrmann, Teheran, Iran, 15 August 2005, 3:27 p.m. local time

Tower2+3.jpg: S. Borrmann, Teheran, at the building site of the Milad Tower, Iran, 17 August 2005, 9:59 a.m. local time

Smog5-11.jpg: S. Borrmann, Teheran, Milad Tower, building site at an elevation of 258 m, Iran, 17 August 2005, 9:37 to 9:42 a.m. local time

BrownCloud30.jpg: S. Borrmann, Geophysical Institute at the University of Teheran, Iran, 17 August 2005, 8:17 a.m. local time

TouchalPanorama.jpg: S. Borrmann, Touchal in the Alborz mountains north of von Teheran, Iran, 17 August 2005, 12:18 p.m. local time

Iran (Esfahan)
Smog1-3.jpg: S. Borrmann, Esfahan, Iran, view from the University and the Meteorological Institute IRIMO, 12/13 August 2005, 6:31 p.m. and 7:47 a.m. local time

Smog4+5.jpg: S. Borrmann, Esfahan, Iran, view from the ruin of the Zoroastric Church situated 8 km to the west of the City, 13th of August 2005, 8:35 a.m. local time