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Have you ever seen this phenomenon pictured below? This view usually can be found easily in busy urban area with lots of mobility or industrial activities. The picture below is showing the photochemical smog phenomenon. What is it and is it dangerous to the environment and people? Let’s see the explanation below!

Sumber: prior-scientific


Smog is a compound word of smoke and fog which means fog that contains air pollutants. Photochemical is a chemical reaction process caused by presence of light. Photochemical smog is mixed pollutants that forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react with sunlight causing brown haze to appear above cities. Those pollutants include ozone, lachrymator (chemical substance that may cause eye irritations), and other hazardous chemical compounds.


There are 3 main compounds needed for the chemical reaction to take place. Those three compounds are ultraviolet light (UV), hydrocarbon, and nitrogen oxides (NOx). These three compounds can come from natural sources or human activities.

Forest fires and microbial process that occurs in nature, especially in the soil, will emit NOx. Meanwhile, VOCs can be produced by naturally-occurring evaporation of organic compounds, such as terpene ((C5H8)n), a hydrocarbon produced by plants and existed in oil as trigger to combustion. Eucalyptus, an Australian tree, also emits this substance in large amount. Furthermore, NOx compounds can be produced in the occurrence of lightning. Nitrogen gas reacts with surrounding oxygen, creating nitrogen monoxide.

Hydrocarbon and NOx are mostly emitted from human activities such as combustion process in vehicle’s engines. In most of urban areas, more than 50% photochemical smog is formed because of vehicle’s emission. Incomplete combustion in engines emits unburned hydrocarbons, carbon monoxide, nitrogen oxide, and sulphur oxide. This incomplete combustion usually occurs when vehicles are going at high speed and in traffic, causing lack of oxygen supply for the combustion process. Incomplete combustion may also occur in industries where oxygen supply is lacking for the processes using engines.

Besides being caused by compounds originating from those sources, photochemical smog is also often caused by calm winds. During winter or colder weather, wind speeds are low, causing smoke and fog to stagnate where smog forms and increasing pollution levels on the surfaces where humans and other living things live.

Influencing Condition

Factor that can influence the occurrence of photochemical smog is thermal inversion. A thermal inversion occurs when a layer of warmer and lighter air occurs over a layer of colder and heavier air, causing no vertical winds which leads to trapping pollutants in the lower atmosphere. Pollutants that should rise higher to be dispersed and diluted remain in the lower layer of atmosphere, causing them to accumulate more. The trapping of these pollutants will cause smog to remain in the area for a long time. If thermal inversion does not occur, the photochemical smog in the area will disappear in short period of time.

The occurrence of thermal inversion can be influenced by several factors, one of which is the location factor. Thermal inversion can occur more easily in valley areas because air movement in there is more restricted and night-time temperatures are colder. In coastal areas, thermal inversion is exceedingly difficult to occur. This is due to the temperature at night which remains warm, caused by high level of humidity. Water vapour which is the biggest contributor to greenhouse effect (50%, calculated from the concentration and ability of substances to absorb solar energy) causes heat to remain trapped in coastal areas at night.


Photochemical smog can have impacts on the environment, human health, and material damage. The main impact that can be seen directly (visually) is the brown haze that is above urban areas. That brown colour is caused by tiny particles of liquid and solid scattering light.

NOx, ozone, and peroxyacetyl nitrate (PAN) compounds can reduce and eventually stop plant growth by reducing photosynthesis process. Small amounts of ozone can cause this easily, but PAN is even more toxic to plants. Smog can cause heart and lung problems, eye irritation, respiratory problems, coughing, and shortness of breath in humans. This is caused by the pollutants contained in the smog itself (NOx, VOCs, ozone, and PAN). Ozone can also damage various material compounds—cracking in rubber, reducing strength of textiles, fading dyed fabrics, and cracking paint.


EPA. (2004). Photochemical smog—what it means for us. Adelaide: EPA South Australia.

Foust, Richard. n.d. Photochemical Smog. Diperoleh dari http://mtweb.mtsu.edu/nchong/Smog-Atm1.htm 

Manahan, Stanly E. (2000). Environmental Chemistry 6th Edition. Florida: CRC Press.

Rani, Bina et al. (2011). Photochemical Smog Pollution and Its Mitigation Measures. Journal of Advanced Scientific Research, 2011; 2(4): 28-33.


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