Smog+(HL)

=__** SMOG **__=

What?
Smog is a portmanteau word, i.e. a word made up of two other words, in this case 'smoke' and 'fog'.



(Following info is extra. If you're just marking to get it over with, ignore this chunk.) Smoke is a mix of airborne solid and liquid particles as well as the air that they occupy. Those particulates are generally unwanted byproducts of combustion, and occur mostly when there is insufficient oxygen for complete combustion. (University of Waikato, 2009)

Fog is basically a cloud that is very close to the ground. There are many different types of fog with different causes, but their formation has the same basic principles as cloud formation. In fog, water vapour condenses when its temperature is within the range of something called the 'dew point' (since dew is just water that condenses on cool surfaces at night), and the water particles condense on dust, ice and other aerosols known as condensation nuclei to form a cloud. (AMS, 1994)

Smog as we learn it in Chem HL is not actually a literal mix of smoke and fog. But it IS like smoky fog in appearance. Or foggy smoke. It's actually the result of the reactions between Primary and Secondary pollutants and sunlight. This is known as **photochemical smog**, or simply put, **smog**.

** How? **
Smog mostly occurs in urban cities that have high numbers of cars and industries, which release Primary pollutants like NO x and other **Volatile Organic Compounds (VOC)**. Nitrogen monoxide, NO, is formed in the internal combustion engines of cars, and this may react with atmospheric oxygen in the reaction:

2NO + O 2 -> 2NO 2

The nitrogen dioxide produced is not only harmful to humans when inhaled, but may react with sunlight in the reaction:

NO 2 + sunlight -> NO + O. (The dot represents a free radical. Wikispace editor does indeed suck.)

The free radical produced is, again, not only harmful and damaging to cells when inhaled, it can also react with more atmospheric oxygen to form ozone in the reaction:

O. + O 2 + M -> O 3 + M Here, the inert catalytic substance M helps to absorb excess energy that might cause the ozone to decay upon formation.

Ozone has numerous negative side-effects, from being harmful to the human body to damaging objects made of rubber to harming plant life.

The oxygen radical doesn't just react with other oxygen molecules, its high reactivity causes it to readily and quickly react with other molecules, forming even more harmful substances.

(All images in this sequence from http://mtsu32.mtsu.edu:11233/Smog-Atm1.htm) 

Equation 4 - The oxygen radical reacts with the moisture in the atmosphere to form hydroxyl radical. Equation 5 - The hydroxyl radical reacts with a hydrocarbon to from a hydrocarbon radical, which then forms a hydrocarbon peroxide as it reacts with oxygen. Equation 6 - The hydrocarbon peroxide reacts with more NO from car exhaust and other anthropogenic sources of NO to form a hydrocarbon oxide radical which reacts with oxygen to form an aldehyde. Equation 7 - Aldehydes react with more hydroxyl radicals to form acyl radicals, which react with oxygen to form acylperoxy radicals. Equation 8 - Finally, the acylperoxy radical reacts with nitrogen dioxide to form peroxyacylnitrates (PANs), which are detrimental to a person's health in similar ways to ozone. This graph shows the relationship between the chemicals involved in the formation of photochemical smog. By observing which compounds increase in quantity, we can see which reactions are occurring at each point in time. For example, aldehydes are at their highest concentration at noon, which should be logical as the temperature and sunlight is generally the highest at midday. This picture is a simplified, pictorial representation of the equations discussed above. (Both pictures are from http://mtsu32.mtsu.edu:11233/Smog-Atm1.htm)

VOCs are volatile organic chemical compounds which are harmful to the environment and to human health, and come from a variety of sources, biological and anthropogenic (man-made) sources, ranging from solvents like paints and coating to CFCs (Chlorofluorocarbons) and similar substances. (USGS, 2010) Benzene and isoprene are examples of VOCs, and they react in sunlight to from other light absorbing, harmful particulates and compounds.

** Where? **
As mentioned at the start of the previous segment, urban cities with many cars and factories will suffer the most from photochemical smog. The production of VOCs and other primary pollutants contributes to the pollution, but the conditions which they accumulate in affects their effect.

An area with high winds and precipitation will suffer less from smog, as the smog is constantly being blown away, and replaced with fresh air, diluting it and reducing its effects. Contrastingly, areas with low wind speeds and precipitation will suffer the most from photochemical smog as the pollutants are able to stagnate and react with sunlight.

Cities situated in a valley, bowl-shaped surroundings, or full of tall buildings suffer from smog due to wind not being able to flow through normally as the air flow is impeded.

The Sun heats up the air near the Earth's surface in the day, and as hot air rises, any low-lying pollutants are carried away with the rising hot air. Cool air then sinks and gets heated up over time, which repeats the 'cleansing' cycle. This makes the air close to the ground cleaner as air currents will constantly move the pollutants there away. However, a phenomenon called 'thermal inversion' sometimes occurs. This is when weather conditions trap a layer of cool, dense air under a layer of less dense, warm air. The pollutants thus stagnate as they cannot be moved by flowing air currents, getting more and more concentrated and becoming increasingly harmful to the inhabitants of the city.

<- Thermal inversion as described above (http://www.olympicsorbust.com/wp-content/uploads/2008/08/_44875197_thermal_inversion466x135.gif)

The Assessment Statements:
10.1 State the source of primary pollutants and the conditions necessary for the formation of photochemical smog. 10.2 Outline the formulation of secondary pollutants in photochemical smog.

Here's a mediocre simulation: http://www.smogcity.com/

==** References ** == University of Waikato, Initials. (2009, November 19).//What is smoke?//. Retrieved from http://www.sciencelearn.org.nz/Contexts/Fire/Science-Ideas-and-Concepts/What-is-smoke

AMS, Initials. (1994). //Ams glossary - fog//. Retrieved from http://amsglossary.allenpress.com/glossary/search?id=fog1

<span style="-webkit-border-horizontal-spacing: 2px; -webkit-border-vertical-spacing: 2px; color: #333333; font-family: 'times new roman',times,serif; font-size: 12px; letter-spacing: 2px; line-height: normal;">USGS, Initials. (2010, June 04). //Volatile organic compounds (voc) definition page//. Retrieved from http://toxics.usgs.gov/definitions/vocs.html