formation of smog

Photochemical smog reactions

“Smog” is a term originally formed by the combination of of the words “smoke” and “fog.” This can seem a little confusing, because Los Angeles, infamous for for its smog, is hardly known for foggy weather. The confusion arises because there are actually two kinds of smog: London smog (well know for its fog) and L.A. smog (also know as photochemical smog). Photochemical smog is driven by the u.v. energy from the sun, and Los Angeles is better know for its sunny weather. The differences between the two types of smog are summarized in the table below.

Name: London smog

(New York smog, gray smog)

Photochemical smog

(L.A. smog, Denver smog, brown smog)

Weather: cool, damp sunny
Content: particulates, sulfur oxides NOx, ozone, hydrocarbons.
Sources: coal, etc. gasoline, combustion.

On a smoggy day, there are literally thousands of reactions that occur in the atmosphere. Fortunately, there are a few that can help us to initially understand the formation of photochemical smog. In the reactions listed below, the most important consitituents are described with a larger, bolder font.

1. NO + O2 —> NO2 + u.v. —> O + NO

In this first reaction, we start with Nitric Oxide (NO), which we already know is emitted from various combustion processes. It combines with oxygen in the atmosphere to form nitrogen dioxide ( NO2 ), which has a characteristic brown color that should be familiar to anyone who has lived in a smoggy region. When the u.v. rays of sunlight strike the NO2, it breaks off a single oxygen radical (O) that triggers many subsequent reactions of photochemical smog.

2. O + O2 —> O3

In this second reaction, we see how the single oxygen radical helps form ozone ( O3 ). A variety of molecules can act as catalysts for this reaction.

3. O3 + NO —> O2 + NO2

This third reaction is called a scavenging reaction, and it happens normally in the evening. Because it converts ozone to O2, the net result is a drop in the ozone concentration in the evenings.

4. RC + O —> RCO + O2 —> RCO3

The fourth reaction shifts our attention to the hydrocarbons (represented here as RC). When combined with the oxygen free radical, it forms RCO, which represents a variety of aldehydes and ketones. Some of these constiutents can combine with oxygen to form peroxide readicals ( RCO3 ).

5. O2 + RCO3 —> O3 + RCO2

The fifth reaction demonstrates the importance of these peroxide radicals ( RCO3 ) — it enhances the formation of ozone.

6. NO + RCO3 —> NO2 + RCO2

The last reaction shows a more subtle role of the peroxide radicals — by enhancing the formation of nitrogen dioxide, we know that the nitrogen dioxide will go on to form more ozone.