Number Of Ozone Molecules Per Cm3

Now 40.7 mol X 6.02 X 1023 molecules/mol = 2.45 X 1025 molecules, or 2.45 X I016 billion molecules of air.

Thus the S02 concentration is

3.01 X 1018 molecules of SP2 2.45 X 1016 billion molecules of air = 123 ppb

Note that the conversion of moles to molecules was not strictly necessary, as Avogadro's number cancels from numerator and denominator. As stated previously, ppb refers to the ratio of the number of moles as well as to the ratio of the number of molecules.

It is vital in all interconversions to distinguish between quantities associated with the pollutant and those of air.


Convert a concentration of 32 ppb for any pollutant to its value on

(b) the molecules per cm3 scale, and

(c) the molarity scale.

Assume 25 °C and a total pressure of 1.0 atm.


Convert a concentration of 6.0 X 1014 molecules/cm3 to the ppm scale and to the moles per liter (molarity) scale. Assume 25°C and 1.0 atm total air pressure.


Convert a concentration of 40 ppb of ozone, 03, into

(a) the number of molecules per cm3, and

Assume the air mass temperature is 27°C and its total pressure is 0.95 atm.


The average outdoor concentration of carbon monoxide, CO, is about 1000 jug/m3. What is this concentration expressed on the ppm scale? On the molecules per cm3 scale? Assume that the outdoor temperature is 17°C and that the total air pressure is 1.04 atm.

react with gaseous water to abstract one hydrogen atom from each H20 molecule:


The average tropospheric lifetime of a given hydroxyl radical is only about one second, since it reacts quickly with one or another of many atmospheric gases. Because the lifetime of hydroxyl radicals is short and sunlight is


Some Important Gases Emitted into the Atmosphere from Natural Sources

Main Natural



Main Natural








Anaerobic biological decay



Hydrogen sulfide

Anaerobic biological decay



Hydrogen chloride

Anaerobic biological decay, volcanoes


Sulfur dioxide




Nitric oxide




Carbon monoxide

Fires; CH4 oxidation




Anaerobic biological decay



Methyl chloride



OH ¡Br

Methyl bromide




Methyl iodide


required to form more of them, the OH concentration drops quickly at nightfall. Recall from Problem 1 of Box 1-2 that because the corresponding reaction involving unexcited atomic oxygen atoms is endothermic, its activation energy is high and consequently it occurs far too slowly to be a significant source of atmospheric OH. Although OH participates in many atmospheric reactions, it has been found recently that its concentration is directly proportional to the O* concentration at any given time.


In one study, the concentration of OH in air at the time was found to be 8.7 X 106 molecules per cubic centimeter. Calculate its molar concentration and its concentration in parts per trillion, assuming that the total air pressure is 1.0 atm and the temperature is 15°C.

The hydroxyl free radical is reactive toward a wide variety of other molecules, including the hydrides of carbon, nitrogen, and sulfur listed in Table 3-1, and many molecules containing multiple bonds (double or triple bonds), including CO and S02. Although suspected for decades of playing a pivotal role in air chemistry, the presence of OH in the troposphere was confirmed only recently since its concentration is so very small. The great importance of the hydroxyl radical to tropospheric chemistry arises because it, not 02, initiates the oxidation of all the gases in Table 3-1 other than HC1. Without OH and its related reactive species HOO, most of these gases would not be efficiently removed from the troposphere, nor would most pollutant gases such as the unburned hydrocarbons emitted from vehicles. Indeed, OH has been called the "tropospheric vacuum cleaner" or "detergent." The reactions that it initiates correspond to a flameless, ambient-temperature "burning" of the reduced gases of the lower atmosphere. If these gases were to accumulate, the atmospheric composition would be quite different, as would the forms of life that are viable on Earth. Interestingly, hydroxyl is unreactive to molecular oxygen—in contrast to the behavior of 02 with many other free radicals— and to molecular nitrogen, so it survives long enough to react with so many other species.

An example of the reactions initiated by hydroxyl radical is the net oxidation of methane gas, CH4, into the completely oxidized product carbon dioxide, ("X)>:

OH catalyst

CH4 + 2 02-*C02 + 2 HzO

As we shall see in Chapter 5, this overall reaction occurs by a sequence of reactions, the first of which involves the reaction of hydroxyl radical with methane, and the next-to-last of which involves the reaction of OH with carbon monoxide. Indeed, this pair of reactions accounts for the fate of most hydroxyl radicals in a clean atmosphere. However, since a new hydroxyl radical is also produced eventually in the multistep reaction sequence, it is acting as a catalyst. Since the OH is originally produced from 03, the case can be made that it is really ozone that causes the oxidation of most atmospheric gases. Diatomic molecular oxygen reacts with some of the free-radical species produced by OH reactions, so it does appear in the overall equation as the substance that oxidizes the reactants.

The hydrogen halides (HF, HC1,11 Br) and fully oxidized gases such as carbon dioxide are relatively unreactive (from the oxidation-reduction point of view) in the troposphere because no further oxidation occurs with them; they eventually are deposited on the Earth's surface, often as a result of dissolving in raindrops.

Continue reading here: Urban Ozone The Photochemical Smog Process The Origin and Occurrence of Smog

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  • Susanne
    How to convert ppb to moleculules per cm^3?
    1 year ago
  • Thorsten
    HOW TO FIND THE Number of ozone molecules per cubic centimeter?
    3 years ago