Physics 137 - Introduction to the Atmosphere and Weather                                                                                        Homework Key - Assignment #18
Revised 11/02/2009

18-QT1. (a) Suppose clouds of nitrogen dioxide drift slowly from a major industrial complex over a relatively unpopulated area. If the area is essentially "free" of hydrocarbons, would you expect high levels of tropospheric ozone to form? Explain. (b) Now suppose that the clouds of nitrogen dioxide drift slowly over an area that has a high concentration of hydrocarbons (VOCs), from both natural and industrial sources. Would you expect high levels of tropospheric ozone to form under these conditions? Explain your reasoning.

    (a) No. Ozone is produced by the photodissociation of nitrogen dioxide (NO2) into nitric oxide (NO) and atomic oxygen (O) followed by the reaction O2 + O / O3, wherein the molecular oxygen (O2) comes directly from the atmosphere. However the ozone is converted back to ordinary molecular oxygen under normal circumstances by the reaction O3 + NO / O2 + NO2. Hence ozone concentrations do not change appreciably.
    (b) Yes. Under the second set of conditions ozone concentrations would increase because the nitric oxide (NO) produced in the first reaction described above would tend to be removed from the atmosphere by reactions with reactive hydrocarbons which would tend to convert it back to nitrogen dioxide thus making it unavailable to convert the ozone back into ordinary molecular oxygen.

18-QT2. For least-polluted conditions, what would be the best time of day for a farmer to burn agricultural debris? Explain why you chose that time of day.

    Mid- to late afternoon would be the best time for burning since that is the warmest time of day and therefore the time when mixing convective currents would tend to be strongest.

18-QT3. Why are most severe air pollution episodes associated with subsidence inversions rather than radiation inversions?

    Subsidence inversions persist for days or even weeks while radiation inversions last only for several hours.

18-QT5. Table 18.4, p.511, shows that cloudiness is generally greater in urban areas than in rural areas. Since clouds reflect a great deal of incoming sunlight, they tend to keep daytime temperatures lower. Why then, during the day, are urban areas generally warmer than surrounding rural areas?

    There are several reasons: (1) dark asphalt and building surfaces tend to absorb more of the radiation that falls on them than does plant cover or soil; (2) the evaporative cooling of plant surfaces which tends to cool rural areas is greatly reduced in urban areas; (3) urban structures tend to reduce surface winds, reducing any possible advective cooling of urban areas; and (4) vehicles, factories and building heating and/or cooling units give off more heat in urban areas than in rural areas.

18-PE3. Suppose the air temperature at the surface is 30°C. Further suppose that the air temperature decreases at the dry adiabatic rate (10°C/km) up to the base of a strong subsidence inversion, situated about 2000 meters above the surface. If the surface air temperature increases to 40°C, and the air temperature continues to decrease at the dry adiabatic rate up to the base of the inversion, will the mixing depth increase or decrease? Explain your answer with a diagram. Will pollutants found within the mixing layer be more concentrated or less concentrated? Explain

     The mixing depth will increase as the inversion base will tend to become higher but not to be destroyed. Because of the greater height of the inversion base and the consequent greater thickness of the mixing layer, pollutants will become less concentrated.

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18-PE4. Rain with a pH of 2 is how much more acidic than rain with a pH of 5?

    Since a difference of 1 on the pH scale corresponds to a factor of ten ratio in acidity the lower pH rain is 10D(pH) = 105-2 = 103 = 1000 times more acidic.