Discussion
Ventilation coefficients through a day.
| Time | Mixing height (m) | Wind speed (m/s) | Ventilation coefficient (m2/s) |
|---|---|---|---|
| 06:00 | 120 | 1.5 | 180 |
| 10:00 | 600 | 3.0 | 1800 |
| 14:00 | 1500 | 5.0 | 7500 |
| 22:00 | 200 | 2.0 | 400 |
At which time is dispersion potential poorest, and by roughly what factor compared with the best?
Answer: B
Correct answer: 06:00, poorer by a factor of about 42
Dispersion potential is worst where the ventilation coefficient is smallest, which is \( 180 \) square metres per second at 06:00 - a shallow \( 120 \) m mixing height left over from the night combined with a wind of only \( 1.5 \) m/s. The best is 14:00 at \( 7500 \), when surface heating has deepened the mixed layer to \( 1500 \) m and the wind has risen to \( 5 \) m/s.
\[ \frac{7500}{180} \approx 42 \]
So the atmosphere disperses a given emission some forty times less effectively at dawn than in mid-afternoon. Both factors work the same way through the day, which is why the diurnal swing in the coefficient is so large.
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