State whether the following is true or false.
Adding condensation nuclei to a cloud always increases the precipitation it produces.
Answer: C
More nuclei give smaller droplets, which coalesce less efficiently and delay the onset of rain. The net effect on total precipitation depends on the cloud type and can go either way. Shallow warm clouds and deep convection respond in opposite directions.
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Consider the following statements about aerosol and precipitation.
1. Smaller droplets collide and coalesce less efficiently.
2. Suppressed rain can prolong the lifetime of a cloud.
3. More nuclei reliably increase the rainfall from any cloud.
Answer: D
The first two are correct and are the basis of the second indirect effect. The third overstates a relationship that reverses in deep convective clouds where invigoration can occur instead. The sign of the effect depends on the regime the cloud belongs to.
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Ship tracks visible in satellite imagery of marine stratocumulus are caused by
Answer: A
The added nuclei produce more and smaller droplets along the track, brightening the cloud in a narrow line. They are among the clearest natural demonstrations of the first indirect effect available. The tracks persist for hours and can be traced for hundreds of kilometres.
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Three of the following are natural sources of condensation nuclei. Which is the odd one out?
Answer: B
Correct answer: sulphate from coal fired power stations
Power station sulphate is anthropogenic. The other three are natural: mineral dust from arid land, sea salt from breaking waves, and the sulphate formed by oxidation of dimethyl sulphide released by marine plankton - the last being the basis of the proposed feedback linking ocean biology to cloud brightness.
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The accumulation mode of the atmospheric aerosol size distribution covers particles of roughly
Answer: C
The name reflects that particles in this range are removed only slowly and so accumulate in the atmosphere. It is also the range that contributes most of the effective condensation nuclei and most of the light scattering.
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Assertion (A): Accumulation mode particles dominate the population of effective condensation nuclei.
Reason (R): They are large enough to activate readily yet small enough to persist for many days.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives both halves. Particles below about 0.1 micrometre need supersaturations clouds do not reach, and particles above about a micrometre settle out or are scavenged within hours. The accumulation mode between them activates readily and has the longest atmospheric lifetime - days to a week - so it both dominates the population and is the mode transported furthest from its source.
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Arrange the aerosol size modes in order of increasing particle size.
1. accumulation mode
2. nucleation mode
3. coarse mode
4. Aitken mode
Answer: A
The sequence runs from freshly nucleated clusters through to mechanically generated dust and sea salt. Particles move up through the smaller modes by coagulation and condensational growth over time. The coarse mode is generated mechanically rather than reached by growth.
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A cloud condensation nucleus counter measures the number of particles that
Answer: B
The instrument exposes the sample to a controlled supersaturation and counts the droplets that grow. Reporting a nucleus concentration without stating the supersaturation is therefore meaningless. Instruments are commonly operated at a set of fixed reference values.
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Assertion (A): A condensation nucleus concentration must be quoted with a supersaturation.
Reason (R): The number of particles that activate depends on the supersaturation applied.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the necessity. Every particle has its own critical supersaturation, so the number that activate rises with the supersaturation applied - the count at 1 per cent may be several times the count at 0.1. A figure quoted without the supersaturation is therefore uninterpretable, which is why instruments report a spectrum rather than a single number.
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State whether the following is true or false.
Every aerosol particle in the atmosphere can act as a cloud condensation nucleus.
Answer: D
Very small particles and insoluble hydrophobic particles require supersaturations the atmosphere does not reach. Only a fraction of the total aerosol population activates in any given cloud. The activated fraction varies with both composition and updraught speed.
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