Write NO MORE THAN TWO WORDS AND/OR NUMBERS from the passage for each answer.
Will Antarctic Ozone Bite the Dust?
This year’s ozone hole above Antarctica may be deeper than ever as a result of the eruption of Mount Pinatubo. This is the conclusion of researchers who have combined observations of the effect of debris from the eruption on the stratosphere with the latest measurements of stratospheric ozone over the equator and the Antarctic.
A team led by W. B. Grant of the NASA Langley Research Center in Hampton, Virginia, monitored the influence of Mount Pinatubo, in the Philippines, before and after its eruption last June, using balloon-borne instruments launched from Brazzaville in the Congo (4 South, 15 East) and Ascension Island (8 South, 14 West).
The scientists found that most of the volcanic material injected into the stratosphere settles out at a height of between 18 and 28 kilometres, with the greatest concentrations between 24 and 25 kilometres. At precisely these altitudes, the amount of ozone in the stratosphere declined between three and six months after the eruption. The greatest loss — of 20 per cent — was at an altitude of 24 kilometres (Geophysical Research Letters, Vol. 19, p.1109).
The loss of ozone at a height of between 16 and 28 kilometres was between 13 and 20 Dobson units — about 5 per cent. There was a slight increase in the ozone concentration above 28 kilometres, but the researchers could not measure ozone beyond 31 kilometres because their balloons burst.
Grant and his team suggest that reactions in the stratosphere involving the Pinatubo material release chlorine in the active form which destroys ozone. Stratospheric chlorine originates chiefly from the manufactured compounds known as CFCs, but is usually locked up in stable form as chlorine nitrate. In the presence of sulphuric acid droplets from Mount Pinatubo, ozone-destroying chlorine monoxide is released.
Chlorine monoxide, which is produced by a similar series of reactions involving icy particles in the stratosphere, is blamed for the springtime depletion of ozone above Antarctica. In 1991, this depletion was as severe as in any year since the discovery of the ozone hole in the early 1980s, according to B. L. Johnson, T. Deshler and R. A. Thompson of the University of Wyoming, Laramie. Balloon flights from McMurdo Station, which lies at 78 South, revealed severe ozone depletion in October 1991 at altitudes between 12 and 20 kilometres with a 93 per cent loss of ozone between 17 and 18 kilometres.
The minimum amount of ozone above McMurdo, reached on 10 October 1991, was 146 Dobson units — almost exactly the same as the minimum reached in 1987 and 1990. The figure for the column between 12 and 20 kilometres altitude was a record low of 18 Dobson units, representing a loss of 83 per cent of the ozone in that layer.
“The 1991 volcanic eruptions will substantially increase the aerosol loading within the polar vortex for 1992, both below and above 20 kilometres," the team predicts. “These extra particles may enhance the heterogeneous reactions, leading to ozone destruction, and this ozone depletion above Antarctica may worsen in 1992 by spreading to altitudes beyond the 12 to 20 kilometres layer.”
| Year | 1987 | 1991 |
| Altitude | km | km |
| Percentage of loss | % | % |
AI explanationShows the answer. Tap to open.
Question Focus: Finding the altitude layer mentioned for ozone measurements in 1987
Correct Answer: 12-20 km
Why this is correct: For 1987, the table asks about the altitude where ozone was measured. The passage states that the ozone column discussed was between 12 and 20 kilometres, so this matches the table entry.
Passage Reference: The minimum amount of ozone above McMurdo, reached on 10 October 1991, was 146 Dobson units - almost exactly the same as the minimum reached in 1987 and 1990. The figure for the column between 12 and 20 kilometres altitude was a record low of 18 Dobson units, representing a loss of 83 per cent of the ozone in that layer.
Common Mistake: Students may choose 17-18 kilometres because it appears nearby in the passage, but that refers to a specific loss layer in 1991, not the general column for 1987. This is a wrong section trap.
Paraphrasing: Passage: 'column between 12 and 20 kilometres altitude' = Table: 'Altitude'
Tips: When filling tables, always check which year or column the gap belongs to before choosing numbers.
Key Vocabulary:
- altitude = height
- column = vertical layer
Question Focus: Identifying the specific altitude with the greatest ozone loss in 1991
Correct Answer: 17-18 km
Passage Reference: Balloon flights from McMurdo Station, which lies at 78 South, revealed severe ozone depletion in October 1991 at altitudes between 12 and 20 kilometres with a 93 per cent loss of ozone between 17 and 18 kilometres.
Why this is correct: For 1991, the passage clearly states that the most severe ozone loss happened between 17 and 18 kilometres. This is the exact altitude range needed for the table.
Common Mistake: Some students write 12-20 kilometres because it appears earlier, but that describes a wider layer, not the most severe loss. This is a partial truth trap.
Paraphrasing: Passage: 'between 17 and 18 kilometres' = Table: 'Altitude (1991)'
Tips: Look for phrases like 'revealed severe ozone depletion' to find key data points.
Key Vocabulary:
- severe = very serious
- depletion = loss
Question Focus: Percentage of ozone loss for 1987
Correct Answer: 83%
Passage Reference: The figure for the column between 12 and 20 kilometres altitude was a record low of 18 Dobson units, representing a loss of 83 per cent of the ozone in that layer.
Why this is correct: The passage states that the ozone loss for this column was 83 per cent. This directly matches the percentage required for 1987 in the table.
Common Mistake: Students may choose 93 per cent, but that percentage refers to a different altitude and year. This is a context shift trap.
Paraphrasing: Passage: 'loss of 83 per cent' = Table: 'Percentage of loss (1987)'
Tips: Always match percentages carefully with the correct year and altitude.
Key Vocabulary:
- representing = showing
- loss = reduction
Question Focus: Percentage of ozone loss for the most depleted layer in 1991
Correct Answer: 93
Passage Reference: revealed severe ozone depletion in October 1991 at altitudes between 12 and 20 kilometres with a 93 per cent loss of ozone between 17 and 18 kilometres.
Why this is correct: The passage says there was a 93 per cent loss of ozone at this altitude in 1991. This is exactly what the table is asking for.
Common Mistake: Some students write 83 per cent, confusing it with the column loss figure. This is a wrong section trap.
Paraphrasing: Passage: '93 per cent loss of ozone' = Table: 'Percentage of loss (1991)'
Tips: If two numbers look similar, re-read the sentence to check which one matches the exact detail asked.
Key Vocabulary:
- loss = decrease
- revealed = showed
General Tips:
- Summary and table completion questions follow passage order. Use earlier answers to help locate later ones.
- Always copy numbers and units exactly as they appear in the passage.
- Check which year, location, or condition the table column refers to before choosing an answer.
- Percentages and measurements are common traps. Double-check you are matching the correct figure to the correct category.
- Keep an eye on the word limit. Numbers usually count as one word, but extra words will make the answer wrong.
The rest of this explanation is for members. Sign in free to read why the other options are wrong, key words and tips.
Sign in to read the full explanation