Paper 1 · Comprehension
Passage
According to a study published recently in the journal Nature Climate Change, while black carbon has a large effect on snow darkening and the resultant melting of snow, dust particles transported from as far as Saudi Arabia that get deposited in the Western Himalayan Region (WHR) have a large role to play in the melting of snow, particularly at higher elevations. Dust transported as elevated aerosol layers gets deposited at 1 to 5 km elevations, while black carbon emission is mostly a surface phenomenon and influences the melting of snow from the surface to about 3 km elevation. Based on remote sensing data of the spatial distribution of dust aerosol concentration over the Indian subcontinent, and on dust-induced snow albedo reduction over the Himalayas during 2011 to 2016 and simulations, it has been inferred that the relative impacts of dust and black carbon vary with the surface elevation of the snow pack. This is in addition to snow-melt caused by warming due to climate change. Earlier studies have shown that the magnitude of snow mass decrease is about 1 mm per year at 1 km elevation, about 5 mm per year at 4.5 km elevation, and about 3 mm per year at 6 km elevation. Though black carbon has a larger snow albedo darkening effect than dust, due to a larger mass absorption efficiency, the study found that the radiative effects of dust deposited on snow are comparable to black carbon in the WHR at higher elevations. This is mainly because the deposition of dust by mass is 100 to 1000 times more than black carbon. As the elevation increases, the influence of dust becomes greater than black carbon, and this coincides with the maximum intensity of snow melt reduction seen at 3 to 5 km elevation. Between these two, black carbon mainly contributes to snow melt at lower elevations, while dust is the major contributor at higher elevations. Westerlies transport dust particles as elevated aerosol layers at maximum intensities mostly during the pre-monsoon period, and this gets deposited at higher elevations in the WHR. Due to global warming, snow cover at lower elevations in the Himalayas will occur less frequently or totally disappear compared with snow cover at higher elevations. The annual contribution of dust to snow melt will therefore likely increase in future, as the black carbon effect at lower elevations weakens with the dwindling snow pack.
Given below are two statements. Assertion A: The reduction in snow albedo is more in the case of black carbon than dust. Reason R: Black carbon has a higher mass absorption efficiency compared to dust. Choose the correct answer.
ABoth A and R are true and R is the correct explanation of A ✓ Correct
BBoth A and R are true but R is NOT the correct explanation of A
CA is true but R is false
DA is false but R is true
Correct answer: (A) Both A and R are true and R is the correct explanation of A — Both A and R are true and R correctly explains A, so that is the answer.
Explanation
★Both A and R are true and R correctly explains A, so that is the answer.
★The passage says black carbon has a larger snow albedo darkening effect than dust, so assertion A is true.
★It gives the reason as its larger mass absorption efficiency, so reason R is true.
★It states the two together, linking the greater darkening directly to the higher absorption efficiency.
★So R is not merely true but is the actual cause given for A.
★Hence both A and R are true and R is the correct explanation of A.
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