Which of the following is used to measure and monitor air pollution?

Concept — RADAR, SONAR, LIDAR and SODAR are all active remote-sensing techniques built on a single idea: transmit a pulse of energy, wait for the fraction that comes back, and read the delay as distance. What separates them is the carrier — the kind of wave that is sent out — because a carrier travels only through a medium that supports it, and it reveals only those constituents its own physics lets it interact with. Sound is returned by density and temperature structure in a fluid; radio waves are returned by objects large compared with a raindrop; light is scattered by suspended particles and is absorbed in the characteristic bands of individual gas molecules. The carrier therefore fixes both where an instrument can operate and what it is able to see.

Application — Air pollution consists of fine particulate matter a few micrometres across (PM10 and PM2.5) together with trace gas molecules dispersed through the atmosphere. Sensing it therefore demands a carrier that propagates through air and that interacts with both of those constituents.

1. The medium is air, so the carrier must propagate through the atmosphere rather than through water.

2. Particulate matter is a few micrometres across, comparable to the wavelength of ultraviolet, visible and infrared light (roughly 0.25 to 10 micrometres). Light is therefore scattered strongly by such particles, and the strength of that scattering tracks how much of them the beam passed through.

3. Trace gases are sensed by a different mechanism. Each gas absorbs light in its own characteristic wavelength bands, so the amount of light removed from a beam at one of those wavelengths measures how much of that gas lies along the path.

4. LIDAR (Light Detection And Ranging) exploits both effects. It fires laser pulses skyward and records the light returned along the beam; the delay of each return fixes the height of the layer that produced it, and the strength of that return fixes the particulate loading at that height, giving a vertical profile of the aerosol column.

5. In the differential-absorption configuration (DIAL), two laser wavelengths are fired — one on a gas absorption line and one just off it — and the ratio of the two returns gives the concentration of that gas, such as ozone, sulphur dioxide or nitrogen dioxide, layer by layer.

Cross-check — Setting the four carriers side by side shows which of them can reach an airborne pollutant at all:

Result — LIDAR is the technique used to measure and monitor air pollution, because it is the only one of the four that both travels through air and interacts directly with the pollutants themselves — scattering off suspended particles and being absorbed by pollutant gas molecules. SODAR is the nearest miss: it also works in the atmosphere, but its acoustic return responds to temperature and turbulence structure, so it reports the wind and mixing-layer conditions that disperse pollution rather than the pollutant load itself. The National Testing Agency final answer key for UGC NET Paper 1, 27 August 2024 Shift 2, marks LIDAR.