In recent years, there has been some concern over the threat posed by the Mathura Oil Refinery and the thermal power plants to the Taj Mahal in Agra. The scientific basis of any possible damage to the Taj is mainly

The principal documented causes of Taj Mahal deterioration are acidic deposition from sulphur and nitrogen oxides and oxidant pollution (ground‑level ozone). Acid rain chemically attacks calcium carbonate in marble, producing gypsum and sulfates that lead to surface erosion, black crusts, and flaking — directly implicated in damage to monuments like the Taj Mahal [2]. In addition, tropospheric (ground‑level) ozone, formed from NOx and VOCs emitted by vehicles, refineries and power plants, is a reactive oxidant that contributes to surface soiling and material degradation and is a recognized component of urban smog affecting outdoor materials [4]. These mechanisms match option (2) — acid precipitation and tropospheric ozone.

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Impact of Acid Rain on Precipitation > p. 10
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > f) Acid rain damage on Materials > p. 105
3. [3] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > Ozone. > p. 64
4. [4] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 19: Ozone Depletion > Effects on air quatity > p. 272