Final answer:
Methane gas is the least likely pollutant emitted from smokestacks; instead, emissions typically include sulfur oxides, nitrogen oxides, carbon monoxide, carbon dioxide, and particulate matter.
Step-by-step explanation:
The pollutant least likely to be emitted from a smokestack is methane gas. Smokestack emissions primarily include sulfur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO₂), and particulate matter. Methane is a relatively minor component of smokestack emissions because it is typically released from other sources such as agriculture and natural gas systems, rather than the combustion processes at work in smokestacks.
Sulfur oxides and nitrogen oxides are significant pollutants emitted from the combustion of fossil fuels in power plants and industrial facilities, causing acid rain and contributing to respiratory issues. Carbon monoxide and carbon dioxide are produced by the incomplete and complete combustion of carbon-containing fuels respectively. Particulate matter can originate from various combustion processes, including those within smokestacks, affecting respiratory health.