Question

1. which of the conventional pollutants has decreased in the recent past?

a. carbon monoxide
c. sulfur oxides
b. nitrogen oxides
d. methane

Explanation:

To determine which conventional pollutant has decreased, we analyze each option:

• Option a (carbon monoxide): Due to improved vehicle emission standards, better combustion technologies, and stricter industrial regulations, carbon monoxide emissions from sources like vehicles and industries have decreased significantly in recent years.
• Option b (nitrogen oxides): While efforts are made to reduce NOₓ, their reduction rate is not as high as that of carbon monoxide in many regions, and NOₓ emissions still pose significant challenges (e.g., from vehicle exhaust and industrial processes).
• Option c (sulfur oxides): Regulations like the Clean Air Act (in the US) and similar policies globally have reduced SOₓ, but the decrease in carbon monoxide is more notable in terms of the rate and scale of reduction, especially from mobile sources.
• Option d (methane): Methane is a greenhouse gas, and while some efforts to reduce it exist (e.g., from the oil and gas industry), it is not a conventional pollutant in the same category as the others here, and its emissions (from natural sources and human activities like agriculture) have not shown the same consistent decrease as carbon monoxide from anthropogenic sources.

Among these, carbon monoxide has seen a more significant decrease in the recent past due to advancements in emission control for mobile and stationary sources.

Answer:

a. carbon monoxide