In environmental chemistry, which metal is a major air pollutant in big cities, mainly due to vehicle emissions and industrial activities?

Introduction / Context:
Air pollution in large cities is caused by a mixture of gases and particulate matter, including harmful metals. Certain metals are especially dangerous because they can accumulate in the environment and in living organisms. Historically, lead has been one of the most important toxic metals associated with urban air pollution, particularly when leaded petrol was widely used. This question tests whether you can identify the metal that has been a notorious air pollutant in big cities around the world.

Given Data / Assumptions:
- The question asks which metal pollutes the air of big cities. - Metals listed include chromium, lead, copper, and zinc. - We consider typical urban pollution sources such as vehicle exhaust and industrial emissions.

Concept / Approach:
For many years, petrol contained tetraethyl lead as an antiknock compound. When petrol was burned in engines, lead particles were released into the air and spread widely through exhaust gases. This made lead a serious urban air pollutant. Even after the phase out of leaded petrol in many countries, lead can still enter the air from industrial processes, smelting, battery recycling, and other activities. Lead is highly toxic to the nervous system, especially in children, and can cause developmental problems. Other metals like chromium, copper, and zinc can also cause pollution, but lead has been particularly associated with big city air.

Step-by-Step Solution:
Step 1: Recall historical sources of metal pollution in cities. Leaded petrol was once widely used in vehicles. Step 2: Understand that burning leaded petrol released lead particles into the air, contributing to high lead levels in urban environments. Step 3: Note that lead continues to be released from some industrial and recycling activities. Step 4: Compare lead with other listed metals. While chromium, copper, and zinc may cause pollution in specific industrial settings, lead is most prominently known as a widespread urban air pollutant. Step 5: Conclude that lead is the correct answer.

Verification / Alternative check:
Environmental reports and public health studies from the twentieth century often highlight high blood lead levels in children living near busy roads or industrial zones. These findings were strongly linked to airborne lead originating from vehicle exhaust. Many countries introduced unleaded petrol and strict regulations to address this issue. Even today, monitoring of air quality often includes lead concentrations as an important indicator. These facts confirm that lead is a key metal pollutant in big cities.

Why Other Options Are Wrong:
Option A (Chromium) can be a pollutant near certain industries such as electroplating and tanning, but it is not as universally recognised as the main urban air pollutant as lead is. Option C (Copper) is used in electrical wiring and alloys, and although it can appear in industrial waste, it is not the classic metal associated with widespread city air pollution. Option D (Zinc) is present in some industrial emissions and corrosion processes but is again not the primary metal pollutant historically linked with vehicle exhaust in big cities.

Common Pitfalls:
Students sometimes confuse water pollution and air pollution when thinking about metals. They might recall chromium or other metals from discussions of industrial wastewater rather than urban air. Another pitfall is focusing only on present day conditions and ignoring the historical impact of leaded petrol. For general knowledge examinations, lead remains the textbook answer because of its long history as a major urban air pollutant and its serious health effects, especially on children.

Final Answer:
The correct answer is: Lead.