Question

The concentration of which air pollutant is commonly measured in parts per billion (ppb)?

• A. Carbon Monoxide (CO)
• B. Sulphur Dioxide (\(SO_2\))
• C. Nitrogen Dioxide (\(NO_2\))
• D. Lead (Pb)

Hint:

Air pollutants like \(NO_2\) and \(SO_2\), which exist in trace amounts, are often measured in parts per billion (ppb) for precision in air quality monitoring. In contrast, pollutants like CO, which have higher concentrations, are typically measured in ppm, while heavy metals like lead are reported in µg/m³.

Answer 1

The Correct Option is C

Step 1: Identify the given data.
The question asks which air pollutant’s concentration is commonly measured in parts per billion (ppb). The options are:
(1) Carbon Monoxide (CO)
(2) Sulphur Dioxide (\(SO_2\))
(3) Nitrogen Dioxide (\(NO_2\))
(4) Lead (Pb)
Step 2: Recall the units used for measuring air pollutants.
Air pollutant concentrations are measured in various units depending on the pollutant and its typical concentration in the atmosphere:
Parts per million (ppm): Used for pollutants present in higher concentrations.
Parts per billion (ppb): Used for pollutants present in trace amounts.
Micrograms per cubic meter (µg/m³): Used for particulate matter and some heavy metals.
We need to determine which pollutant is typically measured in ppb.
Step 3: Analyze each pollutant.
Option 1: Carbon Monoxide (CO)
CO is a common air pollutant from vehicle exhausts and combustion processes. Its concentration in ambient air is typically higher, often in the range of 1–10 ppm in urban areas. Air quality standards (e.g., WHO, EPA) usually report CO in ppm (e.g., 9 ppm for an 8-hour average), not ppb.
Option 2: Sulphur Dioxide (\(SO_2\))
\(SO_2\) is emitted from industrial processes and burning fossil fuels. Its ambient concentration is relatively low, often in the range of 10–100 ppb, but air quality standards sometimes report it in ppb or µg/m³. For example, the WHO guideline for \(SO_2\) is 20 µg/m³ (approximately 7.6 ppb) for a 24-hour mean, so ppb is used, but it’s also commonly reported in µg/m³.
Option 3: Nitrogen Dioxide (\(NO_2\))
\(NO_2\), a key pollutant from vehicle emissions and power plants, is present in trace amounts in the atmosphere, typically 10–100 ppb in urban areas. Air quality standards commonly report \(NO_2\) in ppb. For example, the WHO guideline for \(NO_2\) is 200 µg/m³ for a 1-hour mean, which is approximately 106 ppb, and annual averages are often 10–40 ppb. \(NO_2\) is frequently measured and reported in ppb, making it a strong candidate.
Option 4: Lead (Pb)
Lead in the air (e.g., from historical use of leaded gasoline) is typically measured as a particulate (PM) in µg/m³, not ppb. For example, the WHO guideline for lead is 0.5 µg/m³ (annual mean). Since lead is a heavy metal, its concentration is not typically expressed in ppb.
Step 4: Determine the pollutant commonly measured in ppb.
Both \(SO_2\) and \(NO_2\) can be measured in ppb, but \(NO_2\) is more commonly reported in ppb in air quality monitoring due to its lower ambient concentrations and the sensitivity required for its measurement. CO is typically in ppm, and lead is in µg/m³.
Step 5: Select the correct option.
Nitrogen Dioxide (\(NO_2\)) is commonly measured in parts per billion (ppb), matching option (3).
\[ \boxed{\text{Nitrogen Dioxide (NO}_2\text{)}} \]