1. Conceptual Framework & CPCB AQI Architecture

A. Definition & Core Objective

National Air Quality Index (AQI) is a simplified system used to communicate the overall status of ambient air quality to the public.

• Launched: October 2014

• Developed by: Ministry of Environment, Forest and Climate Change (MoEFCC) and Central Pollution Control Board (CPCB)

• Purpose: To convert complex air-quality data from multiple pollutants into:

  • One Number

  • One Colour

  • One Description

The AQI therefore converts technical monitoring data into an easily understandable indication of air quality and associated health effects. CPCB's AQI system considers 8 pollutants.

B. Basic AQI System Architecture

8 Tracked Pollutants
        ↓
Measure pollutant concentrations
        ↓
Calculate individual sub-index for each pollutant
        ↓
Use CPCB concentration breakpoints
        ↓
Linear interpolation
        ↓
Compare all sub-indices
        ↓
Highest sub-index = Overall AQI
        ↓
AQI Category + Colour + Health Description

C. Minimum Data Requirement

For calculation of the overall AQI:

• Data for a minimum of 3 pollutants must be available.

• At least one pollutant must be either PM₂.₅ or PM₁₀.

• Otherwise, the data are considered insufficient for AQI calculation.

• CPCB documentation also specifies a minimum of 16 hours of data for calculating a pollutant sub-index.

Exam Trick:

Minimum 3 pollutants + at least 1 particulate pollutant (PM₂.₅ or PM₁₀).

2. The 8 Pollutants Used in National AQI

The National AQI uses eight pollutants for which short-term ambient air-quality standards are prescribed.

Important Averaging-Time Pattern

24-hour pollutants:

PM₁₀, PM₂.₅, NO₂, SO₂, NH₃ and Pb

8-hour pollutants:

CO and O₃

Important Exclusion

CO₂ and CH₄ are NOT included in the National AQI calculation.

They are primarily important as greenhouse gases in the context of climate change rather than as parameters in the CPCB National AQI system.

3. CPCB AQI Categories

The National AQI has 6 categories, covering a scale of 0–500. CPCB uses these categories to communicate the likely health impact associated with different levels of air pollution.

A. AQI Category Spectrum

0       50        100        200        300        400        500
│────────│─────────│──────────│──────────│──────────│──────────│
  Good     Satisfactory   Moderate     Poor     Very Poor    Severe

 Green      Light Green    Yellow      Orange      Red       Dark Red

B. AQI Categories and Health Effects

Memory sequence:

Good → Satisfactory → Moderate → Poor → Very Poor → Severe

Green → Light Green → Yellow → Orange → Red → Dark Red

4. CPCB AQI Breakpoint Concentration Matrix

A breakpoint is a concentration value corresponding to a particular AQI value.

The measured concentration of a pollutant is first located between two concentration breakpoints. Linear interpolation is then used to calculate its sub-index.

CPCB AQI Breakpoints

Note: The O₃ values marked with * involve the special treatment specified in the CPCB AQI documentation for mathematical calculation.

Important

The above values are concentration breakpoints, not the AQI values themselves.

For example:

PM₂.₅ = 75 µg/m³

falls within:

61–90 µg/m³ → Moderate AQI band → AQI 101–200

The exact sub-index must then be calculated by interpolation.

5. Core Mathematical Framework

The AQI calculation has two major steps:

Pollutant concentration
        ↓
Find corresponding concentration breakpoint
        ↓
Calculate pollutant sub-index
        ↓
Repeat for available pollutants
        ↓
Highest sub-index
        ↓
Overall AQI

Formula 1: Pollutant Sub-Index

The sub-index is calculated using linear interpolation:

Iᵢ = I_low + [(I_high − I_low) / (C_high − C_low)] × (Cᵢ − C_low)

Where:

• Iᵢ = Sub-index of pollutant i

• Cᵢ = Measured concentration of pollutant i

• C_low = Lower concentration breakpoint

• C_high = Upper concentration breakpoint

• I_low = AQI value corresponding to C_low

• I_high = AQI value corresponding to C_high

How to use the formula

Always follow these steps:

Step 1: Identify the measured concentration.

Step 2: Find the concentration range containing it.

Step 3: Identify the corresponding AQI range.

Step 4: Substitute the four breakpoint values into the interpolation formula.

Step 5: Calculate the pollutant sub-index.

6. Overall AQI

After calculating the sub-index for every available pollutant:

Overall AQI = Maximum of all pollutant sub-indices

Or:

AQI = max(I₁, I₂, I₃, ...)

The pollutant producing the highest sub-index is the dominant/prominent pollutant for that monitoring period.

Example

Suppose:

• PM₂.₅ sub-index = 180

• PM₁₀ sub-index = 235

• NO₂ sub-index = 120

• SO₂ sub-index = 60

Then:

Overall AQI = 235

and

Dominant pollutant = PM₁₀

Key Concept

AQI is NOT the average of pollutant sub-indices.

It is determined by the worst/highest sub-index.

7. GRAP and AQI — Delhi-NCR

The Graded Response Action Plan (GRAP) is a separate air-pollution response mechanism for Delhi-NCR.

For basic exam understanding, the AQI-based stages are commonly remembered as:

Important distinction

AQI category ≠ GRAP stage

They are related, but they are not the same classification system.

For example:

AQI = 336

→ AQI category = Very Poor

→ Delhi-NCR GRAP = Stage II, subject to the applicable GRAP framework and trigger conditions.

8. Solved Numerical Problems

Problem 1: PM₂.₅ Sub-Index

Question

A Continuous Ambient Air Quality Monitoring Station (CAAQMS) records a 24-hour average PM₂.₅ concentration of:

75 µg/m³

Calculate:

1. The relevant CPCB breakpoints

2. PM₂.₅ sub-index

3. AQI category and colour

Solution

Step 1: Identify concentration range

PM₂.₅ = 75 µg/m³

From the CPCB table:

61–90 µg/m³ → AQI 101–200

Therefore:

• C_low = 61 µg/m³

• C_high = 90 µg/m³

• I_low = 101

• I_high = 200

Step 2: Apply interpolation

I_PM₂.₅ = 101 + [(200 − 101) / (90 − 61)] × (75 − 61)

= 101 + (99 / 29) × 14

= 101 + 47.79

= 148.79

Therefore:

PM₂.₅ sub-index ≈ 149

Step 3: Classification

149 lies within:

101–200 → Moderate

Colour:

Yellow

Answer

PM₂.₅ sub-index = 149

Category = Moderate

Colour = Yellow

9. Problem 2: PM₁₀ Sub-Index

Question

A monitoring station records:

PM₁₀ = 300 µg/m³

Calculate the PM₁₀ sub-index.

Solution

Step 1: Identify breakpoint range

PM₁₀ = 300 µg/m³

From the table:

251–350 µg/m³ → AQI 201–300

Therefore:

• C_low = 251 µg/m³

• C_high = 350 µg/m³

• I_low = 201

• I_high = 300

Step 2: Apply interpolation

I_PM₁₀ = 201 + [(300 − 201) / (350 − 251)] × (300 − 251)

= 201 + (99 / 99) × 49

= 201 + 49

= 250

Answer

PM₁₀ sub-index = 250

Category = Poor

Colour = Orange

10. Problem 3: Overall AQI and Dominant Pollutant

Question

The following air-quality measurements are recorded:

Determine:

1. Overall AQI

2. Dominant pollutant

3. AQI category

4. Delhi-NCR GRAP stage

Solution

Step 1: Compare all sub-indices

Sub-indices:

• PM₂.₅ = 331

• PM₁₀ = 336

• NO₂ = 135

• SO₂ = 44

• CO = 118

Therefore:

Overall AQI = maximum sub-index

AQI = max(331, 336, 135, 44, 118)

AQI = 336

Step 2: Identify dominant pollutant

Highest sub-index = 336

Corresponding pollutant = PM₁₀

Therefore:

Dominant pollutant = PM₁₀

Step 3: AQI category

336 falls within:

301–400 → Very Poor

Colour:

Red

Step 4: GRAP

AQI 336 corresponds to:

GRAP Stage II

Answer

Overall AQI = 336

Dominant pollutant = PM₁₀

AQI category = Very Poor

Colour = Red

GRAP stage = Stage II

11. Problem 4: CO Sub-Index

Question

An 8-hour average CO concentration is:

12.0 mg/m³

Calculate the CO sub-index.

Solution

Step 1: Identify concentration range

CO = 12.0 mg/m³

From the CPCB table:

10.1–17.0 mg/m³ → AQI 201–300

Therefore:

• C_low = 10.1 mg/m³

• C_high = 17.0 mg/m³

• I_low = 201

• I_high = 300

Step 2: Apply interpolation

I_CO = 201 + [(300 − 201) / (17.0 − 10.1)] × (12.0 − 10.1)

= 201 + (99 / 6.9) × 1.9

= 201 + 27.26

= 228.26

Therefore:

CO sub-index ≈ 228

Step 3: Classification

228 falls within:

201–300 → Poor

Colour:

Orange

Answer

CO sub-index = 228

Category = Poor

Colour = Orange

12. Quick Reference Sheet

Main Formula

Iᵢ = I_low + [(I_high − I_low) / (C_high − C_low)] × (Cᵢ − C_low)

Overall AQI

AQI = Maximum of all available pollutant sub-indices

13. Six AQI Categories — Memorize This

One-line memory

Good → Satisfactory → Moderate → Poor → Very Poor → Severe

0–50 → 51–100 → 101–200 → 201–300 → 301–400 → 401–500

14. Exam-Focused Rapid Recall

AQI basics

Q. How many pollutants are used in National AQI?
A. 8

Q. Name them.
A. PM₁₀, PM₂.₅, NO₂, SO₂, CO, O₃, NH₃ and Pb

Q. Minimum number of pollutants required?
A. 3

Q. Which particulate pollutant must be present?
A. At least one of PM₂.₅ or PM₁₀

Q. Overall AQI is based on what?
A. Maximum sub-index

Q. Is AQI the average of all pollutant sub-indices?
A. No

Q. Which pollutant becomes the dominant pollutant?
A. The pollutant having the highest sub-index

Q. Which AQI category is 301–400?
A. Very Poor

Q. Which colour represents Very Poor?
A. Red

Q. Which AQI category is 401–500?
A. Severe

Q. Which pollutants use 8-hour averaging?
A. CO and O₃

Q. Which pollutants use 24-hour averaging?
A. PM₁₀, PM₂.₅, NO₂, SO₂, NH₃ and Pb

Q. What mathematical method is used for sub-index calculation?
A. Linear interpolation

Q. What happens if fewer than 3 pollutants are available?
A. AQI cannot be calculated; data are insufficient.

Q. What happens if 3 pollutants are available but neither PM₂.₅ nor PM₁₀ is included?
A. Data are insufficient for overall AQI calculation.