Delhi’s air is being “re-polluted” at roughly 8–11 tonnes of particulate matter every single hour in winter

Delhi’s air is being “re-polluted” at roughly 8–11 tonnes of particulate matter every single hour in winter.

Any solution that does not first slow this replenishment rate is mathematically doomed.

Executive Summary

This white paper presents a quantitative, physics-based analysis of Delhi’s winter air pollution problem. It estimates the total particulate mass suspended over Delhi during severe pollution episodes, calculates the hourly replenishment of fresh emissions, and evaluates the feasibility of large-scale industrial air filtration as a mitigation strategy. The analysis demonstrates that outdoor filtration is mathematically and physically infeasible at the city scale, and that effective air quality improvement is only achievable by reducing emission and resuspension rates.

1. Background and Context

During winter months, Delhi frequently experiences AQI levels above 500. These episodes are driven by high emissions, low wind speeds, temperature inversions, and reduced atmospheric mixing height. Public discourse often suggests large-scale air filtration as a solution. This paper evaluates that idea using first-principles mass balance.

2. Total Particulate Mass in Delhi’s Air

Assumptions used:

• Area of Delhi (NCT): ~1,484 km²

• Effective winter mixing height: 500–1000 m

• Severe winter concentrations: PM10 ≈ 350–700 µg/m³, PM2.5 ≈ 220–450 µg/m³

Estimated suspended mass during severe episodes:

• PM10: ~350–700 tonnes

• PM2.5: ~220–450 tonnes

Total particulate mass in air: ~600–1,100 tonnes

3. Replenishment Speed of Fresh Emissions

Based on winter emission inventories and observed activity patterns:

• PM10 emissions: ~140–200 tonnes/day (≈ 6–8 tonnes/hour)

• PM2.5 emissions: ~45–65 tonnes/day (≈ 2–3 tonnes/hour)

Total replenishment rate: ~8–11 tonnes of particulate matter per hour

This replenishment is continuous and does not include additional regional inflow from NCR or secondary aerosol formation, both of which can further increase observed concentrations.

4. Industrial Filtration Capacity Analysis

Particle removal rate per filtration unit is governed by:

Mass removed per hour = Airflow × Concentration × Capture efficiency

Using a high-end industrial unit (≈ 6000 m³/hour, ≈ 99% efficiency):

• Typical winter PM10 concentration: ~300–450 µg/m³

• Removal per unit: ~1.8–2.7 grams/hour

Units required to remove 8–10 tonnes/hour:

• Approximately 3–6 million high-capacity industrial filtration units

This assumes perfect air mixing, zero recirculation losses, and constant exposure to polluted air, conditions that are not achievable outdoors.

5. Why City-Scale Outdoor Filtration Fails

To merely keep pollution levels constant, removal must match replenishment. For Delhi, this means continuously removing 8–11 tonnes of particles every hour from an open, dynamic atmosphere. This would require filtering billions of cubic metres of air per hour at near-perfect efficiency. Such a system is economically, logistically, and thermodynamically infeasible.

6. What Actually Works

Observed improvements in Delhi’s air quality occur only when replenishment slows. Effective levers include:

• Reduction of emissions at source

• Control of road dust and construction dust

• Vehicle and freight restrictions

• DG set regulation

• Regional coordination across NCR

• Favourable meteorological conditions (rain, wind, higher mixing height)

7. Conclusion

Delhi’s winter air quality crisis is fundamentally a replenishment problem, not a removal problem. Any intervention that does not first reduce the 8–11 tonnes per hour addition of particulate matter will fail regardless of scale or cost. Mathematics and Design Thinking, not ideology, defines the solution space.
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