NEW DELHI: A new study suggests that fine particulate matter (PM1) in Delhi may be absorbing moisture from the humid winter air, leading to an underestimation of pollution levels by up to 20%.
The research, published in npj Clean Air, highlights how hygroscopic growth, the swelling of particles due to moisture absorption, reduces the efficiency of pollution monitoring devices, potentially distorting air quality assessments. Conducted by researchers from the University of Birmingham, UK, the study addresses a key gap in understanding PM1 pollution in New Delhi, the world’s most polluted megacity.
According to the 2024 World Air Quality report by IQAir, the Swiss air quality organisation, New Delhi remains the world’s most polluted capital city. PM1 particles, which are smaller than a micron in diameter, pose severe health risks as they can infiltrate lung tissue and enter the bloodstream. Lead author Ying Chen emphasised the need to account for hygroscopic growth when assessing pollution levels, especially in humid conditions. The research found that the most significant underestimation occurs during winter morning rush hours in December and January when humidity is the highest.
During these peak hours, PM1 levels may be underestimated by up to 50 micrograms per cubic metre on average. In contrast, pollution estimates during the monsoon season (July-September) show minimal bias.
The researchers explained that heavy rainfall washes out the swollen pollutant particles, preventing underestimation. Similarly, during the driest summer months (April-June), the lack of humidity limits particle swelling, leading to more accurate pollution measurements.
The study also found that the second-largest underestimation, around 8.5%, occurs during spring mornings (February-March), when humidity levels hover around 80%.
However, bias remains negligible in both the monsoon and summer months.
