Hygroscopic growth causes fine particulate matter (PM1) to swell, reducing sampling devices efficiency and leading to underestimation, with greatest underestimation in estimated concentrations happening during winter morning rush hours, when humidity is highest and pollution is most severe.

In contrast, research shows that the monsoon season shows negligible bias due to the washout of hygroscopic particles by heavy rainfall -- the bias is more pronounced during high pollution episodes, with underestimation increasing exponentially with humidity.

Publishing the findings today (12 Mar) inNPJ Clean Air, study author Dr Ying Chen, from the University of Birmingham, concludes that PM1 pollution in the city is worse than previously thought, but provides correction tools for future studies to better estimate particulate levels.

Dr Chen commented: "This study highlights the true extent of air pollution in New Delhi and offers a framework for more accurate future assessments that better inform public health strategies and mitigation efforts. When carrying out air quality assessments, it is crucial to consider hygroscopic growth and the potential for significant underestimation of pollution levels in humid conditions."

Biased measurements due to hygroscopic growth are significant in New Delhi due to the high-water content of its aerosol particles, which can contain up to 740 µg/m³ of water and is the highest in megacities all over the world.

The study suggests that controlling emissions from biomass burning and residential sources, which emits highly hygroscopic chlorine species, could effectively improve air quality and reduce this bias, therefore improve air quality understanding as well. Dr Chen calls for more in-situ observations of PM2.5 and PM10 to gain a more rounded understanding of air pollution in New Delhi.

There are several seasonal variations identified in the study:

• Winter (December to January): The most significant underestimation (up to 20%) occurs during the morning rush hour (8-9 am) due to high humidity (90%) and a shallow planetary boundary layer -- the lowest part of the atmosphere, where the surface of the Earth affects the wind, temperature, and moisture.

• Spring (February to March): The second highest variance occurs during the morning rush hour with an average relative humidity (RH) of 80%, leading to an 8.6% underestimation.

• Monsoon (July to September): Despite high humidity (85%), bias is minimal due to the washout of hygroscopic particles by frequent rain.

• Summer (April to June): The driest season with RH between 28% and 50%, resulting in negligible bias due to limited hygroscopic growth.

New Delhi is recognized as the world's most polluted capital city, with severe air quality issues threatening the health of its 33 million residents. Particulate matter is the dominant pollutant, responsible for approximately 10,000 premature deaths annually. The World Health Organization (WHO) reports that New Delhi's PM1 levels are 24 times higher than recommended safe levels, even though based on the underestimated observations.

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