Air pollution in New Delhi is more severe than observed due to hygroscopicity-induced bias in aerosol sampling.

New Delhi, India, is suffering from one of the worst air quality in the world, estimated to be responsible for 10,000 premature deaths per year. Although the high pollution level of fine particulate matter (PM) in New Delhi has attracted global attention, the true level of PM pollution could still be underestimated due to the inherent sampling bias associated with particle hygroscopic growth. This study compiles a comprehensive in-situ observation dataset from a series of recent studies in New Delhi, to quantify hygroscopicity-induced bias for the first time, and found that the more severe pollution the larger underestimation, and report the underestimate can be up to 20% (or 50 µg/m) of PM concentration on average in humid winter morning rush hours. This study fills in the gap of the understanding of PM pollution in the most polluted megacity in the world, and provides a calibration approach for future studies to develop better understanding of air quality in New Delhi.