Significant Climate Impact of Highly Hygroscopic Atmospheric Aerosols in Delhi, India

Hygroscopicity of aerosol (κ _chem) is a key factor affecting its direct and indirect climate effects, however, long-term observation in Delhi is absent. Here we demonstrate an approach to derive κ _chem from publicly available data sets and validate it (bias of 5%–30%) with long-term observations in Beijing. Using this approach, we report the first estimation of κ _chem in Delhi and discuss its climate implications. The bulk-averaged κ _chem of aerosols in Delhi is estimated to be 0.42 ± 0.07 during 2016–2018, implying a higher activation ability as cloud condensation nuclei in Delhi compared with Beijing and continental averages worldwide. To activate a 0.1-μm particle, it averagely requires just a supersaturation of ~0.18% ± 0.015% in Delhi but ~0.3% (Beijing), 0.28%–0.31% (Asia, Africa, and South America) and ~0.22% (Europe and North America). Our results imply that representing κ _chem of Delhi using Asian/Beijing average may result in a significant underestimation of aerosol climate effects.