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Hygroscopicity of aerosol (kappa(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 kappa(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 kappa(chem) in Delhi and discuss its climate implications. The bulk-averaged kappa(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-mu m particle, it averagely requires just a supersaturation of similar to 0.18% +/- 0.015% in Delhi but similar to 0.3% (Beijing), 0.28%-0.31% (Asia, Africa, and South America) and similar to 0.22% (Europe and North America). Our results imply that representing kappa(chem) of Delhi using Asian/Beijing average may result in a significant underestimation of aerosol climate effects.

Plain Language summary Hygroscopic water uptake of aerosols can enhance its light extinction and cloud activation. Therefore, hygroscopicity of aerosol (kappa(chem)) is a key factor affecting its direct and indirect climate effects; however, long-term observation of kappa(chem) in Delhi is absent. Here we demonstrate an approach to retrieve kappa(chem) from publicly available data sets of PM 2.5 and meteorology and report the first long-term estimation of kappa(chem) in Delhi is 0.42 +/- 0.07 during 2016-2018. This value indicates only a supersaturation of similar to 0.18% +/- 0.015% is required to activate a particle with 0.1-mu m diameter in Delhi, in contrast to similar to 0.3% supersaturation is required for Beijing and Asian average. It implies a higher water uptake and cloud activation ability for Delhi aerosols. Therefore, using Asian/Beijing averaged kappa(chem) to represent Delhi aerosols would lead to a significant underestimation of aerosol climate effects.