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

Hygroscopicity of aerosol (κchem) 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 κchem 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 κchem in Delhi and discuss its climate implications. The bulk‐averaged κchem 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 κchem 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 (κchem) is a key factor affecting its direct and indirect climate effects; however, long‐term observation of κchem in Delhi is absent. Here we demonstrate an approach to retrieve κchem from publicly available data sets of PM2.5 and meteorology and report the first long‐term estimation of κchem in Delhi is 0.42 ± 0.07 during 2016–2018. This value indicates only a supersaturation of ~0.18% ± 0.015% is required to activate a particle with 0.1‐μm diameter in Delhi, in contrast 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 κchem to represent Delhi aerosols would lead to a significant underestimation of aerosol climate effects. Key Points: For the first time, we estimate the hygroscopicity of aerosols in Delhi (κ = 0.42 ± 0.07), based on 3‐year (2016‐2018) ground observationsHygroscopicity of aerosols in Delhi is much higher than Beijing and Asian average, therefore leading to remarkable climate effectsWe demonstrate a valuable method for deriving bulk‐averaged hygroscopicity of aerosol based on publicly available data sets [ABSTRACT FROM AUTHOR]