Anthropogenic Emissions in Coastal India Strongly Influence New Particle Formation and Cloud Condensation Nuclei Activity.

Aerosols influence Earth's energy balance and hydrological cycle as cloud condensation nuclei (CCN), yet uncertainties persist in how anthropogenic emissions alter their abundance and climate-relevant properties. Abrupt, large-scale reductions in human activities provided a natural experiment to quantify anthropogenic impact on aerosol-cloud-climate interactions in coastal India. Combining chemical and microphysical measurements under drastically reduced and subsequently reintroduced emission scenarios, we reveal that CCN concentrations increased by 80-250% postlockdown. This surge coincided with increased new particle formation (NPF) event frequency and enhanced particle growth rates. Postlockdown air masses shifted from marine to continental sources, revealing that anthropogenic organic matter (OM), despite lower hygroscopicity, dominated particle growth to CCN-active sizes, offsetting hygroscopicity limitations. These findings demonstrate how shifts in anthropogenic activity can strongly impact aerosol-cloud interaction potential, even under varying air mass influences, and provide a reference for understanding the atmospheric effects of future air quality interventions.