Accurately quantifying aerosols' role in climate forcing is crucial for understanding anthropogenic impacts. Aerosol climate forcing, particularly its influence on cloud albedo, depends on both anthropogenic and natural aerosol concentrations. Under cleaner conditions, cloud condensation nuclei (CCN) concentration and cloud albedo are more sensitive to anthropogenic emissions increases. However, our understanding relies heavily on modeling due to pervasive pollution, especially in India. The COVID-19 lockdown in India provided a rare opportunity to study aerosol behavior under relatively cleaner conditions with significantly reduced emissions from traffic and industrial sources. The Neyveli coal-fired power plant in southern India continued full operation during the lockdown, offering a chance to isolate its impact on aerosol formation and growth, particularly concerning cloud-forming potential, with minimal interference from other anthropogenic sources.

Existing literature highlights the importance of understanding aerosol-cloud interactions and their impact on climate. Studies have emphasized the challenges in accurately quantifying pre-industrial aerosol concentrations and the resulting uncertainties in climate forcing estimations. Research into aerosol-cloud interactions has shown the non-linear response of cloud properties to aerosol concentration, highlighting the need for measurements under varying conditions. Previous studies on new particle formation events have demonstrated the role of different precursor emissions, aerosol formation and growth, and their impact on CCN concentrations. The COVID-19 lockdown provided a unique opportunity to observe aerosol behavior under relatively cleaner conditions, which is rarely available in highly polluted regions like India.

Aerosol measurements were conducted in Chennai, India, between April 11 and May 6, 2020, during the COVID-19 lockdown. The study site experienced a substantial reduction in local pollution due to the lockdown, while the Neyveli coal-fired power plant (approximately 200 km south) continued full operation. Comprehensive measurements included: * **Non-Refractory PM1 (NR-PM1) and chemical composition:** An Aerosol Chemical Speciation Monitor (ACSM) measured the chemical composition of NR-PM1, including organics, sulfate, nitrate, ammonium, and chloride. * **Particle number-size distribution and PM0.5 mass derivation:** A Scanning Mobility Particle Sizer (SMPS) measured the aerosol number size distribution, from which PM0.5 mass concentration was derived. * **Size-resolved CCN measurements:** A continuous flow stream-wise thermal gradient CCN counter (CCNC) provided size-resolved CCN measurements, allowing for the calculation of the hygroscopicity parameter (κ). * **Black carbon measurements:** An Aethalometer (AE-33) measured black carbon (BC) concentrations. * **Meteorological data:** Automatic weather station data and backward trajectories were used to understand the meteorological conditions during the event. * **Source identification:** The Stochastic Time-Inverted Lagrangian Transport (STILT) model and HYSPLIT dispersion modeling were used to investigate the origin of the sulfate particles observed. TROPOMI satellite imagery was also used to provide information on the distribution of SO2. Detailed descriptions of the measurement site, data analysis techniques and the calibration procedures are provided in the supplementary information.

On May 1, 2020, a significant new particle formation (NPF) and rapid particle growth event was observed in Chennai. This event coincided with a shift in wind direction, bringing air masses from the south, which had passed over the Neyveli power plant. The following key findings were observed: * **High sulfate concentration:** A marked increase in sulfate concentration in PM1 was observed during the event, significantly exceeding the organic fraction, contrary to typical observations in heavily polluted environments. * **Rapid particle growth:** Ultrafine particles rapidly grew to CCN-active sizes (50-100 nm and above), with growth rates exceeding those reported in previous studies. * **High cloud-forming potential:** The sulfate-rich particles exhibited high hygroscopicity (κ), and significantly increased CCN concentrations across a wide range of supersaturations. * **Source attribution:** Back-trajectory analysis, STILT modeling, and TROPOMI satellite data indicated the Neyveli power plant as the most likely source of the high sulfate concentration, showing that the SO2 plume directly impacted the observation site. * **Impact of reduced emissions:** The reduced concentration of pre-existing aerosol particles during the lockdown was likely a major factor in the enhanced NPF and growth observed, indicating that under cleaner conditions, SO2 emissions from power plants can have a disproportionately significant impact on aerosol formation and cloud formation. Specific quantitative data points included high sulfate concentrations (reaching approximately 60% in NR-PM1), high hygroscopicity parameter (κ) values (reaching 0.53 at Seff = 0.15%), and rapid growth rates of newly formed particles (approximately 20 nm/h for particles above 50 nm). These findings are extensively illustrated with figures showing time series of various aerosol properties, size distributions, and spatial modeling results.

The findings demonstrate the significant impact of SO2 emissions from coal-fired power plants on aerosol formation and cloud-forming potential, particularly under conditions of reduced anthropogenic emissions. The high sulfate concentrations and rapid particle growth observed highlight the importance of considering the specific impacts of point sources like power plants even during periods of relatively lower overall pollution. The study's results highlight the potential for significant enhancements in CCN activity from power plant plumes, even in a relatively clean background environment. The unusual dominance of sulfate in the aerosol growth during this event indicates a shift in aerosol formation mechanisms under the cleaner conditions created by the lockdown. The observed rapid growth of sulfate-rich particles to CCN sizes highlights the potentially large impact of power plant emissions on cloud formation processes and subsequent climate impacts, suggesting that policies aimed at reducing PM2.5 pollution need to carefully consider the effects of point source emissions on aerosol growth.

This study presents direct observational evidence highlighting the significant impact of SO2 emissions from coal-fired power plants on CCN activity, particularly under conditions of reduced anthropogenic emissions. The rapid particle growth event associated with high sulfate concentration demonstrates the importance of considering power plant emissions when assessing their contribution to cloud formation and climate forcing. The findings suggest that reducing SO2 emissions from these sources could have a disproportionately large impact on aerosol properties and cloud formation. Further research should explore the influence of various meteorological factors on such NPF events and investigate the impact on regional and global climate.

The study is a case study based on measurements during a specific period and location. The generalizability of the findings to other regions or times of the year may be limited. The inability to directly measure the chemical composition of particles smaller than ~50 nm using the ACSM prevents a complete understanding of nucleation-mode particle composition. The study focuses on a specific power plant and may not represent all coal-fired power plants.