Meteorology Induced New Particle Formation over India - A modelling approach

Atmospheric particles strongly influence air quality, climate, and human health, and understanding how new particles form in the atmosphere is essential for improving predictions of these impacts. New particle formation (NPF) is one of the key processes that creates fresh aerosol particles, but its behaviour varies widely across regions and seasons. Winter conditions, with shallow boundary layers and high aerosol loading, usually suppress NPF. However, in Hyderabad, India, we observed frequent winter NPF events that occur despite these unfavourable conditions occurring on about 30% of the days. This study combines long-term particle size distribution measurements with meteorological reanalysis and high-resolution WRF-Chem simulations to investigate the meteorological drivers behind these winter events.

Particle number size distributions (10–600 nm) were collected at the University of Hyderabad between 2019 and 2022. Large-scale atmospheric conditions were analysed using ERA-5 and MERRA-2 reanalysis, and a detailed WRF-Chem simulation was conducted for a one-week period in February 2020, during which three consecutive NPF events occurred. The reanalysis data show that a persistent high-pressure system developed during these days, producing calm winds, stable conditions, and strong vertical subsidence. Under these conditions, we observed both high aerosol loading and high levels of precursor gases, which helped support NPF even during winter pollution episodes.

WRF-Chem results reveal elevated SO₂ and PM_2.5 concentrations up to 2–3 km altitude on NPF event days. Despite high PM_2.5, the SO₂/PM_2.5 ratio was much higher compared to non-event days, indicating a more favourable chemical environment for nucleation. Importantly, the simulations also show that these favourable meteorological and chemical conditions extend over a large spatial region around Hyderabad covering around 500 Kms, suggesting that winter NPF is not only a local phenomenon but part of a wider regional process. Overall, the findings highlight that winter NPF in Hyderabad is strongly controlled by high-pressure-driven meteorology, vertical subsidence, enhanced precursor availability, and large-scale regional influence. These results improve our understanding of particle formation mechanisms in polluted urban environments.