Influence of meteorological variability on aerosol size distribution during the winter fog campaign over Delhi: a case study

The aerosol size distribution, particularly the number and mass distributions, plays a crucial role in understanding changes in optical properties due to hygroscopic growth, which affects visibility and radiative forcing on a regional scale. The Indo-Gangetic Plain (IGP), including National Capital Region (NCR) of Delhi, experiences severe fog and haze with reduced visibility during the post-monsoon to winter months (October–February) every year. This study reports aerosol mass size distribution over Delhi during a winter fog campaign (December 15, 2015–February 15, 2016) using a ground-based optical particle counter. The fine and coarse mode aerosols were contributed to ∼85% and 15% to the total aerosol mass concentration during the campaign period. The characteristic changes in aerosol size distribution, effective radius, and the influence of meteorological factors, particularly relative humidity (RH) and temperature, under three visibility conditions: Vis-1 (<600 m), Vis-2 (600–1200 m), and Vis-3 (>1200 m) were investigated. Fine-mode aerosols accounted for ∼85 % of the total aerosol mass, with their concentration increasing by a factor of 3.7 during Vis-1 and 2.3 during Vis-2 compared to Vis-3, when the effective radius of aerosol was lowest (R_eff: 0.44 μm). Fine particle concentrations showed a positive correlation with RH (R = 0.35) and a negative correlation with visibility (R = −0.65), suggesting that the high RH and fine-mode aerosols contribute to fog formation and reduced visibility in Delhi-NCR.