Role of atmospheric aerosols in severe winter fog over Indo Gangetic Plains of India: a case study

Abstract. Winter fog and severe aerosol loading in the boundary layer over north India, especially in the Indo-Gangetic Plain (IGP), cause disruption in the daily lives of millions of people in the region. To understand better the role of atmospheric aerosols on the occurrence, spatial extent, and persistence of winter fog in the IGP, several model simulations have been performed using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem). Results from WRF-Chem represented relative humidity (RH) and fog formation in agreement with observations when using the ERA-Interim reanalysis data as meteorological initial/boundary conditions and soil nudging were applied. WRF-Chem successfully simulates the spatial distribution and magnitude of PM_2.5 when evaluated with observations from the Central Pollution Control Board of India (CPCB) monitoring network. However, the aerosol composition predicted by WRF-Chem was quite different from measurements obtained during the Winter Fog Experiment (WiFEX) in Delhi, with chloride aerosol fraction being strongly underpredicted (~66.6 %). By investigating a fog event on December 23–24, 2017 over central IGP, we found that the aerosol-radiation feedback weakens turbulence, lowers the boundary layer height, and increases PM_2.5 concentrations and RH within the boundary layer. The increase in RH is found to be important for fog formation and it promoted the growth of aerosol size and increased aerosol activation in the polluted environment over IGP. Loss of aerosols through deposition of cloud droplets is found to be a significant aerosol loss process during fog. The internal mixing of absorbing aerosols and hygroscopic growth reduces the single scattering albedo impacting aerosol-radiation feedbacks. Aqueous-phase chemistry increases the PM_2.5 concentrations during fog events which subsequently participates in aerosol-radiation feedback. With aerosol-radiation interaction and aqueous phase chemistry, fog formation began 1–2 hours earlier and caused a longer fog duration than when these processes were not included in the WRF-Chem simulation. These processes were also found to increase RH, stabilize the boundary layer, increase PM_2.5 promoting aerosol activation, and thus increasing the fog water content over IGP. This suggests that the aerosol-radiation feedback and secondary aerosols play an important role in the air quality and in the intensity and lifetime of fog over IGP.