Understanding the role of atmospheric circulations and dispersion of air pollution associated with extreme smog events over South Asian megacity.

The winter fog/haze events in northeastern Pakistan and surrounding regions of India are often mixed with pollutants to form smog, and consequently damage human health and hampers daily life in the form of fatalities through road accidents, road blockages, and flight delays. The persistent anti-cyclonic conditions can further trigger the temperature inversion and prolong the smog event from days to weeks. The present study provides characteristics and lasting mechanisms of two persistent winter fog events (2016-2017) in Lahore, Pakistan, by using the fifth generation of European Center for Medium-Range Weather Forecast (ECMWF) ERA5 reanalysis data and National Oceanic and Atmospheric Administration (NOAA) Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model simulated with Global Data Assimilation System (GDAS) meteorological data. The results showed the presence of strong low-level anti-cyclonic circulations with wind speed less than 1.5 m/s from November to January over Eastern Punjab for two foggy winter seasons. The deep inversion during the fog events was observed that prevented the natural ventilation of air in the upper atmosphere and ultimately the smoke and heavy pollutant accumulated in the lower atmosphere. Furthermore, high relative humidity greater than 83% near the ground indicates a high condensation rate for water vapors to form fog near the ground. The analysis of the NOAA HYSPLIT trajectory model at different vertical heights revealed that smoke from stubble crop burning in the first week of November 2017 in Punjab and Haryana mixed with fog under favorable stable conditions that lead to intense smog over Lahore. This study will help to understand and to develop a forecasting mechanism of fog events by characterizing the meteorological conditions of the study area and to minimize the adverse impacts of smog on public health.