Modelling of Haze Episode over Delhi Region using a Chemistry-Transport Model: Contribution of Emission and Meteorology

Haze episodes are often shrouded over Delhi-NCR and most of the Northern Indo-Gangetic Plain during the Post-Monsoon and Winter seasons, posing a huge negative burden on human health, the environment, and the economy. The combined influence of local emissions, meteorology, regional transport, and topography results in the complex chemical mechanism, which is difficult to predict based on observational studies, emphasizing the necessity of modelling the episodic haze in a chemistry-transport model. The present study adopted Weather Research and Forecasting (WRF) model, coupled with the CHIMERE chemistry-transport model, to simulate the haze event that happened in November 2019 over Delhi, to understand the chemical mechanism involved and to quantify the contribution of emissions and meteorology towards the increase of PM_2.5 concentration. Temporal variations of the modelled PM_2.5, air quality and meteorology variables are in good agreement with the observed data. Correlation coefficients (R) between simulated and observed values were larger than 0.7, and the normalized mean biases (NMB) were within ± 30%. The evaluations indicate that WRF-CHIMERE is able to capture the trends of haze events. Major fraction of PM_2.5 during the haze was comprised of Organic Matter (OM) followed by secondary inorganic aerosols. It was also found that although OM was high in concentration, the rate of increase of nitrates was higher than OM, indicating an important role of inorganic aerosols in high PM_2.5 concentration. In addition, sensitivity simulations revealed that anthropogenic emissions had a significant contribution for high particulates during the episode. Therefore, adopting the anthropogenic source emissions control startegy could be an effective control measure for reducing the severity of PM_2.5 pollution over Delhi region.