Performance of high resolution (400 m) PM forecast over Delhi.

This study reports a very high-resolution (400 m grid-spacing) operational air quality forecasting system developed to alert residents of Delhi and the National Capital Region (NCR) about forthcoming acute air pollution episodes. Such a high-resolution system has been developed for the first time and is evaluated during October 2019-February 2020. The system assimilates near real-time aerosol observations from in situ and space-borne platform in the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to produce a 72-h forecast daily in a dynamical downscaling framework. The assimilation of aerosol optical depth and surface PM observations improves the initial condition for surface PM by about 45 µg/m (about 50%).The accuracy of the forecast degrades slightly with lead time as mean bias increase from + 2.5 µg/m on the first day to - 17 µg/m on the third day of forecast. Our forecast is found to be very skillful both for PM concentration and unhealthy/ very unhealthy air quality index categories, and has been helping the decision-makers in Delhi make informed decisions.