Hazy Weather Index as a Predictor for Present and Future Indian Metropolitan Air Quality in a Multiyear WRF-Chem Simulation

Major Indian metropolitan areas suffer from severe air pollution accompanied by rapid economical growth in recent years. Over this summer-monsoon dominated subcontinent, a strong meteorological correlation with pollution level was documented in previous observational studies. Based on a multi-year WRF-Chem (Weather Research and Forecasting model coupled to Chemistry) simulation (Kumar et al., submitted), a daily hazy weather index (modified from Cai et al., 2017) was defined to quantify the meteorology-pollution relationship in three largest and most polluted cities: Delhi, Kolkata and Mumbai. The daily index consists of zonal wind near surface (U10) at Western Indian (12°-22°N, 60°-70°E), temperature at 200 hPa (T200) and local planetary boundary layer height (PBLH). The simulated PM2.5 levels during the present-day (1997-2004) demonstrates significant negative correlations between the index and pollution level. The anti-correlation captures the day-to-day variability but is particularly high when the seasonal transitions periods (location dependent) are included. Two future (2046-2054) warming simulations were conducted under scenarios of Representative Concentration Pathway (RCP) 8.5 and 6.0, showing a higher pollution level in mid-21st century, especially under RCP8.5 by ~30%. The future change in hazy weather index (T200 most remarkably), however, works in favor of strong pollution removal and dispersion. Therefore, anthropogenic emission increase is the main contributor a higher PM2.5 level. The chemical compositions of PM2.5 and ozone are also discussed.