A comprehensive study on heavy pollution episode over urban city Hyderabad using observations and WRF-CAMx simulations

Abstract

High pollution Episodes (HPEs) are increasing in urban conglomerations, posing a serious threat to human health and modifying regional weather patterns. One such episode occurred in Hyderabad during 21–27 December 2023, when the air quality index in Hyderabad alarmingly rose to over 200, reaching an “unhealthy” status. In order to understand underlying processes causing such episodes, both observations (ceilometer, automatic weather station, radiosonde data) and air quality model simulations for the period from 15 to 31 December 2023 have been used. Air quality simulations were conducted using CAMx v7.2, driven by meteorological inputs from WRF v4.2 and emissions from the EDGAR-HTAP 3.0 inventory. The daily mean surface PM_2.5 concentrations exceeded 100 $\mu g/m³$ during the HPE days at several locations in Hyderabad. The first two days (21 to 22 December) of the HPE were associated with low surface temperatures, low specific humidity, and weak wind speeds, and the cold dry air being denser remains close to ground, favouring the formation of HPE. Pollutants persisted due to the lowering of PBL due to higher aerosol (aerosol-PBL feedbacks). During the HPE days, the ceilometer backscattered density (BSD) showed increased backscattering counts at night due to aerosol accumulation within the PBL. WRF-CAMx simulations show that transboundary pollutants contribute a major part (30 to 40 %) of pollution in Hyderabad. During the HPE days, the maximum contribution of pollutants was observed from the eastern boundary, and the impact extended to the northwest part of the city. Among local emission sources, the industrial sector contributes 20 to 30 %, followed by the residential sector at 15 to 25 %, the transport sector at 8 to 10 %, while the remainder comes from agriculture and other sectors.