Two heavy haze events over Lumbini in southern Nepal. Enhanced aerosol radiative forcing and heating rates

The Indo-Gangetic Plain (IGP) suffers from recurrent heavy air pollution during the dry season of each year. The AERONET station established in Lumbini, a semi-urban site and historically significant location as the birthplace of Buddha in the northern edge of the central IGP in Nepal, captured two events of heavy air pollution: 29–30 October (Event-I) and 1–2 December (Event-II) of 2014. Daily aerosol optical depth (AOD) was found to be 2.66, 2.44, 2.09 and 2.15 respectively with Ångström exponent (AE) > 1 during both events. The daily AOD during two events was 3.26–4.43 times higher than average AOD observed during the monitoring period 2013-2014. Based on various aerosol parameters we concluded that the fine-mode aerosols sourced from biomass-burning and urban/industrial activities were responsible for the haze events. Aerosol absorbed 205-240 Wm-2 of the solar radiation in the atmosphere, resulting in aerosol-induced atmospheric heating rate of 5.76–7.28 K day−1 during the hazy days. Back-trajectory analysis of air masses reaching Lumbini showed that the air masses either originated or travelled over the regions with open fires both prior and during the events. In addition to this, meteorological conditions (e.g. prevailing winds) also played role in initiation/persistence of the haze events.