Transported aerosols regulate the pre-monsoon rainfall over north-east India: a WRF-Chem modelling study

Abstract

Abstract. The study differentiates and quantifies the impacts of aerosols emitted locally within the north-east (NE) India region and those transported from outside this region to ascertain whether local or transported aerosols are more impactful in influencing this region's rainfall during the pre-monsoon season (March–April–May). Due to the existence of a declining pre-monsoon rainfall trend in NE India, the study also quantified the role of different aerosol effects in radiative forcing (RF) and rainfall. The study has been carried out using the WRF-Chem model by comparing simulation scenarios where emissions were turned on and off within and outside the NE region. The impact of all emissions as a whole and black carbon (BC) specifically was studied. Results show that aerosols transported primarily from the Indo-Gangetic Plain (IGP) were responsible for 93.98 % of the PM10 mass over NE India's atmosphere and 64.18 % of near-surface PM10 concentration. Transported aerosols contributed >50 % of BC, organic carbon, sulfate, nitrate, ammonium and dust aerosol concentration and are hence a major contributor to air pollution. Hence, the aerosol effects were much greater with transported aerosols. An indirect aerosol effect was found to be the major effect and more impactful, with transported aerosols that dominated both rainfall and RF and suppressed rainfall more significantly than the direct and semi-direct effect. However, the increase in direct radiative effects with an increase in transported BC counteracted the rainfall suppression caused by relevant processes of other aerosol effects. Thus, this study shows atmospheric transport to be an important process for this region, as transported emissions, specifically from the IGP, were also found to have greater control over the region's rainfall. Thus, emission control policies implemented in the IGP will reduce air pollution as well as the climatic impacts of aerosols over the NE India region.