Investigating the convective transport possibilities of lower-atmospheric pollutants to the UTLS region using rainwater and aerosol chemical characterization

Abstract

Various attempts using different platforms to understand the chemical composition of the Asian Tropopause Aerosol Layer (ATAL) have concluded that it primarily consists of nitrate (NO₃⁻) aerosols. Recent in-situ measurements from aircraft flying through ATAL have suggested that ammonia (NH₃) pollution from Asia could serve as the precursor for nitrate aerosols in ATAL through gas-to-particle formation after undergoing convective uplift to the Upper Troposphere and Lower Stratosphere (UTLS) regions. In this study, we aimed to investigate the potential pathways of convective transport of lower-atmospheric pollutants by analysing the chemical composition of rainwater and aerosols in a highly convective region, Kolkata (Head Bay of Bengal), India. We utilized the PILS-IC system established at Kolkata Camp Observatory of the National Atmospheric Research Laboratory (KCON). The analysis revealed a significant dominance of NH₄⁺ and NO₃⁻ ions in both rainwater and aerosol samples during the monsoon and post-monsoon seasons, with the dominance increasing during the post-monsoon season. Air mass trajectories indicated a clear influence of the Indo-Gangetic Plain (IGP) region during the post-monsoon season, which is well-known for its dense agricultural activities and industries. High concentrations of NH₃ even at higher altitudes, have been observed in Atmospheric Infrared Sounder (AIRS) measurements. Moreover, using Outgoing Longwave Radiation (OLR) and vertical wind as proxies for tropical deep convection and updrafts/downdrafts, respectively, we found evidence supporting the possibility of convective transport of these lower-atmospheric pollutants to the UTLS regions during the monsoon season. In conclusion, this study provides evidence for the potential convective transport of lower-atmospheric pollutants, including NH₃ and other precursor gases, to higher levels. These pollutants could partially serve as the source of nitrate aerosols in the ATAL through gas-to-particle formation processes.