Source apportionment of rainwater chemical composition to investigate the transport of lower atmospheric pollutants to the UTLS region

Abstract

Efforts to understand the chemical composition of Asian Tropopause Aerosol Layer (ATAL) in the Upper Troposphere Lower Stratosphere (UTLS) region have revealed the dominance of nitrates in the samples collected from ATAL layer during the recent balloon campaigns. Potential sources have been thought to be in-situ formation, convective uplift and long-range transport. Rainwater chemical composition consists water-soluble chemical ions that are wet scavenged during rain events and gives an indirect indication of lower atmospheric pollutants. Keeping this in focus, total monsoon precipitation chemistry at Gadanki (13.5°N, 79.2°E) has been studied to understand the convective uplift possibilities to the UTLS region. About 32 rainwater samples collected during July to December 2017 were analysed for their chemical composition using Ion Chromatography. Total 16 ions comprising of 5 anions (F−, Cl−, NO3−, SO42− and PO43−), 6 cations (Na+, K+, Ca2+, Mg2+, Li+ and NH4+) and 5 trace metals (Cd2+, Ni2+, Co2+, Mn2+ and Zn2+) have been detected in different rainwater samples. Rainwater chemical composition data has been subjected to the Principal Component Analysis (PCA) to understand the correlations between different chemical species and to identify the possible sources of origin qualitatively. It has been observed that the chemical composition of the rainwater is very different from the chemical composition of the ATAL layer indicating non-existence of convective transport of lower level pollutants to the UTLS region at Gadanki. This observation is also well supported by the vertical distribution of CALIPSO derived aerosol types and ERA interim vertical pressure velocities during the sampling period.