Source evaluation of aerosols measured during the Indian Ocean Experiment using combined chemical transport and back trajectory modeling

This work presents an analysis of aerosol measurements made during the Oceanographic Research Vessel Sagar Kanya cruise of January-March 1999, in the Indian Ocean Experiment intensive field phase (INDOEX-IFP), with regard to the aerosol chemical constituents and identification of source regions of their origin. This is done through a hybrid approach which uses an Eulerian forward transport calculation in a general circulation model (GCM) with region-tagged emissions along with an analysis of Lagrangian back trajectories and emission inventory information, for overlapping time periods. Back trajectory analysis showed that the ship was mainly influenced by air masses from the Indo-Gangetic plain, central India, or south India during the early part of its cruise with the GCM-predicted aerosol species composed of mainly sulfate and organic matter, whereas dust species dominated during its cruise in late February and early March over the Arabian Sea when the ship was influenced by air masses from Africa-west Asia or northwest India. However, a typical clean marine aerosol dominated by sea salt was encountered during February when the ship cruised in the tropical Indian Ocean and was mostly influenced by marine air masses. The high aerosol optical depth was due to roughly equal parts of organic matter and sulfate. Region-tagged GCM estimates showed the presence of distinct transport at surface and higher layers for, e.g., DOY 56-61 and 65-70, indicating strong signals of emissions of black carbon, organic matter, and sulfate originating in central and northwest India, whereas elevated transport channels of black carbon and organic matter from Africa-west Asia. This is consistent with the back trajectory analysis and in corroboration with INDOEX measurement studies which observed different aerosol properties from aircraft and ship attributed to different transport pathways in surface and elevated flows. However, back trajectory analysis is not sufficient to evaluate the major source regions contributing to the transported aerosol. The fractional contribution of a source region also depended upon the emission flux from the region and its proximity to the receptor domain.