Thesis of Smita Panda

Defense of thesis Shivange Srivastava  - Laboratory LOA

Insight into the conversion of SO2 to sulphate aerosols in volcanic plumes from the joint analysis of hyperspectral OMI and multi- angular polarimetric POLDER satellite observations

Abstract :

The atmospheric evolution of SO2 from volcanic emissions into sulfate aerosols is a complex process involving numerous chemical and physical stages. Within the tropospheric volcanic plumes these transformations and impacts of these secondary aerosols on climate and environment are not yet fully understood and evaluated. In this study, we utilize observational data from the Aura/OMI space spectrometer to trace SO2 emissions and analyze the dispersal patterns of sulfur-rich volcanic plumes. Concurrently, we leverage the PARASOL/POLDER space imager's capacity for detecting fine mode particles, enhancing our understanding of the developmental path of volcanic sulfate aerosols. The aim is to quantify the transformation rate of SO2 gas into particulate sulfate.Central to our analysis is the retrieval of POLDER measurements by GRASP algorithm. This algorithm helps accurate identification of the AOD of fine mode particles, offering insights into aerosol absorption, and distinguishing various aerosol components, particularly in the GRASP/Components version. It provides detailed assessments of both soluble and insoluble fractions of aerosols in fine and coarse modes, as discerned through their complex refractive indices and standard optical properties. Our investigation centers on the degassing episodes from the Kilauea volcano in Hawaii, covering a period from 2006 to 2012. This timeframe includes phases of both passive and explosive degassing. We observed a spatial co-occurrence between areas with high SO2 concentrations, as identified by OMI data, and regions rich in fine, non-absorbing aerosols, as detected by POLDER. This correlation is quantitatively supported by fine AOD values ranging from 0.1 to 0.4 and SSA values between 0.95 and 1.0 at 440 nm.This distinct collocation allows us to differentiate these volcanic particles from other fine absorbing anthropogenic and natural aerosols that originate from Asia, based on their absorption characteristics. Consequently, we attribute these fine mode particles to sulfate aerosols formed from the SO2 emitted by Kilauea.The dispersion of aerosol plumes from Kilauea shows a broader spatial distribution compared to the SO2 plumes, characterized by higher fine AOD and SSA values. Irrespective of the degassing intensity, a consistent pattern is observed, aligning with the oxidation of SO2 into secondary sulfate aerosols. This process is marked by a gradual decrease in SO2 concentration alongside an increase in fine AOD, peaking approximately 500 to 3000 km from the volcanic source. The temporal scale of SO2 oxidation and the geographic spread of Kilauea's aerosol plumes were found to be influenced by the volcano's degassing intensity, as well as seasonal variations and prevailing meteorological conditions. The study also estimates the atmospheric lifetimes of SO2 and sulfate aerosols within the volcanic plume. For the eruptive degassing period of April to November 2008, the effective SO2 lifetime was evaluated to range from 16 to 57 hours. For volcanic sulfate aerosols, the study presents the first determination of their atmospheric lifetime, estimated between 58 to 93 hours,as inferred from POLDER observations. The estimation of the atmospheric lifetime of volcanic sulfate aerosols, ascertained from the fine AOD measurements by POLDER, is considered accurate and aligns well with the established global average for sulfate aerosol lifetime. This accuracy is largely due to the heightened sensitivity of the POLDER system in effectively distinguishing of fine non-absorbing particles within volcanic plumes. Furthermore, the complex refractive index of Kilauea aerosol particles, as determined by the GRASP/Component algorithm, reveals the aerosols are not solely composed of sulfate but may include absorbing elements and potentially indicate complex aerosol composition and structure such as presence of fine ash or sulfate-coated ash within the plume.

Keywords : remote sensing, sulfate aerosol particles, sulfur dioxide gas, ash particles, Volcanic plumes, atmospheric aerosol composition