Effects of forest fire smoke and volcanism on the stratospheric aerosol

Abstract: This thesis explores the stratospheric aerosol through both in-situ measurements and remote sensing. The background stratospheric aerosol is complex and is perturbed by injections of forest fire smoke particles and by particles formed from volcanic SO2. Climate models need accurate description of the stratospheric aerosol in order to have sound radiation budgets. Using the remote sensing technique of satellite borne lidar, it was seen that volcanic eruptions increased the stratospheric optical depth on average by 40 % in the period between 2006 and 2015. Forest fires also increased the stratospheric optical depth but their effect was found to disappear faster than the effect from volcanic eruptions. By investigating in-situ samples, the background aerosol was found to contain a carbonaceous fraction that seems to be produced in the stratosphere. In order to better portray fresh volcanic emissions of SO2, a new method of compiling SO2 datasets with high vertical resolution was developed. This method combined many lidar observations of the aerosol formed from SO2 to provide vertical distributions of the SO2 gas. The lidar used in this thesis is CALIOP which is aboard the CALIPSO satellite. The satellite was launched in 2006 and CALIOP is still operational. This lidar measures the radiation scattering from the aerosol at a high vertical resolution. Measurement of elemental concentrations and other in-situ measurements were done using the IAGOS-CARIBIC aircraft platform. Both CALIOP and IAGOS-CARIBIC have long successful histories of measurements which allowed long-term effects to be studied. A comparison between the in-situ measurements from IAGOS-CARIBIC and the remote sensing measurements by CALIOP was also made. If the measurements are taken sufficiently above the tropopause, then the calculated scattering based on in-situ IAGOS-CARIBIC measurements of sulphur, water and carbon are similar to the scattering measured by CALIOP. In the vicinity of the tropopause, additional aerosol components and water uptake are needed to explain the scattering from stratospheric aerosol.