Quantifying the influence of meteoric smoke particles and fragments on stratospheric aerosol and chemistry

A large amount of total cosmic dust enters the Earth’s atmosphere every day. ~90% of large meteoroids undergo fragmentation. Meteoric ablation also occurs as cosmic dust particles enter the atmosphere. A variety of metals (e.g. Na, Fe, Mg, K, Ca, Ni, and Si) are produced as the meteoroids ablate in the mesosphere and lower thermosphere (MLT, ~70-120 km), generating layers of metal atoms between 80 and 105 km. These meteoric metal layers can be observed by the ground-based lidar (laser radar) techniques, as well as by satellite-borne optical spectroscopy. Metallic ions are also measured by mass spectrometry on sub-orbital rockets, each metal provide a unique tracer of the physics and chemistry of the atmosphere at the interface with geospace.

The metal vapors generated by this meteoroid ablation (principally Fe, Mg, and Si) then oxidize and condense into tiny particles with the size typical around 0.5-2 nm in radius. These are termed Meteoric Smoke Particles (MSPs), which has been observed by the satellite measurement SOFIE spectrometer on the AIM satellite and are transported down to the stratosphere inside the polar vortex.

The dominant influence from the meteoric material, is the way both types of meteoric particle (MSPs and meteoric fragments, MFs) may trigger the nucleation of polar stratospheric clouds (PSCs). This heterogeneous nucleation of PSCs on meteoric particles has been inferred from a range of different observations from recent field campaigns.

It has long been hypothesised that stratospheric aerosol particles nucleate also heterogeneously on MSPs, and lab experiments at Leeds measuring the dissolution of MSP analogues in sulphuric aerosol in the lab and also the uptake of nitric acid and H2O2. The Leeds stratospheric composition modelling group recently adapted the interactive stratospheric aerosol configuration of the UK composition-climate model to represent this heterogeneous nucleation of MSP-sulphuric particles.

This PhD project will involve global chemistry-climate model experiments to understand how meteoric particles (both MSPs and MFs) affect the stratospheric aerosol layer, forming two distinct sub-classes of stratospheric sulphuric aerosol particles, these preferentially freezing to form nitric acid PSCs in the cold Arctic, with subsequent effects on polar ozone loss via heterogeneous chemistry. The project will assess the simulated mix of stratospheric aerosol and PSCs for recent Arctic winters to satellite measurements from the CALIOP lidar, which provides an unique record of PSC occurrence for the period 2006-present, with recently also a new CALIOP stratospheric aerosol type product.

The composition-climate models include the influences from dynamics, transport, microphysics, photochemistry, radiation and their influences on stratospheric ozone depletion. A current community research focus is to understand why satellite observations show HCl is essentially completely depleted inside the dark, midwinter Antarctic polar vortex but most models significantly overestimate HCl in this region. The project will also investigate the influence of MSPs on Antarctic PSCs and explore whether the uptake of HCl on MSPs and/or meteoric fragments could potentially be causing this apparent discrepancy.

The goal of this project is to answer a number key questions: How do MSPs form and how are they transported to the stratosphere; and where they deposit at the surface? Why do current global models fail to capture the observed surface deposition of MSPs? How do MSPs and fragments affect stratosphere aerosol, PSCs and chemistry, and thus climate change in the middle atmosphere? Which important processes are missing in the current whole atmosphere models?

It is becoming increasingly apparent and important to consider the coupling processes among the lower, middle and upper atmosphere to understand the weather and climate changes. However, it is still not clear and fully understood the coupling mechanism and the impact on the lower atmosphere. This project addresses the coupling of the atmosphere to the geospace environment by developing self-consistent models for us to better understand the meteor astronomy, chemistry and transport processes that control atmospheric composition and aerosol in the middle atmosphere. The project is therefore likely to have significant impacts in a number of fields, including global atmospheric modelling, aeronomy, and aerosols.