MeteorStrat (Meteoric influences on stratospheric aerosol and clouds)

Co-investigators: Professor John Plane (School of Chemistry), Professor Ben Murray, Professor Martyn Chipperfield

Volcanic injections of sulphur brighten the stratospheric aerosol layer with major eruptions inducing periods of strong cooling within global mean surface temperature trends. The stratospheric ozone layer shields us from harmful UV radiation, and accurately predicting how it will recover relies on predicted changes in stratospheric aerosol and polar clouds.

The MeteorStrat project is addressing two key knowledge gaps that limit current predictive capability of composition-climate models, both associated with the effects from the continual supply of meteoric material entering the upper atmosphere.

Firstly, recent in-situ observations have confirmed findings from the late 1990s that most particles in the stratospheric aerosol layer contain refractory core of meteoric origin, posing a major challenge to the current generation of interactive stratospheric aerosol models.

Secondly, how the polar stratospheric clouds, that provide the medium by which emissions of compounds such as CFCs leads to polar ozone loss, form in the Arctic has remained a persistent uncertainty, limiting the confidence of model predictions for how the ozone layer will recover.

The MeteorStrat team have made two breakthrough research findings that are uniquely enabling us to address long-standing questions in stratospheric aerosol and PSC science.

Firstly, our global model “meteoric smoke interaction experiments” show major effects from extra-terrestrial material, the meteoric inclusions radically altering the vertical distribution of sulphuric particles, challenging how models predict changes in the stratospheric aerosol layer.

Secondly, our laboratory PSC freezing experiments reveal that rather than ablation-generated smoke particles (which were found to be poor NAT nuclei), it is an inclusion of non-ablated meteoric fragment particles that may explain how many NAT particles nucleate in the Arctic. The project builds on these exciting research findings with two hypotheses addressing the overarching aim to assess how cosmic dust influences the composition of the stratosphere.

A. That meteoric-sulphuric particles are larger, with shorter stratospheric residence times, has important consequences for how models predict decay from volcanic enhancement

B. The mechanism by which solid nitric acid PSCs form in the Arctic can finally be explained by the non-ablated meteoric fragments providing the preferential NAT nuclei We are combining our internationally-leading laboratory and modelling capabilities to test these hypotheses.

Our workplan is addressing the following 5 related science questions:

1. How far do the high-latitude source meteoric-sulphuric particles extend to lower latitudes and how variable is their mid-latitude abundance across different seasons and years?
2. Do the larger meteoric-sulphuric particles effect a faster volcanic decay timescale and if so what are the implications for the surface cooling attributed to volcanic eruptions?
3. What is the source flux and size distribution of the non-ablated “meteoric fragment” input and how do smoke or fragments transform into Junge layer meteoric-sulphuric particles?
4. How do the distinctly composed meteoric fragments facilitate NAT freezing and what are the implications of the laboratory findings compared to smoke-driven parameterizations?
5. How frequently do meteoric-enacted PSCs occur and how is Arctic ozone loss enhanced?

A key philosophy of the project involves gathering in situ and satellite measurement datasets to ensure model predictions are observationally-constrained and calibrated to maximise confidence in research findings. The project is providing the UK Earth System Model with vital capability to simulate future changes to the stratosphere, in particular for the effects from volcanic and potential stratospheric sulphur or particle geoengineering.

Partners and collaborators

1. Dr Margaret Campbell-Brown (University of Ontario, Canada; meteoroid fragmentation)      
2. Dr Dan Murphy (NOAA ESRL, Boulder, Colorado, USA; strat-aerosol composition measurements from high-altitude aircraft) 
3. Prof. Terry Deshler (Univ. Wyoming, Laramie, USA; balloon-borne aerosol and PSC measurements) 
4. Prof. James C. Wilson (Univ. Denver, Colorado, USA; aerosol particle number and size distribution measurements from high-altitude aircraft)
5. Prof. Stephan Borrmann (Max Planck Institute for Chemistry, Mainz, Germany; aerosol particle number and type measurements from high altitude aircraft)
6. Dr Larry Thomason (NASA Langley Research Center, Hampton, Virginia, USA; satellite measurements of stratospheric aerosol)
7. Dr Michael Pitts (NASA Langley Research Center, Hampton, Virginia, USA; satellite measurements of PSCs)
8. Dr Reinhold Spang (Forzschung Zentrum Juelich Research Centre, Juelich, Germany; satellite measurements of PSCs and nitric acid)
9. Dr Claudia Timmreck (Max Planck Institute for Meteorology, Hamburg, Germany; microphysical modelling of the stratospheric aerosol layer)

Impact

MeteorStrat is focussed on two key features of the Earth's atmosphere, the stratospheric aerosol layer and the stratospheric ozone layer. The research will quantify how both layers are influenced by cosmic dust particles, which are continuously entrained down into the upper atmosphere as small meteoroids burn up during atmospheric entry. The research will also lead to better understanding of natural influences on climate, principally from volcanic injections of sulphur.

Our science will inform upcoming international assessments of climate (CMIP6) and of the stratospheric aerosol (SSiRC) and will be of direct interest to government departments, chiefly the BEIS "Science and Innovation for Climate and Energy" directorate and the DEFRA "Strategic Evidence and Analysis Team". Results from MeteorStrat will feed into the upcoming IPCC AR6 assessment and any future WMO/UNEP ozone or stratospheric aerosol assessments. We have been heavily engaged with these in the past, with co-I Chipperfield serving as convenor or lead authors for several WMO/UNEP assessments.

We have budgeted to fund a 1-day workshop during year 3 of the project (autumn 2020) to engage with other leading scientists, media and government civil servants on the effects of cosmic particles on stratospheric aerosol and PSCs. MeteorStrat will further develop community modelling tools for chemistry-climate and Earth System Modelling studies and the wider public will benefit from more robust predictions of future stratospheric change.

Our existing collaborations with the UK Met Office and the European Centre for Medium-range Weather Forecasts, and the inclusion of the GLOMAP aerosol scheme within the UK Earth System Model and the flagship atmospheric monitoring system C-IFS provides guaranteed impact from the model improvements.

The general public has a keen interest in the environment, and climate change in particular. It remains extremely important to engage with the public and to provide latest scientific evidence related to these issues. We already do this and MeteorStrat will increase our efforts in this regard. As such, policy makers, national and European meteorological services and the general public will all benefit from the research findings from this project.