Transport, Atmospheric Interactions and Deposition of Meteoric Smoke Particles (MSPs)
Sandip Dhomse, Russell Saunders, Wenshou Tian, John Plane and Martyn Chipperfield
Background
Nano-sized meteoric smoke particles (MSPs) with iron-magnesium silicate composition are formed in the upper mesosphere as a result of meteoric ablation. These particles may remove sulphuric acid from the gas-phase above 40 km and may also affect the composition and behaviour of supercooled H2SO4-H2O droplets in the stratospheric aerosol (Junge) layer. These particles are ultimately transported into the troposphere and deposited at the Earth’s surface. We use our global 3-D models to study the interaction of MSPs with stratospheric sulphuric acid particles and how the MSPs are deposited at the Earth’s surface.
Results
Saunders et al. (2012) described a time-resolved laboratory spectroscopic analysis of the evolution of the ferric (Fe3+) ion originating from amorphous ferrous (Fe2+)-based silicate powders dissolved in varying Wt % sulphuric acid (30–75 %) solutions over a temperature range of 223–295 K. Complete dissolution of the particles was observed under all conditions. The first-order rate coefficient for dissolution decreases at higher Wt % and lower temperature, which is consistent with the increased solution viscosity limiting diffusion of H2SO4 to the particle surfaces. Dissolution under stratospheric conditions should take less than a week, and is much faster than the dissolution of crystalline Fe2+ compounds.
We then used the UMSLIMCAT chemistry-climate model to study the transport of MSPs through the middle atmosphere. A series of model experiments were performed with different uptake coefficients. Setting the concentration of 1.5 nm radius MSPs at 80 km to 3000 cm-2 (based on rocket-borne charged particle measurements), the model matches the reported Wt % Fe values of 0.5–1.0 in Junge layer sulphate particles, and the MSP optical extinction between 40 and 75 km measured by a satellite-borne spectrometer, if the global meteoric input rate is about 20 tonnes per day. The model indicates that an uptake coefficient >0.01 is required to account for the observed two orders of magnitude depletion of H2SO4 vapour above 40 km.
UMSLIMCAT was then used to simulate the transport and deposition of Plutonium (Pu)-238 oxide nanoparticles (238PuO2) formed after the burn up of a Systems Nuclear Auxiliary Power (SNAP) unit in the upper stratosphere in 1964. In Dohmse et al. (2013) we showed that the model is able to reproduce both the observed hemispheric asymmetry (greater deposition in the southern hemisphere) and timescale of appearance of Pu-238 at the surface (around 2-6 years). Sensitivity simulations suggest that the SNAP burn up was likely to have occurred outside the stratospheric tropical pipe (poleward of 12oS) and at approx. 35 km, which is significantly lower than earlier estimates (40-60 km). The same CCM set up was then used to investigate transport of MSPs from the mesosphere to the surface.
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Our simulations suggest that strongest deposition of MSP occurs at mid-latitudes and might be providing a significant source of Fe-rich fertilization in SH mid-latitudes. As discussed in Dhomse et al. (2013), the deposition of Fe from MSPs into the southern ocean may be between 50% and 400% of the soluble Aeolian dust input. The model also predicts more deposition in Greenland than Antarctica, in agreement with ice core observations. This implies that climate proxy measurements from just a few sites used to determine natural forcings must be carefully interpreted.
Publications
Dhomse, S.S., R.W. Saunders, W. Tian, M.P. Chipperfield and J.M.C. Plane, Plutonium-238 observations as a test of modeled transport and surface deposition of meteoric smoke particles, Geophys. Res. Lett., 40, x-x, doi:10.1002/grl50840, 2013.
Saunders, R. W.; Dhomse, S.; Tian, W. S.; Chipperfield MP; Plane, JMC, Interactions of meteoric smoke particles with sulphuric acid in the Earth's stratosphere, Atmos. Chem. Phys., 12, 4387-4398, doi:10.5194/acp-12-4387-2012, 2012.