Alexander D. James

Stratospheric Aerosol--Observations, Processes, and Impact on Climate

Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfate matter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes.

A novel instrument to measure differential ablation of meteorite samples and proxies: The Meteoric Ablation Simulator (MASI)

On entering the Earths atmosphere, micrometeoroids partially or completely ablate, leaving behind layers of metallic atoms and ions. The relative concentration of the various metal layers is not well explained by current models of ablation. Furthermore, estimates of the total flux of cosmic dust and meteoroids entering the Earths atmosphere vary over two orders of magnitude. To better constrain these estimates and to better model the metal layers in the mesosphere, an experimental Meteoric Ablation Simulator (MASI) has been developed. Interplanetary Dust Particle (IDP) analogs are subjected to temperature profiles simulating realistic entry heating, to ascertain the differential ablation of relevant metal species. MASI is the first ablation experiment capable of simulating detailed mass, velocity, and entry angle-specific temperature profiles whilst simultaneously tracking the resulting gas-phase ablation products in a time resolved manner. This enables the determination of elemental atmospheric entry yields which consider the mass and size distribution of IDPs. The instrument has also enabled the first direct measurements of differential ablation in a laboratory setting.

The uptake of HO2 on meteoric smoke analogues

The kinetics of heterogeneous HO2 uptake onto meteoric smoke particles (MSPs) has been studied in the laboratory using analogues of MSP aerosol entrained into a flow tube. The uptake coefficient, γ, was determined on synthetic amorphous olivine (MgFeSiO4) to be (6.9 1.2) × 10 2 at a relative humidity (RH) of 10%. On forsterite (Mg2SiO4), γ= (4.3 0.4) × 10 3 at RH= 11.6% and (7.3 0.4) × 10 2 at RH= 9.9% on fayalite (Fe2SiO4). These results indicate that Fe plays a more important mechanistic role than Mg in the removal ofHO2 from thegasphase. Electronic structure calculations show that Featomsexposedat theparticle surface provide a catalytic sitewhereHO2 is converted toH2O2 via an Eley-Ridealmechanism, but this does not occur on exposed surfaceMg atoms. The impact of this heterogeneous process in themiddle atmosphere was then investigated using a whole atmosphere chemistry-climate model which incorporates a microphysical treatment of MSPs. Using a global MSP production rate from meteoric ablation of 44 t/day, heterogeneous uptake (with γ=0.2) on MSPs significantly alters the HOx budget in the nighttime polar vortex. This impact is highly latitudedependent and thus couldnotbe confirmedusing currently available satellitemeasurements of HO2, which are largely unavailable at latitudes greater than 70°.

Meteoric Smoke Deposition in the Polar Regions: A Comparison of Measurements With Global Atmospheric Models

The accumulation rate of meteoric smoke particles (MSPs) in ice cores – determined from the trace elements Ir and Pt, and superparamagnetic Fe particles - is significantly higher than expected from the measured vertical fluxes of Na and Fe atoms in the upper mesosphere, and the surface deposition of cosmic spherules. The Whole Atmosphere Community Climate Model (WACCM) with the Community Aerosol and Radiation Model for Atmospheres (CARMA) has been used to simulate MSP production, transport and deposition, using a global MSP input of 7.9 t d-1 based on these other measurements. The modeled MSP deposition rates are smaller than the measurements by factors of ~32 in Greenland, and ~12 in Antarctica, even after reanalysis of the Ir/Pt ice core data with inclusion of a volcanic source. Variations of the model deposition scheme and use of the United Kingdom Chemistry and Aerosols (UKCA) model do not improve the agreement. Direct removal of MSP-nucleated polar stratospheric cloud particles to the surface gives much better agreement, but would result in an unfeasibly high rate of nitrate deposition. The unablated fraction of cosmic dust (~35 t d-1) would provide sufficient Ir and Pt to account for the Antarctic measurements, but the relatively small flux of these large (> 3 μm) particles would lead to greater variability in the ice core measurements than is observed, although this would be partly offset if significant fragmentation of cosmic dust particles occurred during atmospheric entry. Future directions to resolve these discrepancies between models and measurements are also discussed.