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.

[1] To test the applicability of Mie theory in climate models and remote sensing data retrievals, we have studied the scattering phase function and linear polarization of representative mineral dust aerosol components at a wavelength of 550 nm. The mineral components investigated include the silicate clays, kaolinite, illite, and montmorillonite, and non-clay minerals, quartz, calcite, gypsum, and hematite, as well as Arizona road dust. In each case the aerosol size distribution was simultaneously monitored with an aerodynamic particle sizer. Particle diameters in this study fall in the accumulation mode size range characteristic of long-range transport aerosols. Our results show significant discrepancies between the experimental and Mie theory phase functions. The model shortcomings are due to particle shape effects for these non-spherical mineral dust particles. We find intriguing differences in the scattering between the silicate clay and non-clay components of mineral dust aerosol in this size range. For the non-clay minerals the most significant errors are found at large scattering angles where Mie theory substantially overestimates the backscattering signal. For the silicate clay minerals, there is more variability in the comparison to Mie theory. These findings have important consequences for the radiative forcing component of global climate models and remote sensing measurements that rely on Mie theory to characterize atmospheric dust. We also present experimentally based synthetic phase functions at 550 nm, for both silicate clay and non-clay mineral dust aerosols in the size parameter range X = 2-5, which may be useful for empirical models of the scattering due to particles in the accumulation mode size range.

[1] A combined infrared spectroscopy and visible light scattering study of the optical properties of quartz aerosol, a major component of atmospheric dust, is reported. Scattering phase function and polarization measurements for quartz dust at three visible wavelengths (470, 550, 660 nm) are compared with results from T-matrix theory simulations using a uniform spheroid model for particle shape. Aerosol size distributions were measured simultaneously with light scattering. Particle shape distributions were determined in two ways: (1) analysis of electron microscope images of the dust, and (2) spectral fitting of infrared resonance extinction features. Since the aerosol size and shape distributions were measured, experimental scattering data could be directly compared with T-matrix simulations with no adjustable parameters. c 2 analysis suggests that T-matrix simulations based on a uniform spheroid approximation can be used to model the optical properties of irregularly shaped dust particles in the accumulation mode size range, provided the particle shape distribution can be reliably determined. Particle shape distributions derived from electron microscope image analysis give poor fits, indicating that two-dimensional images may not give an accurate representation of the shape distribution for three-dimensional particles. However, simulations based on particle shape models inferred from IR spectral analysis give excellent fits to the experimental data. Our work suggests that correlated IR spectral and visible light scattering measurements, together with the use of theoretical light scattering models, may offer a more accurate method for characterizing atmospheric dust loading, and aerosol composition, size, and shape distributions, which are of great importance in climate modeling.

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