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Pergamon

www.elsevier.eom/locate/j aerosci

Session 7A -Atmospheric aerosols: optical properties I

PROPERTIES OF PARTICLES IN THE TROPOPAUSE REGION

B. KARCHER 1 and S. SOLOMON 2

1 DLR Institut fur Physik der Atmosphfire, D-82234 Wessling, Germany.

2 NOAA, Aeronomy Laboratory R/E/AL8, Boulder, CO 80303, U.S.A.

Keywords: aerosol growth processes, aerosol optical properties, aerosol chemical composition, heterogeneous chemistry, aerosol-cloud interactions.

INTRODUCTION

High-altitude cirrus clouds have been identified as important components of the Earth's radiation budget. In the tropical tropopause region, subvisible cirrus clouds probably play an important role in the dehydra- tion of air entering the upper troposphere and lower stratosphere. Besides their well-documented presence in the upper tropical troposphere, subvisible cirrus can form, albeit less frequently, near the midlatitude tropopause. Aerosols and cirrus clouds may perturb chlorine chemistry and play a role in ozone chemistry at midlatitudes. Characterising these particles in terms of chemical composition and physical properties and understanding the link between aerosol particles and ice crystals in cirrus are key steps in assessing the impact of these particles on ozone chemistry and their effect on the radiative forcing of climate. We study the properties of particles in the midlatitude tropopause region in terms of size distributions, chemical composition, and optical extinction and relate our results to extinction measurements from the Stratospheric Aerosol and Gas Experiment (SAGE).

RESULTS AND DISCUSSION

Aqueous sulphate aerosols exhibit a markedly enhanced hygroscopicity and related particle growth in the presence of other condensable species. We investigate aerosol growth and chemical composition by em- ploying a thermodynamic equilibrium model, in which the coupling between gas and aerosol phase is taken into account.

Under temperature and humidity conditions typical for the midlatitude tropopause, we demonstrate that ni- tric acid (HNO3) can dissolve into liquid sulphuric acid (H2SO4/I-I20) near the ice frost point, affecting both specific surface area and extinction of the aerosol population. Species like ammonia amplify condensational growth above the frost point by enhancing the solubility of HNO3. The general aerosol extinction level is determined by the amount of particulate sulphate and other condensable species. We show that aerosols in the tropopause region display a larger spread of extinction and extinction ratios than background strato- spheric aerosols. Further, our results indicate that even pure H2SO4/H20 aerosols can exceed the extinction values marking the transition region between aerosol and aerosol/cloud mixtures in satellite occultation data at low temperatures and water vapour mixing ratios of 10-50 ppmvthat are not uncommon at the tropopause.

Aerosol surface areas greater than 10 #m 2 cm -3 are unlikely to be present at the tropopause. This suggests that the high average surface areas (exceeding 10 #m 2 cm -3) estimated from SAGE II data cannot be fully attributed to liquid particles. We find that only small cloud fractions detected along the line of sight of the SAGEII sensors suffice to satisfy the extinction data. Hence, midlatitude aerosol/cloud surface area densities in the tropopause region may be partly attributed to the presence of subvisible cirrus containing small (effective radius 5 - 10 #m) ice crystals. In the figure, we demonstrate that adding small fractions of such clouds to aerosols in the measuring volume of the SAGE II or other sensors dramatically changes

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Abstracts of the 2000 European Aerosol Conference $595

microphysicai properties of the particle mixture. Besides leading to higher surface areas, mixtures of aerosol and subvisible cirrus particles are also characterised by higher extinction efficiencies and lower extinction

ratios, consistent with observations.

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Figure l : Extinction E at 1.02/zm, 0.525 #m versus 1.02 #m extinction ratio ER, and specific surface area A of a simulated aerosol/cloud mixture. The value of n denotes the fraction of the SAGE II measuring volume ascribed to subvisible cirrus; the remaining fraction (1 - ,~) is filled with sulphate aerosol. The vertical bars indicate the ranges of E, ER, and A at selected t~ values upon variation of the cloud's ice water content at fixed aerosol parameters, whereby all curves have common starting points in the cloudless case. The bottom shaded area marks the transition range in terms of E(1.02 #m) between aerosols and aerosol/cloud mixtures. The dashed horizontal line marks the threshold ER value below which particles are classified as cloud in conjuction with the l-/~m extinction range. The top shaded area gives the range of estimated A values (45 ° N, 45 ° S) based on SAGE II subvisible cloud occurrence frequencies.

The described partitioning of particle types in aerosol/cloud mixtures also impacts heterogeneous reaction rates. We show how the efficiency of halogen-induced heterogeneous chemistry depends on the relative abundance of liquid and ice particles in a given volume of air. For example, the reaction of CIONO2 with HCI on ice particles is characterised by a high reactive uptake coefficient (~ 0.2 - 0.3) but is limited by gas phase diffusion when the reaction involves ice crystals in subvisible cirrus. On the other hand, similarly high uptake rates on smaller liquid particles are achieved at low temperatures (,-~ 205 K).

CONCLUSIONS

This work highlights the need for detailed microphysical and chemical modelling of aerosols and subvisible cirrus in order to quantify their role in atmospheric chemistry and radiative forcing, in concert with dedi- cated field and laboratory measurements. A better understanding of the role of ammonia and organics in particle formation and growth, ice nucleation mechanisms, the impact of particles generated by aircraft on ice formation and heterogeneous chemical processing, the influence of small-scale waves on the tempera- ture variability, and amplitude and frequency of occurrence of related cooling events is required before the chemical and climatic impact of particles in the tropopause region can be fully addressed.

ACKNOWLEDGEMENTS

This work was partly supported by BMBF within the research programme Atmospheric Aerosols.

REFERENCES

Kfircher, B. and S. Solomon (1999). On the composition and optical extinction of particles in the tropopause region. J. Geophys. Res., 104, 27,441.