THE DISTRIBUTION OF RELATIVE HUMIDITY IN CIRRUS CLOUDS AND ITS IMPACT ON THE NITRIC ACID CONTENT OF INTERSTITIAL AEROSOL PARTICLES M. KR ¨ AMER a , A. MANGOLD a , I. GENSCH a , S. SCHLICHT a , C. SCHILLER a , H. ZIEREIS b H. BUNZ c , H. SAATHOFF c , O. M ¨ OHLER c a Inst. f¨ ur Chemie und Dynamik der Geosph¨ are I: Stratosph¨ are, Forschungszentrum J¨ ulich, 52428 J¨ ulich, Germany b Inst. f¨ ur Physik der Atmosph¨ are, DLR Oberpfaffenhofen, 82234 Weßling, Germany c Inst. f¨ ur Meteorologie und Klimaforschung - AAF, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany KEY WORDS: CIRRUS CLOUDS, IN-CLOUD RELATIVE HUMIDITY, INTERSTITIAL AEROSOL, NITRIC ACID INTRODUCTION, AIM AND METHODS Recent studies (e.g. Ovarl` ez et al., 2002) have shown that the relative humidities with respect to ice inside cirrus clouds (in–cloud RH ice ) can substantially exceed the saturation value RH ice =1.0 and reach values up to RH ice =1.8. A consequence of these in–cloud ice supersaturations may be a growth of the interstitial aerosol particles subsequently leading to uptake of nitric acid (HNO 3 ). This process could be important in the frame of the discussion on denitrification of the upper troposphere via sedimenting cirrus cloud ice particles carrying previously adsorbed HNO 3 . Up to now, the mechanisms controlling the HNO 3 uptake on ice are not completely understood. In particular the amount of HNO 3 taken up by the interstitial aerosol particles –which is no longer available for adsorption on ice particles– has hitherto been neglected. In this study, we investigate the distribution of in–cloud RH ice from both laboratory measurements at the aersol chamber AIDA as well as from aircraft field measurements. Furthermore, the HNO 3 uptake by two different types of interstitial aerosol particles (ternary, H 2 O-H 2 SO 4 - HNO 3 , and quaternary, H 2 O-H 2 SO 4 - HNO 3 - NH 3 , solutions) in dependence on the in–cloud RH ice is determined using the thermodynamic equilibrium model AIM. Relaxation times for HNO 3 uptake are calculated using a detailed kinetic model. Combining the measurements of in–cloud RH ice and model simulations of HNO 3 uptake in interstitial aerosol particles, we discuss the importance of HNO 3 uptake onto interstitial particles for the partitioning of HNO 3 in cirrus clouds between ice particles, gas phase and interstitial aerosol. RESULTS Measurements of in–cloud RH ice : The in–cloud RH ice is shown in Figure 1, left panels, for two phases of the AIDA ice cloud life cycles. In the upper panel the RH ice during ice onset and cooling phase is plotted, showing that the in–cloud RH ice is very high (120-180%) at low temperatures and ranges from about 100- 140% for higher temperatures. During warming and dissipation phases (lower left panel) RH ice still reaches 150% for low temperatures while ranging around 100% for higher temperatures. Model simulations of HNO 3 uptake in interstitial particles: The HNO 3 uptake in interstitial particles is deter- mined for in–cloud RH ice = 1.0, 1.3 and 1.6 (see Figure 1, right panels) for ternary and quaternary solution interstitial aerosol. For ternary solution interstitial aerosol, a strong decrease of the HNO 3 uptake with tem- perature is found for RH ice = 1.0 and 1.3. In case of quaternary solutions, more HNO 3 resides in the interstitial particles, because the additional ammonia causes a stronger HNO 3 uptake. At RH ice = 1.6, almost all HNO 3 is taken up in the interstitial aerosol independently on temperature and aerosol type. The relaxation times to reach 50, 90 and 99% of the equilibrium HNO 3 (for ternary solutions, see tables in Figure 1) are about 5min, 15min and 6.5h at 190K, respectively. At 230K the uptake is much faster: only about 10sec, 30sec and 2min are needed. Obviously, increasing with increasing temperature, 50-100% of the equilibrium HNO 3 can be taken up by the interstitial aerosol on the timescale of a cirrus life cycle. Abstracts of the European Aerosol Conference 2004 S861