4.1. Properties of Larger Particles in the Upper Troposphere
 In this section, we will discuss the particles with Dp > 3.6 μm measured in the upper troposphere over the Tibetan Plateau during the 17 August flight. The Moderate Resolution Imaging Spectroradiometer (MODIS) data have shown that the mean top pressure of cirrus clouds over the Tibetan Plateau is ∼150 hPa [Fu et al., 2006], suggesting that the condition around this altitude is favorable for ice nucleation. The satellite data indicated that ice crystals in the clouds were in the size range of about 10–90 μm in diameter. Unfortunately, the OPCs used in this study cannot display information on size-resolved particle concentrations in the size range of ice crystals because the largest integration limit is set to 3.6 μm. However, it is expected that background aerosols in the upper troposphere rarely contained particles with Dp > 3.6 μm because these larger particles were rarely detected at <400 hPa (Figure 1), with the exception of 17 August 1999. This means that the background aerosols are considerably smaller than 3.6 μm. In fact, it is well known that the background aerosols are usually submicron aerosols, consisting of solutions (mixtures of H2SO4, HNO3, NH3, H2O, or organics), followed by carbon, mineral dust, fly ash, etc. [e.g., Murphy et al., 1998]. For these reasons, we consider that the particles above the 3.6-μm detection threshold were predominantly ice crystals.
 To further investigate the possibility of ice formation around the cloud layer, we referred to the RHice values estimated from the radiosonde temperature and the specific humidity from the European Centre for Medium-range Weather Forecasts (ECMWF) data, which has a temporal resolution of 6 hours, horizontal resolution of 2.5° and vertical resolution of 23 pressure levels from 1000 to 1 hPa (not shown here). According to this result, relatively high water vapor and supersaturated or subsaturated conditions with respect to ice were reproduced from about 250 to 150 hPa over the Tibetan Plateau. However, the ECMWF meteorological fields are quite problematic and in-situ measurements of water vapor are needed in the future work.
4.2. Processes Responsible for Cloud Formation
 Figure 2b shows the number fractions of submicron aerosols and particles with Dp > 3.6 μm (i.e., ice crystals)' to the total number concentrations of particles with Dp > 0.3 μm. The observed total particles were dominated by submicron aerosols and the percentage of ice crystals was less than that of submicron aerosols even at ∼150 hPa. This implies that not all particles in the cloud layer took part in ice nucleation, and most of the background aerosols were not removed by ice nucleation and scavenging.
 The selectivity of the ice nucleation process may depend closely on aerosol size, because larger droplets freeze prior to smaller ones [Koop et al., 2000]. Here we estimated the relative humidity required for homogeneous freezing of supercooled solution droplets between 0.3 and 1.2 μm in diameter by using the parameterization given by Koop et al. . As shown in Figure 3, around 150 hPa or 210 K, the relative humidity required for homogeneous freezing is about 150–160% RHice, and make little difference in the size of the droplets. Also, given that the relative humidity is much lower than the liquid water saturation, submicron solution droplets in the background aerosols probably do not show exponential growth via adsorption as well as ice nucleation until the critical RHice is reached [e.g., Möhler et al., 2003]. For this reason, we consider that ice nucleation processes other than homogeneous ice nucleation (i.e., heterogeneous ice nucleation) are required to explain the coexistence of submicron aerosols and ice crystals in cirrus clouds.
Figure 3. Examples of the critical RHice required for heterogeneous ice nucleation on uncoated mineral dust (Arizona test dust, Asian dust, and Saharan dust) [Möhler et al., 2006], uncoated and H2SO4 coated soot [Möhler et al., 2005], and solid ammonium sulfate [Abbatt et al., 2006] from the AIDA cloud chamber. The shaded region indicates the critical RHice required for orographic cirrus formation derived from field measurements over the Rocky Mountains [Heymsfield and Miloshevich, 1995]. The double solid line indicates the critical RHice required for homogeneous ice nucleation in supercooled solution droplets with Dp = 0.3–1.2 μm using a freezing rate of 1 min−1 [Koop et al., 2000]. The dashed line indicates the liquid water saturation; i.e., relative humidity with respect to liquid water (RHliq) = 100%. These vertical profiles are based on the radiosonde temperature and pressure on 17 August 1999.
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 The particles with Dp > 3.6 μm in the continental cirrus clouds observed over the Tibetan Plateau may reflect natural and anthropogenic inputs of ice nuclei into the upper troposphere via deep convection, orographic lifting or advection. During the observing period, a synoptic scale monsoon anticyclone (i.e., Tibetan anticyclone) dominated the large-scale circulation in the upper troposphere over the Tibetan Plateau [Tobo et al., 2007]. The temperature profiles from the 17 August flight indicated a warm core up to ∼150 hPa compared to those from the other two flights (Figure 1) and the zonal mean temperature profiles from the ECMWF data (not shown here). It is well known that the Tibetan anticyclone occurs primarily as a response to diabatic heating associated with deep convection over the Tibetan Plateau [Randel and Park, 2006]. Recent model studies by Li et al.  suggested that anthropogenic aerosols, such as sulfate, black carbon, and organic carbon, emitted from South Asia extend to the upper troposphere and then are entrained by the Tibetan anticyclone for several days in deep convective systems. These points raise the possibility of the widespread existence of continental aerosols in the upper troposphere.
 In addition, we speculate that a temperature-dependence of effective ice nuclei is kind of responsible for the cirrus formation. For example, Rangno and Hobbs  indicated extremely low concentrations of effective ice nuclei at warmer temperatures (warmer than 254 K), which are several orders of magnitude less than the concentrations of the particles with Dp > 3.6 μm at cirrus temperatures (∼210 K) presented in this study. On the other hand, the concentrations of the cloud particles with Dp > 3.6 μm are similar to the concentrations (∼0.01 cm−3) of effective ice nuclei at lower temperatures (less than 235 K), which have been measured in the free troposphere over the Rocky Mountains [DeMott et al., 2003a].
 As illustrated in Figure 3, recent laboratory experiments with the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber have suggested that deposition nucleation (the formation of ice from a supersaturated gas environment directly on a solid surface) or immersion freezing (the formation of ice at the surface of a nucleus suspended in a supercooled liquid) involving mineral dust [Möhler et al., 2006], soot [Möhler et al., 2005] or solid-phased soluble species (solid ammonium sulfate) [Abbatt et al., 2006] is efficient at temperatures of about 210–220 K, corresponding approximately to the altitude ranges from 150 to 200 hPa over Lhasa. It seems that the critical RHice required for heterogeneous ice nucleation on these continental aerosols is approximately consistent with that required for orographic cirrus formation indicated by Heymsfield and Miloshevich . The temperature-dependent changes in the ice nucleation threshold are still largely uncertain, but suggest that these aerosols act as efficient ice nuclei at cirrus temperatures.