David C. Rogers, Paul J. DeMott, Sonia M. Kreidenweis, Yalei Chen
Department of Atmospheric Science, Colorado State University,
Fort Collins, CO
Abstract. Instruments on NASA's DC-8 Airborne Laboratory were used to obtain measurements of ice nucleating aerosols (IN) and CN (total particle number) in background and aircraft-affected air during the April-May 1996 SUCCESS project. IN concentrations ranged from ~0.1 to ~500 per liter, being generally greater at colder temperatures and higher supersaturations. Overall, there was wide variability of IN concentration. Shorter time periods showed closer grouping of the data than did entire flights or the entire data set. IN and CN did not generally correlate. Day to day variations were substantial. During penetrations of aircraft exhaust plumes, CN exhibited a very strong response, but there was no evidence that exhaust is a significant IN source for the temperature and supersaturation conditions of our measurements.
IN are a special subset of total aerosol, characterized only by their ability to nucleate the ice phase. Measurements of IN are difficult because the activity of IN depends on both temperature and humidity, there are multiple nucleation mechanisms, and there is no clear association with size or chemical properties of the IN aerosol. Very little information exists about IN concentrations at high altitudes (one paper by Bigg 1967) or about the potential of aircraft exhaust as IN (Popoff et al. 1958). The goals of our research are to estimate the concentrations, spatial distributions, sizes and chemical composition of IN and non-IN aerosols in aircraft exhaust plumes and in background air, and from these, to derive quantitative descriptions of IN and CN suitable for use by numerical and conceptual models (i.e., to describe the heterogeneous IN activity based on temperature, humidity and whether the aerosols have been affected by aircraft exhaust).
This paper describes the measurement technique and presents an overview of the results, including the dependence on supersaturation and the IN activity of aircraft exhaust. Companion papers in this issue describe the size and chemical composition of IN (Chen et al. 1997) and an assessment of the role of IN for upper tropospheric clouds (DeMott et al. 1997).
The project's emphasis was on high altitude clouds, so most flights were spent upwards of ~10km where the air was already quite cold (-35 to -60°C). The effect of ice nuclei in those conditions is expected to be slight, since homogeneous freezing of droplets should dominate the ice formation process. Modeling studies, however, suggest that haze/solution droplets will freeze more readily if they contain ice nuclei, with the consequence that IN-affected cirrus clouds can have substantially different microphysical and radiative properties (DeMott et al. 1997).
The IN and CN instruments were located inside the NASA DC-8 aircraft as shown in Figure 1. A forward facing 5cm diameter tube was mounted on the port fuselage near the trailing edge of the wing ~24m aft of the nose. The inlet was 38cm from the aircraft skin, outside the flow boundary layer. Inside the aircraft cabin the tube was reduced to 2cm, made a short loop, and was vented back outside; our instruments sampled from this loop. Ambient air flowed continuously through the tube (residence time <0.5s). The inlet system affected our sampling in several ways, including non-isokinetic flow at the inlet, diffusion losses in the smaller piping, air deceleration and heating, and flow perturbations at this location during strong aircraft maneuvers. Non-isokinetic effects were not evaluated; they are expected to be substantial for particles >~1µm (e.g., Davies 1968), but their significance for these measurements is reduced because we used an impactor at the upstream end of our piping to remove particles >2µm. Particle losses at the inlet were not measured. Losses in the piping were measured at ~20% for 0.07<Dp<0.2µm, increasing to 50% at ~0.03µm. CFD and CN measurements were continuous and at a pressure slightly above ambient due to ram pressure.
CN concentration was measured with a butanol type instrument (TSI 3010) at 0.4s intervals and recorded on the CFD computer. Minimum detectable CN was 0.012µm. Performance tests indicated there was no significant reduction in sensitivity for pressures down to 150mb. The PIXE impactor made timed collections of size-resolved total aerosol on transmission electron microscope (TEM) grids for later single particle analysis (Chen et al. 1997).
The CFD is a thermal gradient diffusion chamber of concentric cylinder design (Rogers 1988; 1994). Figure 2 shows the operating principles. Air flows vertically downward in the annular space between two ice-coated cylinders that are at different temperatures. Sample air occupies the central 10% of the flow, sandwiched between particle-free sheath flows and is exposed to a supersaturation determined by the temperature difference. Active IN aerosols form ice crystals that grow to ~3µm diameter or larger, depending on temperature, humidity and residence time. The droplet evaporation section starts at ~2/3 through the chamber, where the warm ice vapor source is removed, and the humidity drops to ice saturation. Particles >3µm (ice crystals) are counted at the chamber outlet by a Climet 7350A optical particle counter (OPC). An inertial impactor was located immediately downstream of the OPC in order to capture crystals (containing IN) on TEM grids. Typically, several hundred crystals were collected on a grid, and single particle analysis techniques were used to determine the number, size, morphology and chemical composition of the nuclei (Kreidenweis et al. 1997).
The airborne CFD is a second-generation instrument and has a number of refinements over the laboratory model described in Rogers (1988). The instrument and performance will be described in a separate paper (Rogers and DeMott 1997). In brief, the principal changes are:
· chamber is 50% longer, giving more time for crystal growth and droplet evaporation
· a computer-based system manages temperature control and data logging
· the temperature control is based on direct expansion of refrigerant from a compressor with digital control valves
· sampling conditions (temperature, supersaturation, growth time, IN concentration and IN collected number) are calculated and displayed in real time
· the chamber and piping operate at outside pressure in a pressurized cabin
· air flows are measured with mass flow meters
During SUCCESS, IN measurements were made over a wide range of conditions, 13 to 38°C and -25% to +20% water supersaturation (SSw),as shown in Figure 3. Entries from 19 flights are shown. The lower SS limit is ice saturation. The upper SS limit is based on wall temperatures, but modeling simulations suggest that CCN will ultimately limit the actual SS by serving as vapor sinks. Thus, the real maximum limit is perhaps ~10-15%. The CFD samples at only one temperature and SS at a time; changing the conditions (wall temperatures) takes a few minutes. Two sampling approaches were used: (1) constant temperature and SS; and (2) varying temperature and SS. The second approach is an attempt to estimate the thermodynamic response of IN (spectra), but the inherent limitation is the continuously changing sample air. As operated during SUCCESS, the CFD had no sensitivity to detect homogeneous freezing or contact-freezing.
A large quantity of IN measurements were collected during SUCCESS. The results shown here represent some aspects of the larger data set and illustrate the type of data obtained. Overall, the IN concentration ranged from the lower detection limit ~0.1 to over 100 per liter, as shown in Figure 5. For this figure, all measurements were first integrated into ~1 minute averages (~1 liter of air). There were also many zeroes and a few periods with very high concentration. Notice that some high concentration values were below water saturation. Perhaps this indicates a deposition mechanism, or immersion freezing of a haze/solution droplet. Observational studies in winter clouds have noted a lack of evidence to support the existence of deposition nuclei (e.g., Cooper and Vali 1981), but the generality of that conclusion is unknown.
A specific example of IN measurements is shown in Figure 3. The CFD sample temperature was -32 to 36°C, and SS was adjusted to oscillate above and below water saturation. Notice that IN concentration generally oscillated in phase with SS, suggesting that condensation-freezing was a strong mode. The same data plotted as IN versus SS show a simple dependence of the form: N~3exp(SS/10). The area around water saturation is especially interesting region for ice nucleation because different nucleation modes come into play, i.e., deposition and condensation-freezing. Observational studies of wave clouds (e.g., Cooper and Vali 1981; Heymsfield and Milosevich 1993) suggest there is a strong increase in crystal production above water saturation. IN studies have also suggested a rapid increase there (Rogers 1993; Mizuno and Fukuta 1996). During this period, the aircraft was skimming the top of a cirrus cloud at ~210mb, -60°C.
A specific focus of these studies was to assess the potential of aircraft exhaust as a source of ice nuclei. On several days, the DC-8 followed other project aircraft, penetrating contrails and exhaust regions for extended periods at distances from ~10 to 30km (40 to 120s travel time). Figure 6 shows IN and CN measurements during one 20 minute period on May 3. The DC-8 was at 10.6km altitude and trying to stay in the exhaust plume from the NASA B-757. Wake vortexes were strong enough that the DC-8 was pushed out of the plume often and abruptly. Large variations in CN were measured as the DC-8 crossed back and forth through the plume, from background values of about 100/cc to peaks of over 1000/cc; it is likely that total CN were much higher, but our CN counter had limited response to particles smaller than 0.012µm. IN data in the second panel are plotted as counts instead of concentration to show the nature of the measurement, i.e., many zeros with occasional non-zero values. IN counts were sampled every 0.4s, so there are 3000 points shown, of which 2893 are zeroes and 107 are ones. The plot shows no obvious strong correlation between CN (exhaust) and IN. During this period, the CFD sample temperature was cooling slightly, from -33 to -35°C, and SS increased from -4 to +5%. The average IN concentration was 10.5 per liter, 9.9 when below water saturation and 10.9 above. The IN fraction was about 35 per million CN.
To evaluate the possibility that exhaust is a weak source of IN, an integrated analysis was done for six days that had exhaust study periods. CN concentrations were used to stratify the IN data as background or exhaust, and data in 4s records were accumulated and compared. The results showed that IN concentrations were slightly greater in exhaust air for only one of the six cases (May 7). On the other five other days (April 18, 27 and 30 and May 3 and 4), the exhaust regions were not different from background regions for IN content. Our measurements thus indicate that exhaust is not a strong source of ice nuclei, for the temperature and supersaturation conditions used in the CFD.
Ice nuclei concentrations ranged from ~0.1 to ~500 per liter, with greater IN concentrations at colder temperatures and higher supersaturations. Overall, there was wide variability of IN concentration. Shorter time periods showed closer grouping of the data than did entire flights or the entire data set. IN and CN concentration did not generally correlate. IN increased with colder sampling temperatures and higher SSw. Single flights (days) generally showed the most consistency of IN; day to day variations were substantial. During penetrations of aircraft exhaust plumes, CN exhibited a very strong response, but there was no evidence for a strong IN presence.
This research was sponsored by NASA grant NAG-2-924 and NSF grant ATM93-11606. We also wish to acknowledge the technical support from the Medium Altitude Missions Branch at NASA Ames Research Center for helping us install the instruments.
Bigg, E.K., Cross sections of ice nucleus concentrations at altitude over long paths. J. Atmos. Sci., 24, 226-229, 1967
Cooper W.A. and G. Vali, The origin of ice in mountain cap clouds. J. Atmos. Sci., 38, 1244-1259, 1981.
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Figure 1. Aerosol sampling arrangement. CFD, CN and PIXE sample
from continuously ventilated tube. In CFD, ice nuclei activate,
grow to detectable size and are counted by OPC. At selected times,
they were also collected on 3µm impactor. CN measures total
particle concentration. MF is mass flow meter.
Figure 2. Operating principles of CFD. Air flows between ice-covered
walls held at two different temperatures. Particles nucleate
and grow to detectable size in supersaturated region. Particles
>3µm (ice crystals) are counted by OPC. With walls -20
and -36°C, sample temperature is -30°C and 1% supersaturation.
Typical residence time ~3-5s.
Figure 3. Range of temperatures and supersaturation conditions
in CFD for all ice nuclei measurements during 19 SUCCESS flights.
Letters identify flight and represent one minute averages.
Figure 4. Cumulative frequency distribution of IN concentrations
during SUCCESS, for all temperature and supersaturations. Measurements
were first integrated into ~1 minute samples (~1 liter). Also
shown is ratio of IN to CN measured at the same time.
Figure 5. Ice nuclei measurements on 24 April 1996 for ~25 minutes,
showing oscillating control of supersaturation and response of
IN. Sampling temperature -32 to 36°C. Values are
1-minute averages. Same data are plotted to show dependence on
supersaturation.
Figure 6. CN and IN measurements on 3 May 1996 for ~20 minutes
during numerous exhaust plume penetrations of B-757 aircraft ~40s
ahead of DC-8. Values at 0.4s interval.
