Norman T. Kjome

Backscattersonde: a new instrument for atmospheric aerosol research

A relatively simple balloonborne device for measuring the local aerosol backscatter at multiple wavelengths has been developed and field tested. The instrument produces detailed profiles which are inherently similar to those generated from lidar soundings. It is also sensitive, being able to measure the 20-km stratospheric aerosol background layer with a signal-to-noise ratio of approximately 100:1. An important feature of this device is its ability to be calibrated in an absolute sense. Theoretical considerations show that the measurements can be accurately converted to aerosol mass loading in the stratosphere for conditions ranging from background to heavy volcanic influence. The instrument is not expected to replace other observational techniques; rather it provides highly complementary information as well as furnishes a cost-effective alternative to measurement systems available to only a few scientists.

Temperature histories in liquid and solid polar stratospheric cloud formation

Polar stratospheric clouds (PSCs) have been observed by balloonborne backscatter sondes from Alert, Thule, Heiss Island, Scoresbysund, Sodankyla, Sondre Stromfjord, and Ny Alesund during winters 1989, 1990, 1995, and 1996 in 30 flights. The observations can be categorized into two main groups: type 1a and type 1b PSC particles. Type 1b PSCs show the characteristics expected from liquid ternary solution (HNO3/H2SO4/H2O) particles, consistent with model simulations. Type 1a PSCs are observed at all temperatures below the condensation temperature TNAT of nitric acid trihydrate (NAT), consistent with solid NAT composition. Air parcel trajectories have been calculated for all observations to provide synoptic temperature histories of the observed particles. A number of cases have been identified, where the particles have experienced temperatures close to or above the sulfuric acid tetrahydrate melting temperatures within 20 days prior to observation. This assures a knowledge of the physical phase (liquid) of the particles at this time, prior to observation. The subsequent synoptic temperature histories, between melting and the time of observation, show pronounced differences for type 1a and type 1b PSC particles, indicating the qualitative temperature conditions, necessary to generate solid type 1a PSCs. The temperature histories of type 1b particles show smoothly, in most cases monotonie, decreasing temperatures. The temperature can apparently decrease to the frost point without causing the particles to freeze. The type 1b PSC particles are mostly observed shortly after entering a cold region. The observed type 1a particles have spent several days at temperatures close to or below TNAT prior to observation, often associated with several synoptic temperature oscillations around TNAT, and the particles are observed in aged clouds. It appears that the PSC particles may freeze, if they experience synoptic temperatures below TNAT with a duration of at least 1 day, possibly accompanied by several temperature oscillations. However, liquid particles that experience a smooth cooling, even to very low temperatures, or single smooth cooling/heating below TNAT without synoptic temperature fluctuations do not seem to freeze.

Dehydration and sedimentation of ice particles in the Arctic stratospheric vortex

Balloon borne frost-point hygrometers and backscatter sondes were launched at Sodankyla, Finland in January and February of 1996. These instruments measure water vapor and the backscatter ratio of light due to polar stratospheric clouds in the Arctic stratospheric vortex. Here we report the results of a hygrometer sonde and a backscatter sonde launched within 3.5 hours of each other on January 22/23. Together these soundings show a strong loss of water vapor due to the formation of ice clouds as a result of record cold temperatures in the Arctic stratosphere. The separation of the upper edge of the layer showing water vapor loss and the upper edge of the PSC layer indicates sedimentation of the ice particle layer, possibly leading to a permanent dehydration in the upper part of the layer exhibiting water vapor loss.

Decay of Mount Pinatubo aerosol at midlatitudes in the northern and southern hemispheres

Conjugate in situ observations of the stratospheric aerosol following the 1991 Mount Pinatubo eruption were made over Laramie, Wyoming (41°N), and Lauder, New Zealand (45°S), using balloon-borne backscattersondes. The results show similar aerosol loading and decay rates occurring over both midlatitude locations. A comparison with the 1982 El Chichon eruption as observed over Laramie indicates only somewhat higher mass mixing ratios for the Pinatubo eruption and very similar decay rates. As in the past, a strong correlation between local tropopause pressure and stratospheric aerosol mass loading was observed at the stations involved. Such a feature would be potentially useful for putting local measurements into a global prospective.

Simultaneous ozone and polar stratospheric cloud observations at South Pole station during winter and spring 1991

Simultaneous polar stratospheric cloud (PSC) and ozone measurements were made over South Pole Station using a two-wavelength backscattersonde. This instrument produces aerosol profiles similar to those obtained with a ground-based lidar system but with higher vertical resolution. In one sounding, depolarization of the PSCs was also measured. The backscattersondes were supplemented with occasional frost point soundings. The measurements made before the appearance of PSCs do not show clear evidence of particle deliquescence, suggesting that the background sulfate particles may be frozen solids rather than liquids. PSCs began appearing at ∼20 km when the temperature at that altitude dropped to −80°C (193 K). Initially, there was apparent evidence of supersaturation (with respect to nitric acid trihydrate) associated with some type I PSCs, while other examples indicated that the condensation of nitric acid was in quantitative agreement with that expected from the saturation vapor pressure and available nitric acid vapor. The apparent supersaturated layers (which occurred within the first 2 weeks of the onset of PSCs) can alternatively be interpreted as denitrified regions. The wavelength dependence of the backscatter is used to deduce rough particle sizes, and in particular, type Ia and Ib population types can be readily identified by the backscattersonde when not occurring as mixed systems. The mode radius of the first observed PSCs of the season was ∼0.5 μm. In the polarization sensitive sounding, two varieties of type I PSCs were observed, one of which exhibited significant depolarization and another which produced very little depolarization. This observation would be consistent with the classification of types Ia and Ib, respectively. At the precise time that sunlight was returning to the stratosphere near South Pole Station, a strong inverse correlation in the structure of PSCs and ozone mixing ratio was observed. Using trajectory analysis, it is argued that the effect is probably the result of chemical depletion rather than transport processes. This chance observation is consistent with enhanced ozone depletion occurring in association with sunlit PSCs during the early spring.

Comparison of analyzed stratospheric temperatures and calculated trajectories with long‐duration balloon data

The Polar Vortex Balloon Experiment (POVORBEX) has flown eight long-duration flights 1992 to 1995 in the arctic winter stratosphere at about 50 mbar. European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are compared with the balloon temperatures and winds, which are accurate and independent, and substantial differences are found. For example, the average temperature difference is 2.4 K. These temperature differences have important implications for the potential for polar stratospheric clouds, which play a vital role in the destruction of ozone. Large differences are also found between the observed and calculated trajectories. For the longest flight, which lasted almost 6 days, the calculated trajectory end point is off by 2700 km (23% of the trajectory length). For the other seven POVORBEX flights, which lasted about 1/2-3 days, the calculated end point is off by 156-544 km (5-19% of the trajectory length for six of the flights). The main causes of these differences are discrepancies between real and ECMWF analyzed winds and temperatures and the large distance between the model levels at this altitude.

Penetration of Mt. Pinatubo aerosols into the north polar vortex

Observations of the Mount Pinatubo cloud as it moved northward and into the winter polar vortex were made with a balloon borne two-wavelength backscattersonde. Some volcanic debris had arrived at far northerly latitudes below 20 km by October and was apparently incorporated into the initial vortex. Subsequent measurements did not show a significant increase in the central vortex aerosol until after a mid January disturbance, at which time the backscatter profiles began to increase. Above 20 km and near the center of the vortex there was no significant increase in aerosol through mid March. The column stratospheric aerosol mass loading as calculated from individual soundings at the wall of the vortex during March 1992 ranged from 18 to 24 megatonnes per unit area where the unit of area is that of the earth. This indicates that a significant amount of material was transported northward by the end of the winter.

Polar stratospheric cloud threshold temperatures in the 1995-1996 arctic vortex

Balloon-borne backscattersondes have been used to study the relationship between particle scattering and ambient temperature near the vertical edge of arctic polar stratospheric clouds (PSCs) as well as to delineate the cloud type occurrence probability as a function of temperature. The observed typical threshold temperatures as a function of altitude are about1°K warmer than the temperature TSTS expected for rapid growth of supercooled ternary solution aerosols. A more descriptive analysis shows that the threshold temperatures occur over a definable range of temperatures and tend to cluster near, but somewhat warmer than, TSTS. Considering the experimental and theoretical uncertainties, this difference may not be significant. The probability of type Ib PSC occurrence shows a dramatic increase at TSTS±1°K, while for type Ia PSCs the probability is roughly constant at 10% for temperatures below the formation point of nitric acid trihydrate (TNAT).

Chemical, microphysical, and optical properties of polar stratospheric clouds

A balloonborne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 near Kiruna/Sweden. Besides an aerosol composition mass spectrometer, the gondola carried optical particle counters, two backscatter sondes, a hygrometer, and several temperature and pressure sensors. A mountain wave induced PSC was sampled between 20 and 23 km altitude. Strongly correlated PSC particle properties were detected with the different instruments. A large variability of particle types was measured in numerous PSC layers, and PSC development was followed for about two hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of nitric acid trihydrate (NAT) particles with a molar ratio H 2 O:HNO 3 close to 3 at temperatures near and below the equilibrium temperature T NAT . The median radius of the NAT particle size distribution was between 0.5 and 0.75 μm at concentrations around 0.5 cm -3 . Below the NAT layers and above T NAT , thin cloud layers containing a few large particles with radii up to 3.5 μm coexisted with smaller solid or liquid particles. The molar ratio in this region was found to be close to two.

Springtime aerosol layers in the free troposphere over Australia : Mildura Aerosol Tropospheric Experiment (MATE 98)

A field campaign focused primarily on free tropospheric aerosol measurements over Mildura, Australia, at 34°S (Mildura Aerosol Tropospheric Experiment (MATE 98)) was conducted in the austral spring of 1998 to test for the current presence of a seasonal aerosol layering activity observed in the 1970s and to obtain additional characteristics that would lead to a better understanding of the phenomenon. Ground-based lidar as well as balloon-borne optical particle counters and backscattersondes with ozone sensors were employed. The results indicate that large horizontal scale layers are present and show that their structure is highly correlated with excess ozone. Particle concentrations in the layers were sufficient to measurably affect aerosol optical depth. The probable source is distant biomass burning regions, but a detailed understanding of associated smoke transport and evolution as observed over Mildura is incomplete.