Division Atmospheric Physics

Aerosols in Polar Stratospheric Clouds

Investigations of Aerosols and Polar Stratospheric Cloud particles
Supported by EU and BMBF

Polar stratospheric clouds (PSCs) play an important role in the destruction of the ozone layer, which protects life on earth from high energetic UVradiation. Only during the winter in polar regions, temperatures in thestratospherecan decrease to extreme low values, leading to the formationof this specialtype of clouds at altitudes between 15 and 25 km. Passivechlorine speciesof mainly anthropogenic origin can react in and on thesecloud particles.The reaction products are released into the gas phase,photolyzed by solarradiation and thus become activated. Only the activatedhalogen species candestroy ozone. The activation of the halogen speciesdepends strongly on particlecomposition and stratospheric temperatures.Despite their importance in theozone destroying cycle, the compositionof polar stratospheric clouds hasnever directly been measured.
Laboratory studies of PSC particle simulations are difficult to perform.Often thermodynamic and kinetic parameters are measured on macroscopicsubstrates which are considerably different from actual microscopic particles.A large kryoaerosol chamber provides the opportunity to simulate realisticallyover days many stratospheric parameters like aerosol particle growth atlow temperatures.
Precise and accurate calibrations are a prerequisite for the chemicalanalysis of PSCs in the stratosphere and laboratory studies.
 

Balloon-borne measurements of Polar Stratospheric Clouds

First results of in-situ measurements of the chemical composition ofPSCs are presented, which have been performed over Kiruna, Sweden, in theearly morning of January 25, 1998. PSCs at 21.5 and 23 km and temperaturesbetween 187 and 192K have been encountered with a balloon-borne experiment,a combination of a PSC mass spectrometer and a backscatter sonde (operatedby N. Larsen, Danish Meteorological Institute, Copenhagen, Denmark) [1].In the clouds,a simultaneous increase of condensed water, nitric acid,and backscatter ratios has been observed. The measured molar ratios ofwater to nitric acid indicate the presence of liquid mixtures of water,nitric acid, and sulfuric acid,so-called supercooled ternary solutionparticles (STS).
 

 
PSC mass spectrometer. 
The PSC mass spectrometer has a specially designed particle inlet, an aerodynamic lens, which focuses particles to an aerosol beam [2]. Behind the lens,gas molecules spread out and are pumped away, whereas the particle beamcrosses a differentially pumped vacuum system. Particles are stopped ina gold-plated sphere, evaporate, and the chemical composition of the resultinggas is measured with a quadrupole mass spectrometer. This system providesan effective separation of the particles from ambient air.

The mass spectrometer has been calibrated before and after the flight; see calibration .
Results. The balloon was launched on January 25th at 00.30 UTC fromKiruna, passed south-eastwards and landed four hours later in Finland nearthe boarder of the Baltic Sea. During that night, PSCs appeared behindthe Scandinavian mountains due to a strong leewave situation.

The particle inlet was opened at 7300 s. During the measurement period, two PSCs have been encountered near 21.5 and 23 km at temperatures between 187 and 192 K. The clouds had a vertical thickness of 700 and 300 m, respectively. The mass spectrometer signals show a sharp increase of water (mass 18,H2O+) and nitric acid (mass 30, NO+, a fragment ion of HNO3)  whilecrossing the clouds. A strong correlation between elevated mass spectrometersignals and large backscatter ratios is evident. Sulfuric and hydrochloricacids have not been detected due to low sensitivity and high instrumentalbackgroundsignal. Between the clouds and towards the end of the measurement,an increasein the water signal indicates an uptake of water into stratosphericbackgroundaerosol with decreasing temperature. During this time, nitricacid remainednear or below the detection limit.
 

 
Count rate at mass 18 (dots) and mass 30 (triangles), backscatter ratio (grey line in the lower panel), altitude and temperature (upper panel) during the flight.
The count rates of water and nitric acid have been converted to molar ratios H2O:HNO3 in the condensed phase using calibration data. Molar ratios H2O:HNO3 in both clouds indicate the presence of supercooled ternary solution particles (STS). The measured molar ratios (black lines with error bars) are compared to equilibrium STS-model calculations [3]. Assuming an error of ?1 K inthe temperature measurement, the model (grey shaded area) agrees with themeasurements.
 
3 3  
Molar ratios of water to nitric acid in the condensed phase (left: first PSC, right: second PSC), lines with dots and error bars are the measured values, the grey shaded area indicates model results.
The variation of the molar ratios in the second PSC may be explained byrapid temperature fluctuations, which can lead to nonequilibrium particlecompositions. Nitric acid trihydrate (HNO3·3H2O) particles witha molar ratio ofthree, also suggested to exist at these temperatures byCrutzen and Arnold[4] and Hanson and Mauersberger [5], have not been observeddirectly. Nevertheless, a small fraction of these particles might havebeen present in the cloud due to temporal and spatial integration in themass spectrometer. Ice particles have not been measured.

[1] Rosen, J.M. and N.T.Kjome: Backscatter sonde: a new instrument for aerosol research. Appl. Optics 30, 1552-1561 (1991).
[2] Schreiner, J. et al.: Aerodynamic lens systemfor producing particle beams at stratospheric pressures. Aeros. Sci. Tech.29, 50-56 (1998).
[3] Carslaw K.S., et al.: An analytic expressionfor the composition of aqueous HNO3-H2SO4 stratospheric aerosols includinggas phase removal of HNO3. Geophys. Res. Lett. 22, 1877-1880 (1995).
[4] Crutzen, P.J. and F. Arnold: Nitric acidcloud formation in the cold Arctic stratosphere: A major cause for springtime"ozone hole". Nature 324, 651-655 (1986).
[5] Hanson, D.R. and K. Mauersberger: Laboratorystudies of the nitric acid trihydrate: Implications for the south polarstratosphere. Geophys. Res. Lett. 15, 855-858 (1988).
 


 

Calibration of the aerosol analysis systems

Introduction. Data analysis and comparison with model calculations require an accurate calibration of our aerosol analysis experiments. At the moment, two different calibration methods exist: One possibility is the generation of aerosol particles with a well-defined chemical composition. This hasalready been done for sulfuric acid/water aerosols and successfully appliedto the calibration of the measurements at the AIDA chamber. Another methodwhichworks without aerosols, is a gasphase calibration. The idea is toproducewell-known partial pressures of the vapour of the substances expectedto becondensedin PSC particles inside the evaporation sphere. This methodhasalready beenused to calibrate the balloon-borne PSC analysis experiment.
Experimental. For the second method, a calibration system has beendesigned and constructed during the last two years to determine the sensitivityofthe mass spectrometer for vapours of substances expected to be condensedinPSC particles. Most of these compounds – mainly water and nitric acid,as well as traces of sulfuric, hydrochloric, and hydrobromic acids –stickto surfaces or tend to decompose. Furthermore, the pressures generatedby evaporation of PSC particles in the analysis system are in the very-high-vacuumrangeand therefore difficult to establish with sufficient precision. Toovercomethese difficulties, the calibration is based on a constant gasflow throughtwo chambers with a dynamic pressure reduction taking placebetween them.
 

 
Calibration system for the PSC analysis experiment 
The pressure pM in the first chamber is in the fine-vacuum range and canbe measured accurately and gas-independently with a capacitance manometer.Water vapour (H2O), hydrochloric acid (HCl) and nitrogen (N2) enter thischamber through a variable leak valve which enables us to control pM. Thevessel for nitric acid (HNO3) is surrounded by a cooling jacket filledwith siliconefluid which is held at a constant temperature down to –60°Cby acryostat. The resulting vapour pressure of nitric acid is so low thatit canbe measured directly with the capacitance manometer. The mass spectrometerto be calibrated is connected to the second – nearly spherical –chamberdownstream of the small orifice. This chamber is operated at the sametemperatureT as the evaporation sphere in the balloon-borne system. The bigorificeat itsexit has a much higher pumping speed than the small orificeat theentrance.Consequently, for a constant gas flow from the left to theright,the calibrationpressure p in the second chamber is several ordersof magnitudelower thanpM. From the well-known pumping speeds and the temperaturesof the two chambers,the ratio of the pressures p and pM can be calculated.

The calibration method mentioned first was established during the last year. The principle consists of using liquid water/sulfuric acid aerosolparticles with a definite composition to calibrate the aerosol beam massspectrometer.
The aerosol particles pass a flow reactor. By varying the concentrationof the sulfuric acid solutions in this flow reactor, the partial pressureofthe water is changed. Therefore the composition of the particles isaltered,because the liquid particles will come into equilibrium with thewater gasphase by uptaking or releasing water. In equilibrium the particleswill reachthe composition of the sulfuric acid bath inside the flow reactor.The dimensionsof the flow reactor were chosen such as to ensure that theparticles willreach the equilibrium state before entering the aerodynamiclens.

The sulfuric acid aerosols are produced by atomizing a sulfuric acidsolution with nitrogen. The particles enter a conditioner which is a simpleglass tube filled with 98 wt% sulfuric acid. This is necessary to dry theaerosols. With an electrostatic classifier and a condensation-nuclei counterit is controlled that the particles are in a size range of 0.2 to 1 µm.The aerosol particles enter the glass tube surrounded by a cooling jacket.The cooling of the flow reactor is necessary to decrease the contributionof the background gas phase to the water signal and temperatures of –60°Ccan be achieved. Inside the reactor a rotating glass cylinder touches thesulfuric acid bath. Theglass cylinder rotates with approximately 1 to5 rpm. For this reason theinner and outer side of the cylinder are wettedhomogeneously and thereforeprovide a homogeneous water gas phase. Theparticles enter the aerodynamiclens in the cold area to be certain thatthe composition remains unchangeduntil they reach the evaporation sphereof the aerosol-beam mass spectrometer.
 

 
Scheme of the flow reactor with a rotating glass cylinder inside. At the KF-16 flanges pressure and temperature sensors and a pump can beconnected.
As a calibration result we obtain a linear relationship between the count rate ratio of water (mass 18) to sulfuric acid (mass 98) and the molarratio of the aqueous sulfuric acid solution. The possible reason for thepositive y-axis offset might be dissociation of H2SO4 in the evaporationsphere ofthe aerosol-beam mass spectrometer. With this calibration methodwe obtainan absolute calibration error of about 5 wt% in a range of 15to 65 wt% sulfuricacid.
 
 
Diamonds show the experimental data points with experimental error. The curve with error bars represents the calculated fit.  The concentrations of the sulfuric acid solutions  are expressed in molar ratios.

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last change: 02.01.2001        questions and comments to:  Dr.Jochen Schreiner

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