Aerosol particles in the atmosphere between thetropopause and an altitude of approximately 30 kmare composed mainly of aqueous sulfuric acid.1 Theparticles provide surfaces for processing chemicalspecies involved in stratospheric ozone depletion atmid-latitudes and in polar regions.2–8 In addition,he aerosols can scatter and absorb short- and long-ave radiation, altering the temperature of the lower
tratosphere and reducing the radiant energy reach-ng the surface–troposphere system.9,10 The num-er density, size, phase, and acidity of the aerosolarticles determine their effects on stratospherichemistry and radiative transfer.
In typical stratospheric conditions, water is the
When this study was performed, A. E. Heathfield, D. A. Newn-ham, and J. Ballard were with the Space Science Department,Rutherford Appleton Laboratory, Chilton, Didcot, OxfordshireOX11 OQX, UK. A. E. Heathfield is now Specialist Assistant tothe Select Committee on Science and Technology at the House ofLords, London, UK. The e-mail address for D. A. Newnham [email protected]. When this study was performed, R. G.Grainger and A. Lambert were with the Department of Atmo-spheric, Oceanic, and Planetary Physics, Clarendon Laboratory,Parks Road, Oxford OX1 3PU, UK. R. G. Grainger is now withthe Department of Physics and Astronomy, University of Canter-bury, Private Bag 4800, Christchurch, New Zealand. A. Lambertis now with the National Center for Atmospheric Research, P. O.Box 3000, Boulder, Colorado 80307-3000.
Received 23 February 1999; revised manuscript received 28June 1999.
6408 APPLIED OPTICS y Vol. 38, No. 30 y 20 October 1999
dominant component of the vapor above sulfuricacid–water mixtures. Water vapor is exchanged be-tween the aerosol particles and the surrounding at-mosphere, causing the particle size and thecomposition to change.11,12 The sulfuric acid loadingof the stratosphere alters the total mass and sizedistribution of the aerosol particles, as evidenced byobserved changes to the aerosol in the stratospherefollowing major volcanic eruptions.13,14 In the labo-ratory the exchange of water vapor between aerosolparticles and the surrounding air has been used tocontrol the composition of the aerosol phase. Flow-ing the particles over a bath of liquid sulfuric acid ofknown composition will cause a net uptake or evap-oration of water vapor from the particles until theyreach the same composition as the liquid acid.15
Sulfuric acid aerosols have been studied in thestratosphere with ground-based, airborne, and satel-lite remote sensing.16,17 Lidar and atmosphericmeasurements of the visible, near-IR, and mid-IRextinction are often used to derive information aboutthe size distribution and composition of the particles.The successful interpretation of lidar and aerosol ex-tinction measurements relies on accurate knowledgeof the optical properties of the aerosol.18
In recently reported laboratory measure-ments,19–22 spectroscopic methods have been used toinvestigate the freezing behavior of sulfuric acid–water aerosol particles at low temperatures. Theabsorption and the scattering features in the mid-IRextinction spectra depend on particle-size distribu-tion and acid composition. Phase changes from liq-uid to solid ~frozen! states are indicated by theappearance of sharp features in the mid-IR extinction
emp
satl
toae
mthsctdprc
20,23,24
s
spectrum that correspond to absorptions from crys-talline hydrates of sulfuric acid or water ice.20,21,23,24
Previous laboratory studies19–22 have shown thatven at temperatures of less than 200 K all but theost dilute sulfuric acid droplets remain in the liquid
hase for long periods of time.The liquid aerosol droplets are assumed to be
pherical, and consequently their optical absorptionnd scattering properties can be described by Mieheory.25 The measured extinction spectra ~eitheraboratory or remotely sensed data! can be fitted to a
computer model based on Mie theory, where the mod-eled parameters are the size distribution, chemicalcomposition, and refractive indices of the particles.If the refractive indices are constrained to literaturevalues, the composition and size distribution can bedetermined from iterative nonlinear least-squaresfitting of the model to give the best fit to the measuredspectra. In many cases the chemical composition ofthe aerosol may be determined independently of theMie fitting ~e.g., through ancillary water vapor andemperature measurements for remote-sensing datar by chemical or optical techniques in the laboratory!nd the iterations used to retrieve the best-fit param-ters of the size distribution.A log-normal distribution26 is commonly used to
characterize the size distribution; four variables arerequired and the distribution has the form
dN~r!
dr5
N0
r ln sÎ2pexpF2ln2~ryrg!
2 ln2 s G , (1)
where N0 is the total number of particles per unitvolume, r is the particle radius, rg is the geometricmean radius, and s is the geometric mean standarddeviation ~i.e., the standard deviation of ln r!.
The refractive-index data most commonly used tomodel the extinction from sulfuric acid aerosols arethose published by Palmer and Williams.27 Thesedata cover a wide range of wave numbers and acidcompositions for sulfuric acid solutions but were mea-sured at room temperature only. The application ofthese room-temperature data to measurements onaerosols at temperatures ~185–250 K! encountered inthe lower stratosphere has been necessary in the ab-sence of measured low-temperature refractive-indexdata.
The main absorption bands of sulfuric acid ~deter-ined by the imaginary part of the complex refrac-
ive index, k! occur in the mid-IR spectral region. Atigher wave numbers the extinction is dominated bycattering since the values of k are close to zero andonsequently the absorption is small. When theemperature is reduced, the expected change in theensity of the droplets can be used to correct the realart of the complex refractive index ~n! in spectralegions where no absorption occurs ~Lorentz–Lorenzorrection28!. However, at wave numbers where the
sulfuric acid absorbs light strongly, reducing the tem-perature may alter the extinction not only throughthe density but also as a result of changes to the bondlengths and speciation of the ions within the drop-
lets. Alterations to the bond lengths and ionicspeciation will produce changes in the refractive in-dices that are not accounted for by the Lorentz–Lorenz correction. The lack of knowledge of low-temperature sulfuric acid refractive indices has beenidentified13,18,20,29 as an impediment to the correctinterpretation of satellite and laboratory data.
The experiments described in this paper allowedthe extinction spectra of aqueous sulfuric acid aero-sols to be measured between 750 and 23,000 cm21 attemperatures of 230 and 294 K. The broad spectralrange enabled the Mie fitting to include the purelyscattering part of the spectrum as well as those re-gions containing absorption bands. The spectrawere recorded in pairs by using aerosols of the samecomposition but with two particle sizes to providespectral data that could be fitted by using the samerefractive indices but with different parameters de-scribing the size distribution. One pair of spectrawas recorded at 294 K to test that the experimentaland the fitting procedures produced results consis-tent with the existing room-temperature data. An-other pair of spectra was recorded at 230 K todetermine how well the room-temperature data de-scribe the aerosol extinction at a lower temperaturemore typical of the stratosphere.
Laboratory measurements on actual aerosol parti-cles rather than bulk liquids or thin films providedata that are more representative of real atmosphericaerosol systems. In its aerosol form, the sulfuricacid–water system is free from surface interactionsthat occur between a bulk solution and its containeror between a thin film and its substrate. Anotheradvantage over bulk solutions arises from the possi-bility of measuring the extinction spectra of smallvolumes of sulfuric acid: The mid-IR absorption isso strong that, to avoid spectroscopic saturation of thebands, either a thin liquid sample or a small volumedistributed as an aerosol is required.
However, laboratory experiments in which aerosolsare generated and observed are complicated by theflow patterns and the gravitational settlement of theparticles. This can lead to measurements havingmore diverse sources of error than is the case in stan-dard gas or liquid phase spectroscopy. Here we de-scribe these additional sources of error and theireffects on spectral measurements of aerosols.
Extinction is used throughout this study to denotethe combined processes of absorption and scattering.For any given wave number the extinction might bedue entirely to one process or the other. In the ex-amples of spectra shown here, the ordinate is opticaldepth defined as
optical depth 5 2lnFItr~n!
I0~n!G , (2)
where Itr~n! is the transmitted light intensity ~aerosolample present! and I0~n! is the background light
intensity ~no aerosol sample present!. In the ab-sence of scattering, optical depth is equivalent to the
20 October 1999 y Vol. 38, No. 30 y APPLIED OPTICS 6409
Btt
6
quantity of Napierian absorbance that is widely usedin molecular spectroscopy.
The experiments and data fitting fall into threesections:
The main spectroscopic measurements and dataprocessing ~Sections 2 and 3!.
Benchtop and computer modeling investigations ofmultiple scattering and the spatial distribution of theaerosol in the spectroscopic cell ~Sections 4 and 5!.
The fitting of the experimental data by using Mietheory ~Section 6!.
2. Spectroscopic Measurements and Data Treatment
A. Overview of Spectroscopic Experiments
The main components of the experimental equipmentare shown in Figs. 1 and 2. Nitrogen was flowedthrough separate mass-flow controllers ~MFC’s! togive two streams of gas. One stream was flowedthrough a bubbler containing water ~BDH AnalaRgrade! to add H2O vapor to the flow and the otherthrough a bubbler containing oleum ~30% free SO3,
DH AnalaR grade! to entrain SO3 vapor. Whenhe outputs from the two bubblers were combined,he reaction between SO3 and H2O produced sulfuric
acid particles. The aerosol was passed over a bath ofaqueous sulfuric acid at room temperature to allowthe exchange of water vapor between the liquid andthe aerosol phase acid and to stabilize the composi-tion of the aerosol. The aerosol was then flowed intoa coolable gas cell for spectroscopic analysis. Spec-tra fully covering the 750–23,000-cm21 spectral re-gion were collected in four wave-number sections,measured sequentially while the aerosol flowed con-tinuously through the cell. When all the aerosol
Fig. 1. Aerosol generation system.
Fig. 2. Spectroscopic gas cell.
410 APPLIED OPTICS y Vol. 38, No. 30 y 20 October 1999
sample spectra had been recorded, the flow wasstopped, and the particles remaining in the cell wereallowed to settle out before a series of empty-cellbackground spectra were taken.
B. Gas-Handling System and Coolable SpectroscopicCell
The presence of both SO3 vapor and liquid sulfuricacid required the aerosol generation system to becorrosion resistant. The bubblers were made of bo-rosilicate glass, with input and output gas lines ofperfluoroalkoxy ~PFA! tubing with PFA vacuumseals. The aerosols were formed in a glass tube of380-mm length and a 9-mm internal diameter thatcould be isolated with polytetrafluoroethylene~PTFE! taps. This tube was connected to a widerdiameter glass vessel of 180-mm length and 65-mminternal diameter, which contained approximately 50cm3 of the bulk sulfuric acid solution. The aerosolflowed into and out of the spectroscopic cell throughPFA and glass tubing, with an adjustable PFA valvelocated at the cell outlet.
The spectroscopic cell was constructed from adouble-walled stainless-steel cylinder of 250-mmlength and 34-mm inner diameter, and was a modi-fication of a collision cooling cell described previous-ly.30 Two ports, located 120 mm apart on one side ofthe cell, were used for the input and the output of theaerosol flow. PTFE-covered stainless-steel spring Orings ~Furon Omniseal! formed vacuum seals be-tween the inlet and the outlet tubing and the cellports and between the inner faces of the optical win-dows and each end of the cell. Barium fluoride wasfound to be a suitable window material, providingresistance to acid corrosion and high transmittancebetween the IR ~from a low-wave-number cutoff at750 cm21! and the UV. A 1° wedge across the face ofeach window prevented channeling interference pro-ducing artifacts in the recorded spectra.
The cell could be maintained at any temperaturebetween 180 and 300 K by circulating liquid nitrogen-cooled ethanol between the double walls of the cell.The cell temperature was determined from the resis-tance measurements of nine two-wire platinum re-sistance thermometers ~Fisher Rosemount, PT-100grade B! and five thermistors ~RS Components, neg-ative temperature coefficient type, resistance-temperature curve matched! in thermal contact withthe outer surface of the cell. The pressure of gas inthe flow system was measured with a Baratron ca-pacitance gauge ~1000-Torr full scale, MKS Type 390!located between the spectroscopic cell and the adjust-able PFA outlet valve.
C. Aerosol Generation and Characterization
The size, composition, and number density of aerosolparticles generated in the system were varied bychanging the flow rates of nitrogen through the twobubblers. The flow rates were controlled withMFC’s ~MKS Type 1179a! with maximum flow ratesof 2000 and 500 standard cubic centimeters perminute ~sccm! for N2. For the measurements re-
2waartrti
tlatsttsuab
maiwss
Table 1. Optical Components used for the Four Sections of the
ported here, two combinations of flow rates wereused: ~a! 400 sccm through the oleum bubbler with80 sccm through the water bubbler and ~b! 130 sccmthrough the oleum bubbler with 15 sccm through thewater bubbler. For both combinations the higherflow rate ~through the oleum! was controlled with the000-sccm MFC, and the lower flow rate ~through theater! was controlled with the 500-sccm MFC. Inll cases we maintained the measured cell pressuret 440 hPa by adjusting the output flow rate to aotary vacuum pump through a liquid nitrogen coldrap. With a constant cell pressure the lower flowates increased the residence times of the aerosol inhe system. The longer residence times permittedncreased droplet aggregation15 and produced parti-
cles with larger mean radii.The removal of SO3 from the oleum sample during
the course of the experiments could reduce theamount of SO3 being carried in the stream of nitrogengas. A significant depletion of the SO3 concentra-tion would lead to time-varying changes in the aero-sol composition or particle-size distribution. Inthese experiments a reasonably large volume ~ap-proximately 80 cm3! of oleum was used so that theconsumption of SO3 would not cause changes in theaerosols generated.
The aerosol was flowed over a bath of ;67-w wt.%H2SO4, prepared from accurately known volumes ofwater and pure sulfuric acid ~BDH AnalaR grade;specific gravity, 1.84; .98% H2SO4! before enteringhe cell. The composition of the aerosol was stabi-ized by the exchange of water vapor between theerosol and the bath of acid, thus ensuring that nei-her the variability in the MFC outputs ~61% of fullcale! nor the switch between the high and the lowotal flow rates changed the composition of the par-icles flowing into the cell. An approximate compo-ition was determined from the measured IR spectrasing the relative intensities of the sulfate and OH IRbsorption bands according to the calibration giveny Anthony et al.19 A further estimate of the com-
position was achieved through optimizing the Miefitting of the spectral data by varying the parameterthat describes the aerosol composition.
D. Spectroscopic Measurements
All aerosol spectra were measured at the RutherfordAppleton Laboratory ~RAL! with a Bruker IFS 120HR Fourier-transform spectrometer in the MolecularSpectroscopy Facility. Full spectral coverage be-tween 750 and 23,000 cm21 was achieved with fourcombinations of source, beam splitter, and detector.The sources were a 150-W water-cooled silicon car-bide globar and a 50-W quartz tungsten halogen~QTH! lamp ~Osram type HLX 64610!. The four de-tectors used were a broadband liquid nitrogen-cooledmercury cadmium telluride ~MCT! detector ~lowwave-number cutoff at 550 cm21!, a liquid nitrogen-cooled indium antimonide ~InSb! detector, and room-temperature silicon ~Si! and gallium phosphide ~GaP!photodiodes. The combinations of the light sources,beam splitters, and detectors used for the four wave-
number regions are shown in Table 1. Table 2shows the spectrometer settings used for recordingspectra of each wave-number region.
All the recorded interferograms were Fouriertransformed with triangular apodization and Mertzphase correction. The choice of apodization and res-olution was found to be relatively unimportant inthese measurements where the observed spectral fea-tures show no sharp structure. The main objectivesin choosing the specifications in Table 2 were toachieve a high signal to noise with rapid recording ofeach spectrum.
For some of the mid-IR measurements all opticalpaths of the spectrometer were evacuated to a pres-sure lower than 0.2 Pa. For the remaining measure-ments the spectrometer was filled with dry nitrogengas. However, during beam-splitter changes somelaboratory air inevitably entered the spectrometer,and consequently water vapor and CO2 bands ap-peared in the recorded spectra.
To check that the composition or the number den-sity of the aerosol particles remained constant withtime, repeat mid-IR spectra were recorded at inter-vals for at least an hour before the main series ofquantitative spectroscopic measurements began.The overall optical depth and the relative intensity ofthe absorption bands in the test spectra were in-spected to check that the aerosol generation systemhad reached a steady state.
Ten repeat spectra ~each composed of 32 or 64 co-added scans! were recorded for the sample spectra~with the aerosol flowing!, and five repeat measure-
ents for the empty-cell background spectra. Afterll sample spectra were recorded for each spectralnterval, the aerosol flow was stopped and the cellas isolated from the vacuum pump. Once the aero-
ol particles had settled out, the sets of backgroundpectra were recorded. The variability and uncer-
Aerosol Spectral Measurements Fully Covering the Range between720 and 23,000 cm21
20 October 1999 y Vol. 38, No. 30 y APPLIED OPTICS 6411
m
csimea
Ortsgtet3aos
T
tlttrt
Table 3. Measured Temperatures for Uncorrected PRT’s, Corrected
t
6
tainty in the extinction over the measurement time ineach spectral region ~typically 10 min! were deter-
ined from the repeat sample spectra.To check that the aerosol composition remained
onstant throughout the measurements in the fourpectral regions, mid-IR spectra were recorded as thenitial and the final sets of each series of measure-
ents. The initial set of mid-IR measurements inach series was made with the spectrometer evacu-ted ~to remove absorption bands arising from CO2
and H2O in the air!. All other sample measure-ments were recorded with the spectrometer filledwith dry nitrogen. The background spectra were re-corded with the spectrometer filled with dry nitrogenuntil the final background. Then the spectrometerwas evacuated again to provide a background thatcould be ratioed against the initial set of mid-IR sam-ple spectra.
3. Spectroscopic Results
A. Cell Temperature
In the low-temperature experiments, readings of thecell temperature were taken either once or twice dur-ing each series of 10 repeat measurements. Thetemperatures determined from resistance measure-ments of the platinum resistance thermometers~PRT’s! ~operated as two wire devices! were generally4 K higher than those determined from the ther-mistor readings. The discrepancy was attributed tothe lead resistance in the PRT measuring system~calculated to be 1.5 V when the room-temperaturePRT and thermistor readings were compared!.
nce the low-temperature PRT data had been cor-ected by the 1.5 V offset, agreement between thehermistor and the PRT data was very good ~ashown by the data in Table 3!. The temperatureradients across the cell were found to be smallest inhose experiments in which the spectrometer wasvacuated. Temperature stability during the timeaken to record each complete set of spectra ~typicallyh! is indicated in Table 3, which shows the mean
verage temperature data together with the value ofne standard deviation over each complete set ofcans.
B. Composition
Flowing the aerosol over a liquid sample of approxi-mately 67-wt.% sulfuric acid ensured that the com-
PRT’s, and Thermistors for the Two Low-Temperature Experiments
AerosolSample
Temperature fromCorrected PRT’sa
~K!
Temperature fromThermistorsa
~K!
Small 228.7 ~2.0! 228.5 ~2.3!Large 229.0 ~2.0! 228.5 ~2.4!
aThe mean values for the cell temperature across the entiremeasurement time of the sample and the background spectra areshown with values of one standard deviation in parentheses.
412 APPLIED OPTICS y Vol. 38, No. 30 y 20 October 1999
position of the aerosol flowing through the cellremained constant throughout each set of measure-ments. Examples of the mid-IR spectra taken ateither end of one series are shown in Fig. 3. Figure3 also shows the water vapor and CO2 bands thatappeared in the spectrum following spectrometerbeam-splitter changes. Comparison of these spectraindicates that the relative intensities of the OH andsulfate absorption bands and the overall band shapesremained constant through the measurements andthat no change in composition occurred ~although asmall change in the overall optical depth was ob-served!.
The acid composition ~in wt.% H2SO4! of the aero-sol was calculated from the relative intensities of theIR extinction in the sulfate absorption region ~820–1470 cm21! and the OH absorption region ~2409–3650 cm21! described by Anthony et al.19 Thepresence of water vapor and CO2 absorption bands inthe spectra recorded with the spectrometer ventedmade integration of the aerosol extinction inaccurate;the spectra with these features removed ~described inSubsection 3.C! were used to produce the values in
able 4.Although all the calculated compositions were close
o the value of approximately 67 wt. % chosen for theiquid acid used to stabilize the aerosol composition,he mid-IR extinction spectra indicated that at lowemperature the particles were more dilute than atoom temperature. This result is expected givenhat the water vapor surrounding the aerosol parti-
Fig. 3. Extinction spectra recorded at the start ~lower curve, nowater vapor or CO2 bands! and the end ~upper curve, water vaporand CO2 bands present in the air in the spectrometer! for room-emperature, large-particle aerosol measurements.
aThe calibration procedureb accurate to 67 wt.%.bRef. 19.
cles at room temperature will have been absorbedinto the cooled droplets. The exact change in com-position in these circumstances depends on cell tem-perature and the relative losses of water vapor to thecell walls and to absorption into the particles. In ourexperiments an unknown amount of sulfuric acidpartially coating the cell walls made it impossible tocalculate the expected change of composition on cool-ing the aerosol from 294 to 230 K, although theamounts of water vapor and sulfuric acid flowing intothe cold cell could be estimated.
C. Spectral Data Treatment
Spectra of the four wave-number sections were re-corded for large- and small-particle aerosol samplesat room temperature and at low temperature. Theaverages and standard deviations of the observed ex-tinction spectra were calculated from each of the re-peat sample spectra and the averages of the repeatbackground spectra.
The ten repeat measurements collected over a 10-min time period produced mean average extinctionspectra for the four intervals together with the stan-dard deviation of the observed spectra about themean, from which 95% confidence limits were calcu-lated. The spectra in adjacent intervals overlappedwithin the 95% confidence intervals in all but onecase. Even in the one exceptional case ~at the 9000-cm21 intersection between the near-IR and visiblemeasurements for the small-particle sample at roomtemperature! the values of the adjacent extinctionsections at 9000 cm21 differed by only 6%—withinthe observed distribution of the repeat measure-ments.
The averaged sections of spectra were normalizedto form continuous spectra between 750 and 23,000cm21 by multiplying each section by a constant sothat the values where the sections intersected ~i.e., at5000, 9000, and 18,000 cm21! matched exactly. Thenear-IR region ~where the data were least noisy! wasused as the section to which the others were normal-ized.
The mid-IR spectra recorded with the spectrometerfilled with nitrogen tended to have slightly higheroptical depths than those recorded with the spec-trometer evacuated and showed better agreementwith the other spectral sections covering 5000–23000cm21. The possible causes of the differences be-tween the vented and the evacuated mid-IR spectrainclude the time taken for the aerosol generation sys-tem to reach equilibrium, small variations in back-ground spectrum signal intensity with spectrometerpressure, and the buildup of sulfuric acid on the cellwindows. ~Each of these sources of errors is dis-cussed in Section 7!. In general, accurate measure-ments of the mid-IR spectra with the spectrometerevacuated were made more difficult than the vented~i.e., filled with nitrogen! ones by the longer elapsedtime between collecting the sample and backgroundevacuated spectra. However, since the ventedmid-IR aerosol spectra contained absorption bandsfrom CO2 and H2O in the spectrometer, the evacu-
ated mid-IR spectra ~multiplied by suitable, largeroffset values! were used to fill in sections of thevented spectra where the CO2 and the H2O absorp-tion obscured the aerosol extinction features. TheH2O absorption bands in the near-IR ~5000–9000cm21! were removed from the spectra simply by fit-ting a straight line across the region of the bands.This removal procedure worked well in the near-IRwhere the aerosol extinction varies as an almost lin-ear function of wave number and the absorptionbands due to H2O are far weaker than those in themid-IR.
D. Spectra
When recording the empty-cell background spectra,it was found that evacuating the cell rapidly after theaerosol had been flowing into the cell for a long periodcaused liquid acid to be transferred onto the cell win-dows, making the background spectra ~containingfeatures that are due to this acid film! unusable. Itwas found that leaving the aerosol to settle out withthe flow switched off provided a better method ofremoving the particles from the optical path, but thisprocedure took considerable time ~longer than 30min! for the small particles at 230 K. In these cir-cumstances the mid-IR background spectra were oflimited quality, and therefore previous backgroundspectra were used to give an extinction spectrum thatoverlapped well with the other spectral sections forthat experiment.
The reproducibility of the background spectra wasassessed by recording five repeat spectra of eachwave-number section for each temperature andparticle-size distribution. The background spectrawere observed to be stable, particularly in the mid-IRwhere the globar source was used. Backgroundspectra of the visible region were the least stable, but,by averaging the five repeat spectra, reliable back-grounds were produced from which the extinctionspectra could be calculated.
The repeat spectra showed that the optical depthvaried with time in each spectral interval, with thelargest variation occurring where the optical trans-mission was lowest. The observed variation wasgenerally larger in the sample spectra than in thebackground spectra, indicating some variation in theaerosol sample during each set of measurements.The optical depths measured in the repeat spectraappeared to oscillate around a mean value ratherthan steadily to increase or decrease, whereas theoverall shape of the spectrum remained the same.Changes in the optical depth without changes in theshape of the extinction spectra indicated a variationin the total number density of particles, but not in theacid composition or in the particle size.
The extremes ~maximum and minimum! of the ob-served optical depths in each section of the room-temperature, large-particle spectra are shown in Fig.4. The standard deviation of the optical depth fromthe mean average value was typically 5%.
The water vapor present around the sulfuric aciddroplets in the optical path gave rise to narrow ab-
20 October 1999 y Vol. 38, No. 30 y APPLIED OPTICS 6413
stwCsn
tr
C
o
o
6
sorption lines in the IR spectra. The resolution atwhich the measurements were taken ~4 cm21! wastoo low to resolve the water lines fully, but water-vapor features did appear in the spectra even at 230K where the equilibrium pressure of water vaporabove the acid is very low. Since the backgroundspectra were collected with the cell isolated from thevacuum pump, water vapor from the aqueous sulfuricacid on the cell walls appeared in both the sample andthe background spectra. Furthermore, because ofthe low spectral resolution used in the measure-ments, the profile of the water-vapor lines was deter-mined by the spectrometer’s instrument resolutionrather than pressure broadening. Consequently thewater-vapor line shape was approximately the samein both the sample and the background spectra. Thewater lines in the sample and the background spectrawere also of approximately the same intensity be-cause the composition of the acid on the cell walls willhave been approximately the same as that of theparticles flowing though the cell. Consequently, thewater-vapor lines in the sample and the backgroundspectra largely ratioed out when the extinction spec-tra were calculated, except in those cases in which theamount of water vapor present in the spectrometerchanged between recording the background and thesample spectra.
For the spectra recorded with the spectrometerevacuated ~to 0.2 Pa!, the aqueous sulfuric acidpresent as an aerosol or on the cell walls was the onlysource of water vapor in the measurements, and sim-ple ratioing of the sample and background spectraremoved the water-absorption features from the ex-tinction spectra completely. However, for spectrarecorded with the spectrometer filled with nitrogen,the intensities of the water-vapor features were in-creased by moist air present in the optical path of thespectrometer ~but outside the cell!. Repeated beam-plitter changes allowed more moist air into the spec-rometer volume, increasing the intensities of theater-vapor lines over the course of the experiments.onsequently, when the sample and the backgroundpectra recorded with the spectrometer filled withitrogen were ratioed to generate extinction spectra,
Fig. 4. Maximum and minimum values of optical depth in therepeat measurements for the large-particle spectrum at room tem-perature.
414 APPLIED OPTICS y Vol. 38, No. 30 y 20 October 1999
he features of the water-vapor absorption were notemoved completely.
The averaged spectra with the water-vapor andO2 bands removed as described in Subsection 3.C
are shown in Figs. 5 and 6 for experiments at 294 and230 K, respectively. The larger-particle spectrashow that the extinction approaches a maximumvalue in the visible region, whereas for the smallerparticles the extinction continues to rise with wavenumber into the near UV. In the low-temperature,small-particle spectrum in Fig. 6, the baseline signalto noise of the data in the spectral region above16,000 cm21 was lower than in the other sectionsowing to the low intensity of light reaching the de-tector. Furthermore the spectral region close to theintersection between the spectra recorded with Siand GaP photodiode detectors showed some level ofdistortion in this experiment ~see Subsection 6.B!.
In the low-temperature experiments, there was thepossibility of water vapor inside the spectrometercondensing onto the cold exterior surfaces of the cellwindows, resulting in ice-absorption features thatcould have increased in intensity with time as moreair was allowed into the spectrometer. However, nospectral features from water ice were observed in any
Fig. 5. Extinction spectra of sulfuric acid aerosols recorded at 294K. The large-particle spectrum ~top! has been offset by four unitsf optical depth for clarity.
Fig. 6. Extinction spectra of sulfuric acid aerosols recorded at 230K. The large-particle spectrum ~top! has been offset by four unitsf optical depth for clarity.
nwinfwwssrwbgiiwtbosmsb
fsmawiflmttc
of the spectra, even for those measurements at coldtemperatures in which the spectra were recordedwith the spectrometer filled with nitrogen.
4. Benchtop Experiments and Computer Simulations
Light is scattered by particles with an angular dis-tribution of intensity determined by the phase func-tion ~which varies with particle size and wave
umber!. In the experiments reported here inhich small particles are illuminated by IR and vis-
ble radiation, the phase function will be predomi-antly isotropic and the radiation will be scatteredairly equally in all directions. With increasingave number and particle size, the phase functionill show a definite forward peak and most of the
cattered photons will propagate with relativelymall angular deviations from the incident light di-ection. Scattered photons arriving at the detectorill cause inaccuracies when one is fitting the spectray using Mie theory because the modeled spectra areenerated on the assumption that the scattered lights removed entirely from the beam. In experimentsn which the optical depth is low, the detected signalill be dominated by directly transmitted light and
he contribution from scattered light will be negligi-le. Where the optical depth is high the proportionf the signal due to scattered photons could becomeignificant. Furthermore at high optical depthsultiple scattering may deflect photons originally
cattered away from the direction of the originaleam back toward the detector.To ensure that the observed spectra were suitable
or fitting with our Mie theory model, it was neces-ary to check the influence of scattered light on oureasurements. As an initial test the aerosol gener-
tion and flow control equipment described aboveas used to carry out some qualitative investigations
nto the scattering behavior of the aerosol. Withow conditions identical to those used in the spectro-etric measurements, the transmission properties of
he aerosol in the visible spectral region were inves-igated by observing the transmission through theell of the collimated output from red ~peak output at
approximately 16,000 cm21! and blue ~peak output atapproximately 21,500 cm21! high-intensity galliumnitride LED’s. The visibility of a fine cross markedon a cell window and the sharpness of the edge of thecollimated beam at the far side of the cell from theLED’s when viewed through the aerosol sample byeye allowed changes in the angular distribution of thephotons on scattering to be detected. The shape ofthe aerosol cloud in the cell at each combination offlow rates was also inspected when benchtop simula-tion of the spectrometric experiments was used.
A more rigorous investigation into the effects ofscattering was provided by Monte Carlo simulationcalculations to model the scattering of photons by theaerosol sample in the cell. The relative intensities oftransmitted and scattered light emerging from thecell along the axis of the input light beam were cal-culated at a variety of optical depths.
5. Results of Benchtop Tests and ComputerSimulations
The benchtop experiments showed that light trans-mitted through the aerosol sample preserved detailedstructure in the light beam in conditions in which theoptical depth approached 8 or 9 ~when the blue LEDwith peak output intensity at approximately 21,500cm21 was used!. These observations indicate thateven though the spectroscopic measurements wereconducted at high optical depths, light reaching thefar side of the aerosol sample from the light sourcewas dominated by directly transmitted light, with atmost a small contribution possible from scatteredlight. The Monte Carlo simulation program con-firmed these observations, showing that, althoughmultiple scattering did occur at the highest opticaldepths encountered in these experiments, the de-tected photons ~i.e., those producing the observed ex-tinction spectra! were almost entirely directlytransmitted ~i.e., unscattered!. The low intensity ofscattered light reaching the detector in these exper-iments was shown to be due to the geometry of thespectroscopic cell. The small diameter of the cellcompared with its length ensures that the majority ofscattered photons are absorbed by the unpolished,gray steel walls of the cell and are not scattered backinto the field of view of the spectrometer’s detector.
The benchtop experiments also demonstrated thatat the higher flow rates used to make the small par-ticles the aerosol sample filled the volume of the cellfairly evenly, whereas at the lower flow rates for thelarger particles, the aerosol sample settled predomi-nately in the lower half of the cell. In the spectrom-eter the light is focused to an image at the center ofthe cell. This configuration of the optical path en-sured that no light passed directly through the cellwithout encountering the aerosol cloud.
6. Data Fitting
The broadband spectra were fitted to model spectracalculated from Mie theory by using the literaturevalues of the refractive indices27 and a single-modelognormal size distribution. The modeled spectrawere normalized to the same integrated extinction asthe measured spectra; then the squares of the devi-ations between the model and the measured spec-trum were calculated for 2000 points regularlyspaced across the spectral interval. The mean ra-dius and the width of the log-normal distributionused in the model were varied to map out a surface ofsummed squared residuals for different size distribu-tions. The best fit was shown by the minimum pointon this surface.
The values of the aerosol acid concentration used inthe fitting were chosen to provide the best simulationof the band shapes observed in the mid-IR spectra~from values within the 67-wt.% error limits of thevalues shown in Table 4!. The spectra recorded at294 K were best fitted with refractive indices for 72-wt.% H2SO4 ~interpolated from the Palmer and Wil-
20 October 1999 y Vol. 38, No. 30 y APPLIED OPTICS 6415
wtm
d
Table 5. Determined Values of the Parameters for the Best-Fit
6
liams data27!; the optimum fit to the spectra at 230 Kwas achieved with 61-wt.% H2SO4 data.
To calculate the best fit to the measured spectra,we generated sets of large-particle and small-particlespectra at the two chosen compositions by using theMie theory model. Large-particle spectra were sim-ulated for mean radii in the 0.15–0.35-mm range,incrementing calculations in 0.01-mm steps, andwidths of the log-normal distribution between 1.2 and1.85, with steps of 0.05 ~making a total of 294 calcu-lated spectra!. Small-particle spectra were calcu-lated for mean radii in the 0.01–0.21 range mm and
ith distribution widths between 1.2 and 2.25, usinghe same step sizes as above ~producing a total of 462odel spectra!.The observed spectra were fitted to the modeled
ata over the spectral range of 750–23,000 cm21,
Single-Mode Log-Normal Size Distribution for the Broadband Spectra
Fig. 7. Results of fitting ~a! the 294-K, small-particle measured spe~rm 5 0.08 mm, sg 5 1.75, N 5 44.4 3 107 cm23!; ~b! the 294-K, larglog-normal distribution ~rm 5 0.28 mm, sg 5 1.40, N 5 2.15 3 161-wt.% H2SO4, single-mode, log-normal distribution ~rm 5 0.11measured spectrum to model: 61-wt.% H2SO4, single-mode, log-n
416 APPLIED OPTICS y Vol. 38, No. 30 y 20 October 1999
apart from the low-temperature, small-particle spec-trum, where data above 16,000 cm21 were excludedfrom the fit owing to their high noise and spectraldistortion. The results of the fitting are shown inTable 5.
As the mean particle radius used in the model isincreased, the change in the fit between the modeland the measured spectra can be partially offset bydecreasing the width of the log-normal distribution;i.e., fits to the distributions provided a small set ofcombinations of values of mean radius and distribu-tion width that fit the data almost as well as thebest-fit values given in Table 5. The fitting de-scribed here did not include a search algorithm toexplore the surface around the minima in the least-squares results; a more detailed analysis is necessaryto optimize retrieval of the size distribution. How-ever, the fitting did provide best-fit parameter valuesfor the broadband spectra that were consistent withthe expected behavior of the aerosol: ~a! the in-creased residence times produced larger particles and~b! the particles grew and became more dilute oncooling ~through absorption of water vapor in theroom-temperature aerosol flow!.
The observed spectra are compared with thebest-fit simulations in Figs. 7. In each figure partthe mid-IR ~750–5000-cm21! sections are expandedand shown in the insets.
Fitting the model to the observed data can be seen
m to model: 72-wt.% H2SO4, single-mode, log-normal distributionticle measured spectrum to model: 72-wt.% H2SO4, single-mode,23!; ~c! the 230-K, small-particle, measured spectrum to model:
sg 5 1.60, N 5 35.2 3 107 cm23!; ~d! the 230-K, large-particle,al distribution ~rm 5 0.32 mm, sg 5 1.35, N 5 1.59 3 107 cm23!.
ctrue-par07 cmmm,orm
otlt2trqedl
tpipsoRfiifc
wtctsns
e
t
to be good in the near-IR and visible spectral regions,and the main discrepancies appear in the mid-IRregion. This result is partly due to fitting the data inextinction, where the maximum ordinate values oc-cur at high wave numbers. When assessing the fitquality by the squares of the differences between themodel and the measured spectra, even a small pro-portional difference at high extinction will give alarge squared value. Conversely, a large propor-tional difference in the mid-IR will not affect the sumof the squares much. The close fitting of the small-particle spectral data recorded at 294 K indicatesthat the quality of the spectral data, and their pro-cessing and fitting routines, is satisfactory.
The fitting highlights two main results: ~1! thebserved mid-IR spectra of the small particles fittedhe model more closely than did the spectra of thearge particles, and ~2! the fit of the observed data tohe model in the mid-IR was better for the particles at94 K than those at 230 K. Lack of knowledge of theemperature dependence of the imaginary part of theefractive indices would be expected to reduce theuality of the fit to the mid-IR data at 230 K, but thisffect ~although observable! is less marked than theifference in the quality of the fits to the small- andarge-particle spectra.
Since the particles grow by aggregation, there ishe possibility that a mode of small unaggregatedarticles may be present in the large-particle exper-ments. A model with a bimodal distribution mayrovide a better fit to the observations and explainome of the observed differences between the qualityf fit for the large- and the small-particle spectra.efinements to the model and the fitting procedures,tting of additional broadband spectra, and compar-
sons with fits generated by using other sets of re-ractive indices are necessary before any furtheronclusions can be drawn.
Spectroscopic measurements on aerosol samplesith the same composition and particle-size distribu-
ions would be necessary to quantify the separateontributions from the particle size and the change ofemperature to the reduced quality of fit. In thistudy, where reduction in temperature is accompa-ied by particle growth and changing composition, nouch quantitative assessments can be made.
7. Discussion of Errors
The critical requirements for the experiments to pro-duce useful data are that the aerosol composition,size distribution, and temperature remain constantthroughout each series of measurements covering thefour spectral regions. If these criteria are achieved,the separately recorded sections of spectra can thenbe combined to produce a continuous spectrum thatmay be fitted by using a single set of parameters inthe Mie theory model. The sources of error in thisresearch fall into three main areas: variations inthe composition and size distribution of the generatedaerosol, spectrometric errors, and errors in the datafitting; each of these areas is considered individuallyin the following subsections.
A. Errors Caused by Variations in the AerosolComposition and Size Distribution
Variations in the mass flow controller outputs ~quot-d accuracy, 61% of full scale, i.e., 620 sccm for the
flow rate of nitrogen through oleum and 65 sccm forthe flow rate of nitrogen through distilled water!would change the relative concentrations of SO3 andH2O vapor present in the initial formation of theaerosol and hence modify the initial composition andsize distribution of the particles generated. In addi-tion, changes in the gas flow through the cell wouldalter the flow patterns and possibly affect the sizedistribution of the particles present in the opticalpath. However, collecting repeat sample spectrashould average out any random variations, and flow-ing the aerosol over the bath of aqueous sulfuric acidbefore it enters the cell should buffer any variationsin the composition. The good overlap between adja-cent spectral sections indicates the absence of signif-icant problems from variations in the composition,total number, or size of the particles.
Although the majority of the acid aerosol passedstraight through the flow system and cell, the sur-faces of the aerosol generating and flow system andthe spectroscopic cell walls became coated with sul-furic acid during the course of the experiments. Thelow vapor pressure of sulfuric acid ensured that theacid coating was not removed by evacuating the sys-tem, although the composition of the acid could havebeen altered by pumping away the more volatile wa-ter vapor from the aqueous acid. The rapid ex-change of water vapor between the aerosol phase andthe liquid acid covering the walls meant that theentire flow system had to reach compositional equi-librium before the repeat measurements of the ex-tinction spectra were consistent.
The presence of sulfuric acid or any contaminantmaterial on the surfaces also affects the number den-sity of the aerosol. As with H2O, SO3 can be ab-sorbed into liquid sulfuric acid, and consequently theSO3 flowing over the surfaces of the gas-handlingsystem had to reach equilibrium with any coating ofacid, or react fully with any contaminants in the sys-tem, before a steady amount was available to reactwith the water vapor. After starting the flow of SO3and water vapor for each set of measurements, theextinction spectrum was observed to change withtime and slowly approached an equilibrium value.Sets of IR spectra collected at intervals for a period of1–2 h were checked to ensure that the extinctionspectra were consistent before starting the mainmeasurements.
The initial change in optical depth with time mayhave been one cause of the lack of overlap betweenthe initial observation of the mid-IR extinction mea-sured with the spectrometer evacuated ~recorded athe start of each set of measurements! and the re-
maining sections ~including the final mid-IR mea-surement!, which were recorded in relatively quicksuccession after the spectrometer had been filled withnitrogen. If the series of measurements were
20 October 1999 y Vol. 38, No. 30 y APPLIED OPTICS 6417
21
Gsssdmniacbmddwis
5
6
started before the system had reached a true equilib-rium ~and fewer aerosol particles were present in thecell!, the first measurement would be expected toshow a lower optical depth than subsequent ones.
The temperature stability of the system both acrossthe cell and during each series of measurements wasdescribed in Subsection 3.A. Any change in the tem-perature will alter the vapor pressure of H2O andchange the aerosol sample by absorption or evapora-tion of water vapor from the droplets. Flow patternswithin the cell may also change with temperature.The effect of the observed temperature fluctuations~at worst 62 K! is not significant in these measure-ments.
Water vapor flowing into the cooled cell with thesulfuric acid droplets can undergo homomolecularnucleation to form pure water particles distinct fromthe aqueous sulfuric acid droplets. Previous inves-tigations carried out at RAL showed that flow ratesand water-vapor concentrations need to be far higherthan those in the sulfuric acid experiments for waterdroplets or ice particles to form in our spectroscopiccell at 230 K.
B. Spectrometric Errors
The possibility of the spectra being significantly dis-torted by multiple scattering was investigated withbenchtop experiments and the Monte Carlo simula-tion described above. Both investigations showedthat the contribution of scattered light to the detectedlight intensity was negligibly small, even at the blueend of the visible spectrum where the optical depth ishighest. Further evidence that the extinction spec-tra were not significantly altered by multiple scatter-ing is given by the good quality of the fitting of thenear-IR and visible spectra with a single-scatteringmodel.
The high optical depths at high wave numbersmeant that low levels of light reached the photodiodedetectors, making the extinction spectra at thesewave numbers noisier than in the mid- and near-IRregions. In addition, the response of the GaP detec-tor was found to be more noisy than that of the Sidetector. Spectral data above 16,000 cm21 for thesmall aerosol particles at 230 K, where the highestoptical depths in all the measurements were ob-served, were the most noisy.
The mid-IR data were subject to a further possiblesource of error through the nonlinear response tolight intensity of the MCT detector and preamplifier.Distortions in the spectrum caused by detector non-linearity are likely to be similar in nature to thebroadband, smooth features found in the mid-IR ex-tinction and so are difficult to distinguish from thecorrect signal. The mid-IR spectra were thereforecorrected for detector nonlinearity by using a soft-ware correction.
The broadband transmission spectra measuredwith the InSb and photodiode detectors were ob-served to become distorted toward the high-wave-number end of their spectral range. Thetransmission tended to drop off rapidly, then rise
418 APPLIED OPTICS y Vol. 38, No. 30 y 20 October 1999
again above 9,000 cm as measured by the InSb, anda similar pattern was seen above 18,000 cm21 withthe Si photodiode and above 28,000 cm21 with the
aP photodiode, although this pattern in the GaPpectra was largely obscured by random noise. Pos-ible causes of the spectral distortions include back-cattering of light into the interferometer and theifficulty of accurately determining very low trans-ission values in broadband spectra ~where the dy-amic range of light intensity arriving at the detector
s large!. These distortions became noticeable onlyt low transmitted light intensity, and the problemsould be obviated by switching to a different detector–eam-splitter combination. However, this solutionakes the study of overlapping spectral regions from
ifferent detector and beam-splitter combinationsifficult, and for this reason the spectral sectionsere normalized simply to the end values of each
nterval ~as described in Subsection 3.C!. Themall-particle spectrum above 16,000 cm21 recorded
at 230 K was most seriously affected by spectral dis-tortions, possibly because transmission in this regionwas extremely low.
Consistent background spectra are required for ac-curate extinction spectra. Sources of possible errorsin the background spectra include variation in thesource output and buildup of either sulfuric acid onthe inside of the windows or water ice on the outsideof the windows with the cell at low temperature.The deposition of sulfuric acid on the cell windowswas not generally a problem in the measurements,except where the cell was evacuated rapidly, causingsulfuric acid at the bottom of the cell to be depositedon the windows. Cases where the amount of acid onthe windows changed significantly between the col-lection of the sample and background spectra werehighlighted by distortions in the mid-IR extinctionspectra ~particularly noticeable between 3500 and,000 cm21!, and the affected measurements were
discarded and the entire experiment was repeatedwith clean windows.
One other possible source of error was identified byan apparent change in the overall intensity of themid-IR background spectra with spectrometer pres-sure. As the spectrometer pressure approached itsminimum value ~;0.2 Pa!, the output intensity of theglobar was observed to change compared with that at100-Pa pressure, typically by 2%. The possiblecauses of this include changes to the globar coolingrate by air molecules at low pressure. Also, move-ment of the wedged cell windows with internal andexternal pressure differences could change the inten-sity of light incident on the detectors. Failure tomatch the pressure conditions exactly between thesample and the background spectra could alter thederived extinction spectra so that the mid-IR spectrarecorded with the spectrometer evacuated do notmatch those recorded with the spectrometer vented.However, since the spectra covering the entire 720–23,000 cm21 region were derived from measurementstaken with the spectrometer vented, the overall data
cttrrndwttfc
picaa5rameP
LfRtW
quality will not have been affected by the effects ofspectrometer pressure.
C. Errors in the Data Treatment and Fitting
Although no water ice features were seen in the spec-tra, absorption ascribed to water vapor and CO2present in the spectrometer did appear in the mid- andnear-IR measurements. The procedure for removingthese features from the spectra by filling in the affectedwave-number intervals with sections taken from thespectra recorded with the spectrometer evacuated wasdescribed in Subsection 3.C. Although this correctionprocedure was carried out carefully, such changes tothe spectra may increase the uncertainty in the extinc-tion values. However, since the fitting calculationsand concentration calibration apply to the aerosolphase components only, filling in the data is preferredto leaving the sharp features of vapor phase absorptionin the spectra.
8. Conclusions and Future Research
Experimental methods have been developed for gen-erating sulfuric acid aerosols in the laboratory at avariety of different acid concentrations, particle-sizedistributions, and temperatures corresponding tothose found in the lower stratosphere. The extinc-tion spectra of these aerosols have been measured togive complete coverage between 750 and 23,000 cm21
~mid-IR, near-IR, and visible spectral regions! withonfidence that the aerosol composition, tempera-ure, and size distribution are constant throughouthe measurement over the entire wave-numberange. Fitting the measured spectra to an existingefractive-index database with a single-mode log-ormal size distribution in a Mie theory model pro-uced values for the mean particle radius and theidth of the size distribution. The quality of the fit
o the model was good in the near-IR and visible, buthe mid-IR spectral region was fitted relatively poorlyor lower temperatures and larger particles ~from in-reased aggregation times!.
Our results indicate possible limitations in the ap-licability of existing refractive-index data to model-ng atmospheric sulfuric acid aerosol particles andonsequently have implications for remote sensingnd laboratory studies involving IR sulfuric aciderosol spectroscopy. There can be as much as a0% bias in determination of the relative extinctionatio between two wavelengths; e.g., at low temper-tures, for a Stratospheric Aerosol and Gas Experi-ent II visible wavelength channel ~0.5 mm! the
xtinction predicted in the IR region by use of thealmer and Williams’s refractive-index data27 is too
high by a factor of ;2.Further studies involving more sophisticated fit-
ting routines and using other refractive-index datasets as well as extending the measurements to in-clude a wider range of temperatures and composi-tions are planned.
We are grateful for the U.K. Natural EnvironmentResearch Council ~NERC! grant ref. GSTy1245y
SAC ~awarded to D. Newnham and J. Ballard! andor access to the Molecular Spectroscopy Facility atAL. The authors acknowledge the technical assis-
ance provided by M. S. Page, P. O’Brien, and R. G.illiams at RAL.
References1. R. P. Turco, R. C. Whitten, and O. B. Toon, “Stratospheric
aerosols: observation and theory,” Rev. Geophys. Space Phys.20, 233–279 ~1982!.
2. J. M. Rodriguez, M. K. W. Ko, and N. D. Sze, “Role of hetero-geneous conversion of N2O5 on sulphate aerosols in globalozone losses,” Nature ~London! 352, 134–137 ~1991!.
3. J. F. Gleason, P. K. Bhartia, J. R. Herman, R. McPeters, P.Newman, R. S. Stolarski, L. Flynn, G. Labow, D. Larko, C.Seftor, C. Wellemeyer, W. D. Komhyr, A. J. Miller, and W.Planet, “Record low ozone in 1992,” Science 260, 523–526 ~1993!.
4. D. W. Fahey, S. R. Kawa, E. L. Woodbridge, P. Tin, J. C.Wilson, H. H. Jonsson, J. E. Dye, D. Baumgardner, S. Bor-rmann, D. W. Toohey, L. M. Avallone, M. H. Proffitt, J. Mar-gitan, M. Loewenstein, J. R. Podolske, R. J. Salawitch, S. C.Wofsy, M. K. W. Ko, D. E. Anderson, M. R. Schoeberl, and K. R.Chan, “In situ measurements constraining the role of sulphateaerosols in mid-latitude ozone depletion,” Nature ~London!363, 509–514 ~1993!.
5. D. R. Hanson, A. R. Ravishankara, and S. Solomon, “Hetero-geneous reactions in sulfuric acid aerosols: a framework formodel calculations,” J. Geophys. Res. 99, 3615–3629 ~1994!.
6. M. A. Tolbert, “Sulfate aerosols and polar stratospheric cloudformation,” Science 264, 527–528 ~1994!.
7. M. T. Coffey, “Observations of the impact of volcanic activity onstratospheric chemistry,” J. Geophys. Res. 101, 6767–6780~1996!.
8. S. Borrmann, S. Solomon, J. E. Dye, D. Baumgardner, K. K.Kelly, and K. R. Chan, “Heterogeneous reactions on strato-spheric background aerosols, volcanic sulfuric acid droplets,and type 1 polar stratospheric clouds: effects of temperaturefluctuations and differences in particle phase,” J. Geophys.Res. 102, 3639–3648 ~1997!.
9. A. Lacis, J. Hansen, and M. Sato, “Climate forcing by strato-spheric aerosols,” Geophys. Res. Lett. 19, 1607–1610 ~1992!.
10. J. T. Houghton, L. G. Meira Filho, J. Bruce, H. Lee, B. A.Callander, E. Haites, N. Harris, and K. Maskell, eds., IPCCClimate Change 1994, Radiative Forcing of Climate Changeand an Evaluation of the Intergovernmental Panel on ClimateControl IS92 Emission Scenarios, ~Cambridge University P.,London, 1995!.
11. H. M. Steele and P. Hamill, “Effects of temperature and hu-midity on the growth and optical properties of sulfuric-acid-water droplets in the stratosphere,” J. Aerosol Sci. 12, 517–528~1981!.
12. A. Tabazadeh, O. B. Toon, S. L. Clegg, and P. Hamill, “A newparameterization of H2SO4yH2O aerosol composition: atmo-spheric implications,” Geophys. Res. Lett. 24, 1931–1934 ~1997!.
13. C. P. Rinsland, G. K. Yue, M. R. Gunson, R. Zander, and M. C.Abrams, “Mid-infrared extinction by sulfate aerosols from theMt. Pinatubo eruption,” J. Quant. Spectrosc. Radiat. Transfer52, 241–252 ~1994!.
14. A. Ansmann, I. Mattis, U. Wandinger, F. Wagner, J.Reichardt, and T. Deshler, “Evolution of the Pinatubo aerosol:Raman lidar observations of particle optical depth, effectiveradius, mass, and surface area over central Europe at 53.4°N,”J. Atmos. Sci. 54, 2630–2641 ~1997!.
15. E. R. Lovejoy and D. R. Hanson, “Measurement of the kineticsof the reactive uptake by submicron sulfuric acid particles,” J.Phys. Chem. 99, 2080–2087 ~1995!.
16. A. Lambert, R. G. Grainger, C. D. Rodgers, F. W. Taylor, J. L.
20 October 1999 y Vol. 38, No. 30 y APPLIED OPTICS 6419
Mergenthaler, J. B. Kumer, and S. T. Massie, “Global evolu-
1
1
1
2
2
2
23. A. M. Middlebrook, L. T. Iraci, L. S. McNeill, B. G. Koehler,
2
2
2
6
tion of the Mount Pinatubo volcanic aerosols observed by theinfrared limb-sounding instruments CLAES and ISAMS onUARS,” J. Geophys. Res. 102, 1495–1513 ~1997!.
7. C. Brogniez, R. Santer, B. S. Diallo, M. Herman, J. Lenoble, andH. Jager, “Comparative observations of stratospheric aerosolsby ground-based lidar, balloon-borne polarimeter and satellitesolar occultation,” J. Geophys. Res. 97, 20805–20823 ~1992!.
8. R. G. Grainger, A. Lambert, F. W. Taylor, J. J. Remedios, C. D.Rodgers, M. Corney, and B. J. Kerridge, “Infrared absorptionby volcanic stratospheric aerosols observed by ISAMS,” Geo-phys. Res. Lett. 20, 1283–1286 ~1993!.
9. S. E. Anthony, R. T Tisdale, R. S. Disselkamp, M. A. Tolbert,and J. C. Wilson, “FTIR studies of low temperature sulfuricacid aerosols,” Geophys. Res. Lett. 22, 1105–1108 ~1995!.
0. A. K Bertram, D. D. Patterson, and J. J. Sloan, “Mechanismsand temperatures of the freezing of sulfuric acid aerosols mea-sured by FTIR extinction spectroscopy,” J. Phys. Chem. 100,2376–2383 ~1996!.
1. M. L. Clapp, R. F. Niedziela, L. J. Richwine, T. Dransfield, andR. E. Miller, “Infrared spectroscopy of sulfuric acidywater aero-sols: freezing characteristics,” J. Geophys. Res. 102, 8899–8907 ~1997!.
2. D. G. Imre, J. Xu, and A. C. Tridico, “Phase transformations insulfuric acid aerosols: implications for stratospheric ozonedepletion,” Geophys. Res. Lett. 24, 69–72 ~1997!.
420 APPLIED OPTICS y Vol. 38, No. 30 y 20 October 1999
M. A. Wilson, O. W. Saastad, M. A. Tolbert, and D. R. Hanson,“Fourier transform-infrared studies of thin H2SO4yH2O films:formation, water uptake, and solid-liquid phase changes,” J.Geophys. Res. 98, 20,473–20,481 ~1993!.
24. R. Zhang, P. J. Wooldridge, J. P. D. Abbat, and M. J. Molina,“Physical chemistry of the H2SO4yH2O binary system at lowtemperatures: stratospheric implications,” J. Phys. Chem.97, 7351–7358 ~1993!.
5. G. Mie, “Beitrage zur optik truber medien speziell kolloidalermetallosungen,” Ann. Phys. 25, 377 ~1908!.
26. M. Kerker, The Scattering of Light and other ElectromagneticRadiation ~Academic, New York, 1969!.
7. K. F. Palmer and D. Williams, “Optical constants of sulfuricacid; application to the clouds of Venus?,” Appl. Opt. 14, 208–219 ~1975!.
8. L. W. Pinkley and D. Williams, “The infrared optical constantsof sulfuric acid at 250 K,” J. Opt. Soc. Am. 66, 122–124 ~1976!.
29. R. G. Grainger, A. Lambert, C. D. Rodgers, F. W. Taylor, andT. Deshler, “Stratospheric aerosol effective radius, surfacearea and volume estimated from infrared measurements,” J.Geophys. Res. 100, 16,507–16,519 ~1995!.
30. D. Newnham, J. Ballard, and M. Page, “Doppler-limited spec-troscopy at cryogenic temperatures: application of collisioncooling,” Rev. Sci. Instrum. 66, 4475–4481 ~1995!.
