Composition of cirrus-forming aerosols at the tropical tropopause

Composition of cirrus-forming aerosols at the tropical tropopause K. D. Froyd, D. M. Murphy, P. Lawson, D. Baumgardner, and R. L. Herman NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO, USA Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO, USA SPEC Incorporated, Boulder, CO, USA Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico City, Mexico Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA


Introduction
Optically thin cirrus are a common feature of the tropical troposphere layer (TTL) (Fueglistaler et al., 2009;Dessler et al., 2006;Liu, 2007).Cirrus with very low optical densities (τ<0.03),often termed subvisible cirrus (SVC), are widespread laminar features that occur primarily in the tropical tropopause region.SVC can form in situ (Pfister et al., 2001;Schwartz and Mace, 2009) or as a consequence of recent convection (Dessler and Yang, 2003;Garrett et al., 2004;Massie et al., 2002).These cirrus regulate the vertical transport of water vapor (Jensen and Pfister, 2004;Jensen et al., 1996;Luo et al., 2003) as well as aerosols and condensable gases into the stratosphere.The radiative properties of thin cirrus can influence the local thermal budget and drive dynamics of the tropopause region (Comstock et al., 2002;Hartmann et al., 2001;Jensen et al., 1996;McFarquhar et al., 2000).
Knowledge of SVC formation mechanisms is hampered by limited field observations of aerosol size, composition, and ice-forming capability in the tropical upper tropo-Introduction

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Full sphere.Relative humidities well above ice saturation are frequently found in clear air in the tropical tropopause region and imply a shortage of effective heterogeneous ice nuclei (IN).Furthermore, relative humidities near the tropopause are often near or may exceed levels required for homogeneous freezing of soluble aerosols (Jensen et al., 2005;Jensen et al., 2001;Kr ämer et al., 2009).The onset of homogeneous freezing would convert a large fraction of accumulation mode aerosols to ice crystals and would be largely independent of aerosol composition.However, in simulations of SVC formation, introducing small concentrations of heterogeneous IN more accurately reproduced thin cirrus occurrence, persistence, and microphysical properties (Jensen et al., 2001;Jensen et al., 2008;Karcher, 2002Karcher, , 2004)).Jensen et al. (2009b) analyzed recent measurements of tropopause level SVC crystal sizes and concentrations using a parcel model and evaluated probable nucleation scenarios.A summary of microphysical properties of SVC is given by Lawson et al. (2008).Single particle information is particularly important for studying ice nucleation since the onset of freezing can vary greatly for different aerosol types that are often externally mixed.Also, direct measurement of ice crystal residue is critical to determining nucleation mechanisms.For instance, by measuring only out-of-cloud aerosols a subset of particularly effective ice-forming aerosols present at the 1 in 500 level could be overlooked yet could still be the dominant ice-forming aerosol type.
Recently, we reported out-of-cloud aerosol composition measurements in the tropical upper troposphere and lower stratosphere (Froyd et al., 2009).Here we present composition measurements of aerosols that froze to form SVC particles near the tropical tropopause.The SVC clouds in this study were not directly associated with recent convective events and are presumed to have formed in situ from aerosols that slowly ascended through the tropical tropopause layer (TTL).The sizes and chemical signatures of the cirrus-forming aerosols are compared to interstitial and out-of-cloud aerosols.From these compositional observations we comment on likely nucleation mechanisms for subvisible cirrus formation.

Measurement techniques
The NOAA PALMS (particle analysis by mass spectrometry) instrument is a single particle mass spectrometer that measures online aerosol composition and size for diameters 0.25-3 µm.Individual particles scatter light from an continuous laser beam, which triggers a pulse from an ionizing laser.Ions are extracted and analyzed with a time-of-flight mass spectrometer, producing one complete mass spectrum per particle.Aerodynamic diameters are measured for most particles.PALMS can distinguish between different aerosol types in an external mixture, and chemical components within aerosols can also be studied.The PALMS instrument was deployed on the NASA WB-57 aircraft during the CR-AVE (Costa Rica Aura Validation Experiment) campaign in Jan-Feb 2006.See Murphy et al. (2006a) for modifications to the original flight instrument (Thomson et al., 2000).
A combination aerosol/counterflow virtual impactor (CVI) inlet (Cziczo et al., 2004b) was used to alternately sample ambient aerosols or cloud particles.The 24 cm long, 2 mm i.d.inlet was housed in the center of a 88.5 cm long, 5 cm i.d.sampling duct extending from the WB-57 nose in which air flowed near aircraft speed.In clear air aerosols were sampled directly with an inlet residence time of 70 ms.In cloudy regions, a counterflow of filtered air was activated to reject interstitial aerosols and selectively sample cloud particles.Cloud particles were then heated to remove water, and remaining cloud "residual" particles (d >0.25 µm) were characterized by PALMS.
Turning off the counterflow within SVC effectively sampled both small ice crystals and predominantly (>90%) interstitial aerosol.
The CVI deployed during CR-AVE was particularly well suited for sampling SVC crystals.Tropopause level cirrus contain smaller crystals than anvil cirrus associated with a convective system (Lawson et al., 2008).Ice crystals 10-50 µm in size comprise the peak of the SVC mass distribution, and crystals >30 µm are <3% by number.The effective sampling range of the CVI was approximately 5-25 µm.The lower sampling size limit is set by the counterflow, and the upper limit is controlled by particle stopping Introduction

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Full distance and evaporation time within the CVI inlet.Cloud particles larger than 25 µm will impact upon a tubing bend that is downstream of the sampling pickoff running to PALMS.
Convective clouds and anvil cirrus can be problematic for in situ sampling from high speed aircraft because large ice crystals impact on inlet walls and leading edges.
Not only do ice crystals shatter and alter the measured cloud particle size distribution (Jensen et al., 2009a), but crystals with enough mass ablate pieces of inlet material that can then be mistaken for cloud residuals (Murphy et al., 2004).These micronsized contamination particles can be identified with composition-resolving instruments such as PALMS or electron microscope analysis provided the technique is sensitive to the contaminant.Stainless steel particles were efficiently generated from crystal impaction on the CVI inlet within anvil cirrus during previous studies.For the current study, CVI inlet surfaces were plated with gold.Since gold is more ductile than stainless steel, secondary particle generation was significantly reduced.Also, gold provides a unique composition tracer to identify particles generated from inlet walls.The majority of contamination particles were generated within the CVI inlet, but in dense ice clouds with number modes sizes >100 µm, a few particles contained signatures identified as fiberglass from the inner walls of the sampling duct or paint from the duct shroud.Examples of aerosols with secondary material are given in Fig. 1.Positive aerosol spectra were flagged as contaminated when any of the following criteria were met: 1) Au + ions were detected, 2) signatures from an internal o-ring were observed as Si where >30% of spectra showed contamination.
One particularly difficult aspect of cloud residual composition analysis is that contamination signatures often appear as minor components of otherwise realistic atmospheric aerosols.Apparently, ice crystal impaction events can generate internal mix-Introduction

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Full tures of the original ice residual with inlet material (Fig. 1c).Signals from many of the contamination metals are 1% of the total ion current and probably represent a similarly small fraction of aerosol mass, yet these are not observed outside clouds in, e.g., mineral dust spectra, and they clearly result from secondary particle generation on inlet surfaces.This has important implications for electron microscopy, ion chromatography, or single particle analysis of captured cloud residuals where minor metal components may be overlooked.Figure 2 shows how contamination from ice crystal shattering is a function of cloud ice water content and ultimately, crystal size.Subvisible cirrus particles are ∼100-1000 times less abundant than TTL aerosols.Our measurement technique relies on a large concentration enhancement (∼x80) by the CVI to achieve reasonable cloud particle sampling rates.Because PALMS detects aerosols by light scattering, detection efficiency is a strong function of size.Aerosols with d =0.25 µm were detected with <1% efficiency, whereas 10-50% of >0.5 µm aerosols were detected.If we assume that cloud residual particles have a size distribution typical of the background aerosol and are larger than 100 nm, then PALMS must be sampling in CVI mode for 3 min to evaluate 10 SVC particles.Ice nucleation can be strongly size dependent, where only the largest aerosols freeze.In these cases the PALMS size range may encompass a large fraction of the cloud-forming aerosol, and heterogeneous IN should be observed with high relative efficiency.
The NASA JPL Laser Hygrometer (JLH) is an open path instrument that measures water vapor absorption from a near-infrared laser to determine water vapor mixing ratio (May, 1998).The laser is scanned at 8 Hz across a strong water vapor absorption line (7299.43cm −1 ), and modulated at 256 kHz for sensitive harmonic detection at 512 kHz.
There is negligible background signal from cloud particles in low density cirrus because this narrow absorption line is specific to water vapor.Furthermore, the harmonic signal is normalized to the power reaching the detector, which accounts for scattering losses due to ice.Absorption data for CR-AVE were calibrated by operating the instrument under similar conditions inside a stainless steel chamber, and flowing a known water vapor/air mixture through the chamber (Troy, 2007).These careful laboratory mea-Introduction

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Full surements led to a revision of the JLH data originally submitted to the CR-AVE data archive, to be presented in a future publication.SVC ice crystal size distributions and ice water content were measured using the Cloud Aerosol and Precipitation Spectrometer (CAPS, d =0.5-1550 µm) (Baumgardner et al., 2001) and the 2D-S two-dimensional stereo optical probe (5-1400 µm) (Lawson et al., 2006).

In situ sampling of subvisible cirrus
Tropopause level cirrus clouds were encountered throughout the CR-AVE campaign.SVC were typically ≤1 km thick and centered around the level of maximum relative humidity located just below the cold point tropopause.SVC did not extend more than a few hundred meters above the tropopause.Most cirrus in the tropopause region were not directly associated with deep convection and are presumed to have formed in situ.Isolated cirrus layers that were remnants of recent convective systems were also observed at lower altitudes in the TTL. Figure 3 details an extensive (>500 km) tropopause subvisible cirrus layer encountered on 2 February between Central America and the Galapagos.
In a recent study (Froyd et al., 2009), we reported measurements of aerosol composition of the tropical upper troposphere and lower stratosphere and discussed source regions and mechanisms.Tropical upper tropospheric aerosol particles were predominantly internal mixtures of sulfate and organic material.Organic material was highly oxidized and often comprised a larger fraction of the aerosol mass than sulfate.Sulfate was partially or fully neutralized, and estimated NH TTL over a period of several weeks (Fueglistaler et al., 2009;Park et al., 2007).Figure 4 and Table 1 compare compositional characteristics of SVC particle residues and unfrozen aerosols during the 2 February encounter.Unfrozen aerosols include interstitial aerosols sampled within SVC as well as aerosols in nearby cloud-free air within the same altitude range.Particle type classifications are based on positive ion mass spectral features described previously (Hudson et al., 2004;Froyd et al., 2009).Subvisible cirrus formed overwhelmingly from freezing of the dominant sulfate-organic aerosol type.
Aerosols with biomass burning signatures were underrepresented in SVC residue.Data from the current study are limited such that we cannot definitively exclude freezing of any biomass burning aerosols.However, these observations are consistent with previous measurements from a cirrus encounter at 14.5 km where potassium-rich particles were also underrepresented in ice residues (Cziczo et al., 2004b).Biomass burning aerosols are highly organic but also contain small fractions of insoluble inorganic material and sometimes inclusions of elemental carbon that might promote heterogeneous freezing.Here we find no evidence that biomass burning aerosols are efficient IN under these conditions.Furthermore, this observation does not support the notion that the sulfate-organic aerosols froze because they contained small elemental carbon inclusions.
Lower tropospheric aerosols such as mineral dust were not enhanced in subvisible cirrus residues.Furthermore, interstitial dust aerosols were occasionally observed within SVC.In contrast, Cziczo et al. (2004b) found large enhancements of both mineral dust and sea salt in anvil cirrus residues (Fig. 4b).For the tropopause region described here, mineral dust did not possess a substantial advantage to freezing over other aerosol types.It is likely that effective IN were removed from the aerosol population lower in the TTL.Many of the dust spectra showed evidence of organic and sulfate coatings, which probably reduced their effectiveness as IN (Mohler et al., 2008;Cziczo et al., 2009).Acidic sulfate aerosols and meteoric material from the stratosphere were not found Introduction

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Full in either the SVC residue or the nearby unfrozen aerosol.As a CR-AVE mission average, 5% of detected aerosols within 1 km below the tropopause originated from the stratosphere.The air in the region of SVC formation appeared to have unusually low stratospheric influence, probably because mixing of dry stratospheric air lowers the relative humidity and suppresses SVC formation.
Figure 5 compares mass spectra and aerodynamic size of the sulfate-organic particle type for cirrus residue and unfrozen aerosol.The sulfate-organic mixtures that froze did not appear chemically unique, and cirrus residue spectra were very typical of TTL aerosol.Sulfate was highly neutralized and probably in the form of ammonium sulfate.Nitrogen content was relatively high due to both ammonium and gas phase nitric acid uptake.Relative mass fractions of sulfate and organic material were 50/50% for unfrozen aerosols and 36/64% for SVC residue.Although these averages are not statistically different (n=197 and 57), they indicate that ice formation was not initiated only by sulfate-rich aerosols.This is in contrast to anvil cirrus residues where aerosols with higher sulfate content preferentially froze (Cziczo et al., 2004b).Signatures of mercury and halogens (Murphy et al., 2006b) provide further evidence that ice residue and background aerosol were not recently injected from the lower troposphere.
Preferential ice formation on larger aerosols has been predicted theoretically (Pruppacher and Klett, 1997;Koop et al., 2000) and observed for both homogeneous (Cziczo et al., 2004a) and heterogeneous mechanisms (Archuleta et al., 2005;Gallavardin et al., 2008).Since cirrus residues were not chemically distinct, one might expect that only the largest sulfate-organic aerosols froze to form SVC. However, SVC residues detected by PALMS were not significantly larger than unfrozen aerosols (Fig. 5c).We presume that aerosol composition is fairly homogeneous across the accumulation mode, and that it is therefore unlikely that aerosols below the detection range would dominate ice nucleation via a different mechanism.This is probably a good assumption since long residence times in the TTL will act to homogenize sulfate-organic particles in the accumulation mode.
Ice residue data from other SVC encounters during CR-AVE are consistent with the Introduction

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Full 2 February measurements (Table 1).To augment the limited residue dataset, particle type classifications were also determined for negative ion spectra, although classification is less selective than for positive spectra.Particles with biomass burning or meteoric material cannot be differentiated from sulfate-organic particles in negative ion mode.Nevertheless, the negative ion particle types are consistent with positive ion data: the vast majority of ice residues and unfrozen aerosols were sulfate-organic internal mixtures with very low abundances of mineral dust and sea salt.

Nucleation near the tropical tropopause
Unlike controlled laboratory experiments of ice nucleation, ice onset conditions for SVC encountered in flight cannot be accurately determined.Assuming that vertical displacement was <1 km since SVC formation, ice initiation occurred in the temperature range 185-200 K. Despite uncertainties regarding ice onset conditions, we can make several definitive statements about the nucleation mechanism.The microphysics of these SVC are not easily explained by either conventional homogeneous or heterogeneous freezing.In situ water vapor measurements indicated ice saturation levels (RHi) up to 130% within and near SVC, below the 160-165% level where homogeneous freezing of soluble aerosols is predicted (Fig. 3).Aerosol concentrations (d >100 nm) within the 2 February SVC, were ∼10 cm −3 compared to ice crystal concentrations of ∼20-50 L −1 , indicating that only 1 in ∼200-500 aerosol particles froze.This selectivity is not consistent with homogeneous freezing that tends to activate a large fraction of accumulation mode aerosols within a very narrow relative humidity range.A more detailed analysis has been performed by Jensen et al. (2009b).In their simulations homogeneous freezing produced far higher fractions of freezing aerosol than were observed during CR-AVE.A low ice crystal number concentration can be formed in SVC if only a small subset of aerosol freezes at ice saturations below the homogeneous threshold (Kr ämer et al., 2009).However, that would normally imply that some particles had unique com-Introduction

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Interactive Discussion positions to serve as effective IN, which was not observed.Our results suggest that at tropopause temperatures some sulfate-organic particles possess an unseen advantage to freezing and can freeze via a heterogeneous mechanism.Furthermore, the sulfate-organic aerosols that did freeze were more effective IN than other aerosol types including mineral dust.We previously estimated the average TTL mineral dust concentration outside cloud during CR-AVE to be ∼5 L −1 (Froyd et al., 2009).Even if all the mineral dust aerosols froze, they could not generate the crystal concentrations in SVC.
Recent laboratory studies have identified novel nucleation mechanisms that may be relevant to freezing of sulfate-organic aerosols in the tropical tropopause region.There is evidence that dry ammonium sulfate aerosols can act as efficient heterogeneous ice nuclei (Abbatt et al., 2006), freezing at ice saturations up to 45% below the homogeneous threshold.Upon gradual ascent through the TTL, it is plausible (Jensen et al., 2009b) that a pure ammonium sulfate aerosol would experience a relative humidity over liquid water RHw <35%, at which point it would effloresce.At temperatures below about 205 K the deliquescence relative humidity for pure ammonium sulfate becomes higher than that required for homogeneous freezing (Fortin et al., 2002) so that efflorescence is effectively irreversible.PALMS measurements suggest that the sulfate in these aerosols was nearly or fully neutralized.All the ice residuals also contained organics, which will likely suppress heterogeneous freezing (Mohler et al., 2008).However, ammonium sulfate aerosols that are only partially coated and have exposed surface sites and may still be IN-active.
In other experimental studies, organic species thought to be representative of upper tropospheric aerosol material formed highly viscous "glasses" at low temperatures (Murray, 2008;Zobrist et al., 2008).These glassy aerosols resisted water uptake and did not freeze at ice saturation levels greater than those for conventional homogeneous freezing.However, recent experiments at the AIDA expansion chamber (Murray et al., 2009) show that small fractions of these glassy aerosols can act as heterogeneous nuclei, forming ice crystals at saturation levels significantly lower than homogeneous  , 2002) and may exert a disproportionally strong influence on aerosol properties in the tropopause region (Froyd et al., 2009).TTL cirrus formed subsequent to these injection events may therefore proceed via different mechanisms than the tropopause level SVC described here.a The metallic components of particles with biomass burning or meteoric material are not detected in negative ion mode, and consequently, these aerosols are usually classified as sulfateorganic.
b Relative mass abundance of sulfate and organic material, and sulfate neutralization are determined from negative ion spectra.Introduction

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Full  and Koop (2005), and the shaded region encompasses the full range of measurements.The homogeneous freezing limit (vertical bar) is shown for soluble 300 nm aerosols (Koop et al., 2000).T and RHi data include regions both within and outside cloud.
+ and SiO + with their respective isotopes in the absence of other metals, 3) very intense Fe+ signals were observed in combination with Mo + , Cr + , or Ni + , indicating stainless steel, 4) combinations of Sn + , Sb + , Ni + , Zn + , and Cu + were observed at >50 times their usual intensities.Additionally, aerosol spectra were rejected in regions Screen / EscPrinter-friendly Version Interactive Discussion and ammonium sulfate (2:1) were the most common forms of sulfate.Other particle types such as biomass burning aerosol, mineral dust, meteoric material, and stratospheric sulfuric acid were present as minor fractions of an external mixture with the sulfate-organic aerosol.Outside of regions influenced by recent continental convection, aerosols showed aging signatures consistent with slow ascent through the

Fig. 1 .Fig. 3 .
Fig. 1.Contamination signatures from CVI sampling in dense ice clouds.(a) Stainless steel.(b) A nearly pure silicon particle probably from an o-ring inside the CVI.(c) An internal mixture of atmospheric mineral dust and inlet metals.* Denotes contamination signatures.
It is clear that the subvisible cirrus observed throughout this study did not form via a "conventional" nucleation mechanism such as ice deposition on mineral dust or homogeneous freezing of sulfuric acid solutions.Accumulation mode aerosols in the TTL are principally mixtures of partially or fully neutralized sulfate with organic material.A small fraction of these aerosols freeze to form ice crystals and generate subvisible cirrus at the tropopause.The ice-forming aerosols do not appear chemically distinct, nor are they larger than background aerosols.In contrast to previous observations, high organic content did not inhibit freezing.Heterogeneous nucleation involving either dry ammonium sulfate or glassy organic aerosols are both plausible mechanisms to initiate ice formation and ultimately generate SVC clouds.It is worth noting that deep continental convection sporadically penetrates the TTL (Alcala and Dessler

Table 1 .
Composition of ice residues and unfrozen aerosols during SVC encounters.