A review of polar stratospheric cloud formation

CHINA PARTICUOLOGY Vol. 4, No. 6, 261-271, 2006

z Review

A REVIEW OF POLAR STRATOSPHERIC CLOUD FORMATION Xihong Wang* and Diane V. Michelangeli Department of Earth and Space Science and Engineering, York University, Toronto, Ontario, M3J 1P3, Canada *Author to whom correspondence should be addressed. E-mail: [email protected]

Abstract

Liquid and solid particles in polar stratospheric clouds (PSCs) have been known to play a crucial role in the chemical loss of stratospheric ozone over the Antarctic and Arctic regions in late winter and early spring. The stratospheric aerosol and cloud particles provide the sites where fast heterogeneous chemical reactions convert inactive halogen reservoir species into potential ozone destroying radicals. The sedimentation of nitric acid-containing PSC particles irreversibly removes HNO3 gas (denitrification) from the lower stratosphere, which slows the return of chlorine to its inactive forms, resulting in more severe stratospheric ozone destruction. Although these clouds have been investigated extensively during the past decade using in situ field observation, laboratory experiment and modeling studies, the detailed microphysics processes under cold stratospheric conditions are still uncertain. This paper reviews the recent advances in our understanding of PSCs.

Keywords

polar stratospheric clouds, ozone loss, cloud microphysics, denitrification, atmospheric modeling

1. Introduction Although the stratosphere is much cleaner than the troposphere, it contains a significant amount of suspended particulate matter, such as aerosol and cloud particles. It is well-known that the stratosphere contains a permanent aerosol layer, which exists at altitudes between the tropopause and about 25 to 30 km, and consists mainly of particles of aqueous sulfuric acid and water with a mean diameter of about 0.1 μm and a concentration from -3 1-10 particles⋅cm (Turco et al., 1982; Wilson et al., 1990; Deshler & Oltmans, 1998). Steele and Hamill (1981) developed the first parameterization of the equilibrium composition and volume of background stratospheric sulfate aerosols (SSAs) as a function of stratospheric water partial pressure and temperature and predicted that H2SO4 concentration in the droplets would vary from 80 wt% at 230 K typical of mid-latitudes to about 45 wt% at 195 K in the winter polar stratosphere. In addition to the ubiquitous SSAs, laboratory experiments, in situ measurements and theoretical studies have shown that cloud particles in both liquid and solid phase exist in the polar stratosphere at very low temperatures (e.g., Toon et al., 1986 & 2000; Crutzen & Arnold, 1986; Hanson & Mauersberger, 1988; Poole & McCormick, 1988; Goodman et al., 1989; Dye et al., 1992 & 1996; Worsnop et al., 1993; Carslaw et al., 1994; Tabazadeh et al., 1994a; Voigt et al., 2000; Schreiner et al., 1999; Fahey et al., 2001; Drdla et al., 2003; Northway et al., 2002; Deshler et al., 2003). Based on their formation temperature and phase, these polar stratospheric clouds (PSCs) have been classified into three main types, namely Type Ia (solid HNO3-containing particles; e.g., Nitric Acid Trihydrate (NAT) or Nitric Acid Dihydrate (NAD)), Type Ib (Supercooled Ternary Solution, (STS)), and Type II (Water Ice Crystals). Although the exact chemical composition, phase, and formation mechanisms

of PSCs are still unclear (Tolbert & Toon, 2001), it is generally believed that the phase and composition of SSAs play a central role in the formation of PSCs. PSCs have been observed for more than a century in the polar regions (Stanford & Davis, 1974). However, little was known about the PSCs until Farman et al. (1985) discovered the Antarctic ozone hole in the mid-1980s. Intensive scientific activity followed the discovery of Farman et al. (1985), leading to the finding that the severe ozone depletion seen in the Antarctic spring, occurring in the lower stratosphere, was the result of chlorine activated from its reservoir species by heterogeneous reactions occurring on (or in) PSCs (Solomon et al., 1986; Solomon, 1988 & 1990). Since PSCs have been linked to ozone loss, the understanding and the quantification of the impact of such clouds on ozone chemistry was one of the objectives of the scientific community. Laboratory studies quickly provided the rates of key heterogeneous reactions on PSCs. A scientific review on these issues has been given in detail by Solomon (1999). Tables 1 and 2 list the current commonly accepted key heterogeneous reactions on (or in) PSCs, where (l) marks the liquid phase species, (s) marks the solid phase species, and γ is the heterogeneous reaction probability provided by laboratory measurements for each of the processes. When PSCs start to form in the polar vortex, fast heterogeneous reactions take place on (or in) PSCs, converting reservoir species (ClONO2, HCl and BrONO2) into more active forms of chlorine and bromine, such as HOCl, HOBr, BrCl and Cl2. These active halogen species (potential ozone-destroying species) will be maintained at high levels during the polar winter at higher latitudes due to limited sunlight to trigger the ozone- destroying catalytic cycles. In spring, when sunshine is available, photolysis of the species leads to the production of halogen atoms (Cl, Br), as well as ClO and BrO. These reactive halogen species then enter the catalytic ozone cycles in

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the lower stratosphere, causing ozone to be destroyed. Table 1

Heterogeneous reactions in STS droplets and the reaction probabilities (γ )

γ STS

Reactions

(R1) N2O5 + H2O (l) → 2HNO3 0.1a γ 2b (R2) ClONO2 + HCl (l) → Cl2 + HNO3 γ 3b (R3) ClONO2 + H2O (l) → HOCl + HNO3 γ 4c (R4) HOCl + HCl (l) → Cl2 + H2O γ 5d (R5) BrONO2 + H2O (l) → HOBr + HNO3 γ 6e (R6) HOBr + HCl (l) → BrCl + H2O a b c Sander et al., 2003; Hanson, 1998; Donaldson et al., 1997 d Hanson, 2003; e Hendricks et al., 1999

and on the character of the particle surface (Ravishankara & Hansen, 1996). Also, denitrification due to particle sedimentation depends largely on particle size which, in turn, depends on particle composition and phase. However, many uncertainties about the clouds still remain. In this paper, we present a short review of the current advances in our understanding of PSCs. Section 2 introduces the PSC types and composition. Section 3 details the PSC formation mechanisms. The impact of PSCs on denitrification is discussed in Section 4. Section 5 discusses the current model development of PSCs and Section 6 presents the summary.

2. PSC Types and Composition Table 2

Heterogeneous reactions on NAT and ice particles and the reaction probabilities (γ ) Reactions

(R7 ) (R8 ) (R9 ) (R10) (R11) (R12) (R13) a

N2O5 + H2O (s) → 2HNO3 N2O5 + HCl (s) → ClNO2 + HNO3 ClONO2 + HCl (s) → Cl2 + HNO3 ClONO2 + H2O (s) → HOCl + HNO3 HOCl + HCl (s) → Cl2 + H2O BrONO2 + HCl (s) → BrCl + HNO3 HOBr + HCl (s) → BrCl + H2O

γ NAT 0.0004 0.003a 0.2ª 0.004ª 0.1a 0.3b 0.1a

γ ice a

0.02a 0.03a 0.3a 0.3a 0.2a 0.3b 0.3a

Sander et al., 2003; b Lary et al., 1996

In addition to providing sites for heterogeneous chemical reactions, gravitational sedimentation of HNO3-containing PSC particles irreversibly moves HNO3 gas from higher to lower altitudes, causing denitrification. Nitric acid can be produced by heterogeneous reactions on PSCs (see Tables 1, 2). It is destroyed by photolysis and by the reaction with OH, (1) HNO3 + hv → OH + NO2 HNO3 + OH → NO3 + H2O (2) The photolysis of HNO3 in the polar spring releases NO2 which deactivates chlorine by the reaction, (3) ClO + NO2 + M → ClONO2 + M Removal of HNO3 from the polar stratosphere by denitrification reduces the concentration of NO2, and thereby slows down chlorine deactivation, resulting in prolonged ozone destruction and more severe stratospheric ozone destruction (Rex et al., 1997; Waibel et al., 1999; Tabazadeh et al., 2000b). Moreover, the sedimentation of Type II PSCs can result in dehydration by removing H2O, with possible consequences for subsequent PSC formation, the hydrogen oxide (HOx) budget, and the stratosphere’s radiation balance (Mancini et al., 1992; Schiller et al., 1996). The knowledge of these clouds has evolved considerably since the discovery of the Antarctic ozone hole. Scientific efforts have focused on determining the PSC composition, phase, size distributions, and formation conditions, as well as their effect on denitrification. This information is crucial in accurately estimating the amount of ozone loss and revealing the mechanism of ozone destruction during polar winter, because of the dependence of heterogeneous reaction rates on the amount of aerosol surface available

Previously, ground-based and airborne lidar backscatter and depolarization measurements have been used to classify the stratospheric particles of relevance for the ozone depletion (World Meteorological Organization (WMO), 1999). Two types of PSCs forming respectively above (Type I PSC) and below (Type II PSC) the ice frost point temperature were originally identified (Poole & McCormick, 1988). Type I PSCs were later subdivided into two subclasses based on lidar depolarization: Type Ia (large depolarization, implying that they are solid particles) and Type Ib PSCs (small depolarization, implying that they probably consist of spherical droplets) (Browell et al., 1990; Toon et al., 1990a). While Type II PSCs are known to be composed of water ice particles and form at temperatures 3-4 K below the ice frost point (Poole & McCormick, 1988; Goodman et al., 1989), the composition of solid Type Ia PSCs still remains unclear (Bertram et al., 2000). It was originally suggested that Type Ia PSCs are composed mainly of HNO3 and H2O, possibly in the form of thermodynamically stable nitric acid trihydrate (NAT) (Crutzen & Arnold, 1986; Toon et al., 1986; Hanson & Mauersberger, 1988). However, kinetic barriers to nucleation and crystal growth may cause the composition to deviate substantially from thermodynamic equilibrium. In many cases, particles may remain in metastable hydrate form (Bertram et al., 2000), such as nitric acid dihydrate (NAD, HNO3⋅2H2O) (Worsnop et al., 1993), nitric acid pentahydrate (NAP, HNO3⋅5H2O) (Marti & Mauersberger, 1994) and a mixed hydrate (MIX, HNO3⋅H2SO4⋅5H2O) (Fox et al., 1995). Recently, NASA ER-2 measurements in the 1999/2000 Arctic winter (Fahey et al., 2001; Northway et al., 2002) have provided conclusive evidence that the large observed HNO3-containing particles are composed principally of NAT and have significant potential to denitrify the lower Arctic stratosphere. On the other hand, laboratory, field and modeling studies have shown that sulfate solutions can remain as supercooled liquid to very low temperatures (Dye et al., 1992; Tabazadeh et al., 1994a & b; Carslaw et al., 1994 & 1995; Drdla et al., 1994). Theoretical equilibrium models of Carslaw et al. (1994 & 1995) and Tabazadeh et al. (1994b) have shown that SSAs absorb large amounts of HNO3 and

Wang & Michelangeli: A Review of Polar Stratospheric Cloud Formation H2O from the gas phase for temperatures between NAT and ice frost points, leading to a steep increase in volume and a change in the composition from almost pure binary H2SO4/H2O to a supercooled ternary HNO3/H2SO4/H2O solution. These thermodynamic models indicated that the observed Type Ib PSCs would be STS. Infrared spectra analysis by Toon and Tolbert (1995), and in situ measurements by Dye et al. (1996), Schreiner et al. (1999), and Larsen (2000) strongly supported the presence of STS PSCs.

3. PSC Formation Mechanisms 3.1 STS It is now generally accepted that partitioning processes of HNO3 into the SSAs with falling temperature can transform liquid binary SSAs into STS. The equilibrium composition of STS particles under polar stratospheric conditions has been modeled well and their thermodynamic properties have been described in detail (e.g., Carslaw et al., 1994, 1995 & 1997; Tabazedeh et al., 1994a & 1994b; Larsen, 2000). When the temperature drops below approximately 195 K, background SSAs absorb significant quantities of HNO3 gas, changing the composition of droplets from a nearly binary sulfuric acid solution into STS. As temperatures plunge further, in particular, at the temperatures roughly 3-4 K below the NAT frost point, the STS particles absorb more H2O and HNO3, forming PSC particles that are essentially binary solutions of HNO3/H2O. Another alternative formation mechanism of STS might be deliquescence upon cooling of frozen Sulfuric Acid Tetrahydrate (SAT) particles. Field measurements (Toon et al., 1993; Larsen et al., 1995; Dye et al., 1992) and laboratory results (Anthony et al., 1995; Carleton et al., 1997; Koop et al., 1997; Imre et al., 1997) indicate that SSAs often remain liquid down to very low temperatures. Calculations using classical nucleation theory predicted that the binary H2SO4/H2O aerosols would not freeze homogeneously to SAT in the stratosphere (Luo et al., 1994). However, other investigations from both the laboratory (Iraci et al., 1995; Koop et al., 1995; Middlebrook et al., 1993; Molina et al., 1993) and the field (Beyerle et al., 2001; Gobbi & Adriani, 1993; Larsen et al., 1995; Nagai et al., 1997; Rivière et al., 2000; Rosen et al., 1993; Sassen et al., 1994) have observed crystalline SAT at stratosphere temperatures. According to the bulk phase diagram for the H2SO4/H2O system (Gable et al., 1950), it is thermodynamically favorable for sulfuric acid particles to be frozen under stratospheric conditions. Currently, two alternative mechanisms have been invoked to account for the SAT particle formation: (1) evaporation of crystalline PSCs (either Type Ia PSCs or Type Ib PSCs) leaving a SAT core (Tolbert, 1996); and (2) very rare heterogeneous nuclei in the stratosphere may cause the background SSAs to form SAT. Candidates for freezing nuclei in the stratosphere

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would be micrometeorites, spacecraft debris, volcanic ash, or particles from the troposphere (Tolbert & Toon, 2001). Once formed, SAT particles remain frozen until temperatures above roughly 216 K (Middlebrook et al., 1993). However, if frozen SAT particles are cooled below a given threshold temperature in the presence of high concentrations of HNO3 vapor, thermodynamic model calculations (Koop & Carslaw, 1996) and laboratory studies (Iraci et al., 1998) have shown that the particles become unstable. Dissolution of the particles will take place, and a film of STS will form on the particle surface by uptake of HNO3 and H2O until the particle is completely dissolved and turned into a liquid STS particle. Iraci et al. (1998) pointed out that this process requires the HNO3 saturation ratio with respect to NAT (SNAT) as high as 15. This process may compete with the nucleation by HNO3 vapor deposition on pre-activated SAT to form NAT, which will take place at a lower saturation ratio.

3.2 Type Ia PSC 3.2.1 Homogeneous nucleation While both the composition and formation mechanism of liquid Type Ib PSCs are clear to some extent, the formation processes of Type Ia PSCs still remain uncertain although selective nucleation mechanism (via homogeneous or heterogeneous freezing) has typically been invoked (World Meteorological Organization (WMO), 2003; Tolbert & Toon, 2001). Currently, two theories involving the nucleation occurring in the volume or on the surface of a supercooled liquid droplet have been proposed to account for the homogeneous freezing of Type Ia PSCs from STS droplets. Traditionally, homogeneous nucleation rates have been based on nucleation initiated in the volume of supercooled water droplets (Pruppacher & Klett, 1997). In this way, the freezing rate of ice becomes proportional to the volume of the droplets. Similarly, almost most of the data from laboratory studies on the freezing of HNO3-containing droplets into nitric acid hydrates have been analyzed assuming that crystalline nuclei form inside the droplet volume (e.g., Salcedo et al., 2001; Prenni et al., 1998; Bertram & Sloan, 1998a & 1998b). Tabazadeh et al. (2001) have extrapolated the laboratory homogeneous freezing rates of Salcedo et al. (2001) to stratospheric conditions and obtained nucleation rates sufficient to produce the observed large HNO3-containing particle populations in a microphysical model. However, Knopf et al. (2002) argued that the nucleation rates of Salcedo et al. (2001) for concentrated solutions of aqueous HNO3 seem not to be linearly extrapolated to more dilute systems, which are representative of the lower stratosphere, implying that the homogeneous nucleation is not responsible for the observed polar denitrification. This leads some current 3-D modeling studies to dismiss the homogeneous nucleation (e.g., Carslaw et al., 2002; Mann et al., 2002). However, some more recent studies, by comparing modeling results with the in situ balloon-borne and satellite observations, re-emphasized

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the importance of the homogeneous nucleation mechanism of Type Ia PSCs out of Type Ib PSCs in the formation of large HNO3-containing particles observed in the Arctic winter (e.g., Irie & Kondo, 2003; Wang & Michelangeli, 2006). Therefore, a proper treatment of the homogeneous nucleation is critical for correctly modeling Type I PSC formation and stratospheric denitrification. Besides the volume-based homogeneous freezing mechanism, some recent experiments by Tabazedeh et al. (2002a & 2002b) and theoretical studies by Djikaev et al. (2002 & 2003) argued that homogeneous freezing of NAD and NAT most likely initiates on the surface of a supercooled cloud droplet rather than within its bulk volume. Moreover, several molecular dynamics simulation studies (Chushak & Bartell, 1999 & 2000) showed that for all kinds of supercooled liquid clusters, crystal nuclei appear preferentially at, or very close to, the surface. This theory challenges the established traditional volume-based homogeneous nucleation theory. However, the importance of surface-based nucleation in the atmosphere remains unknown. Kay et al. (2003) proposed that the potential for germs formation at the surface of atmospheric droplets can not be proven or eliminated using existing experimental, thermodynamic, or atmospheric data. In addition, the homogeneous nucleation rates in the work of Tabazadeh et al. (2002a) are most likely not applicable to the stratosphere, because the fitted rates may only be valid for concentrated binary solutions of nitric acid and water. Therefore, more measurements and laboratory experiments are indeed needed to make further progress in this area. 3.2.2 Heterogeneous nucleation Besides the homogeneous nucleation, heterogeneous nucleation may occur if the suitable freezing nuclei exist in the stratosphere. Modeling studies by Drdla et al. (2003) showed that heterogeneous freezing can produce large particles if only a very small fraction of stratospheric aerosol particles contained an effective freezing nucleus. Thus, the importance of heterogeneous nucleation on the freezing nuclei cannot be neglected. Previously, it was thought that the most likely mechanism involves the heterogeneous nucleation of nitric acid hydrates on the surfaces of ice particles (e.g., Tolbert, 1994; Koop et al., 1995). In this case, Type Ia PSC formation would first require the occurrence of water ice crystals. However, evidence from the current observations in the Arctic winter indicates that solid nitric acid clouds in the stratosphere often form independently of ice clouds (World Meteorological Organization (WMO), 2003; Tabazadeh et al., 2000b). Also, modeling studies by Drdla et al. (2003) have shown that ice supersaturation is not necessary for NAT formation. On the other hand, Dye et al. (1992) and Turco et al. (1989) proposed that NAT might condense directly on SAT to yield crystalline Type Ia PSCs. Although some subsequent investigations (e.g., MacKenzie et al., 1995; Iraci et al., 1995) have shown that SAT is not a good surface for

the condensation of NAT from gaseous HNO3 and H2O, other laboratory studies (e.g., Zhang et al., 1996) have shown that NAT may nucleate onto preactivated SAT at a NAT-saturation ratio around 7-13, providing an alternative heterogeneous way to generate Type Ia PSC particles above the ice frost point. Based on the current laboratory studies, classical heterogeneous nucleation theory given by Pruppacher and Klett (1997) has been improved and widely used in the microphysical model (e.g., Larsen, 2000) to describe the nucleation rates of NAT on preactivated SAT.

3.3 Type II PSC (ice crystal) The ice nucleation mechanism to form Type II PSCs is important for controlling the ice particle size and hence the possible dehydration in the polar winter stratosphere. One mechanism that has been suggested to initiate Type II PSC nucleation in the stratosphere is volume-based homogeneous freezing of H2SO4/H2O, H2SO4/HNO3/H2O, or HNO3/H2O liquid aerosols. Extensive freezing experiments on micron-sized supercooled aqueous solutions such as H2SO4/H2O, H2SO4/HNO3/H2O, or HNO3/H2O (Bertram et al., 1996; Koop et al., 1997 & 1998; Chang et al., 1999; MacKenzie et al., 1998) and the in situ lidar observations of a stratospheric lee wave ice cloud (Carslaw et al., 1998) have demonstrated that the supercooled liquid particles will freeze homogeneously into water ice at temperatures a few kelvin below the ice frost point. On the basis of these laboratory data, classical volume-based nucleation theory for homogeneous freezing has been employed to calculate the freezing rate of ice in supercooled binary H2SO4/H2O solution (Tabazadeh et al., 1997a), HNO3/H2O solution (Tabazadeh et al., 1997b), and ternary H2SO4/HNO3/H2O solution (Tabazadeh et al., 2000a). However, due to the lack of laboratory data from H2SO4/HNO3/H2O freezing experiments, in particular the data for diffusion activation energy in STS, classical homogeneous nucleation theory is insufficient to describe the properties of ice freezing from STS solution. Recently, Koop et al. (2000) found that the homogeneous nucleation of ice from supercooled aqueous solutions is independent of the chemical nature of the solute, but depends only on the water activity of the solution. Thus, the ice nucleation rate in solutions of different chemical composition and solute concentration is almost the same for solutions of the same water activity at a given temperature. This work provides an activity-based parameterization for treating homogeneous ice freezing in atmospheric models, which expresses the nucleation rate as a function of water activity and pressure. Theoretical calculations by using activity-based parameterization of Koop et al. (2000) suggested that ice freezing from ternary H2SO4/HNO3/H2O aerosols has strong temperature dependence and occurs in a very narrow temperature interval at temperatures roughly 3-4 K below ice frost point for the different aerosol sizes, and requires saturation ratios with respect to ice of up to 1.6, which are very close to observation results (Carslaw et al., 1998). Since the homoge-

Wang & Michelangeli: A Review of Polar Stratospheric Cloud Formation neous nucleation rate is proportional to the volume of the particles, larger droplets have higher homogeneous freezing temperatures than smaller droplets. The larger particles freeze first during cooling. Once the frozen particles occur, fast condensation of H2O vapour to these particles will decrease the H2O partial pressure and decrease the freezing temperature furthermore for the remaining smaller droplets, inhibiting further freezing of the smaller particles. Heterogeneous ice nucleation is also believed to be a major pathway for the formation of Type II PSCs. Previous studies have shown that NAT particles could act as effective nuclei for Type II clouds (e.g., Toon et al., 1989; Drdla & Turco, 1991). However, the theory of heterogeneous nucleation is not well-developed. Thus far, classical heterogeneous nucleation theory (Pruppacher & Klett, 1997) has been widely used in the microphysical model (e.g., Toon et al., 1989; Drdla & Turco, 1991) to describe the nucleation rate. Due to the lack of laboratory and field data, some physical quantities in classical heterogeneous nucleation theory, to which the results are very sensitive, still remain uncertain. For example, as a strong dependent parameter of the nucleation rate, compatibility parameter, m, which indicates the compatibility of the crystal lattice of the nucleus with that of the condensing germ, has not been measured in the laboratory. When m equals to 1, it indicates that there will be no energy barrier for nucleation, except for the Kelvin effect of the smaller particles. Toon et al. (1989) performed a thorough set of computations for Type II PSC nucleation, using m-values between 0.9 and 1.0, and recommended m=0.95 as a reasonable guess. In addition to ice nucleation on top of NAT, some other theoretical works suggested that Type II PSCs could be formed by heterogeneous nucleation of ice out of liquid aerosol containing insoluble nuclei, such as mineral oxides or soot (DeMott et al., 1997; Jensen & Toon, 1997; Sassen & Benson, 2000). However, limited laboratory data exists to support these claims (Chen et al., 2000; Zuberi et al., 2001), all of which have focused on conditions and heterogeneous nuclei that are relevant to the troposphere. More recently, Fortin et al. (2003) has suggested a new alternative heterogeneous formation mechanism for Type II PSCs, the vapor deposition of ice on top of SAT particles. Their laboratory results showed that SAT is an efficient ice nucleus with a critical ice saturation ratio of Sice = 1.3-1.02 over the temperature range 169.8-194.5 K. This corresponds to a necessary supercooling of 0.1-1.3 K below the ice frost point. By incorporating this nucleation scheme into a microphysical and photochemical model, the simulations showed that even a very small number of SAT particles (e.g., 10-4 cm-3) would result in ice nucleation on SAT as the dominant mechanism for Type II PSC formation. As a result, Type II PSC formation is more widespread, leading to larger-scale dehydration. Therefore, a proper treatment of SAT is critical for correctly modeling Type II PSC formation and stratospheric dehydration.

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4. Effect of PSCs on Denitrification Denitrification is believed to be an important factor controlling the magnitude of ozone loss in the polar winter. Modeling studies have shown that Arctic ozone loss caused by denitrification can be as much as 30% (Chipperfield & Pyle, 1998; Waibel et al., 1999; Tabazadeh et al., 2000b). At present, satellite and in situ measurements have shown that extensive denitrification does occur in both the Antarctic and Arctic winter stratosphere (e.g., Fahey et al., 1990; Santee et al., 1999 & 2000; Waibel et al., 1999; Dessler et al., 1999; Kondo et al., 2000; Popp et al., 2001; Kleinböhl et al., 2002). In comparison with the Antarctic denitrification, Arctic denitrification is typically weaker. However, future climate changes may cause a decrease in polar stratospheric temperature, leading to more frequent PSC formation in the Arctic and more widespread Arctic denitrification, thereby, enhancing future ozone loss (Waibel et al., 1999; Tabazadeh et al., 2000b). It is therefore important to understand the conditions and processes required for denitrification to occur. The main factor in understanding denitrification is the PSC composition, size, and nucleation mechanisms. STS particles grow by condensation of HNO3 and H2O on background SSAs as ambient temperatures decrease below 200 K (Carslaw et al., 1997). This process is not impeded by a nucleation barrier, and thus HNO3 and H2O condense on all of the sulfate aerosols. However, the amount of available HNO3 and H2O in the lower stratosphere limits the particles reaching sizes large enough for sedimentation and denitrification (Tolbert and Toon, 2001). In contrast, laboratory studies have indicated that there is a substantial nucleation barrier to NAD and NAT formation (Worsnop et al., 1993; Prenni et al., 1998). Selective nucleation of a small number of solid HNO3-containing particles can result in the transfer of a large fraction of the total available nitric acid to just a few particles, which can then grow to sizes large enough to denitrify the atmosphere (Salawitch et al., 1989; Toon et al., 1990b; Waibel et al., 1999). Indeed, intensive PSC studies during the 1999/2000 Stratospheric Aerosol and Gas Experiment III (SAGE III) Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone (SOLVE/THESEO 2000) have observed large HNO3-containing particles with diameters of 10 to 20 μm (mean diameter of 14 μm) and number densities in the -5 -3 -3 range of 10 to 10 cm (average number density is -4 -3 o o 2×10 cm ) over large horizontal areas (60 N-85 N) and over a large altitude range (15-21 km) (Fahey et al., 2001; Northway et al., 2002). Simple calculations presented by Fahey et al. (2001) indicated that these large HNO3-containing particles can grow to their observed sizes in about 5 to 8 days and could be responsible for the denitrification observed in the Arctic (Popp et al., 2001). Most recent model simulations of particle growth suggested that the large HNO3-containing particles are likely either nucle-

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ated as NAT, or, if they nucleated as NAD, they likely converted to NAT shortly after nucleation (McKinney et al., 2004). Ice particles have long been thought to play a role in the mechanism of denitrification (Wofsy et al., 1990; Toon et al., 1990b; Waibel et al., 1999). Previously, it was assumed that denitrification could be caused by ice particles absorbing HNO3 as they fall through the NAT-supersaturated air (Wofsy et al., 1990). However, as we discussed in section 3.2.2, this mechanism requires the pervasive existence of water ice crystals before NAT formation, and the temperatures must be below the ice frost point. In fact, in the Arctic, temperatures are infrequently low enough to allow ice formation. This constrains the possible contribution of ice to the nucleation mechanisms of the large NAT particles. Also, modeling studies (Drdla et al., 2003; Waibel et al., 1999; Davies et al., 2002; Carslaw et al., 2002) have shown that the long-accepted mechanism of HNO3 removal on sedimenting ice particles cannot account for observed levels of denitrification (World Meteorological Organization (WMO), 2003). In summary, denitrification has been studied extensively in both polar regions using in situ and remote observations and modeling techniques. Some of the PSC puzzles are clear, but several important details of the processes are still not well-understood. In particular, the composition and formation mechanism of the particles responsible for the denitrification have still not been established (Tolbert & Toon, 2001), leading to the uncertainties in predicting denitrification accurately in ozone loss models.

5. Modeling Studies on PSCs Many models ranging from process-based microphysical box models to 3-D Chemical Transport Models (CTMs) have been developed to investigate the formation and evolution of PSC particles, as well as their effect on denitrification. The majority of PSC microphysics studies have used box models (e.g., Peter et al., 1992; Meilinger et al., 1995; Tsias et al., 1997; Carslaw et al., 1998; Larsen, 2000; Irie & Kondo, 2003). Box models following isentropic air parcel trajectories are ideally suited to track the evolution of individual particles influenced by microphysical processes. Such models including detailed representation of aerosol and PSC microphysics have been used to interpret and validate observations. However, an analysis of an isentropic air parcel may be inappropriate when low number densities of large particles are involved, since the particles larger than several microns in diameter achieve significant fall velocities and quickly sediment to the lower stratospheric layers. Thus, denitrification by such large particles presents a particular challenge in box modeling along isentropic air parcel trajectories. Earlier 3-D CTMs included highly simplified representations of PSC denitrification (Chipperfield et al., 1993; Lefevre et al., 1994; Eckman et al., 1996). These models assumed that HNO3 condenses on ice particles and is then

carried downward wherever ice occurs in the model. However, the evidence from observations showed that denitrification can occur without significant dehydration. This implies that denitrification can occur without the sedimentation of particles containing large amounts of ice. Some current 3-D models have included more-sophisticated PSC schemes (e.g., Waibel et al., 1999; Davies et al., 2002), but these models assume that PSCs are at thermodynamic equilibrium. In fact, the behavior of large particles cannot be analyzed with equilibrium calculations of the hydrate phases (Carslaw et al., 2002). Thus, comprehensive models must account for the nonequilibrium growth and evaporation of sedimenting large particles. Currently, some modelers (e.g., Tabazadeh et al., 2001; Jensen et al., 2002; Wang & Michelangeli, 2006) have developed 1-D comprehensive cloud models which take into account the time-dependent growth and sedimentation of HNO3-containing particles to explore the factors that control PSC evolution and denitrification. Furthermore, Carslaw et al. (2002) have developed a novel 3-D comprehensive model of particle growth and sedimentation based on domain-filling particle trajectories calculations and used the model to simulate the evolution of several thousand individual particles along trajectories. Since the horizontal motion is calculated by using isentropic trajectories and the horizontal motion is determined from the sedimentation speed, such a 3-D model overcomes the drawback of previous box models in simulating large particle behavior. Mann et al. (2002) applied the 3-D microphysical model of Carslaw et al. (2002) to quantify the effect of the large HNO3-containing particles on Arctic denitrification. These new nonequilibrium growth and sedimentation models are able to reproduce particles with sizes typical of those observed, and reproduce the observed levels of denitrification caused by sedimentation of HNO3-containing particles (World Meteorological Organization (WMO), 2003). However, many uncertainties still remain in current modeling studies due to unclear PSC particle composition, size and nucleation mechanism.

6. Summary The spring-time ozone destruction in the polar stratosphere depends on the amount of chlorine and other active halogen species which are converted from inactive reservoir species via heterogeneous reactions occurring on (or in) PSCs (Solomon, 1999). The activated rates, as well as the effect on ozone depletion due to denitrification, depend on the cloud type (Ravishankara & Hanson, 1996). Therefore, identification of composition and revealing the formation mechanism of PSCs are necessary to develop a quantitative understanding of polar ozone loss. During the past decade, extensive laboratory experiments, in situ field measurements, satellite observations and theory studies have improved our understanding of PSC properties and their effect on denitrification. However, many details of the clouds remain unsolved, in particular regarding the com-

Wang & Michelangeli: A Review of Polar Stratospheric Cloud Formation position, size, and nucleation mechanisms of the large HNO3-containing particles and their effect on denitrification. Removing these present uncertainties will permit more careful modeling of the amount and extent of chlorine activation and denitrification. Furthermore, a more complete quantitative understanding of polar stratospheric ozone loss requires further work in investigations of the frequency, duration, and timing of PSC occurrences in the stratospheric polar vortex.

Acknowledgments The authors are grateful to the reviewers for their constructive comments. Financial support for this work was provided by the Natural Science and Engineering Research Council of Canada (NSERC) and the ACE and GCC programmes.

Acronyms CTM NAD NAT PSC SAT SSA STS

chemical transport model nitric acid dihydrate nitric acid trihydrate polar stratospheric cloud sulfuric acid tetrahydrate stratospheric sulfate aerosol supercooled ternary solution

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