Polar vortex concentricity as a controlling factor in Arctic denitrification

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D22, 4663, doi: /2002jd002102, 2002 Polar vortex concentricity as a controlling factor in Arctic denitrification G. W. Mann, S. Davies, K. S. Carslaw, and M. P. Chipperfield Institute for Atmospheric Science, School of the Environment, University of Leeds, Leeds, UK J. Kettleborough British Atmospheric Data Centre, Rutherford Appleton Laboratory, Didcot, UK Received 16 January 2002; revised 21 May 2002; accepted 7 June 2002; published 30 November [1] Recent in situ observations in the Arctic stratosphere have detected nitric acid containing particles with sizes up to 10-mm radius and number concentrations between 10 5 and 10 3 cm 3. Here we quantify the effect of these particles on Arctic denitrification by using a new three-dimensional (3-D) model which can couple particle growth and sedimentation with the full dynamics of the Arctic polar vortex. We show that the very long growth times of large nitric acid trihydrate (NAT) particles lead to a highly nonlinear dependence of Arctic denitrification on the growth and evaporation cycles of individual particles, thus making denitrification dependent on the precise meteorological conditions in a given winter. Using 3-D wind and temperature fields from December 1999, we identify a period that was optimum for denitrification, in which the cold pool and vortex were nearly concentric and in which a large proportion of the particles were able to sediment over about 8 days through the full depth of the cold pool without evaporating. We then show that small departures from concentric conditions can lead to substantial reductions in denitrification. A case is presented in which denitrification was completely shut off even with over half of the cold pool area remaining within the vortex. Under the same conditions, a model in which the particles were assumed to be in continuous equilibrium with the gas phase caused extensive denitrification. Our results show that low Arctic vortex temperatures in themselves are unlikely to be a reliable indicator of potential denitrification if large NAT particles are involved. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0340 Atmospheric Composition and Structure: Middle atmosphere composition and chemistry; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; KEYWORDS: polar stratospheric clouds, NAT rocks, denitrification, stratosphere, Arctic, vortex Citation: Mann, G. W., S. Davies, K. S. Carslaw, M. P. Chipperfield, and J. Kettleborough, Polar vortex concentricity as a controlling factor in Arctic denitrification, J. Geophys. Res., 107(D22), 4663, doi: /2002jd002102, Introduction Copyright 2002 by the American Geophysical Union /02/2002JD [2] Denitrification is the irreversible removal of HNO 3 from the polar stratosphere due to the gravitational sedimentation of HNO 3 -containing particles that form in the polar winter. Under most Arctic and Antarctic conditions, the reductions in gas phase HNO 3 concentrations lead to increased ozone loss rates in springtime. This increased loss rate results from increased active chlorine concentrations, which would otherwise be buffered by reactive nitrogen compounds released from HNO 3 photolysis or reaction between OH and HNO 3 (see, e.g., Solomon [1999]). Several chemical model simulations show that the additional cumulative ozone loss on a single level caused by denitrification of the Arctic stratosphere can be as much as 30% [Chipperfield and Pyle, 1998; Waibel et al., 1999; Tabazadeh et al., 2000]. [3] Denitrification is a ubiquitous feature of the Antarctic polar winter stratosphere, where temperatures are persistently cold. Denitrification has also been observed in the Arctic in the cold winters 1988/1989, 1994/1995, 1995/1996, 1996/1997 and 1999/2000 [Fahey et al., 1989; Hubler et al., 1990; Rex et al., 1997; Hintsa et al., 1998; Waibel et al., 1999; Kondo et al., 1999, 2000; Dessler et al., 1999; Santee et al., 1999, 2000; Popp et al., 2001; Kleinböhlet al., 2002]. [4] An important parameter in any model of denitrification is the number density and size of the sedimenting particles. Arctic observations during winter 1999/2000 detected very low number concentrations ( cm 3 ) of large (up to 10-mm radius) nitric acid containing particles [Fahey et al., 2001; Northway et al., 2002]. These observations have led to a number of model studies aimed at understanding the evolution of such particles [Carslaw et AAC 13-1

2 AAC 13-2 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION al., 2002] as well as their effect on denitrification [Davies et al., 2002; Jensen et al., 2002]. The model study of Tabazadeh et al. [2001], while not motivated directly by these observations, also explored the effect of large nitric acid trihydrate (NAT) particles on denitrification. [5] At present, it is not known how common such large particles are in the Arctic stratosphere because, aside from the NO y instrument used during the 1999/2000 winter, the observing systems needed for their detection have not been deployed. However, their existence has important implications for the way that NAT particles and denitrification are treated in models. In particular, the particle sizes require continuous growth periods for individual particles of typically 5 8 days [Fahey et al., 2001; Carslaw et al., 2002]. However, the typical meteorology of the Arctic stratosphere is such that sedimenting particles are likely to be advected into regions above the NAT temperature more often than once in 5 8 days, implying that large NAT particles may not be able to form in all winters. At the low number densities of hydrate particles observed, the NAT particles will not be in equilibrium with the gas phase HNO 3 [Carslaw et al., 2002]. Thus models must account for the time-dependent growth and evaporation of sedimenting particles throughout the vortex. At present, all three-dimensional (3-D) chemical models assume polar stratospheric clouds (PSC) to be at thermodynamic equilibrium (see, for example, Chipperfield [1999], Considine et al. [2000], and Davies et al. [2002]). [6] Jensen et al. [2002] used a 1-D column model combined with artificial temperature histories to explore the effect on denitrification of the length of the cooling cycle, together with other parameters such as particle number density, and uptake of HNO 3 by liquid aerosols. As was expected, they found that denitrification was increased if particles were allowed to grow for longer periods to larger sizes, at least when number concentrations were low. [7] Tabazadeh et al. [2000] showed that the magnitude of denitrification could be linked to the length of time that air parcels spend below the NAT temperature. They related the observed denitrification to the length of time that air parcels on the 450 K potential temperature surface spent below the NAT temperature. However, an analysis of isentropic air parcels is inappropriate when low number densities of growing particles are involved, since the particles quickly sediment to lower atmospheric layers. The rapid sedimentation of particles (a 6-mm radius particle has a fall speed of 0.9 km per day compared with 0.1 km per day for a 2-mm radius particle) means that isentropic trajectories are valid only for periods shorter than 1 or 2 days. Rather, it is the temperature histories experienced by the particles themselves that are important, and they differ markedly from isentropic trajectories. It is such individual particle trajectories which we examine here as a controlling factor in denitrification. Such a particle analysis is possible only with a model that tracks individual particles in three dimensions [Carslaw et al., 2002]. [8] The idealized 1-D model study of Jensen et al. [2002] was a useful exploration of many of the factors that control denitrification. However, the simulations were restricted to single columns of air in the vortex and hence do not represent the full complexity of coupled particle and air motion in three dimensions. The 3-D model study of Carslaw et al. [2002] focused on the evolution of the large NAT particles and demonstrated that the change in particle characteristics observed in the period January to March 2000 [Northway et al., 2002] could be explained in terms of changes in the 3-D temperature and flow fields. However, they did not calculate denitrification. [9] Here we apply the 3-D microphysical model of Carslaw et al. [2002] to understand how the variable meteorology of the Arctic stratosphere controls denitrification by large NAT particles. In section 2 we describe the model structure and the general setup of the model runs. In section 3 we describe the motivation for and design of the numerical experiments. In section 4 the denitrification results are presented and interpreted in terms of the behavior of the NAT particles in the model. 2. Model Structure [10] The model is described in detail by Carslaw et al. [2002]. Briefly, the model simulates the time-dependent growth and evaporation of several thousand individual nitric acid hydrate particles, with their 3-D motion calculated by using isentropic trajectories combined with vertical motion due to gravitational sedimentation, which depends on particle size. Particle growth by diffusive transfer of HNO 3 removes HNO 3 from the gas phase in each model time step. Gas phase HNO 3 is then calculated (with chemistry switched off) on a 3-D grid and advected each time step using the off-line global Eulerian chemical transport model SLIMCAT [Chipperfield et al., 1996]. For all runs in the present study we have used a SLIMCAT grid with 36 vertical levels between 350 K and 2720 K, giving a resolution of K in the lower stratosphere. [11] The equation for particle growth is where G ¼ D HNO 3 M NAT 2r NAT RT r 2 ¼ r 2 0 þ Gt; ð1þ p HNO3 p NAT HNO 3 : ð2þ D HNO3 is the diffusion coefficient of HNO 3 molecules in air, modified to account for mass transfer noncontinuum effects for particles with size similar to the mean free path. M NAT is the molar mass of NAT and r NAT is the NAT crystal mass NAT density, while p HNO3 and p HNO3 are the ambient partial pressure of HNO 3 and the vapor pressure of HNO 3 over NAT, respectively. Particles are assumed to be in the Stokes drag regime with the sedimentation velocity given by where dz dt ¼ Sr2 ; S ¼ 2gr NAT C C 9h is the sedimentation factor. C C is the Cunningham slip flow correction factor [Pruppacher and Klett, 1997] and h is the viscosity of air. ð3þ ð4þ

3 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION AAC 13-3 [12] For very small particles with negligible sedimentation speed, the combined Lagrangian advection of the particulate HNO 3 and the Eulerian advection of the gas phase HNO 3 conserves total HNO 3 and is equivalent to the straightforward isentropic advection of a tracer in an Eulerian model. In the case of large particles that sediment, the separate advection of gas and particulate HNO 3 leads to 3-D fields of denitrification and renitrification. At each time step, the gas phase HNO 3 is used to calculate subsequent particle growth, so that localized gas phase HNO 3 removal by the hydrate particles feeds back into a reduced particle growth rate. By simulating the motion of the gas phase HNO 3 and particles simultaneously in 3-D space, the model does not make any simplifying assumptions about particle temperature histories or available HNO 3, which are typical deficiencies of column models. [13] In this study we take account only of the HNO 3 partitioning to NAT particles and neglect the uptake into liquid HNO 3 /H 2 SO 4 /H 2 O droplets, which occurs at temperatures typically 4 K below the NAT condensation temperature [Carslaw et al., 1995]. This simplification is justified in the present study since temperatures remain marginally above the temperature at which uptake into droplets occurs. [14] Nucleation of new NAT particles is treated in a straightforward manner. At each time step (here 30 min), new NAT particles are created with a radius of 0.1 mm inall NAT-supersaturated grid boxes of a cosine-weighted horizontal grid and at randomly chosen altitudes. The average nucleation rate (particles per unit air volume and time) is uniform throughout the NAT-supersaturated region. Random altitude levels of nucleation ensure that coherent structures in the final particle size distributions at any one level are smoothed out. Other nucleation mechanisms can also be used [Carslaw et al., 2002]. We have chosen an average nucleation rate corresponding to particle cm 3 s 1, which produces a total number concentration of large NAT particles (radius of >2 mm) equal to cm 3 along the ER-2 flight track on 20 January 2000 [Carslaw et al., 2002], which compares well with the observations of Fahey et al. [2001]. The model transports approximately 20,000 particles at any one time; thus one model particle represents a much higher number of real particles. [15] So long as the nucleation mechanism for very low number concentrations of NAT particles remains unknown, our spatially uniform formation mechanism is the most straightforward method of producing vortex-scale populations of large NAT particles with sizes and number concentrations in good agreement with observations [Fahey et al., 2001; Northway et al., 2002; Carslaw et al., 2002]. Although details of the results may differ with a different nucleation mechanism, our conclusions are unlikely to be sensitive to the precise nucleation mechanism, particularly as our present study focuses on the evolution of the particles in the several days after spatially uniform populations have been generated. 3. Description of the Simulations 3.1. Wind and Temperature Fields [16] The aim of this study is to understand how the characteristic meteorology of the Arctic stratosphere controls denitrification. In the Antarctic polar stratosphere the wind and temperature fields are normally concentric about the pole. In such a state, the region of low temperatures in which PSCs can form (the cold pool) is normally contained within the vortex. In contrast, the Arctic wind and temperature fields are often significantly offset (a strongly baroclinic state) [Pawson et al., 1995]. This characteristic feature of the Arctic stratosphere is likely to exert a strong influence on denitrification by low number densities of slowly growing NAT particles. When the cold pool and the vortex are concentric, NAT particles will tend to remain inside the cold pool as they are advected by the wind, resulting in large particles, but when the cold pool and the vortex are offset, NAT particles are more likely to be advected out of the cold pool and into warmer air before they have sufficient time to grow large enough to sediment out. It is this link between the meteorology of the Arctic stratosphere, the growth period of individual particles, and hence the amount of denitrification that we investigate here in a carefully controlled manner. [17] Pawson et al. [1995] analyzed 30-hPa and 50-hPa geopotential height and temperature fields from winters 1965/1966 to 1993/1994 and described several typical configurations of the Arctic stratospheric flow. These configurations range from near-concentric cases when the cold pool center is colocated with the vortex center to disturbed, nonconcentric cases when the cold pool is displaced considerably from the center of the vortex. The analysis of Pawson and Naujokat [1999] clearly shows that such nonconcentric vortex states do occur in the Arctic stratosphere sufficiently frequently and for sufficient periods of time to influence particle temperature trajectories and, hence, denitrification. [18] In order to isolate the influence of variations in Arctic flow on denitrification, we have used a very controlled set of meteorological conditions and eliminated variations in other factors that influence denitrification. Idealized model runs simulating a development over 10 days were carried out by using fixed wind and temperature fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) as the model forcing. Simulations of 10 days are long enough for particles nucleated in the uppermost level of the cold pool to sediment to the bottom of the stratosphere. In our baseline run we used ECMWF-analyzed wind and temperature fields from 23 December 1999 chosen as a good example of where the cold pool and vortex are nearly concentric or Antarctic-like. We will refer to this configuration of the Arctic stratosphere as the concentric state. Nonconcentric states were produced by artificially shifting the cold pool center away from the vortex center but retaining an identical cold pool shape and area. By doing this, cycles of high and low temperatures experienced by the NAT particles could be influenced without affecting the total area of PSC formation. The shift in the cold pool was achieved by using solid body rotations of between 2.5 and 20, the axis of rotation being the line connecting latitude/longitude points [0, 0 ] and [0, 180 ] on opposite sides of the sphere. The 0 rotation represents a real near-concentric vortex state, while the 20 rotation of the temperature field produces a temperature field representing an idealized cold strongly baroclinic state of the Arctic stratosphere.

4 AAC 13-4 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION Figure 1. Potential vorticity and temperature fields used in the 3-D simulations. Fields are shown for the 585 K potential temperature level because it is the height of the temperature minimum. The angles of each solid body rotation of the temperature field are indicated on each plot. The thick black line shows the vortex edge, as defined by a modified potential vorticity of 40 PV units (i.e., km 2 kg 1 s 1 ). The thick white line is the isopleth of T NAT. Within this region the air is supersaturated with respect to NAT. The 0 case is referred to in the text as the concentric state. Initial fields of HNO 3 and H 2 O were assumed spatially uniform at 8 ppbv and 5 ppmv. [19] Figure 1 shows the temperature fields for 0, 10, 15, and 20 rotations on the 585 K potential temperature surface. This height was chosen because it was the height of the temperature minimum for this ECMWF analysis. For reference, the vortex edge is marked on each plot as a thick black line. Throughout this paper, we define the vortex edge as where the modified potential vorticity (MPV) is 40 PV units (i.e., km 2 kg 1 s 1 ). Modified potential vorticity, is defined as Ertel s potential vorticity (EPV) multiplied by the scaling factor ðq=q 0 Þ 9 2 ; where q0 is a reference potential temperature (taken as 475 K), hence removing much of the density dependence of EPV [Lait, 1994]. We have taken the value of 40 PV units as it is seen to correspond well with the maximum PV gradient, the formal definition of the vortex edge. The white line marks the saturation temperature for NAT, T NAT, calculated by using the expression from Hanson and Mauersberger [1988]. The centroids of the vortex and NAT region are shown for this q surface as black and white squares, respectively. For the 0 run, the entire NAT region is inside the vortex and the centroids are approximately colocated, while for the 20 run, the centroid of the NAT region lies approximately on the vortex edge, and only around half of the NAT region is inside the polar vortex on this q surface. We use the separation of the vortex and NAT region centroids as a measure of vortex concentricity. The centroid separations are normalized to the effective vortex radius at each level (R eff =(A/p) 1/2, where A is the vortex area). A normalized centroid separation equal to 0 implies that the vortex and cold pool centroids are colocated (a highly concentric vortex), and a normalized separation equal to 1 implies that the cold pool centroid sits on the vortex edge (a nonconcentric vortex). [20] Our method of shifting the temperature field means that the wind, temperature, and pressure fields are no longer in thermal wind balance in the rotated runs. However, our intention is to carefully control the cooling/heating cycles of the particles being advected around the vortex, and the nonphysical nature of the meteorological fields that we use are of no consequence in this respect. Fixing the wind, pressure, and temperature fields for 10 days is also somewhat unrealistic. In reality, the cold pool will, to some extent, move around; thus the effect of nonconcentricity on the temperature histories of particles may be smaller in

5 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION AAC 13-5 Figure 2. Relative location of the NAT region and vortex as a function of altitude and rotation of the temperature field, as indicated by the separation of their centroids. reality. However, Pawson et al. [1995] show time series of the magnitude of the geostrophic wind velocity from winters 1984/1985 to 1993/1994 which show numerous periods (e.g., February 1986, January 1987, and February 1993) where the cold center is at the edge of the vortex (the geostrophic wind is high and the flow is strongly baroclinic) and the cold center has T < 195 K for long continuous periods. Consequently, these artificially produced vortex/ cold pool states do represent realistic cases of stable polar vortices of differing baroclinicity. [21] Because NAT particles move in the vertical, it is also important to characterize the vertical structure of the vortex and cold pool. Figure 2 shows a vertical profile of the separation between the cold pool and vortex centroids on each potential temperature surface for the 0, 5, 10, 15, and 20 rotated temperature fields. There is little vertical variation of centroid separation between the 500 K and 600 K potential temperature surfaces for all rotations of the temperature field. Consequently, it is valid to consider the centroid separation at one particular q level in that range as representative of the vortex as a whole. The profile for the nearly concentric 0 case has a different shape than the other fields because the separation distance is in the meridional direction, while for the others, the distance is in a more zonal direction HNO 3 and H 2 O Fields [22] The initial gas phase nitric acid field was assumed uniform throughout the model domain at 8 ppbv. This value is representative of aircraft measurements made in the lower stratosphere in January 2000 [Kleinböehl et al., 2002]. The initial H 2 O field was also assumed uniform at 5 ppmv. This is also a typical value [see Schiller et al., 2002] Equilibrium and Nonequilibrium Simulations [23] In order to understand better the influence of timedependent particle growth on denitrification, we compared the results of our full nonequilibrium model with a 3-D model in which the NAT particles were assumed to be in thermodynamic equilibrium. [24] The equilibrium model is SLIMCAT, as used as the basis for the nonequilibrium model, but with its own much simpler treatment of NAT particles. Denitrification in the equilibrium model is controlled by fixing a particle size (and hence sedimentation speed) in all grid boxes with temperatures below T NAT and calculating a particle number concentration such that the amount of condensed HNO 3 results in an HNO 3 partial pressure in equilibrium with NAT [Davies et al., 2002]. A NAT particle radius of 2 mm was chosen so that the vortex average denitrification for the equilibrium model 0 run was approximately equal to that of the nonequilibrium model 0 run after 10 days. This particle size is much smaller than the mean size in the nonequilibrium model. Using comparable particle sizes in the equilibrium model would lead to almost complete denitrification at some levels, which is undesirable, since denitrification would then be limited by the total available HNO 3 and therefore make the results difficult to compare with the nonequilibrium model. In these equilibrium model runs, the wind and temperature fields were again held fixed for the duration of each of the 10-day runs. 4. Model Results 4.1. Denitrification Fields [25] Figure 3 shows maps of the volume mixing ratio (VMR) of HNO 3 at q = 545 K for the nonequilibrium model runs with temperature field rotations of 0,10, 15, and 20. The 545 K potential temperature level was chosen because it was the level with maximum vortex-averaged denitrification. Figure 4 shows equivalent maps for the equilibrium model run. In this case, the results refer to the 585 K surface, which has the peak denitrification. The vertical profile of the vortex-averaged denitrification and renitrification is shown as a function of time in Figure 5 and as a function of rotation of the temperature field in Figure 6. The vortex average refers to all air within MPV = 40 PV units. [26] The denitrification at the 545 K level for the nonequilibrium 0 case reaches a maximum of about 3 ppbv near the centroid of the vortex and decreases to zero at the vortex edge (Figure 3). With the cold pool shifted by 10, the peak denitrification is reduced to 2 ppbv, and with a 15 rotation the region with HNO 3 > 1 ppbv has diminished to a small patch in the center of the vortex. With a 20 rotation of the temperature field, approximately half of the NAT region remains within the vortex, but the peak denitrification has reduced to just 0.25 ppbv. These large differences in denitrification occur in response to a change in the configuration of the wind and temperature fields, since the temperature field is identical in each model run. [27] In the 0 equilibrium model run (Figure 4) the peak denitrification of between 3 and 4 ppbv is close to the vortex center, as in the nonequilibrium model run (however, no significance should be attached to the magnitude of

6 AAC 13-6 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION Figure 3. HNO 3 VMR on the 545 K potential temperature surface on day 10 of the nonequilibrium model simulations. The different rotations of the temperature field are indicated in each plot (see Figure 1). denitrification, since particle sizes were fixed in order to produce denitrification comparable to the nonequilibrium run). As the temperature field is shifted, the change in predicted denitrification is very different from that in the nonequilibrium model. For the equilibrium model, the area of significant denitrification increases as the vortex-cold pool separation increases, showing that denitrification is occurring equally efficiently in all air below T NAT, irrespective of the configuration of the temperature and wind flow fields. The peak denitrification does decrease as the cold pool is shifted, but this is because the denitrified air is distributed over a larger area. [28] Figure 5 shows the temporal variation of the vertical profiles of vortex-averaged denitrification for the nonequilibrium and equilibrium 0 simulations. In the nonequilibrium simulation (Figure 5a), denitrification begins initially around 590 K after day 2 and then grows to a pronounced region of significant denitrification by day 10 between 450 and 600 K with a maximum vortex-averaged denitrification at 545 K. By day 10 there is a significant region of renitrification between 400 and 450 K where the large denitrifying NAT particles evaporate in warmer air. [29] It is interesting to note that the magnitude of denitrification decreases with decreasing altitude, even though the HNO 3 mixing ratio is constant with altitude. Popp et al. [2001] observed that the magnitude of denitrification matched the profile of available HNO 3 in their observations in January Our results show that such a straightforward interpretation of the denitrification profile will not always be appropriate. The change in denitrification with decreasing altitude below 545 K in our model is caused by a balance between increasing particle size (leading to more efficient denitrification) and an increasing particle loss rate by evaporation (causing renitrification). This balance is discussed further in section 4.2. [30] The vertical profile of HNO 3 in the equilibrium model (Figure 5b) is quite different from the nonequilibrium case. First, the denitrification begins much higher up in the equilibrium model, at around q = 650 K. This is due to the fact that in the equilibrium model, the particles begin sedimenting at a velocity governed by the Stokes drag on a2-mm particle as soon as it is nucleated. No account is taken of the time required to grow to that size, since the assumption in this model is that all particle phase HNO 3 is in the form of NAT particles of radius 2 mm. Second, the shape of the denitrification region is different. The denitrified section of the vortex-averaged profiles from the equilibrium model runs is symmetric about the maximum denitrification, while the nonequilibrium model runs have an asymmetric profile. This is caused by there being no increase in particle size in the equilibrium model; so the first of the two competing effects described above is not present. For the equilibrium model runs, the shape is soleley governed by where and over how wide an area the temper-

7 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION AAC 13-7 Figure 4. HNO 3 VMR on the 585 K potential temperature surface on day 10 of the equilibrium model simulations. The different rotations of the temperature field are indicated in each plot (see Figure 1). ature is below T NAT. Third, denitrification remains insignificant for the first 2 days of the nonequilibrium model, while the denitrification starts immediately where T < T NAT in the equilibrium model. This again reflects the time taken for the particles to grow large enough to acquire an appreciable sedimentation speed in the nonequilibrium model. [31] The change in the vertical profile of denitrification with the shift in the cold pool is also interesting to compare in the nonequilibrium and equilibrium runs (Figure 6). The vortex-averaged denitrification in the nonequilibrium run decreases with successively larger temperature field rotations until, at 20 rotation, denitrification is negligible. In Figure 5. Time dependence of the vertical profile of vortex-averaged denitrification in the (a) nonequilibrium and (b) equilibrium simulations. The vortex edge is defined as MPV = 40 PV units.

8 AAC 13-8 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION Figure 6. Vertical profiles of vortex-averaged denitrification in the (a) nonequilibrium and (b) equilibrium simulations as a function of the rotation of the temperature field. The vortex edge is defined as MPV = 40 PV units. contrast, in the equilibrium model, the denitrification profile is only marginally reduced for an increased baroclinicity Factors Controlling Denitrification [32] We now describe the way in which the meteorology of the Arctic vortex controls the evolution of the particles and the impact that this has on the calculated denitrification shown above. [33] The Lagrangian nature of the nonequilibrium model enables the behavior of individual particles to be tracked over a complete trajectory from nucleation to complete evaporation. Diagnostics such as the particle lifetime and change in particle altitude with time can be used to understand better the link between meteorology, particle size, and resulting denitrification Particle Lifetimes [34] Figure 7 shows the time between particle nucleation and complete evaporation and how this time varies as the vortex concentricity is changed. The particle lifetimes refer to those particles nucleated during the first day of the simulation and are presented as averages over all such model particles. The particle lifetime is a measure of how well the vortex wind field can keep particles within the cold pool. For the near-concentric 0 case, a bimodal distribution exists with one peak at 6 days and the overall maximum being in the smallest lifetime bin. For this case a significant proportion of particles grow for continuous periods of 8, 9, and even 10 days (these particles still have not completely sublimated by the end of the 10-day simulation). [35] For the model run with a 5 rotated temperature field, the second peak in the bimodal distribution has shifted to around 4.5 days, and the proportion of particles which last longer than 5 days is reduced significantly. For the 10 run the second peak has effectively vanished with the average particle lifetime reduced significantly. For the 20 run, around 99% of the particles have a lifetime of under 3 days, illustrating the controlling influence it has on the denitrification. [36] Note that the particle lifetimes in Figure 7 are shorter than the typical time spent by isentropic air parcels below T NAT [Tabazadeh et al., 2001]. This difference exists because particle growth is often terminated by falling into warm air, thus highlighting the importance of examining particle trajectories rather than air parcel trajectories as a measure of potential denitrification Particle Sizes [37] Related to the particle lifetime is the particle size, which is shown in Figure 8. The size distributions are presented as an average over all model particles existing at the end of day 10. For the 0 and 5 cases, which are both nearconcentric and have a large proportion of particles which exist for a long time, 50% of particles have a radius greater than 4 mm, whereas for the 20 case only 2.5% are in this large size range. In the 20 case, the vast majority of particles are between 0 and 2 mm, which is in accord with their short lifetimes. Figure 7. Distribution of particle lifetimes in the model for different rotations of the temperature field. Lifetimes indicate the time between particle formation and complete evaporation and refer to particles nucleated on the first day of the simulation.

9 MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION AAC 13-9 Figure 8. Distribution of particle sizes in the model for different rotations of the temperature field. The average was calculated over all particles in the model at the end of day 10; so it includes all q levels Particle Sedimentation Distances [38] The lifetime of an individual particle determines the size to which it can grow and hence the vertical distance over which it can sediment before it evaporates. Figure 9 shows the vertical distribution of all particles that were nucleated during the first day of the simulation and the height they reach when they eventually evaporate completely. For the near-concentric 0 run, the majority of particles sediment to the bottom of the vortex between 380 and 450 K, leading to denitrification at upper levels and renitrification below. In contrast, for the strongly baroclinic 20 rotated run, the altitude distribution at complete evaporation has only changed a little from that at nucleation, resulting in a much reduced downward transport of HNO 3. Also note the increasing loss rate of particles with decreasing altitude below about 500 K in the concentric case, which is important in determining the profile of denitrification (see section 4.1) The Importance of Vortex Concentricity [39] Together, Figures 7, 8, and 9 show that a shift toward a less concentric vortex state reduces the lifetime and hence size and net downward transport of NAT particles. This, in turn, reduces significantly the amount of denitrification. Thus the relative position of the cold pool and vortex is a key parameter in determining the extent of denitrification. Denitrification by large NAT particles in these simulations is essentially switched off once approximately half of the NAT region remains within the vortex. It is the slow growth of the NAT particles that is responsible for this sensitivity. Allowing the particles to instantaneously reach equilibrium sizes removes this sensitivity entirely. In these simulations, because the cold pool size was kept constant, the equilibrium Figure 9. Vertical distributions of the model particles for different rotations of the temperature field. Dashed line, distribution of initial particles during the first day of the simulation; solid line, distribution of the same particle altitudes at complete evaporation. Insets show the relative positions of the vortex edge and NAT region.

10 AAC MANN ET AL.: VORTEX CONCENTRICITY AND ARCTIC DENITRIFICATION Figure 10. Vortex-averaged denitrification as a function of the relative location of the cold pool and vortex centers. The relative positions are expressed in terms of the normalized separation of the NAT region and vortex centroids. A normalized separation of 0 indicates that the cold pool and vortex centers are colocated, while a value of 1 indicates that the centroid of the NAT region sits on the vortex edge. The vortex edge is defined as MPV = 40 PV units. model runs produced almost identical total denitrification, regardless of the relative position of the vortex and cold pool. [40] To summarize the sensitivity of denitrification to the relative locations of the vortex and cold pool, Figure 10 shows the vortex-averaged denitrification as a function of the separation of the cold pool and vortex centroids. The vortex-averaged denitrification was calculated at each level for nonequilibrium model runs assuming the vortex edge is given by MPV = 40 PV units. The near-concentric 0 run has the shortest distance between the two centroids and has the maximum denitrification. As the vortex cold pool separation increases, the denitrification decreases dramatically as seen in section 4.1, so that when the vortex cold pool separation is around 1 vortex effective radius for the 20 run, the denitrification is negligible. In contrast, the equilibrium model run shows almost no change in denitrification as the cold pool is shifted. 5. Summary [41] We have presented the first 3-D nonequilibrium model simulations of Arctic denitrification by low number densities of large NAT particles. The model captures the growth and sedimentation of particles which occurs when such particles are not in equilibrium with gas phase HNO 3. Model NAT particle number densities are in the range 10 5 to 10 3 in the lower stratosphere, similar to those observed during the winter 1999/2000 [Fahey et al., 2001] (for direct comparisons, see Carslaw et al. [2002]). [42] Denitrification by low number densities of NAT particles presents a particular modeling challenge because of the long growth times of the particles [Fahey et al., 2001; Carslaw et al., 2002] and, hence, the highly nonlinear dependence of denitrification on the time that individual particles have available for growth [Jensen et al., 2002]. [43] The idealized model simulations shown here used fixed forcing fields over 10 days where the temperature field had been rotated with respect to the vortex to simulate changes in vortex concentricity. These simulations were designed to explore how the meteorology of the Arctic stratosphere controls the growth period of individual particles and how this in turn affects denitrification. We have shown that the growth period of NAT particles is controlled in the Arctic stratosphere by the relative location of the vortex and cold pool. When the cold pool and vortex are colocated, NAT particles can spend long continuous periods below the NAT temperature and therefore reach large sizes. Such concentric, or Antarctic-like, conditions were shown to be optimum for denitrification, since particles are able to grow for continuous periods of 8 days or more. When the cold pool and vortex positions are offset, the period spent below the NAT temperature is shortened. Such nonconcentric or strongly baroclinic states of the Arctic vortex result in a considerable decrease in the amount of denitrification compared with the near-concentric situation. We showed that denitrification could be switched off entirely even with half of the NAT region remaining within the vortex. Under these conditions, NAT particles were able to grow for continuous periods of only 2 days or less and so reached much smaller sizes. [44] The relative location of the cold pool and vortex is a feature of the Arctic stratosphere that displays great variability, both within one winter and interanually (unlike in the Antarctic) [Pawson et al., 1995]. Because of the strong control exerted by the relative location of the cold pool and vortex, it is unlikely that denitrification in different winters will correlate well with the minimum temperature, although the minimum temperature will also be an important parameter, particularly if the frost point is reached. Although a highly concentric vortex state is optimum for denitrification, a less concentric state is optimum for chemical ozone loss [O Neill et al., 1994; Manney et al., 1996]. Further studies are needed to understand the coupling between vortex concentricity, denitrification, and ozone loss during an entire winter/spring season. [45] Ideally, the trajectories of particles need to be considered by using a nonequilibrium growth and sedimentation model in order to predict denitrification. An equilibrium model cannot take account of the varying times available for growth which result from different arrangements of the cold pool in relation to the vortex. An approximate indicator of the amount of denitrification could be provided by the separation of the cold pool and vortex centroids, as shown here. For the case that we considered,

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