Chlorine activation on stratospheric aerosols

Introduction Conclusions References

rates doubling for each Kelvin increase in temperature. However, differences between the parameterizations are negligible. For Nitric Acid Trihydrate particles (NAT) the major factors of uncertainty are the number density of nucleated particles and different parameterization choices. These two factors induce an uncertainty that covers several orders of magnitude on the reaction rate. But as predicted reaction rates on liquid 10 aerosols always exceed those on NAT the overall uncertainty is small. In-situ observations of ClO x from Arctic winters in 2005 and 2010 are used to validate the heterogeneous chemistry parameterizations. The ambient conditions for these measurements proved to be very different between those two winters with HCl being the limiting reacting partner for the 2005 measurements and ClONO 2 for the 2010 measurements. 15 Modeled levels of chlorine activation are in very good agreement with the in-situ observations and the surface area provided by Polar Stratospheric Clouds (PSCs) has only a limited impact on modeled chlorine activation. This indicates that the parameterizations give a good representation of the processes in the atmosphere. Backtrajectories started on the location of the observations in 2005 indicate temperatures 20 on the threshold for PSC formation, hence the surface area is mainly provided by the background aerosol. Still, the model shows additional chlorine activation during this time-frame, providing cautionary evidence for chlorine activation even in the absence of PSCs. Vortex-averaged satellite observations also show no definite connection between chlorine activation and PSC formation. The inter-and intra-annual variability of

Introduction
During polar night in the stratosphere the main chlorine reservoir species HCl and ClONO 2 are converted to photo-labile species through heterogeneous reactions on liquid and solid particles (Solomon et al., 1986), a process known as chlorine activation. Under the influence of sunlight, these photo-labile species are converted to ClO x 10 (ClO + 2 · Cl 2 O 2 ), which is referred to as "active chlorine", as ClO x drives the catalytic ozone loss cycles which lead to severe depletion of ozone in polar spring (Molina and Molina, 1987;Solomon, 1999). The liquid and solid particles acting as reactions sites for heterogeneous chemistry are provided by Polar Stratospheric Clouds (PSCs) and the ubiquitous background sulfate aerosol layer. PSCs are composed of mixtures of Su- 15 percooled Ternary Solution (STS), Nitric Acid Trihydrate (NAT) and ice (e.g. Peter and Grooß, 2012). Clouds composed of STS and low number density NAT (3 × 10 −4 cm −3 -10 −3 cm −3 ) are the most common type in cold Arctic winters and STS with high number density NAT (> 10 −3 cm −3 ) are more common for the Antarctic . Ice formation is usually only observed over the Antarctic as the polar vortex over the Arctic Introduction number densities can be constrained to values between 10 −1 and 10 −4 cm −3 . Background aerosol surface area density (SAD) depends on the COS and SO 2 fluxes into the stratosphere and is significantly enhanced only after large volcanic eruptions (Robock, 2000). The influence of the sulfate aerosol on stratospheric ozone was particularly observable after the eruptions of El Chichón (Hofmann and Solomon, 1989) and 5 Mt. Pinatubo (Portmann et al., 1996;Tilmes et al., 2008b). The importance of PSCs and the sulfate aerosol layer on heterogeneous chemistry is well established (e.g. Solomon, 1999), with the impact of an unperturbed aerosol layer on heterogeneous processing first discussed by Rodriguez et al. (1988). But recently Drdla and Müller (2012) suggested that even during volcanic quiescent times chlorine activation in polar 10 night is dominated by reactions on cold binary sulfate aerosol. However, elevated levels of active chlorine usually coincide with the presence of PSCs which makes it difficult to attribute chlorine activation to heterogeneous processing on a particular aerosol or cloud type. Kawa et al. (1997) reported in-situ measurements of active chlorine from the Antarctic during the ASHOE/MAESA campaign in 1994 where backtrajectories in- 15 dicated that during the ten days prior to the measurements, temperatures had been too high for PSC formation and therefore concluded that the observed chlorine activation must have occurred on the background aerosol. With the current high chlorine loading in the stratosphere and a deliberate enhancement of the stratospheric aerosol layer being discussed to counter a temperature increase at the surface (e.g. Crutzen, 2006; 20 Rasch et al., 2008;Tilmes et al., 2008a), understanding the role of the background aerosol for chlorine activation is essential.
The following sections will discuss the existing parameterizations for heterogeneous chemistry on the various aerosol types and their uncertainty. These parameterizations are validated with in-situ measurements and the role of the background aerosol is 25 constrained with model simulations and satellite observations.

Chlorine activation
Following Portmann et al. (1996) the evolution of chlorine species in the polar regions can be divided into four phases. The "setup phase" preceding the polar winter when Cl y is partitioned between the reservoir species (HCl and ClONO 2 ), the "activation phase" during polar night when the reservoir species are partly converted into ClO x , the "main-5 tenance/further activation phase" when activation and deactivation are in competition and further activation occurs if temperatures are sufficiently low and finally the "termination phase" when active chlorine is converted back into the reservoir species. The three heterogeneous reactions mainly responsible for chlorine activation are (Solomon, 1999;Peter and Grooß, 2012): 10 ClONO 2 + HCl → HNO 3 + Cl 2 (R1) In the Arctic active chlorine is principally deactivated into ClONO 2 , the speed of deactivation is thus limited by the availability of NO x (NO + NO 2 + NO 3 ) (Müller et al., 1994;15 Douglass et al., 1995) ClO + NO 2 → ClONO 2 (R4) The formation of NO x is controlled by solar radiation intensity and thus, it is a function of the solar zenith angle and the availability of gas-phase HNO 3 : Large NAT particles can effectively remove HNO 3 from the lower stratosphere (Fahey et al., 2001) which results in suppressed deactivation and prolongs the availability of ClO x into late winter and spring (Harris et al., 2010). In the illuminated polar vortex in spring, ClO x rapidly depletes ozone, causing the creation of the ozone hole over the Introduction Antarctic and severe depletion of ozone over the Arctic for very cold winters. Extensive denitrification of the lower polar stratosphere through sedimentation of NAT particles is more common in the Antarctic than in the Arctic. Temperatures in the Antarctic are lower and remain below the NAT equilibrium temperature (T NAT , Hanson and Mauersberger, 1988) for longer periods than in the Arctic and thus, allow NAT particles to 5 grow and sediment. In the Arctic temperatures are higher and more variable than in the Antarctic which results in less pronounced denitrification and faster deactivation of chlorine through Reaction (R4). In the Antarctic, the almost complete destruction of ozone in polar spring leads to an increase in Cl concentrations, which allows the deactivation reaction 10 Cl + CH 4 → HCl + CH 3 (R7) to occur (Douglass et al., 1995). Recently, Grooß et al. (2011) reported that in the Antarctic, for very low ozone values (< 0.5 ppmv) a balance is maintained for a certain period between rapid gas-phase production of HCl and HOCl and rapid heterogeneous reaction between these two compounds. This period ends by very rapid (on the order 15 of one day) irreversible Cl deactivation into HCl and almost complete destruction of ozone. The speed of the heterogeneous Reactions (R1) to (R3) is described by the rate constant k which depends on the uptake coefficient γ, aerosol surface area density SAD and mean gas velocity c gas .
The uptake coefficient γ, which describes the fraction of collisions of gas molecules with the particle surface which lead to a reaction, and the surface area density are the main factors controlling the heterogeneous reaction rate and both strongly depend on temperature.

25
In this work we use parameterizations of the uptake coefficient to model heterogeneous chemistry on liquid aerosols derived by Shi et al. (2001) and Hanson (1998), and for NAT particles the work of Carslaw and Peter (1997) Hanson and Ravishankara (1993) and Abbatt and Molina (1992). NAT reaction probabilities based on the scheme by Hanson and Ravishankara (1993) represent an upper limit while the scheme by Abbatt and Molina (1992) represents the lower limit. For liquid aerosols the parameterizations yield very similar reaction rates, with Shi et al. (2001) reporting an uncertainty of 40 % for Reaction (R1) and 32 % for 5 Reaction (R2). As the parameterization by Shi et al. (2001) for liquid aerosols has only been derived for binary aerosols it has been extended to STS particles by assuming that there is no difference in the uptake coefficient for STS particles and binary aerosol particles that would exist in the absence of HNO 3 , as shown by Elrod et al. (1995). Liquid aerosol surface area depends on the stratospheric H 2 SO 4 content and increases 10 significantly only for temperatures below about 192 K with the uptake of HNO 3 and the formation of STS (Carslaw et al., 1994). The surface area density for NAT particles depends primarily on the assumed particle density. The main reaction channel for chlorine activation is Reaction (R1) (Fig. 1a). Other reactions are less important for the initial chlorine activation phase as ClONO 2 constitutes the second largest reservoir and 15 is quantitatively removed (Müller et al., 1994;Douglass et al., 1995;Portmann et al., 1996). When ClONO 2 starts to regenerate and HCl falls below ClONO 2 mixing ratios, Reaction (R2) gains importance if temperatures are low enough. Figure 1 shows the first order loss rates in these two species for typical stratospheric conditions for Reactions (R1) and (R2) on liquid aerosols and NAT particles. NAT sur-20 face area density is calculated assuming a uniform distribution of spherical particles and the liquid aerosol surface area assuming a log-normal distribution. The various heterogeneous chemistry parameterizations and possible number densities for NAT particles cover several orders of magnitude in first order loss rates, indicating a large uncertainty concerning their ability to act as reaction sites for chlorine activation. How-25 ever, over most of the temperature range heterogeneous reaction rates on NAT are slower than on the background aerosol, even when a high NAT number density of 10 −1 cm −3 (Fig. 1)  aerosol. With the formation of STS or a NAT number density less than 10 −1 cm −3 , reaction rates on the liquid aerosol always exceed those on NAT. While chlorine activation on liquid aerosols is not sensitive to the parameterizations, it is very sensitive to temperature. The reaction rate doubles for every Kelvin cooling and increases tenfold over a 2 K temperature range around 192 K with the uptake of HNO 3 on the background 5 aerosol (Carslaw et al., 1994;Peter, 1997). This causes high sensitivity of simulated chlorine activation to small variabilities in the temperature field. Therefore, for heterogeneous reactivity on liquid aerosols even a small bias in the temperature field has larger effects than the uncertainty of the uptake coefficients itself. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | a homogeneous airmass was sampled although the flight covered 65 • -85 • equivalent latitude. From these measurements we derive the total inorganic chlorine loading Cl y using the CH 4 -Cl y tracer correlation reported in Grooß et al. (2002) as a function of potential temperature (Θ) and equivalent latitude (Φ). ClONO 2 , HCl, and HNO 3 mixing ratios are taken from ACE-FTS v2.2update (Bernath et al., 2005)  of HNO 3 and the chlorine species is estimated by interpolating the satellite measurements on the potential temperature of the Geophysica observations and subsequently calculating the range of observations of the three nearest neighbors in the equivalent latitude space. Typically, this results in an error of less than 0.2 ppbv. As heterogeneous chemistry is highly sensitive to temperature, accurate knowledge 20 of it is necessary to model the activation of chlorine on stratospheric aerosols. For example, Brakebusch et al. (2012) have shown that temperatures in the specified dynamics version of the Whole Atmosphere Community Climate Model tend to be warm biased compared to MLS observations and that the agreement between model and satellite observations improves when a −1.5 K bias is applied to the heterogeneous 25 chemistry calculations. Ambient temperature on board Geophysica is measured by the Thermodynamic Complex (TDC, Shur et al., 2006). These measurements similarly, show a ∼ 1.5 K bias when compared to ERA-INTERIM temperatures (Fig. 2). Between 455-470 K ERA-INTERIM temperatures are consistently higher than the observations. ACPD 12,2012 Chlorine activation on stratospheric aerosols ERA-INTERIM also shows better agreement at higher temperatures and the warm bias appears to be limited to temperatures below 205 K. In Fig. 2b the vertical profile for the temperature bias is shown. The greatest warm bias is located at 460 K, on flight altitude, with 1.5 K. No in-situ temperature measurements exist over the course of the trajectories so we assume the warm bias exists for the whole trajectory length and 5 adjust the temperatures below 205 K according to the kernel shown in Fig. 2. Modeling PSCs along the trajectories indicates ( Fig. 3) the presence of NAT and STS 24 h prior to the measurements. On the trajectories ending between 08:45 and 09:30 UTC typically less than 0.1 ppbv HNO 3 condenses on STS. This enhances the surface area density by about a factor of 1.5 compared to the background aerosol. 10 The maximum calculated enhancement is about a factor of 2. For trajectories ending before 08:45 and after 09:30 all available HNO 3 is predicted to condense to STS, with a maximum enhancement of the surface area density by a factor of 5. However, these maximum enhancements only occur for a few hours along the trajectories. For most of the time when the trajectories experience temperatures below T NAT , PSCs enhance 15 the surface area by less than 10 % over the background aerosol. As temperatures are below T NAT for only about 20 h modeled NAT particles cannot reach thermodynamic equilibrium. With temperatures below T NAT for such a short time, NAT number density is on the order of 10 −3 cm −3 , which leads to a negligible increase in surface area density by NAT. Therefore, any modeled increase in surface area density is caused by STS and 20 not NAT particles.

In situ observations
To assess the ability of the model to reproduce the measured extent of chlorine activation two simulations are performed. The first calculates the heterogeneous reaction rates with a surface area density that includes all PSCs (Full PSC) and the second uses only the surface area density of the background aerosol without any enhance-25 ment due to STS or NAT (Binary only). Temperatures are well above the frost point so ice PSCs do not form in either of these simulations. Figure 4 shows that the difference between initialized and measured ClO x on the flight path is on average about 1 ppbv. Maximum activation is simulated for trajectories ending before 08:45 and after 20570 ACPD 12,2012 Chlorine activation on stratospheric aerosols teorological fields. However, overall both model simulations show good agreement with measured ClO x , within the uncertainty of initialization and measurements. For trajectories ending before 08:45 modeled chlorine activation tends to be at the upper limit of uncertainty and afterwards at the lower limit. The difference between both simulations is minimal with the "Full PSC" simulation activating slightly more chlorine than 10 the "Binary only" simulation. However, this difference is on the order of 10 % indicating that the heterogeneous reactivity provided by the binary aerosol in the model is sufficient to produce an activation that corresponds to most of the observed activation of chlorine within the considered time-frame. Even though modeled PSCs increase the surface area density by up to 500 % the effect on chlorine activation is limited, as this 15 maximum increase only lasted for a short time-frame. In the model, neither HCl nor ClONO 2 are completely depleted thus, chlorine activation along the trajectories is limited by the heterogeneous reaction rates and not by the availability of either reservoir species. As the model results overlap with the observations, within the uncertainties of this simulation and measurements, the temperature dependence of the current param-20 eterizations for heterogeneous chlorine activation is in agreement with the processes in the real atmosphere. In addition to the Geophysica flight from 7 March 2005 we examined three Geophysica flights in January 2010 from the RECONCILE campaign in the Arctic (Fig. 5). The temperature histories along 7 day back-trajectories ending on the flightpath of these 25 flights indicate temperatures low enough for efficient heterogeneous chemistry. Temperatures are below T NAT at the beginning of the trajectories and during the flights. In between temperatures exceed 200 K so all PSCs that formed initially very likely evaporated. Prior to the flights temperatures for most trajectories are below T NAT for about ACPD 12,2012 Chlorine activation on stratospheric aerosols take from the gas-phase. Measurements during the three considered flights in January 2010 from the up-and downward facing LIDAR MAL (Matthey et al., 2003) on-board Geophysica show that backscatter ratios are elevated by a factor of 3-4 and a depolarization of less than 4 %. The high backscatter combined with the low depolarization is an indicator that PSCs mostly consisted of STS droplets and that no or only very few 10 NAT particles were present. Chlorine activation is simulated by running box-model calculations along these 7 day trajectories. The initialization is taken directly from a hemispheric 3-D-CLaMS simulation. Total Cl y is estimated from measured CH 4 by HAGAR via the CH 4 -Cl y correlation (Grooß et al., 2002). Model HCl and ClONO 2 are then scaled accordingly so that model Cl y agrees with observations. Generally, model Cl y   Pitts et al. (2009Pitts et al. ( , 2011 have shown that the first PSCs forming in Arctic winter are mainly composed of STS and low number density NAT (< 10 −3 cm −3 ). Therefore, the observed 10 decrease in gas-phase HNO 3 in December can mostly be attributed to condensation of HNO 3 on STS and a lesser extent to NAT and, possibly irreversible denitrification. In early winter, HNO 3 shows, in contrast to HCl, high inter-annual variability indicating that the additional surface area provided by PSCs has no detectable direct influence on the rate of chlorine activation on a vortex wide scale. lent latitude, assuming that no irreversible denitrification or dehydration has occurred yet. We show the fraction of trajectories that fall into 6 different bins. The first bin contains the fraction of trajectories with a maximum enhancement of the surface area 5 density of less than 1.1, i.e. showing no significant enhancement over background aerosol levels. For the next two bins (maximum enhancement factor 1.1-1.5 and 1.5-2) only a small fraction of gas-phase HNO 3 has condensed on STS but has already significantly enhanced the surface area density. In the bins with an enhancement factor of 2-5 and 5-10 most available gas-phase HNO 3 is condensed on STS and in the last At 500 K more than 50 % of the trajectories already show an enhanced surface area density by more than a factor of 2. The difference between these three winters becomes most obvious for the trajectories started on 29 December. In 2004 the distribution has 25 completely shifted from trajectories only showing minor enhancement of surface area density on 15 December to the majority showing an enhancement of a factor greater than 10. On the 450 and 475 K isentropes more than 80 % of the backward trajectories exhibit temperatures which lead to a more than tenfold increase in surface area density. 12,2012 Chlorine activation on stratospheric aerosols This is also visible in the sharp drop in MLS gas-phase HNO 3 at the end of December 2004 (Fig. 6). Nonetheless, MLS HCl measurements show no drastic change in the loss rate suggesting that the heterogeneous loss of HCl is not connected to the available surface area density. The change in probability density for 2008 and 2009 towards the end of December is less pronounced. Most trajectories show only a minor increase 5 in surface area density but still their distribution is shifted towards higher surface area densities. This is more apparent in 2008 than in 2009, on 29 December 2009 the maximum increase in surface area density for almost all trajectories was less than a factor 1.5. These three winters show the huge variability in PSC occurrence and HNO 3 uptake 10 for December in the Arctic. Nevertheless, HCl loss rates observed by MLS are fairly constant throughout December with little inter-and intra-annual variability. Only the vertical extent of HCl loss shows inter-annual variability which is correlated with the vertical extent of the low temperatures necessary for efficient heterogeneous chemistry.

15
We have examined the importance of the stratospheric background aerosol for chlorine activation. The evaluation of heterogeneous chemistry parameterizations has shown that uncertainties in NAT microphysics contribute most to the overall uncertainty in modeling heterogeneous chemistry. The uncertainties for NAT reactivity cover several orders of magnitude. However, using the most commonly observed NAT number den-20 sities, heterogeneous processing on these particles is significantly slower than on the background aerosol. For modeling chemistry on the binary background aerosol the greatest uncertainty results from temperature uncertainties as the reaction rate doubles for every Kelvin and increases tenfold with the formation of STS over a 2 K temperature range. To study heterogeneous chemistry on a synoptic timescale we analyzed derived from satellite data, simulating chlorine activation along trajectories ending on the flightpath allows the influence of the various aerosols on modeled chlorine activation to be assessed. Even though NAT and STS could form under the prevailing conditions the additional surface area provided by PSCs does not significantly enhance chlorine activation. 90 % of additionally activated chlorine during this time-frame originate 5 from heterogeneous chemistry on a surface area provided by the background aerosol. This shows that for modeling heterogeneous chemistry the increase in reaction rate with decreasing temperature is more important than the increase of surface area density and that to correctly model heterogeneous chemistry on synoptic timescales accurate knowledge about the prevailing temperatures is essential. This flight also showed 10 that a considerable amount of chlorine can be activated on a timescale of hours when both HCl and ClONO 2 are available. The three flights from January 2010 corroborate these results albeit under very different ambient conditions. For the 2005 flight neither HCl nor ClONO 2 are completely depleted in the model and therefore, the temperature is the decisive factor determining the level of chlorine activation. In 2010, however, 15 ClONO 2 is the limiting factor as it is completely depleted while temperatures remain low enough for efficient heterogeneous chemistry. Thus, heterogeneous chemistry on the background aerosol surface area yields identical results as calculations with full PSC surface area and both simulations show excellent agreement with observations. Not only can heterogeneous processing on the background aerosol surface area 20 explain the bulk of chlorine activation on synoptic timescales along individual trajectories, but based on satellite observations we could demonstrate that the vortex-average chlorine activation rate for the last seven Arctic winters is not correlated with the occurrence of PSCs and the associated uptake of HNO 3 from the gas-phase. The observed HCl loss rate in December is similar for all considered Arctic winters despite MDB for their support of the Geophysica flights and supply of avionic data, Ralf Weigel for COPAS, Nicole Spelten and Cornelius Schiller for FISH data, to ECMWF for meteorological analyses and to the ACE-FTS and EOS MLS teams for their high-quality data products. The Atmospheric Chemistry Experiment (ACE), also known as SCISAT, is a Canadian-led mission mainly supported by the Canadian Space Agency. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Carslaw, K. S., Luo, B. P., Clegg, S. L., Peter, T., Brimblecombe, P., and Crutzen, P. J.: Stratospheric aerosol growth and HNO 3 gas phase depletion from coupled HNO 3 and water uptake by liquid particles, Geophys. Res. Lett., 21, 2479-2482, 1994: An analytic expression for the composition of aqueuos HNO 3 -H 2 SO 4 stratospheric aerosols including gas phase removal of HNO 3 , Geophys. Res.  Meteor. Soc., 137, 553-597, doi:10.1002/qj.828, 2011 and Massie, S. T.: Interhemispheric differences in springtime production of HCl and ClONO 2   25 in the polar vortices, J. Geophys. Res., 100, 13967-13978, doi:10.1029/95JD00698, 1995. 20565, 20566, 20567 Drdla, K. and: Temperature thresholds for chlorine activation and ozone loss in the polar stratosphere, Ann. Geophys., 30, 1055-1073, doi:10.5194/angeo-30-1055-2012, 2012.  , 100, 269-278, 1995. 20567 20579 ACPD 12,2012 Chlorine activation on stratospheric aerosols  von Hobe, M., Salawitch, R. J., Canty, T., Keller-Rudek, H., Moortgat, G. K., Grooß, J.-U., Müller, R., and Stroh, F.: Understanding the kinetics of the ClO dimer cycle, Atmos. Chem. Phys., 7, 3055-3069, doi:10.5194/acp-7-3055-2007, 2007. 20573 von Hobe, M., Stroh, F., Beckers, H., Benter, T., andWillner, H.: The UV/Vis absorption spectrum of matrix-isolated dichlorine peroxide, ClOOCl, Phys. Chem. Chem. Phys., 11, 1571-5 1580 and Borrmann, S.: Experimental characterization of the COndensation PArticle counting System for high altitude aircraft-borne application, Atmos. Meas. Tech., 2, 243-258, doi:10.5194/amt-2-243-2009 Werner, A., Volk, C. M., Ivanova, E. V., Wetter, T., Schiller, C., Schlager, H., and Konopka, P.: Quantifying transport into the Arctic lowermost stratosphere, Atmos. Chem. Phys., 10, 11623-11639, doi:10.5194/acp-10-11623-2010 Schmidt, U., Tan, V., Tuitjer, F., Woyke, T., and Schiller, C.: Fast in situ stratospheric hydrom-Introduction  (Figure 1, panel a) portant for the initial chlorine constitutes the second largest removed (Müller et al., 1994;mann et al., 1996). When ClO HCl falls below ClONO 2 mix importance if temperatures are Figure 1 shows the first o species for typical stratospheri and R2 on liquid aerosols an       12,2012 Chlorine activation on stratospheric aerosols

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ACPD 12,2012 Chlorine activation on stratospheric aerosols  ACPD 12,2012 Chlorine activation on stratospheric aerosols    annual variability which is co of the low temperatures neces chemistry.

Conclusions
We have examined the impor ground aerosol for chlorine a erogeneous chemistry param certainties in NAT microphy all uncertainty in modeling uncertainties for NAT reactiv nitude. However, using the number densities, heterogen cles is significantly slower th For modeling chemistry on the greatest uncertainty resu ties as the reaction rate dou creases tenfold with the form perature range. To study het