Nitric Acid Trihydrate Research Articles

Denitrification inside the stratospheric vortex in the winter of 1999–2000 by sedimentation of large nitric acid trihydrate particles

Synoptic‐scale polar stratospheric clouds (PSCs) have been measured with an aircraft‐borne lidar south of Spitsbergen in early February 2000. The PSCs of moderately depolarizing stratified layers extended from near the tropopause (10–11 km) up to 20 km altitude. Their internal structure in backscatter ratio reflects the variability of the particles' concentration rather than manifold sizes and phases of different optical efficiency. Actually, their size distribution was nearly monodisperse. According to T‐matrix calculations, low backscatter ratios (γ < 3), color ratios of 1–1.5, and moderate depolarization ratios (δa ≈ 0.15–0.2) indicate that the observed layers were composed of aspherical nitric acid trihydrate (NAT) particles larger than 2 μm. During three turning point missions into the cold vortex core a significant settling of the particle layers was observed between aligned outgoing and incoming legs. Low stratospheric winds and quasi‐Lagrangian flight heading minimized horizontal drifts between the probed volumes. A NAT particle sedimentation velocity of 69 ± 14 m/h is derived by cross‐covariance analysis of corresponding pairs of two‐dimensional backscatter sections of the layers. This implies a particle size of rp ≈ 7–8 μm and a mixing ratio of condensed HNO3 near 2 ppbv. The downward NOy flux below 19 km altitude amounts to j ≈ 3 ppbv km/d. Based on particle back trajectory analysis, this flux, derived from the observations in different regions of the Northern European Ice Sea, has a significant denitrification potential.

Relating inferred HNO3 flux values to the denitrification of the 1999–2000 Arctic vortex

Extensive populations of large HNO3‐containing particles were observed in situ in the 1999–2000 Arctic winter vortex. Assuming nitric acid trihydrate (NAT) composition, the associated values of instantaneous gravitational flux of HNO3 are between 1·108 and 5·1010 molec cm−3 km day−1 and are the largest ever derived from in situ measurements. The relationship between these observed populations and observed denitrification in the 1999–2000 Arctic vortex is developed using constraints of NAT formation and sedimentation. This analysis of the 1999–2000 winter provides a useful guide to interpreting future observations of polar stratospheric clouds and denitrification in the polar regions.

A Theoretical Study of the Interaction of HCl with Crystalline NAT

Using density-functional-plane-wave-based and localized-orbital computational methods, we systematically examine the binding of molecular HCl at a variety of surface sites on crystalline nitric acid trihydrate (NAT), a step preceding the chlorine activation reactions that contribute to the depletion of stratospheric ozone at high latitudes. We pay particular attention to the role played by surface dangling (non-hydrogen-bonding) OH groups. After optimizing six low index faces, we find that NAT(001) and (001) faces are thermodynamically the most stable. Only one surface site on the (001) face, with one nearby dangling OH group, exhibits a high affinity for HCl. At this binding site, adsorbed HCl forms a strong H...O hydrogen bond with an NO 3 - ion and a weaker Cl...H hydrogen bond with a nearby H 2 O molecule. The interaction energy and enthalpy at 190 K corrected for zero-point energies are 23 and 25 kJ/mol, respectively. The presence of one strong binding site per simulation cell, versus at least three previously reported on the (0001) face of ice Ih (Mantz, Y. A.; Geiger, F. M.; Molina, L. T.; Molina, M. J.; Trout, B. L. J. Phys. Chem. A 2000, 105, 7037), leads to a prediction of a lower HCl surface coverage on NAT than on ice, qualitatively consistent with experiments conducted on these surfaces. Additionally, we present kinetic and thermodynamic evidence that molecular HCl, adsorbed near one or two dangling OH groups, does not dissociate on NAT. By contrast, molecularly adsorbed HCI likely dissociates when interacting strongly with two dangling OH group on the ice Ih (0001) face as reported in previously published theoretical studies (Svanberg, M.; Pettersson, J. B. C.; Bolton, K. J. Phys. Chem. A 2000, 104, 5787; Mantz, Y. A.; Geiger, F. M.; Molina, L. T.; Molina, M. J.; Trout, B. L. Chem. Phys. Lett. 2001, 348, 285).

Large NAT particle formation by mother clouds: Analysis of SOLVE/THESEO‐2000 observations

During the SOLVE/THESEO‐2000 Arctic stratospheric campaign in the winter 1999/2000 widespread occurrences of very large HNO3‐containing particles, probably composed of nitric acid trihydrate (NAT), were observed in situ by instruments on board the ER‐2 stratospheric research aircraft. These large NAT particles were found with low number densities (n ≈ 10−4 cm−3) in vast regions, in air generally supersaturated with respect to NAT. Within the same campaign other instruments have performed airborne and ground‐based measurements of polar stratospheric clouds (PSCs), often showing the existence of type 1a and type 1a‐enh clouds. Such PSCs often occur on the mesoscale with particle number densities n ≳ 10−2cm−3 and are also most likely composed of NAT. We use forward trajectories for the path of NAT particles, which are advected by winds based on ECMWF analyses and sediment due to gravity, to show that high number density NAT PSCs (mother clouds) could give rise to low number density NAT particle populations several days downstream.

A Lagrangian approach to studying Arctic polar stratospheric clouds using UARS MLS HNO3 and POAM II aerosol extinction measurements

We assess the viability of diagnosing polar stratospheric cloud (PSC) composition and denitrification using existing satellite measurements. A Lagrangian approach is used to track PSC evolution from formation through dissipation. Upper Atmosphere Research Satellite Microwave Limb Sounder observations of gas‐phase HNO3 and Polar Ozone and Aerosol Measurement II observations of aerosol extinction from the Arctic late winter of 1995/1996 are correlated and compared to results from PSC composition models along air parcel trajectories. This approach is successful in capturing the broad patterns of PSC development. That is, a strong correlation is found between low‐temperature, low‐HNO3, and high‐extinction points. In most cases the observed behavior along the Lagrangian paths falls within the range predicted by equilibrium liquid ternary aerosol and nitric acid trihydrate composition models. At no time along the trajectories (when there is data coverage) do the models display large changes in either HNO3 or aerosol extinction that are not reflected in the data. In general, however, there is a large degree of overlap in the comparisons between the models and the data. Unmeasured quantities and large uncertainties in both measurements and model calculations preclude conclusive determination of PSC composition or phase in most cases. In the majority of PSC events studied, gas‐phase HNO3 fully recovers to pre‐PSC abundances following cloud evaporation. We conclude that while severe denitrification (50% or greater) may have occurred in highly localized regions in the Arctic in 1996, it did not occur over spatial scales comparable to or larger than the MLS field of view (∼400 km × 200 km × 6 km), even though it was a relatively cold winter. Improved measurements from upcoming satellite missions, such as Earth Observing System Aura, will ameliorate many of the difficulties in diagnosing PSC composition and denitrification encountered in this study.

NAT-rock formation by mother clouds: a microphysical model study

Abstract. Polar stratospheric clouds (PSCs) of type 1a or 1a-enh containing high number densities of nitric acid trihydrate (NAT) particles, can act as mother clouds for extremely large NAT particles, termed NAT-rocks, provided the air below the clouds is supersaturated with respect to NAT. Individual NAT particles at the cloud base fall into undepleted gas phase and rapidly accelerate due to a positive feedback between their growth and sedimentation. The resulting reduction in number density is further enhanced by the strong HNO3 depletion within a thin layer below the mother cloud, which delays subsequent particles. This paper introduces the basic microphysical principles behind this mother cloud/NAT-rock mechanism, which produces 10-4 cm-3 NAT-rocks with radii around 10 mm some kilometers below the mother cloud. The mechanism does not require selective nucleation and works even for a monodisperse particle size distribution in the mother cloud.

Lee‐wave clouds and denitrification of the polar stratosphere

We present a hypothesis that the known formation of nitric acid hydrates in lee‐wave ice clouds produces a “standing crop” of nitric acid trihydrate (NAT) particles, and that growth and sedimentation of these particles efficiently denitrifies the polar stratosphere. Simulations using a simple two‐dimensional model illustrate that the large NAT number concentrations produced in lee waves (>0.1 cm−3) lead to low gas‐phase HNO3 concentration in the cloud layer, limiting subsequent particle growth. Provided the NAT existence temperature is not exceeded, these particles slowly sediment out of the cloud layer, at which point further growth is initiated. As a result of the long residence time and vertical wind shear, NAT particles produced in geographically selective regions over a short duration can cause extensive denitrification throughout the lower stratosphere. Our model illustrates that a single hypothetical lee‐wave nucleation event [10 hours, 1000 km width (along a longitude) × 100 m altitude] is sufficient to produce significant denitrification (∼25%) vortex‐wide.

Tropical nitric acid clouds

The presence of nitric acid clouds near the tropical tropopause was proposed over a decade ago, although little has been learned about them since. Particle extinction measurements at three wavelengths from the Halogen Occultation Experiment offer compelling evidence for the existence of nitric acid clouds near the tropical tropopause. Most of these clouds are consistent with nitric acid trihydrate, and a small fraction can be explained by liquid ternary H2SO4‐H2O‐HNO3 aerosols.

Microphysical modeling of the 1999–2000 Arctic winter: 1. Polar stratospheric clouds, denitrification, and dehydration

The freezing processes that may lead to the formation of solid phase polar stratospheric clouds (PSCs) have been examined to assess their winter‐long effects, especially denitrification, in a coupled microphysical/photochemical model. Trajectory simulations spanned from November 1999 to April 2000, using a large set of trajectories which provided representative coverage of the entire Arctic vortex through the period of PSC formation and ozone depletion. A freezing process occurring at temperatures above the ice frost point is shown to be necessary to explain both the occurrence of solid phase PSCs early in the winter and denitrification, especially without dehydration. If freezing only occurs below the ice frost point the primary contributor to denitrification is actually sedimentation of liquid phase PSC particles. The mechanism of a second freezing process, occurring above the ice frost point, can not yet be conclusively determined. Of the cases considered, heterogeneous freezing of the aerosol to form nitric acid trihydrate (NAT) particles best reproduced solid phase PSC formation and observations of widespread denitrification with limited dehydration. The simulations constrain the number of frozen particles to be near either 0.02% or 1% of the total aerosol number; values in between 0.02% and 1% produce more intense denitrification than observed, demonstrating that small changes in the number of frozen particles could exacerbate denitrification. However, this result was contingent upon assuming that the heterogeneous nuclei remain active, producing PSCs, throughout the winter. An idealized homogeneous freezing process was also able to produce NAT PSCs and denitrification (rates of 106–107 cm−3 s−1 compared favorably with data) but differed from observations in one key aspect: denitrification was more frequently accompanied by dehydration. Nitric acid dihydrate (NAD) particles were less effective than NAT at denitrification, but heterogeneous freezing of 0.1% of the aerosol yielded results marginally consistent with measurements. An important limitation, however, of all the scenarios considered is that they produced more intense and more widespread dehydration than was observed. This suggests that model minimum temperatures (from UK Meteorological Office analyses) were too cold by 1 to 3 K.

Chemical, microphysical, and optical properties of polar stratospheric clouds

A balloonborne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 near Kiruna/Sweden. Besides an aerosol composition mass spectrometer, the gondola carried optical particle counters, two backscatter sondes, a hygrometer, and several temperature and pressure sensors. A mountain wave induced PSC was sampled between 20 and 23 km altitude. Strongly correlated PSC particle properties were detected with the different instruments. A large variability of particle types was measured in numerous PSC layers, and PSC development was followed for about two hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of nitric acid trihydrate (NAT) particles with a molar ratio H2O:HNO3 close to 3 at temperatures near and below the equilibrium temperature TNAT. The median radius of the NAT particle size distribution was between 0.5 and 0.75 μm at concentrations around 0.5 cm−3. Below the NAT layers and above TNAT, thin cloud layers containing a few large particles with radii up to 3.5 μm coexisted with smaller solid or liquid particles. The molar ratio in this region was found to be close to two.

Modeling the effect of denitrification on Arctic ozone depletion during winter 1999/2000

We have used the SLIMCAT three‐dimensional chemical transport model together with observations from the Stratospheric Aerosol and Gas Experiment (SAGE III) Ozone Loss and Validation Experiment (SOLVE) and the Third European Stratospheric Experiment on Ozone (THESEO 2000) to quantify the effect of denitrification on Arctic ozone loss. We have used two different denitrification schemes in the model: one based on the sedimentation of ice particles containing cocondensed nitric acid trihydrate (NAT) and the other based on large NAT particles. The model was forced using both UK Meteorological Office (UKMO) and European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses. In the Arctic lower stratosphere the UKMO analyzed temperatures are similar to the ECMWF, except at temperatures near the ice point where the UKMO analyses are colder by over 2 K. Consequently, the UKMO analyses predicted large regions of ice clouds, in contrast to the ECMWF. The denitrification scheme based on large NAT particles gives the best agreement with ER‐2 NOy observations for both sets of meteorological analyses. Although the ice scheme and UKMO analyses also produce denitrification, the vertical extent of denitrification and renitrification does not agree as well with the observed NOy. Uncertainties in the budget of ClOy observations from the ER‐2 prevent an indirect validation of the best model denitrification scheme based on these data. The denitrified model runs give the best agreement with the observed HCl and ClONO2 reservoirs in mid March. However, UKMO‐forced runs generally overestimate the observed ClOx during the same period. The denitrified model runs indicate that by late March 56–74% O3 loss had occurred at 460 K and that denitrification contributed 21–30% of this loss. The model runs showing the largest O3 depletion (forced by UKMO analyses) agree well with ER‐2 and ozone sonde data, although these runs overestimated ClOx.

Vortexwide denitrification of the Arctic polar stratosphere in winter 1999/2000 determined by remote observations

Denitrification has been studied using measurements of stratospheric HNO3 and N2O by the Airborne Submillimeter Radiometer (ASUR), operated on board the NASA DC‐8 during SOLVE/THESEO 2000. Lidar measurements taken on board the same aircraft have been used to distinguish between temporary uptake of HNO3 in polar stratospheric clouds (PSCs) and denitrification events. To derive an NOy budget, ClNO3 data by balloonborne and ground‐based Fourier transform infrared measurements and a model estimate of NOx + 2N2O5 have been considered. The HNO3 profiles of sporadic ASUR measurements without PSC coverage in January suggest that denitrification had started in the vortex core region by then. Vortexwide denitrification was found in mid‐March 2000. Corrected for diabatic descent using the N2O measurements, a vortex‐averaged NOy deficit between 1.2 ± 0.9 ppb at about 16 km altitude and 5.3 ± 2.7 ppb at about 20.5 km altitude was derived compared to December 1999, based on an observed decrease in HNO3 between 2.2 and 3.5 ppb during this time period. A shift in the NOy partitioning from HNO3 toward ClNO3 of about 0.4 to 0.7 ppb was observed in mid‐March compared to December, indicating that chlorine deactivation was occurring. Comparisons with the SLIMCAT three‐dimensional chemical transport model applying denitrification schemes based on ice and nitric acid trihydrate particles in equilibrium, respectively, reveal agreement within the error bars at higher altitudes (∼19 km) but show discrepancies at lower altitudes (∼16 km). It is suggested that more sophisticated denitrification schemes are needed to generally describe denitrification processes.

Microphysical modeling of the 1999-2000 Arctic winter 2. Chlorine activation and ozone depletion

[1] The effect of polar stratospheric clouds (PSCs) on ozone depletion during the 1999–2000 Arctic winter has been assessed using a coupled microphysical/photochemical model. Scenarios spanned a large range of denitrification levels, with up to 80% vortex-averaged denitrification and localized dehydration. PSC composition was varied, exploring sensitivity to heterogeneous reaction rates. PSC formation during February was critical in causing severe ozone depletion below 500 K. Heterogeneous chemistry on these PSCs was able to continuously reactivate the newly produced ClONO2. Only 30–40% vortex-averaged ozone loss would have occurred without this chlorine reactivation; with it, 21–32% more ozone loss is possible. During February (unlike earlier in the winter), chlorine reactivation and ozone loss were sensitive to heterogeneous chemistry: varying the reactivities altered ozone loss by 11%. An analysis of the critical temperatures for heterogeneous chemistry demonstrates the importance of temperatures near the nitric acid trihydrate condensation point, where many uncertainties influence heterogeneous reaction rates. Chlorine reactivation during February also prevented denitrification from enhancing ozone loss until March: 70% vortex-averaged denitrification only enhanced ozone depletion by 3% on 10 March. The mid-March vortex breakup probably limited the extent of ozone depletion; if the vortex had remained stable until 15 April, 16% ozone loss (out of a total 68% ozone loss) could be caused by 70% denitrification. Ozone loss intensifies nonlinearly with enhanced denitrification: in individual air parcels with 90% denitrification, 40% ozone loss in mid-April can be attributed to denitrification alone.

The Interaction of Methanol, Acetone, and Acetaldehyde with Ice and Nitric Acid-Doped Ice: Implications for Cirrus Clouds

The uptake of methanol, acetone, and acetaldehyde by ice and nitric acid/ice surfaces representative of cirrus clouds has been measured using a Knudsen cell flow reactor equipped with FTIR spectroscopic observation of the condensed phase. Sticking coefficients and total equilibrium coverage for the organics on ice were obtained over the temperature range of 120−200 K. Both the sticking coefficients and coverage were found to decrease with increasing temperature for all three species. The coverage data was fit using a Langmuir adsorption model, and the results were extrapolated to typical cirrus cloud temperatures. By the use of estimates for the gaseous concentrations of the organic species, calculated coverages on ice at 210 K are extrapolated to be on the order of 1010 molecules/cm2 (10-4 ML) for methanol and acetone and <5 × 109 molecules/cm2 (<10-5 ML) for acetaldehyde. Such small coverages constitute an insignificant portion of each gas-phase species being lost to cirrus clouds. Additionally, the surface coverages of ≤10-4 monolayers (ML) are likely to be too small to impact heterogeneous chemistry. Uptake of methanol, acetone, and acetaldehyde was also studied on several forms of nitric acid-coated ice at temperatures from 190 to 200 K. Studies on nitric acid trihydrate (NAT) and ice with monolayer coverage of HNO3 showed no measurable uptake in the examined temperature range for all three species. While no uptake was observed on a supercooled HNO3/H2O solution for acetone or acetaldehyde at T = 200 K, this solution did show enhanced uptake of methanol at 200 K relative to pure ice.

Role of temperature history in polar stratospheric cloud sightings

[1] In this paper, aerosol data from the Polar Ozone and Aerosol Measurement (POAM) satellites between 1994 and 1998 are used to explore the existence of polar stratospheric clouds (PSCs) in the Antarctic winter stratosphere. Multiwavelength aerosol extinction data are combined to yield aerosol volume density which is compared with that generated from theoretical equilibrium models for nitric acid trihydrate (NAT) particles and supercooled ternary solution (STS) droplets at 30 mb. As winter progresses, PSC volume falls, a strong indication of stratospheric denitrification or dehydration. Six-day isentropic back trajectories are generated from each of the POAM observation locations and are used to examine the recent temperature history of the aerosols. Statistical studies are carried out using multiple regression to determine whether temperature history plays a role in NAT cloud formation. We are unable to identify any factors which unquestionably affect the formation of PSCs at and just below the NAT condensation temperature, although the probability of PSC occurrence at this temperature is higher in air that had previously been colder. There is no significant evidence found for preferential cloud occurrence based upon the length of time that the aerosols were below the NAT condensation point. We conclude from this study that factors, other than temperature and temperature history, are partially responsible for the formation of NAT PSCs.

Quantifying the rate of heterogeneous processing in the Arctic polar vortex with in situ observations of OH

[1] We present simultaneous in situ observations of OH, HO2, ClONO2, HCl, and particle surface area inside a polar stratospheric cloud undergoing rapid heterogeneous processing. A steady-state analysis constrained by in situ observations is used to show that concentrations of OH calculated during a processing event are extremely sensitive to the assumptions regarding aerosol composition and reactivity. This analysis shows that large perturbations in the abundance of OH are consistent with the heterogeneous production of HOCl via ClONO2 + H2O → HOCl + HNO3 and removal via HOCl + HCl → Cl2 + H2O in a polar stratospheric cloud. If the cloud is composed of supercooled ternary solution (STS) aerosols and solid nitric acid trihydrate (NAT) particles, comparison with observations of OH show that modifications to surface reactivity to account for high HNO3 content in STS aerosols and low HCl coverage on NAT particles are appropriate. These results indicate that with the low HCl levels in this encounter and in a processed polar vortex in general, reactions on STS aerosols dominate the total heterogeneous processing rate. As a consequence, the formation of NAT does not lead to significantly faster reprocessing rates when HCl concentrations are low and STS aerosols are present. Model calculations that include these modifications to uptake coefficients for STS and NAT will lead to significantly slower reprocessing and faster recovery rates of chlorine in the springtime Arctic polar vortex.

Redistribution of nitric acid in the Arctic lower stratosphere during the winter of 1996–1997

Vertical profiles of HNO3, N2O, O3, and the aerosol extinction coefficient at 780 nm were observed by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) during the Arctic winter of 1996–1997. Irreversible redistribution of HNO3 is evaluated using HNO3‐N2O and HNO3‐O3 correlations. Denitrification and nitrification started to be observed just after the Arctic vortex cooled to below the ice frost point (TICE) on February 10. Trajectory analyses show that denitrification occurred only in air masses, which were once cooled to near TICE and were kept at temperatures below the nitric acid trihydrate saturation threshold continuously for more than 4 days. Such a temperature history provides the necessary conditions for nucleation and growth of particles causing denitrification. The average extent of denitrification at 19 km reached 43% at the center of the vortex, suggesting that stratospheric ozone could be affected by denitrification deep inside the vortex. Denitrification (&gt;2 ppbv) and nitrification (&gt;1 ppbv) covered 40±10% and 35±10% of the vortex area, respectively. Redistributed numbers of HNO3 molecules at each altitude were calculated by integrating the area‐weighted changes in the HNO3 concentration. The decreases in total HNO3 concentration at 17–21 km in late February and early March agreed with the increases at 12–15 km to within 25%, confirming conservation of HNO3 during sedimentation and evaporation of HNO3‐containing polar stratospheric cloud particles.

Nonequilibrium coexistence of solid and liquid particles in Arctic stratospheric clouds

Observations of polar stratospheric clouds (PSCs) from Ny Ålesund, Spitsbergen, have been examined to quantify the occurrence of solid particles. The polarized backscatter ratio was found to be a more sensitive indicator of the presence of non‐spherical (solid) particles in a PSC than the aerosol depolarization, which approaches zero for clouds containing a large volume of liquid droplets. The analysis corroborates our previous finding that type Ia PSCs cannot be composed of nitric acid trihydrate particles in equilibrium with the gas phase, which would lead to a much too high backscatter ratio. Conversely, type Ib PSCs, previously thought to be composed solely of liquid droplets, often contain a very small fraction of solid particles. These clouds develop a high parallel backscatter, arising from Mie scattering by droplets, and a nonvanishing perpendicular signal due to the few solid particles. Other type Ib PSCs appear to contain only liquid particles. The mixed liquid/solid clouds are observed on the smallest spatial and temporal scale resolvable by the lidar instrument (30 m, l min), implying that their properties are not a result of spatial averaging of different cloud types. Comparison of the observations with optical calculations shows that such nonequilibrium particle distributions are to be expected, as temperature changes are sufficiently rapid to prevent the particles from assuming equilibrium sizes. The observed optical characteristics of type Ia and type Ib clouds can be reproduced in a model by assuming that a very small fraction of the particles are composed of nitric acid hydrate, with the majority being binary H2SO4/H2O or ternary HNO3/H2SO4/H2O droplets.

CRISTA‐2 observations of the South Polar Vortex in winter 1997: A new dataset for polar process studies

During the second mission of the Cryogenic Spectrometers and Telescopes for the Atmosphere (CRISTA) in August 1997 considerable and highly variable cloud cover in the stratosphere was seen over Antarctica. Two large areas of polar stratospheric clouds (PSC) could be observed. One region of PSCs near the vortex edge was caused by a meteorological situation as it is typical for an ozone minihole event and dissolved within two days. The temperature dependence of the HNO3 measured by CRISTA suggests that outside of their center the clouds consists of type I particles. However, a further discrimination between nitric acid trihydrate (NAT) or supercooled ternary solutions (STS) is not possible, due to the uncertainties in the meteorological temperatures and the amount of water vapor.

The detection of large HNO3-containing particles in the winter Arctic stratosphere.

Large particles containing nitric acid (HNO3) were observed in the 1999/2000 Arctic winter stratosphere. These in situ observations were made over a large altitude range (16 to 21 kilometers) and horizontal extent (1800 kilometers) on several airborne sampling flights during a period of several weeks. With diameters of 10 to 20 micrometers, these sedimenting particles have significant potential to denitrify the lower stratosphere. A microphysical model of nitric acid trihydrate particles is able to simulate the growth and sedimentation of these large sizes in the lower stratosphere, but the nucleation process is not yet known. Accurate modeling of the formation of these large particles is essential for understanding Arctic denitrification and predicting future Arctic ozone abundances.