Large Nitric Acid Trihydrate Particles Research Articles

Effects of denitrification on the distributions of trace gas abundances in the polar regions: a comparison of WACCM with observations

Abstract. Polar stratospheric clouds (PSCs) play a key role in the polar chemistry of the stratosphere. Nitric acid trihydrate (NAT) particles have been shown to lead to denitrification of the lower stratosphere. While the existence of large NAT particles (NAT “rocks”) has been verified by many measurements, especially in the Northern Hemisphere (NH), most current chemistry–climate models use simplified parameterizations, often based on evaluations in the Southern Hemisphere where the polar vortex is stable enough that accounting for NAT rocks is not as important as in the NH. Here, we evaluate the probability density functions of various gaseous species in the polar vortex using one such model, the Whole Atmosphere Community Climate Model (WACCM), and compare these with measurements by the Michelson Interferometer for Passive Atmospheric Sounding onboard the Environmental Satellite (MIPAS/Envisat) and two ozonesonde stations for a range of years and in both hemispheres. Using the maximum difference between the distributions of MIPAS and WACCM as a measure of coherence, we find better agreement for HNO3 when reducing the NAT number density from the standard value of 10−2 used in this model to 5×10-4 cm−3 for almost all spring seasons during the MIPAS period in both hemispheres. The distributions of ClONO2 and O3 are not greatly affected by the NAT density. The average difference between WACCM and ozonesondes supports the need to reduce the NAT number density in the model. Therefore, this study suggests using a NAT number density of 5×10-4 cm−3 for future simulations with WACCM.

Vortex‐Wide Detection of Large Aspherical NAT Particles in the Arctic Winter 2011/12 Stratosphere

AbstractMicron‐sized HNO3‐containing particles in polar stratospheric clouds are known to denitrify the polar winter stratosphere and support chemical ozone loss. We show that populations of nitric acid trihydrate (NAT) particles with volume‐equivalent median radii of 3–7 μm can be detected vortex‐wide by means of infrared limb sounding. Key for detection are the applied optical characteristics of highly aspherical particles consisting of the β‐NAT phase. Spectroscopic signatures and ambient conditions of detected populations show that these particles play a key role in denitrification of the Arctic winter stratosphere. Complementary gas‐phase HNO3 observations indicate collocated highly efficient HNO3 sequestration within days and are consistent with measured spectral signals of populations of large NAT particles. High amounts of condensed gas‐phase equivalent HNO3 exceeding 10 ppbv and long persistence of detected populations, despite expected gravitational settling, imply that our understanding of the particles is incomplete.

Interannual variations of early winter Antarctic polar stratospheric cloud formation and nitric acid observed by CALIOP and MLS

Abstract. We use satellite-borne measurements collected over the last decade (2006–2015) from the Aura Microwave Limb Sounder (MLS) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) to investigate the nitric acid distribution and the properties of polar stratospheric clouds (PSCs) in the early winter Antarctic vortex. Frequently, at the very start of the winter, we find that synoptic-scale depletion of HNO3 can be detected in the inner vortex before the first lidar detection of geophysically associated PSCs. The generation of "sub-visible" PSCs can be explained as arising from the development of a solid particle population with low number densities and large particle sizes. Assumed to be composed of nitric acid trihydrate (NAT), the sub-visible PSCs form at ambient temperatures well above the ice frost point, but also above the temperature at which supercooled ternary solution (STS) grows out of the background supercooled binary solution (SBS) distribution. The temperature regime of their formation, inferred from the simultaneous uptake of ambient HNO3 into NAT and their Lagrangian temperature histories, is at a depression of a few kelvin with respect to the NAT existence threshold, TNAT. Therefore, their nucleation requires a considerable supersaturation of HNO3 over NAT, and is consistent with a recently described heterogeneous nucleation process on solid foreign nuclei immersed in liquid aerosol. We make a detailed investigation of the comparative limits of detection of PSCs and the resulting sequestration of HNO3 imposed by lidar, mid-infrared, and microwave techniques. We find that the temperature history of air parcels, in addition to the local ambient temperature, is an important factor in the relative frequency of formation of liquid/solid PSCs. We conclude that the initiation of NAT nucleation and the subsequent development of large NAT particles capable of sedimentation and denitrification in the early winter do not emanate from an ice-seeding process. Finally, we investigate the patterns of interannual variability and compare the relative formation frequency of liquid and solid PSCs in the Antarctic lower polar stratosphere using the results of a cluster analysis to synthesize the combined CALIOP and MLS measurements into a relatively small number of interrelated categories.

Denitrification by large NAT particles: the impact of reduced settling velocities and hints on particle characteristics

Abstract. Vertical redistribution of HNO3 through large HNO3-containing particles associated with polar stratospheric clouds (PSCs) plays an important role in the chemistry of the Arctic winter stratosphere. During the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) campaign, apparently very large NAT (nitric acid trihydrate) particles were observed by the airborne in situ probe FSSP-100 (Molleker et al., 2014). Our analysis shows that the FSSP-100 observations associated with the flight on 25 January 2010 cannot easily be explained assuming compact spherical NAT particles due to much too short growing time at temperatures below the existence temperature of NAT (TNAT). State-of-the-art simulations using CLaMS (Chemical Lagrangian Model of the Stratosphere; Grooß et al., 2014) suggest considerably smaller particles. We consider the hypothesis that the simulation reproduces the NAT particle masses in a realistic way, but that real NAT particles may have larger apparent sizes compared to compact spherical particles, e.g. due to non-compact morphology or aspheric shape. Our study focuses on the consequence that such particles would have reduced settling velocities compared to compact spheres, altering the vertical redistribution of HNO3. Utilising CLaMS simulations, we investigate the impact of reduced settling velocities of NAT particles on vertical HNO3 redistribution and compare the results with observations of gas-phase HNO3 by the airborne Fourier transform spectrometer MIPAS-STR associated with two RECONCILE flights. The MIPAS-STR observations confirm conditions consistent with denitrification by NAT particles for the flight on 25 January 2010 and show good agreement with the simulations within the limitations of the comparison. Best agreement is found if settling velocities between 100 and 50% relative to compact spherical particles are considered (slight preference for the 70% scenario). In contrast, relative settling velocities of 30% result in too weak vertical HNO3 redistribution. Sensitivity simulations considering temperature biases of ±1 K and multiplying the simulated nucleation rates by factors of 0.5 and 2.0 affect the comparisons to a similar extent, but result in no effective improvement compared to the reference scenario. Our results show that an accurate knowledge of the settling velocities of NAT particles is important for quantitative simulations of vertical HNO3 redistribution.

Microphysical properties of synoptic-scale polar stratospheric clouds: in situ measurements of unexpectedly large HNO<sub>3</sub>-containing particles in the Arctic vortex

Abstract. In January 2010 and December 2011, synoptic-scale polar stratospheric cloud (PSC) fields were probed during seven flights of the high-altitude research aircraft M-55 Geophysica within the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interaction) and the ESSenCe (ESSenCe: ESA Sounder Campaign) projects. Particle size distributions in a diameter range between 0.46 and 40μm were recorded by four different optical in situ instruments. Three of these particle instruments are based on the detection of forward-scattered light by single particles. The fourth instrument is a grayscale optical array imaging probe. Optical particle diameters of up to 35μm were detected with particle number densities and total particle volumes exceeding previous Arctic measurements. Also, gas-phase and particle-bound NOy was measured, as well as water vapor concentrations. The optical characteristics of the clouds were measured by the remote sensing lidar MAL (Miniature Aerosol Lidar) and by the in situ backscatter sonde MAS (Multiwavelength Aerosol Scatterometer), showing the synoptic scale of the encountered PSCs. The particle mode below 2μm in size diameter has been identified as supercooled ternary solution (STS) droplets. The PSC particles in the size range above 2μm in diameter are considered to consist of nitric acid hydrates, and the particles' high HNO3 content was confirmed by the NOy instrument. Assuming a particle composition of nitric acid trihydrate (NAT), the optically measured size distributions result in particle-phase HNO3 mixing ratios exceeding available stratospheric values. Therefore the measurement uncertainties concerning probable overestimations of measured particle sizes and volumes are discussed in detail. We hypothesize that either a strong asphericity or an alternate particle composition (e.g., water ice coated with NAT) could explain our observations. In particular, with respect to the denitrification by sedimentation of large HNO3-containing particles, generally considered to be NAT, our new measurements raise questions concerning composition, shape and nucleation pathways. Answering these would improve the numerical simulation of PSC microphysical processes like cloud particle formation, growth and denitrification, which is necessary for better predictions of future polar ozone losses, especially under changing global climate conditions. Generally, it seems that the occurrence of large NAT particles – sometimes termed "NAT rocks" – are a regular feature of synoptic-scale PSCs in the Arctic.

The 2009–2010 Arctic polar stratospheric cloud season: a CALIPSO perspective

Abstract. Spaceborne lidar measurements from CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) are used to provide a vortex-wide perspective of the 2009–2010 Arctic PSC (polar stratospheric cloud) season to complement more focused measurements from the European Union RECONCILE (reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) field campaign. The 2009–2010 Arctic winter was unusually cold at stratospheric levels from mid-December 2009 until the end of January 2010, and was one of only a few winters from the past fifty-two years with synoptic-scale regions of temperatures below the frost point. More PSCs were observed by CALIPSO during the 2009–2010 Arctic winter than in the previous three Arctic seasons combined. In particular, there were significantly more observations of high number density NAT (nitric acid trihydrate) mixtures (referred to as Mix 2-enh) and ice PSCs. We found that the 2009–2010 season could roughly be divided into four periods with distinctly different PSC optical characteristics. The early season (15–30 December 2009) was characterized by patchy, tenuous PSCs, primarily low number density liquid/NAT mixtures. No ice clouds were observed by CALIPSO during this early phase, suggesting that these early season NAT clouds were formed through a non-ice nucleation mechanism. The second phase of the season (31 December 2009–14 January 2010) was characterized by frequent mountain wave ice clouds that nucleated widespread NAT particles throughout the vortex, including Mix 2-enh. The third phase of the season (15–21 January 2010) was characterized by synoptic-scale temperatures below the frost point which led to a rare outbreak of widespread ice clouds. The fourth phase of the season (22–28 January) was characterized by a major stratospheric warming that distorted the vortex, displacing the cold pool from the vortex center. This final phase was dominated by STS (supercooled ternary solution) PSCs, although NAT particles may have been present in low number densities, but were masked by the more abundant STS droplets at colder temperatures. We also found distinct variations in the relative proportion of PSCs in each composition class with altitude over the course of the 2009–2010 Arctic season. Lower number density liquid/NAT mixtures were most frequently observed in the lower altitude regions of the clouds (below ~18–20 km), which is consistent with CALIPSO observations in the Antarctic. Higher number density liquid/NAT mixtures, especially Mix 2-enh, were most frequently observed at altitudes above 18–20 km, primarily downstream of wave ice clouds. This pattern is consistent with the conceptual model whereby low number density, large NAT particles are precipitated from higher number density NAT clouds (i.e. mother clouds) that are nucleated downstream of mountain wave ice clouds.

Large nitric acid trihydrate particles and denitrification caused by mountain waves in the Arctic stratosphere

The contribution of stratospheric mountain waves to the formation of large nitric acid trihydrate (NAT) particles and subsequent denitrification of the Arctic polar vortex is calculated for the 1999/2000 winter using a three‐dimensional (3‐D) model. The model production mechanism involves the formation of NAT clouds with high particle number concentrations downwind of mountain wave ice clouds, as has been previously observed. These wave‐induced NAT clouds then serve as “mother clouds” for the release of low concentrations of sedimenting NAT particles, following the mechanism of Fueglistaler et al. (2002a). Our calculations show that wave‐induced NAT mother clouds can occupy up to 5–10% of the volume of air below the NAT temperature. NAT particles that have sedimented from the mother cloud bases can occupy in excess of 60% of the NAT supersaturated region. Integrated over the entire vortex at the end of the 1999/2000 simulation, it is estimated that denitrification due to this mechanism could potentially be responsible for as much as 80% of that observed. These results show that mountain waves may contribute to the occurrence of solid polar stratospheric clouds more than was previously thought and that they may play a significant role in denitrification.

Formation of large NAT particles and denitrification in polar stratosphere: possible role of cosmic rays and effect of solar activity

Abstract. The formation of large nitric acid trihydrate (NAT) particles has important implications for denitrification and ozone depletion. Existing theories have difficulty in explaining the formation of large NAT particles at temperatures above the ice frost point, which has been observed recently over wide Arctic regions. Our analyses reveal that high-energy comic ray particles might induce the freezing of supercooled HNO3-H2O-H2SO4 droplets when they penetrate these thermodynamically unstable droplets. The cosmic ray-induced freezing (CRIF) appears to be consistent with the observed, highly selective formation of NAT particles. We suggest a possible physical process behind the CRIF mechanism: the reorientation of polar solution molecules into the crystalline configuration in the strong electrical fields of moving secondary ions generated by passing cosmic rays. A simple formula connecting the CRIF rate to cosmic ray flux is derived with an undefined parameter constrained by observed NAT formation rates. Our simulations indicate that strong solar proton events (SPEs) may significantly enhance the formation of large NAT particles and denitrification. The CRIF mechanism offers a possible explanation for the observed high correlations between the thin nitrate-rich layers in polar ice cores and major SPEs, and the observed enhancement in the aerosol backscattering ratio at PSC layers shortly after an SPE and the significant precipitation velocity of the enhanced PSC layers. The key uncertainty in the CRIF mechanism is the probability (P) of freezing when a CR particle hits a thermodynamically, unstable STS droplet. Further studies are needed to either confirm or reject the CRIF hypothesis.

SAGE III observations of Arctic polar stratospheric clouds – December 2002

Stratospheric Aerosol and Gas Experiment (SAGE) III aerosol extinction data from the Arctic in December 2002 are presented and used to illustrate the instrument's capability for polar stratospheric cloud (PSC) observations. Numerous PSCs were observed during the month at potential temperatures from about 450–700 K, and Types 1a and 1b PSCs could be discriminated in plots of 1022‐nm extinction versus color ratio (the ratio of 1022‐nm and 449‐nm extinctions). Type 1a (1b) PSCs were observed primarily above (below) a threshold temperature of 4 K below the nitric acid trihydrate (NAT) equilibrium temperature. Mie calculations show that some of the Type 1a PSC observations are consistent with a mixture of background aerosol and very few large NAT particles. This suggests that SAGE III offers the potential for systematic observations of these clouds, which would greatly improve our understanding of how they form and lead to stratospheric denitrification.

Factors controlling Arctic denitrification in cold winters of the 1990s

Abstract. Denitrification of the Arctic winter stratosphere has been calculated using a 3-D microphysical model for the winters 1994/95, 1995/96, 1996/97 and 1999/2000. Denitrification is assumed to occur through the sedimentation of low number concentrations of large nitric acid trihydrate (NAT) particles (as inferred by e.g. Fahey et al., 2001). We examine whether the meteorological conditions that allowed particles to grow to the very large sizes observed in 1999/2000 also occurred in the other cold winters. The results show that winter 1999/2000 had conditions that were optimum for denitrification by large NAT particles, which are a deep concentric NAT area and vortex. Under these conditions, NAT particles can circulate in the NAT-supersaturated air for several days, reaching several micrometres in radius and leading to a high downward flux of nitric acid. The other winters had shorter periods with optimum conditions for denitrification. However, we find that NAT particles could have grown to large sizes in all of these winters and could have caused significant denitrification. We define the quantity "closed-flow area'' (the fraction of the NAT area in which air parcel trajectories can form closed loops) and show that it is a very useful indicator of possible denitrification. We find that even with a constant NAT nucleation rate throughout the NAT area, the average NAT number concentration and size can vary by up to a factor of 10 in response to this meteorological quantity. These changes in particle properties account for a high degree of variability in denitrification between the different winters. This large meteorologically induced variability in denitrification rate needs to be compared with that which could arise from a variable nucleation rate of NAT particles, which remains an uncertain quantity in models. Sensitivity studies show that although denitrification was likely approaching asymptotic minimum values throughout much of the 1999/2000 vortex, decreases in the volume-averaged nucleation rate would have substantially reduced the denitrification.

Polar vortex concentricity as a controlling factor in Arctic denitrification

Recent in situ observations in the Arctic stratosphere have detected nitric acid–containing particles with sizes up to 10‐μm 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.

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.

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.

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.

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.

Nucleation and Growth of HNO3·3H2O Particles in the Polar Stratosphere

Growth of nitric acid trihydrate (NAT) particles on background stratospheric aerosols is examined for an isolated air parcel cooled at a uniform rate. The free energy barrier against nucleation and rates of cooling are varied over a range of probable values. During the process of nucleation, the saturation ratio of HNO3 vapor reaches a maximum value between 2 and 15, corresponding to supercooling by 1–4 K. Significant supersaturation may be maintained after nucleation due to the small surface area of NAT available for vapor deposition. If cooling rates exceed 0.5–1 K day−1, small (<2 μm radius) particles of NAT are produced. A major fraction of the available condensation nuclei is activated and removal of HNO3 by gravitational settling is slow. If cooling rates are less than 0.5–1 K day−1, the number of aerosols that nucleate is reduced, leading to differential growth of large (>2 μm radius) NAT particles. Gravitational settling of NAT particles could result in removal of HNO3 on time scales close to one week. Observations of 5 μm radius particles in clouds at temperatures above the water frost point may reflect condensation of NAT on ice particles that fall through a column of air as it is cooled. Rapid condensation of HNO3 on ice particles is promoted by the high supersaturation attained during nucleation and maintained during subsequent cooling. This process provides a mechanism for irreversible removal of HNO3.