Solid Nitric Acid Trihydrate Research Articles

Arctic stratosphere changes in the 21st century in the Earth system model SOCOLv4

Two ensemble simulations of a new Earth system model (ESM) SOCOLv4 (SOlar Climate Ozone Links, version 4) for the period from 2015 to 2099 under moderate (SSP2-4.5) and severe (SSP5-8.5) scenarios of greenhouse gas (GHG) emission growth were analyzed to investigate changes in key dynamical processes relevant for Arctic stratospheric ozone. The model shows a 5–10 K cooling and 5%–20% humidity increase in the Arctic lower–upper stratosphere in March (when the most considerable ozone depletion may occur) between 2080–2099 and 2015–2034. The minimal temperature in the lower polar stratosphere in March, which defines the strength of ozone depletion, appears when the zonal mean meridional heat flux in the lower stratosphere in the preceding January–February is the lowest. In the late 21st century, the strengthening of the zonal mean meridional heat flux with a maximum of up to 20 K m/s (∼25%) in the upper stratosphere close to 70°N in January–February is obtained in the moderate scenario of GHG emission, while only a slight increase in this parameter over 50 N–60 N with the maximum up to 5 K m/s in the upper stratosphere and a decrease with the comparable values over the high latitudes is revealed in the severe GHG emission scenario. Although the model simulations confirm the expected ozone layer recovery, particularly total ozone minimum values inside the Arctic polar cap in March throughout the 21st century are characterized by a positive trend in both scenarios, the large-scale negative ozone anomalies in March up to −80 DU–100 DU, comparable to the second lowest ones observed in March 2011 but weaker than record values in March 2020, are possible in the Arctic until the late 21st century. The volume of low stratospheric air with temperatures below the solid nitric acid trihydrate polar stratospheric cloud (PSC NAT) formation threshold is reconstructed from 3D potential vorticity and temperature fields inside the stratospheric polar vortex. A significant positive trend is shown in this parameter in March in the SSP5-8.5 scenario. Furthermore, according to the model data, an increase in the polar vortex isolation throughout the 21st century indicates its possible strengthening in the lower stratosphere. Positive trends of the surface area density (SAD) of PSC NAT particles in March in the lower Arctic stratosphere over the period of 2015–2099 are significant in the severe GHG emission scenario. The polar vortex longitudinal shift toward northern Eurasia is expected in the lower stratosphere in the late 21st century in both scenarios. The statistically significant long-term stratospheric sulfuric acid aerosol trend in March is expected only in the SSP5.8-5 scenario, most probably due to cooler stratosphere and stronger Brewer–Dobson circulation intensification. Both scenarios predict an increase in the residual meridional circulation (RMC) in March by the end of the 21st century. In some regions of the stratosphere, the RMC enhancement under the severe GHG scenario can exceed 20%.

A study of optical scattering modelling for mixed-phase polar stratospheric clouds

Abstract. Scattering codes are used to study the optical properties of polar stratospheric clouds (PSCs). Particle backscattering and depolarization coefficients can be computed with available scattering codes once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of supercooled ternary solution (STS) droplets, solid nitric acid trihydrate (NAT) and possibly ice particles, making the assumption of a single refractive index and a single morphology to model the scatterers questionable. Here we consider a set of 15 coincident measurements of PSCs above McMurdo Station, Antarctica, using ground-based lidar, a balloon-borne optical particle counter (OPC) and in situ observations taken by a laser backscattersonde and OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows us to test the performances of optical scattering models when both spherical and aspherical scatterers of different composition and, possibly, shapes are present. We consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if it is above it. The refractive indices are assumed known from the literature. Mie scattering is used for the STS, assumed spherical. Scattering from NAT particles, considered spheroids of different aspect ratio (AR), is treated with T-matrix results where applicable. The geometric-optics–integral-equation approach is used whenever the particle size parameter is too large to allow for a convergence of the T-matrix method. The parameters Rth and AR of our model have been varied between 0.1 and 2 µm and between 0.3 and 3, respectively, and the calculated backscattering coefficient and depolarization were compared with the observed ones. The best agreement was found for Rth between 0.5 and 0.8 µm and for AR less than 0.55 and greater than 1.5. To further constrain the variability of AR within the identified intervals, we have sought an agreement with the experimental data by varying AR on a case-by-case basis and further optimizing the agreement by a proper choice of AR smaller than 0.55 and greater than 1.5 and Rth within the interval 0.5 and 0.8 µm. The ARs identified in this way cluster around the values 0.5 and 2.5. The comparison of the calculations with the measurements is presented and discussed. The results of this work help to set limits to the variability of the dimensions and asphericity of PSC solid particles, within the limits of applicability of our model based on the T-matrix theory of scattering and on assumptions on a common particle shape in a PSD and a common threshold radius for all the PSDs.

Polar Stratospheric Clouds: Satellite Observations, Processes, and Role in Ozone Depletion

AbstractPolar stratospheric clouds (PSCs) play important roles in stratospheric ozone depletion during winter and spring at high latitudes (e.g., the Antarctic ozone hole). PSC particles provide sites for heterogeneous reactions that convert stable chlorine reservoir species to radicals that destroy ozone catalytically. PSCs also prolong ozone depletion by delaying chlorine deactivation through the removal of gas‐phase HNO3 and H2O by sedimentation of large nitric acid trihydrate (NAT) and ice particles. Contemporary observations by the spaceborne instruments Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Microwave Limb Sounder (MLS), and Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) have provided an unprecedented polar vortex‐wide climatological view of PSC occurrence and composition in both hemispheres. These data have spurred advances in our understanding of PSC formation and related dynamical processes, especially the firm evidence of widespread heterogeneous nucleation of both NAT and ice PSC particles, perhaps on nuclei of meteoritic origin. Heterogeneous chlorine activation appears to be well understood. Reaction coefficients on/in liquid droplets have been measured accurately, and while uncertainties remain for reactions on solid NAT and ice particles, they are considered relatively unimportant since under most conditions chlorine activation occurs on/in liquid droplets. There have been notable advances in the ability of chemical transport and chemistry‐climate models to reproduce PSC temporal/spatial distributions and composition observed from space. Continued spaceborne PSC observations will facilitate further improvements in the representation of PSC processes in global models and enable more accurate projections of the evolution of polar ozone and the global ozone layer as climate changes.

The observation of nitric acid-containing particles in the tropical lower stratosphere

Abstract. Airborne in situ measurements over the eastern Pacific Ocean in January 2004 have revealed a new category of nitric acid (HNO3)-containing particles in the tropical lower stratosphere. These particles are most likely composed of nitric acid trihydrate (NAT). They were intermittently observed in a narrow layer above the tropopause (18±0.1 km) and over a broad geographic extent (>1100 km). In contrast to the background liquid sulfate aerosol, these particles are solid, much larger (1.7-4.7 µm vs. 0.1µm in diameter), and significantly less abundant (<10-4 cm-3 vs. 10 cm-3). Microphysical trajectory models suggest that the NAT particles grow over a 6-14 day period in supersaturated air that remains close to the tropical tropopause and might be a common feature in the tropics. The small number density of these particles implies a highly selective or slow nucleation process. Understanding the formation of solid NAT particles in the tropics could improve our understanding of stratospheric nucleation processes and, therefore, dehydration and denitrification.

Large nitric acid particles at the top of an Arctic stratospheric cloud

In early December 2001, balloon‐borne in situ measurements of particle composition, size, number, phase, and backscatter were completed in an Arctic stratospheric cloud composed of three distinct layers between 22 and 26 km. Below 24.5 km, liquid solution droplets of water, nitric acid, and sulfuric acid and a thin ice layer were observed. Above this layer the particles were primarily solid nitric acid trihydrate. Just above 26 km, at cloud top, where temperatures were near or above the equilibrium temperature for nitric acid trihydrate, there was a thin layer of solid particles narrowly distributed around a radius of 2.0 μm at concentrations of <0.001 cm−3. Lidar backscatter and particle phase measurements approximately 200 km upwind of the in situ measurements indicate a similar vertical structure for the cloud. These in situ measurements represent, to our knowledge, the most comprehensive in situ observations of all phases of polar stratospheric cloud particles, while the large particles at cloud top have not been previously observed and may have implications for producing particles large enough to remove reactive nitrogen from the polar stratosphere.

Observations of a distinctive infra‐red spectral feature in the atmospheric spectra of polar stratospheric clouds measured by the CRISTA instrument

Polar stratospheric cloud particles (PSCs) are known to strongly influence the infra‐red emission spectrum of the lower stratosphere in cold polar winters. The characteristics of these infra‐red features have been examined using limb sounding spectra recorded by the CRISTA experiment in August 1997 when many Antarctic PSCs were observed. A distinctive spectral feature centered at 820 cm−1 has been identified in many of these spectra, the first time that a particular band has been observed in the spectra of PSCs in the atmosphere. The feature can be attributed to the ν2 band of the NO3− ion and strongly suggests a condensed nitric acid component to the particles in the form of solid nitric acid trihydrate (NAT) or liquid ternary solution (STS). The spectral signatures belong to a definite sub‐set of the PSC observations recorded by CRISTA. The particles are observed at temperatures well above 192 K, allowing for temperature errors, and it is suggested that NAT particles are the most likely source of the spectral signature. In addition, it is shown that PSC events in which no spectral signature is present are only observed below 192 K and display a very similar HNO3‐temperature relationship to STS particles.

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.

Optical classification, existence temperatures, and coexistence of different polar stratospheric cloud types

Multispectral lidar measurements of polar stratospheric clouds (PSCs) from two winter campaigns in 1994/1995 and 1996'1997 at Sodankylä, Finland, have been evaluated together with temperature data from local radiosondes to find optical parameters for a PSC classification of different particle types and their existence temperatures. Precise depolarization measurements show that both solid and liquid particles exist below the NAT (nitric acid trihydrate) temperature. A comparison of temperatures at the PSC base and at the cloud top shows a good agreement with the NAT‐existence temperature for solid type Ia clouds and a 3–4 K lower temperature for liquid type Ib clouds. The two particle families are therefore consistent with solid NAT particle formation and condensational growth of HNO3, H2O and H2SO4 liquid ternary solutions. The coexistence of solid and liquid particles has been observed by means of the temporal development of parallel and perpendicular polarized lidar signals. These time series of subsequent lidar measurements show stronger and faster fluctuations in the liquid particle mode compared to the solid particles and thus indicate a higher sensitivity toward temperature fluctuations for the liquid PSCs. While the optical properties of most observations are consistent with the definition of PSC type Ia (solid) and type Ib (liquid) clouds, a third type has been observed which does not fit into the current type Ia/Ib optical classification. This cloud type consists of solid particles but has a higher backscatter than type Ia PSC.

On the lidar‐observed sandwich structure of polar stratospheric clouds (PSCs): 2. Numerical simulations of externally mixed PSC particles

Polar stratospheric clouds (PSCs) were simulated via a mixture of (1) liquid‐ternary‐solution particles consisting of H2SO4, HNO3, and H2O, and (2) solid nitric acid trihydrate (NAT) particles in a microphysical box model. The simulations indicated that the scattering ratio increases as the temperature decreases to near the frost point of ice without a concomitant increase in the depolarization. When the decrease in temperature was halted a few degrees above the frost point, the time‐dependent depolarization was subsequently observed to increase. This response was found when the total number of solid particles was in the finite range, 0.001–0.01/cm3. This number range agrees well with the (large size) particle number concentration observed via an optical particle counter.

Polar stratospheric clouds due to vapor enhancement: HALOE observations of the Antarctic vortex in 1993

Aerosol measurements from the Halogen Occultation Experiment (HALOE) during the Antarctic spring of 1993 are compared with calculations of the volume of different types of polar stratospheric clouds (PSCs) at equilibrium. The observed volumes increased by a factor of ∼30 coincident with water vapor enhancements of ∼3 ppmv, suggesting that the enhancement of water vapor was important in determining PSC growth. The enhanced water vapor was coincident with increased methane mixing ratios, and trajectory analysis suggests that the vapor enhancements were consistent with transport from lower latitudes. The nitric acid distribution was not measured and is treated either as constant or as positively correlated with the observed water. Comparing the observed volumes with model calculations assuming constant nitric acid suggests that the PSCs were composed of liquid ternary H2SO4‐H2O‐HNO3 aerosols (LTA) rather than solid nitric acid trihydrate (NAT). However, if the water vapor intrusions were accompanied by enhanced nitric acid, the observations closely match predictions for solid NAT, and those for LTA. These comparisons highlight the importance of the vapor distributions for modeling PSC growth and suggest that vapor resupply is important for late spring PSC growth. This work also highlights some inherent limitations of large field of view limb‐viewing instruments for the observation of PSCs.

Temperature histories in liquid and solid polar stratospheric cloud formation

Polar stratospheric clouds (PSCs) have been observed by balloonborne backscatter sondes from Alert, Thule, Heiss Island, Scoresbysund, Sodankylä, Søndre Strømfjord, and Ny Ålesund during winters 1989, 1990, 1995, and 1996 in 30 flights. The observations can be categorized into two main groups: type 1a and type 1b PSC particles. Type 1b PSCs show the characteristics expected from liquid ternary solution (HNO3/H2SO4/H2O) particles, consistent with model simulations. Type 1a PSCs are observed at all temperatures below the condensation temperature TNAT of nitric acid trihydrate (NAT), consistent with solid NAT composition. Air parcel trajectories have been calculated for all observations to provide synoptic temperature histories of the observed particles. A number of cases have been identified, where the particles have experienced temperatures close to or above the sulfuric acid tetrahydrate melting temperatures within 20 days prior to observation. This assures a knowledge of the physical phase (liquid) of the particles at this time, prior to observation. The subsequent synoptic temperature histories, between melting and the time of observation, show pronounced differences for type 1a and type 1b PSC particles, indicating the qualitative temperature conditions, necessary to generate solid type 1a PSCs. The temperature histories of type 1b particles show smoothly, in most cases monotonie, decreasing temperatures. The temperature can apparently decrease to the frost point without causing the particles to freeze. The type 1b PSC particles are mostly observed shortly after entering a cold region. The observed type 1a particles have spent several days at temperatures close to or below TNAT prior to observation, often associated with several synoptic temperature oscillations around TNAT, and the particles are observed in aged clouds. It appears that the PSC particles may freeze, if they experience synoptic temperatures below TNAT with a duration of at least 1 day, possibly accompanied by several temperature oscillations. However, liquid particles that experience a smooth cooling, even to very low temperatures, or single smooth cooling/heating below TNAT without synoptic temperature fluctuations do not seem to freeze.

On the theories of type 1 polar stratospheric cloud formation

Several mechanisms for the production of polar stratospheric cloud (PSC) particles are investigated using the classical theories of nucleation and freezing and the multicomponent condensation theory. These mechanisms invoke particle compositions ranging from binary (H2SO4/H2O) solution, solid sulfuric acid tetrahydrate (SAT) and ternary (HNO3/H2O/H2SO4) solution to binary (HNO3/H2O) solution and solid nitric acid trihydrate (NAT). Empirical relations, derived from classical nucleation studies, are used to calculate the surface energies required in calculations of nucleation and freezing. Using these data, we calculate that the nucleation of nitric acid solutions or solid phases onto SAT particles is not efficient. Homogeneous freezing of SAT or NAT from ternary solutions does not occur under stratospheric conditions. Homogeneous freezing of water ice can occur at temperatures near the frost point of pure water. Heterogeneous freezing is a strong function of the contact parameter between the emergent crystal and the initiating seed particle. Heterogeneous freezing of the stratospheric aerosol to SAT and NAT at temperatures above the frost point is not ruled out by our calculations. If formed, NAT can deplete the gas phase nitric acid concentration, by condensational growth, more efficiently than ternary droplets. We conclude that the most likely route to type 1 PSC particles is via condensational growth of ternary solution droplets followed by rapid freezing to NAT, SAT, and water ice at temperatures near the ice frost point. The particles formed are then stable and can reduce nitric acid vapor pressures to the saturation vapor pressure over NAT at all temperatures below the NAT point. Such a mechanism is consistent with observations.

Analysis of the physical state of one Arctic polar stratospheric cloud based on observations

During the Arctic Airborne Stratospheric Expedition, simultaneous measurements of aerosol size distribution and NOy, (HNO3 + NO + NO2 + 2·N2O5) were made along ER‐2 flight paths. The flow characteristics of the NOy instrument allow us to derive the condensed NOy amount (assumed to be HNO3) present during polar stratospheric cloud (PSC) events. Analysis of the January 24th flight indicates that this condensed HNO3 amount does not agree well with the aerosol volume if the observed PSCs are composed of solid nitric acid trihydrate (NAT), as is generally assumed. However, the composition agrees well with that predicted for liquid H2SO4/HNO3/H2O solution droplets using a new Aerosol Physical Chemistry Model (APCM). The agreement corresponds in detail to variations in temperature and humidity. The weight percentages of H2SO4, HNO3, and H2O derived from the measurements all correspond to those predicted for ternary, liquid solutions.

The lifetime of leewave‐induced ice particles in the Arctic stratosphere: II. Stabilization due to NAT‐coating

Large ice particles with radii of 1–5 µm and particle number densities up to 1 cm−3 were observed by balloonborne particle counters at altitudes 20–24 km about 250 km SE of Kiruna (northern Sweden). The particles were probably formed during strong adiabatic cooling in leewaves over the Norwegian mountains. At the time of measurement, temperatures were 5‒10 K above the frost‐point. Pure ice particles of this size would evaporate within a few minutes; however, these particles somehow survived the passage of 2‐3 hours from the Norwegian mountains to the Gulf of Bothnia. The slow evaporation could be explained if at some point the particles had acquired a coating of nitric acid trihydrate (NAT). In air cooling slowly the condensation of NAT will precede the condensation of ice. However, if the air cools very rapidly then nitric acid and ice could condense simultaneously, with the nitric acid embedded in the ice crystal in the form of NAT clusters. In particular large HNO3/H2O ratios in the solid phase would be reached if the formation of pure NAT during initial cooling is suppressed, e.g. when solid NAT nuclei are absent. During warming the preferential evaporation of water would expose the clusters, leaving eventually an almost complete surface layer of NAT. Model calculations suggest that for particles undergoing vertical movement in leewaves this could happen several times in succession, greatly prolonging their lifetime. Thus leewave cloud particles may persist much further downstream than has been previously thought.

Stratospheric nitric acid vapour measurements in the cold Arctic vortex: implications for nitric acid condensation

STRATOSPHERIC nitric acid condensation has recently been proposed1, 2 to take place in the cold winter polar vortex, priming it for chlorine-catalysed ozone destruction. This idea has been developed subsequently3–8. Condensation, thought to proceed via a heterogeneous heteromolecular mechanism involving stratospheric water vapour and pre-existing H2SO4/H2O aerosols, which serve as condensation nuclei, leads to solid nitric acid trihydrate (NAT; HNO3.3H2O) aerosols. It is expected1, 2, 8 that the condensation temperature, Tc(NAT), is significantly greater than that of water-ice, Tc(H2O-ice), given the stratospheric abundances of gaseous nitric acid and water and the thermodynamic properties of stratospheric aerosols8. Knowledge of Tc(NAT) is important because it determines the spatial and temporal extent of NAT aerosols and hence possibly of polar ozone destruction. Here we report in situ measurements of the detailed height distribution of gaseous nitric acid in the cold arctic vortex, using a balloon-borne technique9–16, which offers a much better altitude resolution (30 m) than previous satellite measurements17, 18. Our data set an upper limit to Tc(NAT) of 195 K at 23 km, and are consistent with model predictions based on the present gaseous nitric acid data, typical water vapour abundances and thermodynamic data8 of macroscopic NAT mixtures. However, our upper limit to Tc(NAT) is markedly lower than some of the early model estimates1, 2.