scholarly journals Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification

2002 ◽  
Vol 2 (3) ◽  
pp. 669-687 ◽  
Author(s):  
D. A. Knopf ◽  
T. Koop ◽  
B. P. Luo ◽  
U. G. Weers ◽  
T Peter
Keyword(s):  
Experimental Data ◽  
Nitric Acid ◽  
Nucleation Rate ◽  
Homogeneous Nucleation ◽  
Particle Production ◽  
Rate Coefficients ◽  
The Arctic ◽  
Linear Extrapolation ◽  
Acid Hydrate ◽  
Stratospheric Clouds

Abstract. The nucleation of NAD and NAT from HNO3/H2O and HNO3/H2O/H2SO4 solution droplets is investigated both theoretically and experimentally with respect to the formation of polar stratospheric clouds (PSCs). Our analysis shows that homogeneous NAD and NAT nucleation from liquid aerosols is insufficient to explain the number densities of large nitric acid containing particles recently observed in the Arctic stratosphere. This conclusion is based on new droplet freezing experiments employing optical microscopy combined with Raman spectroscopy. The homogeneous nucleation rate coefficients of NAD and NAT in liquid aerosols under polar stratospheric conditions derived from the experiments are < 2 x 10-5 cm-3 s-1 and < 8 x 10-2 cm-3 s-1 , respectively. These nucleation rate coefficients are smaller by orders of magnitude than the value of ~ 103 cm-3 s-1 used in a recent denitrification modelling study that is based on a linear extrapolation of laboratory nucleation data to stratospheric conditions (Tabazadeh et al., Science, 291, 2591--2594, 2001). We show that this linear extrapolation is in disagreement with thermodynamics and experimental data and, therefore, must not be used in microphysical models of PSCs. Our analysis of the experimental data yields maximum hourly production rates of nitric acid hydrate particles per cm3 of air of about 3 x 10-10 cm-3 h-1 under polar stratospheric conditions. Assuming PSC particle production to proceed at this rate for two months we arrive at particle number densities of < 5 x 10-7 cm-3, much smaller than the value of ~ 10-4 cm-3 reported in recent field observations. This clearly shows that homogeneous nucleation of NAD and NAT from liquid supercooled ternary solution aerosols cannot explain the observed polar denitrification.

scholarly journals Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification

2002 ◽  
Vol 2 (3) ◽  
pp. 207-214 ◽  
Author(s):  
D. A. Knopf ◽  
T. Koop ◽  
B. P. Luo ◽  
U. G. Weers ◽  
T. Peter
Keyword(s):  
Experimental Data ◽  
Nitric Acid ◽  
Nucleation Rate ◽  
Homogeneous Nucleation ◽  
Particle Production ◽  
Rate Coefficients ◽  
The Arctic ◽  
Linear Extrapolation ◽  
Acid Hydrate ◽  
Modelling Study

Abstract. The nucleation of NAD and NAT from HNO3/H2O and HNO3/H2SO4/H2O solution droplets is investigated both theoretically and experimentally with respect to the formation of polar stratospheric clouds (PSCs). Our analysis shows that homogeneous NAD and NAT nucleation from liquid aerosols is insufficient to explain the number densities of large nitric acid containing particles recently observed in the Arctic stratosphere. This conclusion is based on new droplet freezing experiments employing optical microscopy combined with Raman spectroscopy. The homogeneous nucleation rate coefficients of NAD and NAT in liquid aerosols under polar stratospheric conditions derived from the experiments are < 2 x 10-5 cm-3 s-1 and < 8 x 10-2 cm-3 s-1, respectively. These nucleation rate coefficients are smaller by orders of magnitude than the value of ~103 cm-3 s-1 used in a recent denitrification modelling study that is based on a linear extrapolation of laboratory nucleation data to stratospheric conditions (Tabazadeh et al., Science, 291, 2591--2594, 2001). We show that this linear extrapolation is in disagreement with thermodynamics and with experimental data and, therefore, must not be used in microphysical models of PSCs. Our analysis of the experimental data yields maximum hourly production rates of nitric acid hydrate particles per cm3 of air of about 3 x 10-10 cm-3 (air) h-1 under polar stratospheric conditions. Assuming PSC particle production to proceed at this rate for two months we arrive at particle number densities of < 5 x 10-7 cm-3, much smaller than the value of ~10-4 cm-3 reported in recent field observations. In addition, the nitric acid hydrate production rate inferred from our data is much smaller than that required to reproduce the observed denitrification in the modelling study mentioned above. This clearly shows that homogeneous nucleation of NAD and NAT from liquid supercooled ternary solution aerosols cannot explain the observed polar denitrification.

scholarly journals Uncertainties in modelling heterogeneous chemistry and Arctic ozone depletion in the winter 2009/2010

2013 ◽  
Vol 13 (8) ◽  
pp. 3909-3929 ◽  
Author(s):  
I. Wohltmann ◽  
T. Wegner ◽  
R. Müller ◽  
R. Lehmann ◽  
M. Rex ◽  
...  
Keyword(s):  
Nitric Acid ◽  
Reaction Rate ◽  
Transport Model ◽  
Particle Number Density ◽  
Rate Coefficients ◽  
The Arctic ◽  
Number Density ◽  
Ozone Loss ◽  
Mixing Ratios ◽  
The Impact

Abstract. Stratospheric chemistry and denitrification are simulated for the Arctic winter 2009/2010 with the Lagrangian Chemistry and Transport Model ATLAS. A number of sensitivity runs is used to explore the impact of uncertainties in chlorine activation and denitrification on the model results. In particular, the efficiency of chlorine activation on different types of liquid aerosol versus activation on nitric acid trihydrate clouds is examined. Additionally, the impact of changes in reaction rate coefficients, in the particle number density of polar stratospheric clouds, in supersaturation, temperature or the extent of denitrification is investigated. Results are compared to satellite measurements of MLS and ACE-FTS and to in-situ measurements onboard the Geophysica aircraft during the RECONCILE measurement campaign. It is shown that even large changes in the underlying assumptions have only a small impact on the modelled ozone loss, even though they can cause considerable differences in chemical evolution of other species and in denitrification. Differences in column ozone between the sensitivity runs stay below 10% at the end of the winter. Chlorine activation on liquid aerosols alone is able to explain the observed magnitude and morphology of the mixing ratios of active chlorine, reservoir gases and ozone. This is even true for binary aerosols (no uptake of HNO3 from the gas-phase allowed in the model). Differences in chlorine activation between sensitivity runs are within 30%. Current estimates of nitric acid trihydrate (NAT) number density and supersaturation imply that, at least for this winter, NAT clouds play a relatively small role compared to liquid clouds in chlorine activation. The change between different reaction rate coefficients for liquid or solid clouds has only a minor impact on ozone loss and chlorine activation in our sensitivity runs.

scholarly journals Nitric Acid Trihydrate (NAT) formation at low NAT supersaturation in Polar Stratospheric Clouds (PSCs)

2005 ◽  
Vol 5 (5) ◽  
pp. 1371-1380 ◽  
Author(s):  
C. Voigt ◽  
H. Schlager ◽  
B. P. Luo ◽  
A. Dörnbrack ◽  
A. Roiger ◽  
...  
Keyword(s):  
Nitric Acid ◽  
High Altitude ◽  
Equilibrium Temperature ◽  
In Situ Measurements ◽  
The Arctic ◽  
Polar Stratospheric Clouds ◽  
Research Aircraft ◽  
Stratospheric Clouds ◽  
Polar Ozone

Abstract. A PSC was detected on 6 February 2003 in the Arctic stratosphere by in-situ measurements onboard the high-altitude research aircraft Geophysica. Low number densities (~10-4cm-3) of small nitric acid (HNO3) containing particles (d<6µm) were observed at altitudes between 18 and 20km. Provided the temperatures remain below the NAT equilibrium temperature TNAT, these NAT particles have the potential to grow further and to remove HNO3 from the stratosphere, thereby enhancing polar ozone loss. Interestingly, the NAT particles formed in less than a day at temperatures just slightly below TNAT (T>TNAT-3.1K). This unique measurement of PSC formation at extremely low NAT saturation ratios (SNAT≤10) constrains current NAT nucleation theories. We suggest, that the NAT particles have formed heterogeneously, but for certain not on ice. Conversely, meteoritic particles may be favorable candidates for triggering NAT nucleation at the observed low number densities.

scholarly journals Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere during the cold winter 2015/2016

2021 ◽  
Author(s):  
Helmut Ziereis ◽  
Peter Hoor ◽  
Jens-Uwe Grooß ◽  
Andreas Zahn ◽  
Greta Stratmann ◽  
...  
Keyword(s):  
Nitric Acid ◽  
Arctic Region ◽  
The Arctic ◽  
Reactive Nitrogen ◽  
Winter Period ◽  
Air Masses ◽  
Heterogeneous Processes ◽  
Research Aircraft ◽  
Polar Stratosphere ◽  
Stratospheric Clouds

Abstract. During winter 2015/2016 the Arctic stratosphere was characterized by extraordinarily low temperatures in connection with the occurrence of extensive polar stratospheric clouds. From mid of December 2015 until mid of March 2016 the German research aircraft HALO (High Altitude and Long–Range Research Aircraft) was deployed to probe the lowermost stratosphere in the Arctic region within the POLSTRACC (Polar Stratosphere in a Changing Climate) mission. More than twenty flights have been conducted out of Kiruna/Sweden and Oberpfaffenhofen/Germany, covering the whole winter period. Besides total reactive nitrogen (NOy), observations of nitrous oxide, nitric acid, ozone and water were used for this study. Total reactive nitrogen and its partitioning between gas- and particle phase are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. The redistribution of total reactive nitrogen was evaluated by using tracer–tracer correlations. In January air masses with extensive nitrification were encountered at altitudes between 12 and 15 km. The excess NOy amounted up to about 6 ppb. During several flights, along with gas–phase nitrification, indications for extensive occurrence of nitric acid containing particles at flight altitude were found. These observations support the assumption of sedimentation and subsequent evaporation of nitric acid containing particles leading to redistribution of total reactive nitrogen. Remnants of nitrified air masses have been observed until mid of March. Between end of February and mid of March also de-nitrified air masses have been observed in connection with high potential temperatures. Using tracer–tracer correlations, missing total reactive nitrogen was estimated to amount up to 6 ppb. This indicates the downward transport of air masses that have been denitrified during the earlier winter phase. Observations within POLSTRACC, at the bottom of the vortex, reflect heterogeneous processes from the overlying Arctic winter stratosphere. The comparison of the observations with CLaMS model simulations confirm and complete the picture arising from the present measurements. The simulations confirm, that the ensemble of all observations is representative for the vortex–wide vertical NOy-redistribution.

scholarly journals Ice nucleation from aqueous NaCl droplets with and without marine diatoms

2011 ◽  
Vol 11 (3) ◽  
pp. 8291-8336 ◽  
Author(s):  
P. A. Alpert ◽  
J. Y. Aller ◽  
D. A. Knopf
Keyword(s):  
Surface Area ◽  
Water Activity ◽  
Nucleation Rate ◽  
Particle Production ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Rate Coefficients ◽  
Freezing Temperatures ◽  
Aqueous Volume ◽  
Heterogeneous Ice Nucleation

Abstract. Ice formation in the atmosphere by homogeneous and heterogeneous nucleation is one of the least understood processes in cloud microphysics and climate. Here we describe our investigation of the marine environment as a potential source of atmospheric IN by experimentally observing homogeneous ice nucleation from aqueous NaCl droplets and comparing against heterogeneous ice nucleation from aqueous NaCl droplets containing intact and fragmented diatoms. Homogeneous and heterogeneous ice nucleation are studied as a function of temperature and water activity, aw. Additional analyses are presented on the dependence of diatom surface area and aqueous volume on heterogeneous freezing temperatures, ice nucleation rates, ωhet, ice nucleation rate coefficients, Jhet, and differential and cumulative ice nuclei spectra, k(T) and K(T), respectively. Homogeneous freezing temperatures and corresponding nucleation rate coefficients are in agreement with the water activity based homogeneous ice nucleation theory within experimental and predictive uncertainties. Our results confirm, as predicted by classical nucleation theory, that a stochastic interpretation can be used to describe this nucleation process. Heterogeneous ice nucleation initiated by intact and fragmented diatoms can be adequately represented by a modified water activity based ice nucleation theory. A horizontal shift in water activity, Δaw, het = 0.2303, of the ice melting curve can describe median heterogeneous freezing temperatures. Individual freezing temperatures showed no dependence on available diatom surface area and aqueous volume. Determined at median diatom freezing temperatures for aw from 0.8 to 0.99, ωhet ~ 0.11+0.06−0.05 s−1, Jhet ~ 1.0+1.16−0.61 × 104 cm−2 s−1, and K ~ 6.2+3.5−4.1 × 104 cm−2. The experimentally derived ice nucleation rates and nuclei spectra allow us to estimate ice particle production which we subsequently use for a comparison with observed ice crystal concentrations typically found in cirrus and polar marine mixed-phase clouds. Differences in application of time-dependent and time-independent analyses to predict ice particle production are discussed.

scholarly journals Heterogeneous formation of polar stratospheric clouds – Part 1: Nucleation of nitric acid trihydrate (NAT)

2013 ◽  
Vol 13 (18) ◽  
pp. 9577-9595 ◽  
Author(s):  
C. R. Hoyle ◽  
I. Engel ◽  
B. P. Luo ◽  
M. C. Pitts ◽  
L. R. Poole ◽  
...  
Keyword(s):  
Nitric Acid ◽  
Companion Paper ◽  
Solid Particles ◽  
The Arctic ◽  
Orthogonal Polarization ◽  
Polar Stratospheric Clouds ◽  
Formation Pathway ◽  
Immersion Freezing ◽  
Heterogeneous Formation ◽  
Stratospheric Clouds

Abstract. Satellite-based observations during the Arctic winter of 2009/2010 provide firm evidence that, in contrast to the current understanding, the nucleation of nitric acid trihydrate (NAT) in the polar stratosphere does not only occur on preexisting ice particles. In order to explain the NAT clouds observed over the Arctic in mid-December 2009, a heterogeneous nucleation mechanism is required, occurring via immersion freezing on the surface of solid particles, likely of meteoritic origin. For the first time, a detailed microphysical modelling of this NAT formation pathway has been carried out. Heterogeneous NAT formation was calculated along more than sixty thousand trajectories, ending at Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) observation points. Comparing the optical properties of the modelled NAT with these observations enabled a thorough validation of a newly developed NAT nucleation parameterisation, which has been built into the Zurich Optical and Microphysical box Model (ZOMM). The parameterisation is based on active site theory, is simple to implement in models and provides substantial advantages over previous approaches which involved a constant rate of NAT nucleation in a given volume of air. It is shown that the new method is capable of reproducing observed polar stratospheric clouds (PSCs) very well, despite the varied conditions experienced by air parcels travelling along the different trajectories. In a companion paper, ZOMM is applied to a later period of the winter, when ice PSCs are also present, and it is shown that the observed PSCs are also represented extremely well under these conditions.

scholarly journals Ice nucleation from aqueous NaCl droplets with and without marine diatoms

2011 ◽  
Vol 11 (12) ◽  
pp. 5539-5555 ◽  
Author(s):  
P. A. Alpert ◽  
J. Y. Aller ◽  
D. A. Knopf
Keyword(s):  
Surface Area ◽  
Water Activity ◽  
Nucleation Rate ◽  
Particle Production ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Rate Coefficients ◽  
Freezing Temperatures ◽  
Aqueous Volume ◽  
Heterogeneous Ice Nucleation

Abstract. Ice formation in the atmosphere by homogeneous and heterogeneous nucleation is one of the least understood processes in cloud microphysics and climate. Here we describe our investigation of the marine environment as a potential source of atmospheric IN by experimentally observing homogeneous ice nucleation from aqueous NaCl droplets and comparing against heterogeneous ice nucleation from aqueous NaCl droplets containing intact and fragmented diatoms. Homogeneous and heterogeneous ice nucleation are studied as a function of temperature and water activity, aw. Additional analyses are presented on the dependence of diatom surface area and aqueous volume on heterogeneous freezing temperatures, ice nucleation rates, ωhet, ice nucleation rate coefficients, Jhet, and differential and cumulative ice nuclei spectra, k(T) and K(T), respectively. Homogeneous freezing temperatures and corresponding nucleation rate coefficients are in agreement with the water activity based homogeneous ice nucleation theory within experimental and predictive uncertainties. Our results confirm, as predicted by classical nucleation theory, that a stochastic interpretation can be used to describe the homogeneous ice nucleation process. Heterogeneous ice nucleation initiated by intact and fragmented diatoms can be adequately represented by a modified water activity based ice nucleation theory. A horizontal shift in water activity, Δaw, het = 0.2303, of the ice melting curve can describe median heterogeneous freezing temperatures. Individual freezing temperatures showed no dependence on available diatom surface area and aqueous volume. Determined at median diatom freezing temperatures for aw from 0.8 to 0.99, ωhet~0.11+0.06−0.05 s−1, Jhet~1.0+1.16−0.61×104 cm−2 s−1, and K~6.2+3.5−4.1 ×104 cm−2. The experimentally derived ice nucleation rates and nuclei spectra allow us to estimate ice particle production which we subsequently use for a comparison with observed ice crystal concentrations typically found in cirrus and polar marine mixed-phase clouds. Differences in application of time-dependent and time-independent analyses to predict ice particle production are discussed.

scholarly journals Factors controlling Arctic denitrification in cold winters of the 1990s

2003 ◽  
Vol 3 (2) ◽  
pp. 403-416 ◽  
Author(s):  
G.W. Mann ◽  
S. Davies ◽  
K. S. Carslaw ◽  
M. P. Chipperfield
Keyword(s):  
Nitric Acid ◽  
Nucleation Rate ◽  
The Other ◽  
The Arctic ◽  
Optimum Conditions ◽  
Flow Area ◽  
Closed Loops ◽  
Particle Properties ◽  
Sensitivity Studies ◽  
High Degree

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.

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