Trajectory studies of large HNO
3
-containing PSC particles in
the Arctic: Evidence for the role of NAT
K. A. McKinney,
1,2
P. O. Wennberg,
1
S. Dhaniyala,
1,3
D. W. Fahey,
4
M. J. Northway,
4
K. F. Ku
̈nzi,
5
A. Kleinbo
̈hl,
5
M. Sinnhuber,
5
H. Ku
̈
llmann
,
5
H. Bremer,
5
M. J. Mahoney,
6
and
T. P. Bui
7
Received 15 August 2003; revised 9 January 2004; accepted 23 January 2004; published 6 March 2004.
[
1
] Large (5 to >20
m
m diameter) nitric-acid-containing
polar stratospheric cloud (PSC) particles were observed in
the Arctic stratosphere during the winter of 1999–2000. We
use a particle growth and sedimentation model to
investigate the environment in which these particles grew
and the likely phase of the largest particles. Particle
trajectory calculations show that, while simulated nitric
acid dihydrate (NAD) particle sizes are significantly smaller
than the observed maximum particle sizes, nitric acid
trihydrate (NAT) particle trajectories are consistent with the
largest observed particle sizes.
I
NDEX
T
ERMS
:
0305
Atmospheric Composition and Structure: Aerosols and particles
(0345, 4801); 0320 Atmospheric Composition and Structure:
Cloud physics and chemistry; 0340 Atmospheric Composition and
Structure: Middle atmosphere—composition and chemistry.
Citation:
McKinney, K. A., et al. (2004), Trajectory studies of
large HNO
3
-containing PSC particles in the Arctic: Evidence for
the role of NAT,
Geophys. Res. Lett.
,
31
, L05110, doi:10.1029/
2003GL018430.
1. Introduction
[
2
] The existence of polar stratospheric clouds (PSCs) in
the high latitude stratosphere in winter is essential for pro-
ducing the Antarctic ozone hole and the large loss of ozone in
some Arctic springtimes [
WMO
, 2002]. Many details of the
cloud formation processes and composition remain unre-
solved, particularly predictive knowledge of PSC particle
phases and size distributions in vortex air parcels [
WMO
,
2002]. Identification of the phases is necessary to develop a
quantitative understanding of polar ozone loss because par-
ticle phase determines the growth and evaporation rates of the
particles and therefore the rate of denitrification.
[
3
] PSC particles larger than several microns in diameter
achieve significant fall speeds, resulting in denitrification,
i.e., the irreversible redistribution of HNO
3
from higher to
lower altitudes [
Fahey et al.
, 1990]. Liquid cloud droplets
consisting of supercooled ternary solutions (STS) of H
2
SO
4
,
HNO
3
, and H
2
O do not affect significant denitrification due
to their small sizes [
WMO
, 2002]. However, nucleation of a
small number of solid particles with a vapor pressure lower
than STS can result in the transfer of a large fraction of the
total available nitric acid to just a few particles, which can
then grow to sizes large enough for denitrification to occur
[
Salawitch et al.
, 1989;
Toon et al.
, 1990]. Solid cloud
particles may be composed of HNO
3
.
3H
2
O (NAT), the
most thermodynamically stable phase under stratospheric
conditions [
Hanson and Mauersberger
, 1988], or metasta-
ble HNO
3
hydrates, particularly HNO
3
.
2H
2
O (NAD),
which may nucleate more easily than NAT in the strato-
sphere [
Worsnop et al.
, 1993].
[
4
] During the SAGE III Ozone Loss and Validation
Experiment (SOLVE) in the winter of 2000, instruments
aboard the NASA ER-2 detected low number densities of
large(5to>20
m
m) HNO
3
-containing cloud particles
throughout much of the Arctic vortex [
Fahey et al.
, 2001;
Northway et al.
, 2002a]. This was the first observation of
PSC particles large enough to explain extensive vortex-wide
denitrification [
Northway et al.
, 2002b]. The large sizes
imply that the particles were likely solids, most probably
NAT or NAD [
Fahey et al.
, 2001]. For particles of this size
to exist at the ER-2 flight altitudes (
20 km), they must
have grown for several days prior to the observations
[
Fahey et al.
, 2001;
Carslaw et al.
, 2002].
[
5
] In this study we model the growth and sedimentation
of solid particles at and above the ER-2 flight level under
various assumptions about their phase. The results show
that in January and early February of 2000, the largest
particles carrying HNO
3
through the lower stratosphere
could not be composed of NAD. In contrast, the observa-
tions can be explained adequately if the particles were NAT.
2. Model Description
[
6
] A standard particle growth model [
Dhaniyala et al.
,
2002] is used to calculate particle size and altitude as a
function of time. Changes in ambient pressure (due to
particle sedimentation) and temperature (based on air parcel
histories) are taken into account. HNO
3
vapor pressures
above NAT, NAD, and STS are calculated using relation-
ships derived by
Hanson and Mauersberger
[1988],
Worsnop et al.
[1993], and
Carslaw et al.
[1995]. In all
calculations a water vapor mixing ratio of 5 ppmv and a
total H
2
SO
4
mixing ratio of 0.5 ppbv are used.
[
7
] Temperature fields (vs.
Q
and time) are derived from
NMC data using the GSFC trajectory model [
Schoeberl and
GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L05110, doi:10.1029/2003GL018430, 2004
1
Division of Geological and Planetary Sciences, California Institute of
Technology, Pasadena, California, USA.
2
Now at Department of Chemistry, Amherst College, Amherst,
Massachusetts, USA.
3
Now at Department of Mechanical and Aeronautical Engineering,
Clarkson University, Potsdam, New York, USA.
4
NOAA Aeronomy Laboratory, Boulder, Colorado, USA.
5
Institute of Environmental Physics, University of Bremen, Bremen,
Germany.
6
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
7
NASA Ames Research Center, Moffett Field, California, USA.
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L05110
1of4
Sparling
, 1994]. 10-day back trajectories (henceforth air
parcel histories) were initialized at locations every
1500 s
along the ER-2 flight track at 10 K intervals between 350
and 600 K. Based on a comparison between the GSFC
temperatures and those measured by the ER-2 Microwave
Temperature Profiler [
Denning et al.
, 1989], we have
adjusted the air parcel temperatures by
1.25 K. Diver-
gence of a set of air parcels due to wind shear was neglected
for the 4 to 6 day particle trajectories, i.e., the parcels
initialized at and above the ER-2 were treated as a column
in which a particle resided for its entire history. This
approach differs from that used in studies by
Tabazadeh
et al.
[2001] and
Drdla et al.
[2003] who obtain vertical
temperature profiles at the locations of an isentropic trajec-
tory on one theta level. It also differs from that used by
Fahey et al.
[2001] and
Carslaw et al.
[2002], where
particle back trajectories were calculated in a 3-D model.
[
8
] Altitude profiles of gas-phase HNO
3
were obtained
by the ASUR instrument [
von Ko
̈nig et al.
, 2000] on three
DC-8 deployments spanning December 1999, January and
March 2000. For each period, profiles within the polar
vortex (using the Nash criterion for the vortex edge [
Nash
et al.
, 1996]), were averaged together to obtain a single
HNO
3
altitude profile (1
s
25%, see supplementary
material, Figure 4
1
). For January, a mean profile was also
obtained from measurements at positions inside the vortex
without PSC coverage to estimate the amount of HNO
3
that
would be available without uptake onto PSCs and denitri-
fication, and to serve as a basis for sensitivity studies.
3. Constraints on Particle Growth
[
9
] Gas-phase HNO
3
never fully equilibrates with the
rapidly sedimenting solid particles. As a result, kinetic
processes controlling particle growth must be explicitly
included to adequately model the growth and sedimentation
of these particles.
[
10
] The presence of liquid STS particles creates a
‘growth window’ for the solid phases, analogous to the
‘nucleation window’ described by
Tabazadeh et al.
[2001].
STS droplets take up significant HNO
3
at low temperatures
(T
193 K), and dominate the total particle surface area.
Hence they equilibrate with the gas phase rapidly (
hours),
and the vapor pressure over STS limits solid particle growth
rates at the lowest temperatures. As a result, the NAT particle
growth rate is significant only over a temperature range of
about 6 K below the NATexistence temperature (T
NAT
); NAD
growth is limited to an even narrower range. As shown in
Figure 1, the NAD growth rates and temperature range are
smaller at all altitudes than are those for NAT. Significant
denitrification occurred during December and January 2000
[
Popp et al.
, 2001;
Kleinbo
̈hl et al.
, 2002], resulting in a
decrease in the HNO
3
mixing ratio, and therefore in the
temperature range and maximum growth rate.
[
11
] Above
26 km, temperatures in the vortex typically
increase with altitude, finally making the existence of PSCs
impossible. Between January 1 and February 3, 2000, the
maximum altitude where nitric acid hydrate particles could
form was 30 km. Upper limits on the sizes of particles
that can grow in this environment can be calculated by
assuming that the particles originate at 28 km and experi-
ence the peak growth rate at all altitudes (Figure 1). For the
December HNO
3
profile, the maximum particle size at
20 km is
22.5
m
m for NAT and
19.5
m
m for NAD. In
January these drop to 20.5 and 17.5, respectively.
4. Particle Trajectory Calculations
[
12
] To test whether the simulated particle sizes are
consistent with the SOLVE ER-2 observations, particle
diameter and altitude are traced backwards in time (i.e.,
from a final size and location) by integrating the growth and
sedimentation equations and accounting for changing strato-
spheric conditions as described in Section 2. For each ER-2
flight segment, the back trajectories of 2 to 26
m
m diameter
particles were calculated assuming either NAT or NAD
composition.
[
13
] Figure 2 shows an example of particle trajectories for
one segment of the 31 January 2000 flight. Two types of
trajectories are observed. In a ‘‘successful’’ trajectory, the
particle remains below the NAT (NAD) existence temper-
ature during most of its history, and can be traced from its
final size and position to the point at which its diameter was
near zero. ‘‘Unsuccessful’’ trajectories, on the other hand,
spend insufficient time in the growth window and cannot be
traced to an origin. These trajectories generally require that
the particles be unrealistically large (>40
m
m) at some time,
or that they enter the model domain (32 km) already at
significant size – clearly unrealistic as temperatures above
this altitude are too warm for PSCs.
[
14
] Typically, all trajectories up to a certain maximum
size are successful because particles smaller than the
maximum require shorter growth times and/or less cold
temperatures. Thus, for each flight segment and phase we
can estimate the maximum particle size. The maximum
Figure 1.
Growth rate (color scale) for a 10
m
m NAT (top)
or NAD (bottom) particle as a function of temperature and
altitude calculated using the January average HNO
3
profile.
Also shown are the mean temperature profile obtained by the
MTP instrument during the 20 January 2000 ER-2 flight
(green line), the mean temperature profile from the air parcel
back trajectory data for the same time period (black line),
and the ice formation temperature (heavy blue line). The
dashed lines show the maximum growth rate vs. altitude.
1
Auxiliary material is available at ftp://ftp.agu.org/apend/gl/
2003GL018430.
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NAT particle size is always larger than the maximum NAD
particle size. In the case shown in Figure 2, where particles
with a maximum size between 16 and 21
m
m were observed,
NAT particles up to 16
m
m could have been present, whereas
NAD particles of only 6
m
m diameter are achievable.
5. Comparison With Observations
[
15
] During SOLVE, three instruments on the NASA
ER-2 detected large HNO
3
-containing particles between
16 and 21 km in the Arctic vortex. The NOAA Aeronomy
Laboratory NOy instrument front inlet channel detected
individual large particles as short duration pulses of NOy
[
Fahey et al.
, 2001;
Northway et al.
, 2002a]. The integrated
amount of NOy in each pulse, minus the amount in the gas
phase and small particles as measured in the rear channel,
can be related to the mass, and therefore diameter, of the
evaporated particle (assuming spherical particles of a known
density and phase) [
Northway et al.
, 2002a]. Similarly, the
Caltech Chemical Ionization Mass Spectrometer (CIMS)
detected short HNO
3
pulses after evaporation of these large
particles in its inlet, verifying that the NOy content of the
large particles was indeed nitric acid. The ER-2 MASP
instrument also measured the number size distributions of
the large particles [
Brooks et al.
, 2003].
[
16
] Large nitric acid containing particles were detected
over significant portions of the flights of 20 and 31 January
and 3 February 2000. We estimate the sizes of the largest
particles observed in each flight segment using an NOy data
analysis approach similar to that described in
Northway et
al.
[2002a] and compare the observed maximum sizes to
those predicted by the back trajectory calculations described
in Section 4.
[
17
] The maximum particle sizes derived from the data
are shown with those from the trajectory calculations in
Figure 3. The January HNO
3
profile, believed to be repre-
sentative of the conditions in the Arctic vortex during the
observation period, is used in the base cases. Simulations
were also performed using the HNO
3
profile obtained from
locations without PSCs as an upper limit, and the vortex-
average January profile
25% as a lower limit. These
profiles range from
5.5 to 10.5 ppbv at the peak of the
HNO
3
profile near 20 km. Maximum size calculations were
performed for the base temperature ±1.5 K, typically
resulting in differences in particle size of <2
m
m. In some
cases, the calculated maximum size actually decreases with
decreasing temperature, as uptake on STS limits gas phase
HNO
3
. Uncertainties in the measured vapor pressures
(
20%) [
Hanson and Mauersberger
, 1988;
Worsnop et
al.
, 1993;
Carslaw et al.
, 1995] result in differences in the
maximum calculated diameters of 1–2
m
m (not included in
the figure.)
[
18
] To test the validity of the temperature fields derived
using the air parcel column assumption, particle back
trajectories were calculated stepwise in one day intervals
for selected flight segments (i.e., using initial air parcel
Figure 2.
Particle back trajectories for NAT (left) and
NAD (right) particles originating at the altitude of the ER-2
between 39040 and 40500 UTS on 31 January 2000. The
temperature fields derived from isentropic air parcel
histories are shown in color scale. Trajectories for final
particle sizes of 2–26
m
m are shown. Solid lines denote
successful and dashed lines unsuccessful trajectories. NAT
particles up to 16
m
m and NAD particles up to 6
m
min
diameter are predicted for these conditions.
Figure 3.
Comparison of observed (red) and calculated
maximum particle sizes assuming NAT (blue) and NAD
(cyan) for three ER-2 flights. The black bar shows the
latitude range covered in each flight. Data is binned vs.
time; the horizontal bars represent the range of latitudes
sampled during each time bin. For the simulations, the
January HNO
3
profile is used to determine the most likely
maximum particle size (markers). The vertical bars
represent the range in maximum particle sizes under the
various total HNO
3
and temperature conditions discussed in
the text. For the observations, the vertical bars represent the
range in possible maximum particle mass obtained by
calculating the net NOy signal due to particles >2
m
m and
correcting the particle signal heights using the average
response function described in
Northway et al.
[2002a] to
find an upper limit for the maximum particle size. To
estimate a lower limit, we assume that due to sampling of
multiple particles in a single time bin, the mass of the largest
particles could be a factor of 2 lower than the upper limit.
The diameter range shown includes the difference in particle
sizes determined assuming NAT (upper limit) or NAD
(lower limit) particle composition, which differ by
10%.
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histories, one day particle trajectories were calculated, then
new air parcel histories were calculated from the new
particle location, and this process was iterated for the
lifetime of the particle.) The resulting maximum particle
sizes were always within the range of sizes determined
using the column assumption and ±1.5 K temperature
uncertainty.
[
19
] At latitudes polewards of 73
N, the calculated max-
imum NAT particle sizes are similar to the observed
maximum particle sizes. In contrast, the calculated NAD
particle sizes are smaller than those observed. At the edge of
the vortex, where temperatures are near the NAT existence
temperature and air parcel histories are less certain, the
predicted maximum size ranges for NAD do sometimes
overlap the observed maximum particle sizes. Hence we can
differentiate between the phases with less certainty. Even so,
the maximum sizes are better reproduced by assuming NAT
rather than NAD, particularly on 20000131.
6. Discussion and Conclusions
[
20
] These results indicate that unless the largest particles
are composed of a crystalline phase not considered here,
they most likely either nucleated as NAT, or if they
nucleated as NAD, they likely converted to NAT shortly
after nucleation. We calculate that for a particle to grow to
20
m
m at 20 km, it must have been NAT for at least the last
2 days of its 6–7 day trajectory. This analysis provides
some of the strongest evidence to date for the existence of
NAT among the large, denitrifying PSC particles observed
in the Arctic during SOLVE. The remainder of the particles
in the large mode experienced conditions similar to the
largest particles, suggesting that all the particles in this
mode may be composed of NAT. Assuming a realistic size
distribution (D = 14.5
m
m,
s
= 2.45 see
Fahey et al.
[2001]),
particles
18
m
m (i.e., NAT particles) account for 25% of
the total HNO
3
flux.
[
21
] In other recent studies
Carslaw et al.
[2002] calcu-
lated particle forward trajectories in a 3-D model and
compared the resulting NAT and NAD size distributions
to the observations. Because the size distributions overlap
one another substantially, they could not exclude NAD. In
their study, HNO
3
uptake on STS was not explicitly
included. Here we are able to differentiate between the
two phases by including STS and focusing only on the
largest particles.
Drdla et al.
[2003] used a column model
along trajectories to trace PSC evolution, comparing the
results with observed PSC extent, denitrification, and de-
hydration. They concluded that, although they could not
rule out NAD, the NAT simulations agreed more favorably
with the measurements. Our conclusions are consistent with
these earlier studies.
[
22
] For large particles to be present, solid particle
nucleation (by an undetermined mechanism) must occur at
the start of the trajectory. The simulations presented here are
independent of the nucleation mechanism. Hence, it is
possible to find conditions under which the growth of large
particles is predicted, but none are observed because nucle-
ation did not occur. It is therefore interesting to note that for
almost all intervals during these flights when the calculated
trajectories were successful large particles were also
observed.
[
23
]
Acknowledgments.
Thanks to Drs. L. Lait, M. Schoeberl, and
P. A. Newman of the Atmospheric Chemistry and Dynamics branch at
NASA GSFC for use of the Goddard Automailer. This work was supported
by NSF Grant No. ATM9871353 and NASA Grant No. NAG5-8922. Work
at JPL, California Institute of Technology, was carried out under contract
with NASA.
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