Recent analysis of long-term balloon-borne measurements of Antarctic stratospheric condensation nuclei (CN) between July and October showed the formation of a volatile CN layer at 21–27 km altitude in a background of existing particles. We use the nucleation model SAWNUC to simulate these CN in subsiding air parcels and study their nucleation and coagulation characteristics. Our simulations confirm recent analysis that the development of the CN layer can be explained with neutral sulfuric acid–water nucleation and we show that outside the CN layer the measured CN concentrations are well reproduced just considering coagulation and the subsidence of the air parcels. While ion-induced nucleation is expected as the dominating formation process at higher temperatures, it does not play a significant role during the CN layer formation as the charged clusters recombine too fast. Further, we derive sulfuric acid concentrations for the CN layer formation. Our concentrations are about 1 order of magnitude higher than previously presented concentrations as our simulations consider that nucleated clusters have to grow to CN size and can coagulate with preexisting particles. Finally, we calculate threshold sulfuric acid profiles that show which concentration of sulfuric acid is necessary for nucleation and growth to observable size. These threshold profiles should represent upper limits of the actual sulfuric acid outside the CN layer. According to our profiles, sulfuric acid concentrations seem to be below midlatitude average during Antarctic winter but above midlatitude average for the CN layer formation.
Atmospheric aerosol particles are of interest due to their various influences
on radiation, clouds, chemistry, and air quality. Condensation nuclei
counters measure the number concentration of aerosol particles by growing
them to optically detectable sizes by condensation (e.g., McMurry, 2000).
Therefore, condensation nuclei (CN) are defined as all aerosol particles that
are large enough to be measured by a CN counter operating at a given
supersaturation, which typically can measure particles with diameters larger
than
Contrary to this expectation, Rosen and Hofmann (1983) first observed an
increase in volatile CN at 25–30 km altitude during winter at Laramie,
Wyoming (41
Based on these observations, modeling studies began to investigate the
formation of the CN layer. Hamill et al. (1990) calculated nucleation rates
indicating that binary nucleation could occur in the polar winter
stratosphere if sulfuric acid concentrations were high enough. Zhao et
al. (1995) developed a one-dimensional (altitude) aerosol model that showed
that the transformation of carbonyl sulfide (OCS) to SO
In summary, and contrary to the initial expectation, nucleation seems to
occur in the polar stratosphere. During polar winter, more SO
More recently, Campbell and Deshler (2014) presented a long-term record of
stratospheric balloon-borne CN measurements that were performed 2–3 times a
year during winter from 1986 until 2010 above McMurdo Station, Antarctica
(78
Campbell and Deshler (2014) describe a method where they derive an Antarctic sulfuric acid profile from the measured CN by inverting the neutral binary nucleation equation. They used the difference between the CN before sunrise and two weeks after sunrise averaged over all years to derive a nucleation rate for all altitudes from which they derived the corresponding sulfuric acid. This profile is useful, for example, for evaluating global models (Campbell et al., 2014) as no Antarctic sulfuric acid measurements exist. However, Campbell and Deshler (2014) and Campbell et al. (2014) also note that their derived profile might be an underestimation as their method does not consider the particles smaller than their experimental CN detection threshold particle size, losses to preexisting particles, and ion-induced nucleation.
We find this approach of deriving a sulfuric acid profile from the measured CN intriguing. Here we use the nucleation model SAWNUC that simulates small particles, ion-induced nucleation, coagulation, and losses to preexisting particles. We model the Antarctic CN layer based on the observations of Campbell and Deshler (2014) and derive Antarctic stratospheric sulfuric acid profiles.
Temperatures
The SAWNUC (Sulfuric Acid Water NUCleation; Lovejoy et al., 2004) model simulates binary sulfuric acid water neutral and ion-induced nucleation. SAWNUC uses thermodynamic stabilities that are based on experimental values and quantum chemical calculations (Lovejoy and Curtius, 2001; Froyd and Lovejoy, 2003a, b; Hanson and Lovejoy, 2006), and it explicitly simulates step-by-step addition of sulfuric acid molecules in linear size bins for cluster sizes below 2 nm. Above 2 nm particle concentrations are collected in geometric size bins. Here we simulate 30 geometric size bins with a scale factor of 1.7, ranging up to about 400 nm for neutral and negatively charged clusters. For each size bin, SAWNUC can simulate condensation and evaporation of sulfuric acid, coagulation with neutral clusters, recombination of negative clusters with positive ions, and losses to preexisting particles. SAWNUC has been previously described and used (among others) by Lovejoy et al. (2004), Ehrhart and Curtius (2013), Kürten et al. (2015), Ehrhart et al. (2016), and its parameterized version, PARNUC (Kazil and Lovejoy, 2007), is used in Kirkby et al. (2011).
For this study, we extended the SAWNUC model. As in Kürten et al. (2015), coagulation rates between neutral clusters are now calculated including van der Waals forces according to Chan and Mozurkewich (2001). We redesigned the model code to allow for changes in ambient conditions during a simulation and added the ability to perform multiple simulations within one program run. We do not use SAWNUC's procedure to represent losses to preexisting particles by a single surface area loss term, but instead we now fully simulate preexisting particles as initial particle concentrations. For this study, the basic processes simulated by SAWNUC are condensation and evaporation of sulfuric acid and coagulation for every size bin. Condensation and evaporation of sulfuric acid are the dominating processes for the formation of new particles, while coagulation and condensation of sulfuric acid, if present, determine growth and number reduction of existing particles.
To perform a regular SAWNUC simulation for a given region of the Antarctic stratosphere, the temperature, pressure, ion pair production rate, relative humidity, and sulfuric acid concentration are required. Particle concentrations and sizes are the model output at every time step. When inverting SAWNUC, the particle concentrations are required to derive the sulfuric acid concentrations.
Temperatures above Antarctica are taken from Campbell and Deshler (2014), and
those temperatures that are below 190 K (maximum 5 K below), which is
SAWNUC's lower limit of the temperature range, are fixed at 190 K. This
introduces some uncertainty which is estimated in our sensitivity test of a
5 K temperature increase (Sect. 3.3). Altitudes are converted to pressures
according to the global modeling of Campbell et al. (2014). The ionization
rate of the Antarctic stratosphere in August–September 2010 was
CN concentrations are taken from Campbell and Deshler (2014). The measured CN are then compared with the simulated CN by summing over all simulated particles with diameters above 20 nm, as Campbell and Deshler (2014) reported a detection limit of their CN counters of 6–20 nm diameter. As we do not know the exact size of the measured CN, we assume the initial preexisting CN to have a diameter of 100 nm (see below), but we also perform sensitivity studies assuming different sizes in Sect. 3.3. We simulate them as pure sulfuric acid–water particles but as temperatures are too low for significant evaporation, they could also include a nonvolatile core.
Air parcel subsidence trajectories
CN layer gaseous sulfuric acid profiles
We start our simulations with a simplified
The nucleation threshold sulfuric acid profiles are shown in Fig. 1b. Their shapes are similar to the temperature profiles because temperature, sulfuric acid, and losses to preexisting particles mainly determine the nucleation rate. As the preexisting particle concentrations are the same and we target almost the same nucleation rate everywhere, the temperature determines the derived sulfuric acid concentration, and the nucleation threshold profiles consequently increase with increasing temperature. However, the water vapor concentration also has a small influence on the derived profiles as can be seen, for example, in July below 27 km, where the temperature is fixed to 190 K but the derived profile still varies slightly.
Combination of the nucleation threshold sulfuric acid profiles from Fig. 1b (solid) and the CN layer sulfuric acid profiles from Fig. 3a (dashed). Additionally, we show sulfuric acid profiles that cause a CN increase in our CN simulation of Fig. 3b (dotted) which should represent upper limits of the Antarctic winter stratospheric sulfuric acid outside the CN layer.
We continue by studying how the measured CN of Campbell and Deshler (2014)
coagulate outside the CN layer. Therefore, we drop the assumption of
10 CN cm
Figure 2b shows the simulated CN without sulfuric acid being present and
therefore no nucleation. The uncertainty ranges of the measured CN from
Campbell and Deshler (2014) are shown for comparison (
In September, above the CN layer at
It is important to understand how much sulfuric acid is necessary to form the CN layer, and thus reproduce the observations. Using the same method as before, we now derive the amount of sulfuric acid needed to match the simulated and measured CN. This sulfuric acid causes nucleation of new particles, growth of these new particles to CN size, and growth of existing CN.
Figure 3a shows the sulfuric acid profiles that are necessary to form the CN
layer and reproduce the observations (termed “
For a complete interpretation of the processes in the CN layer we combine our
nucleation threshold profiles and CN layer profiles in Fig. 4 (solid and
dashed). Additionally, we derive the sulfuric acid concentrations that lead
to a CN increase in our simulation of the observed CN (Fig. 3b) and include
them in Fig. 4 (dotted). We use the same method as for our nucleation
threshold profiles (deriving the amount of sulfuric acid that leads to a
10 % CN increase), but now with the simulated CN as background. Note that
outside of the CN layer, these profiles represent only
In July, August, and September, the upper limit profiles (dotted) show the
sulfuric acid that is necessary for nucleation and growth to CN size and
leads to additional 10 % CN within 1 month. The concentrations are higher
than our nucleation threshold profiles (solid) because we have a higher
concentration of preexisting CN compared to the 10 cm
In October in the area of the CN layer, however, the upper limit profile (dotted) and the CN layer formation profile (dashed) are both lower than the nucleation profile (solid), showing that no new particles have to nucleate. Instead, small particles that still exist from the nucleation event in September can grow above the CN counting threshold, which requires less sulfuric acid than nucleation and growth of new CN. Therefore, the history of the nucleation event in September allows for a CN increase without new particle formation in October.
In the following sensitivity studies we show and discuss only the nucleation threshold and the CN layer profiles to avoid overloaded figures, but the conclusions for the upper limit profiles are analogous to the other profiles.
Comparison of the nucleation threshold sulfuric acid profiles derived including ion-induced nucleation (solid lines) and without simulating ions (dotted lines). At low sulfuric acid concentrations the derived profiles do not change. The CN layer profiles also hardly change (thick dashed lines; grey and light green are without ions and black and green are with ions, but they are almost identical).
Sensitivity studies varying
Comparison of our derived Antarctic sulfuric acid profiles (nucleation threshold: solid; CN layer: long dashed) with the derived profile from Campbell and Deshler (2014) (dark red, short dashed) and midlatitude measurements and modeling of Arnold et al. (1981), Reiner and Arnold (1997), Schlager and Arnold (1987), Viggiano and Arnold (1981), and Mills et al. (2005) (shaded area). The September nucleation threshold profile for nucleation and growth to a lower cutoff of 6 nm from Fig. 6a is also included (black dotted).
Figure 5 shows the impacts of ion-induced nucleation on the derived sulfuric acid profiles by removing all ions from the simulations and then comparing the derived profiles to those that included ions. In areas with low sulfuric acid concentrations, removing the ions has nearly no effect on the derived profiles; however, in areas with higher sulfuric acid concentrations the derived profiles increase by almost an order of magnitude. At low sulfuric acid concentrations, the small clusters are not growing fast enough by condensation. Negatively charged clusters recombine too early with positively charged ions and therefore are too small to overcome the nucleation barrier of neutral nucleation. At higher sulfuric acid concentrations, ion-induced nucleation occurs as expected. The charged clusters grow larger than the critical size before they recombine and increase the nucleation rate. Thus to create the same number of CN without ions, more sulfuric acid is required than if ions are present.
For the nucleation threshold profiles with 10 cm
To study the uncertainty introduced by the assumed particle size threshold of the CN measurements, we derive the sulfuric acid profiles assuming a lower CN counter threshold of 6 nm diameter, which is the CN counter's lower end according to Campbell and Deshler (2014). The lower threshold leads to lower sulfuric acid concentrations as the nucleated CN do not have to grow as large by sulfuric acid condensation to be counted (Fig. 6a). The impact of CN threshold size decreases with increasing sulfuric acid as, at higher concentrations, the clusters grow quickly once they are nucleated. In October, however, there is more sulfuric acid needed in the CN layer as fewer small clusters exist that can grow across the cutoff size and therefore some nucleation of new CN is needed.
Lowering the size of the initial preexisting particles from 100 to 50 nm diameter reduces their coagulation efficiency and they present a smaller loss during nucleation. Therefore, the modeled sulfuric acid concentrations are lower (Fig. 6b). For the same reason there is no sulfuric acid needed in October in the CN layer. If we assume the initial preexisting particles to be a distribution of different sizes (e.g., 40 % of 50 nm, 50 % of 100 nm, and 10 % of 300 nm particles), the coagulation efficiency increases and leads to fewer simulated CN and higher derived sulfuric acid profiles (Fig. S1 in the Supplement).
We study model uncertainties according to Lovejoy et al. (2004) by adding 0.5 kcal to all changes in Gibbs free energy of negatively charged clusters. This only increases the profiles in regions where ion-induced nucleation dominates (see Sect. 3.2 and Fig. 5). A reduction of all coagulation and condensation rates by 20 % increases all profiles a little but leads to a poorer CN simulation in comparison with the observations. The updated neutral sulfuric acid dimer thermodynamic stabilities presented by Kürten et al. (2015), which have a higher relative humidity dependence of the equilibrium constant, lead to higher dimer evaporation rates. Therefore, they increase our profiles at low relative humidities (high temperatures), but only if neutral binary nucleation dominates. A combination of these influences is shown in Fig. 6c. The increase in the September CN layer profile at 24–26 km is mainly due to the updated dimer thermodynamic stabilities. The October CN layer profile mostly decreases as coagulation is less efficient, which requires less growth of additional small particles. At the lowest altitude no nucleation is needed in September, but therefore nucleation of additional CN is necessary in October.
As the derived sulfuric acid profiles are mainly determined by temperature we also test the effect of a 5 K temperature increase (Fig. 6d). We removed the responses at the highest September and October values as there the temperature was too high, so that evaporating particles complicate the situation. A 5 K temperature increase significantly increases the sulfuric acid profiles by a factor of 2 in the coldest regions and up to a factor of 15 in the warmest regions. Fortunately, the temperature measurement uncertainty is only 0.5 K (Campbell and Deshler, 2014). However, this temperature sensitivity shows that our sulfuric acid profiles in July and August at low altitudes are up to a factor of 2 too high as there we had to increase the temperature to SAWNUC's lower temperature limit of 190 K (maximum increase of 5 K; see Fig. 1a).
Our trajectories might descend too fast from July to August as the CN profile of Campbell and Deshler (2014) is representative of June and July. Also, Campbell and Deshler (2014) note that most measurements were performed between late August and early October, while our October simulations reproduce the measured CN as a monthly mean. If we run our simulations from mid-June until mid-October, the simulated CN in August are lower as the preexisting CN have more time to coagulate and in October less sulfuric acid is necessary to reproduce the CN layer (Fig. S2). Note that, in combination with a preexisting particle size distribution, this might necessitate some nucleation already in August.
Additional sensitivity studies (Fig. S3) imply that the exact number of ions or water molecules (e.g., 5 ppm everywhere) has only a small influence on the derived profiles because the ion concentrations are high enough that they are not a limiting factor, and the few parts per million stratospheric water vapor uncertainty is too small to influence the profiles significantly. Also, a formation of 35 % more CN in the layer (CN measurement uncertainty) needs only little additional sulfuric acid (not shown).
In Fig. 7 we compare our derived September CN layer sulfuric acid profile
with the profile derived by Campbell and Deshler (2014). Campbell and
Deshler (2014) derived sulfuric acid concentrations for 15 to 33 km (dark
red, dashed). Our derived sulfuric acid (black, dashed) is only shown between
21 and 26 km as we need no nucleation above and below the CN layer to
reproduce the observations. Our concentrations are about 1 order of
magnitude higher. This is because our CN have to form in a background of
preexisting particles and they have to grow to observable size. As our
sensitivity tests show, both of these effects require more sulfuric acid. In
the nucleation threshold profile with a cutoff of 6 nm and a background of
10 CN cm
We cannot compare our derived sulfuric acid profiles with Antarctic in situ
or remote sensing measurements as such data do not exist to our knowledge.
However, northern midlatitude balloon-borne measurements mainly from
September and October have been published (Arnold et al., 1981; Reiner and
Arnold, 1997; Schlager and Arnold, 1987; Viggiano and Arnold, 1981) and
summarized by Mills et al. (2005). Note that due to the different tropopause
heights (43
We did not derive sulfuric acid profiles above Wyoming according to Fig. 1a and b of Campbell
and Deshler (2014), as these CN are assumed to have nucleated in
the polar region. However, as temperature mainly controls the nucleation
rate, the nucleation threshold sulfuric acid profiles at temperatures
representative of the stratosphere above Wyoming are used for comparison. In
autumn, temperatures above Wyoming lie between
Analysis of over 20 years (1986–2010) of balloon-borne stratospheric CN measurements above McMurdo Station, Antarctica, between July and October reveals the formation of a layer of mainly volatile CN at 21–27 km altitude in a background of preexisting particles (Campbell and Deshler, 2014). Here, we use the nucleation box model SAWNUC to simulate these CN in subsiding air parcels and study the nucleation processes.
The observed CN of Campbell and Deshler (2014) are reproduced by simulating subsiding air parcels with volume compression, coagulation, nucleation, and growth processes. Antarctic CN concentrations outside the CN layer can be explained by coagulation if air volume compression due to air parcel subsidence is considered. Neutral sulfuric acid–water nucleation forms the CN layer in September, while in October growth of small particles maintains the layer. Ion-induced nucleation does not occur at significant levels as sulfuric acid concentrations are too low and charged clusters recombine too fast. Our results complement Campbell and Deshler (2014), who showed that the CN decrease above Laramie, Wyoming, can be explained by coagulation and that almost all CN inside the CN layer are volatile and therefore can be explained by binary nucleation.
Sulfuric acid concentrations in September during the CN layer formation range
from
Finally, we derived gaseous sulfuric acid profiles that show which
concentration would be necessary for nucleation and growth to CN size to
occur, which should represent upper limits of the actual sulfuric acid
outside of the CN layer where neither the observations nor our simulations
indicate nucleation to occur. The upper limits start at 18 km at
concentrations below 10
If stratospheric sulfuric acid increases above our upper limits, e.g., because of volcanic eruptions or geoengineering, nucleation could occur. In the midlatitudes and in some relatively warm areas above Antarctica, this nucleation would be dominated by ion-induced nucleation and therefore would require less sulfuric acid than predicted by neutral binary nucleation theory. Note, however, that our upper limits would increase if there were more preexisting particles present.
In conclusion, our study supports the explanation of the CN layer as presented by Campbell and Deshler (2014). We can reproduce the CN that decrease over time by coagulation in a low sulfuric acid environment during Antarctic winter. In September between 21 and 26 km we can reproduce the observed CN layer only if we assume a higher sulfuric acid concentration that produces volatile CN mainly by neutral binary nucleation.
The data shown in the figures are available from the authors upon request.
The authors declare that they have no conflict of interest.
We thank Edward R. Lovejoy, Karl D. Froyd, Jan Kazil, and Sebastian Ehrhart for providing the SAWNUC code and Andreas Engel for useful discussion. We thank the two anonymous reviewers for numerous helpful comments.Edited by: Holger Tost Reviewed by: two anonymous referees