We took aerosol measurements at Syowa Station, Antarctica, to characterize the aerosol number–size distribution and other aerosol physicochemical properties in 2004–2006. Four modal structures (i.e., mono-, bi-, tri-, and quad-modal) were identified in aerosol size distributions during measurements. Particularly, tri-modal and quad-modal structures were associated closely with new particle formation (NPF). To elucidate where NPF proceeds in the Antarctic, we compared the aerosol size distributions and modal structures to air mass origins computed using backward trajectory analysis. Results of this comparison imply that aerosol size distributions involved with fresh NPF (quad-modal distributions) were observed in coastal and continental free troposphere (FT; 12 % of days) areas and marine and coastal boundary layers (1 %) during September–October and March and in coastal and continental FT (3 %) areas and marine and coastal boundary layers (8 %) during December–February. Photochemical gaseous products, coupled with ultraviolet (UV) radiation, play an important role in NPF, even in the Antarctic troposphere. With the existence of the ozone hole in the Antarctic stratosphere, more UV radiation can enhance atmospheric chemistry, even near the surface in the Antarctic. However, linkage among tropospheric aerosols in the Antarctic, ozone hole, and UV enhancement is unknown. Results demonstrated that NPF started in the Antarctic FT already at the end of August–early September by UV enhancement resulting from the ozone hole. Then, aerosol particles supplied from NPF during periods when the ozone hole appeared to grow gradually by vapor condensation, suggesting modification of aerosol properties such as number concentrations and size distributions in the Antarctic troposphere during summer. Here, we assess the hypothesis that UV enhancement in the upper troposphere by the Antarctic ozone hole modifies the aerosol population, aerosol size distribution, cloud condensation nuclei capabilities, and cloud properties in Antarctic regions during summer.
The Antarctic is isolated from human activities occurring in the mid latitudes. In spite of the slight amount of human activity in the Antarctic, such as research activities at each station and tourism mostly in the Antarctic Peninsula during summer, the source strength of anthropogenic species (e.g., black carbon from combustion processes) is negligible in the Antarctic circle at the moment (e.g., Weller et al., 2013; Hara et al., 2019). Consequently, aerosol measurements in the Antarctic have been taken to ascertain aerosol physicochemical properties, atmospheric chemistry, and their effects on climate change under Earth's background conditions (i.e., cleanest and pristine conditions).
Despite the cleanest conditions prevailing in the Antarctic, concentrations
of condensation nuclei (CN) show clear seasonal variations with a maximum in summer and a minimum in winter (Gras et al., 1993; Hara et al., 2011a; Weller
et al., 2011).
This seasonal variation relates to supply and deposition
processes. In addition to primary aerosol emissions such as sea-salt
aerosols from sea-surface and sea-ice regions (e.g., Hara et al., 2020),
atmospheric aerosol formation (i.e., new particle formation, NPF) is important to supply atmospheric aerosols (Kulmala et al., 2013; Kerminen et
al., 2018) and to affect climate through indirect processes (Asmi et al.,
2010;
Dall'Osto et al., 2017). Measurements of aerosol size distributions
were taken continuously to elucidate and discuss (1) NPF, (2) particle
growth, (3) volatility as indirect information of aerosol constituents, and
(4) hygroscopicity and ability of cloud condensation nuclei (CCN) using a
scanning mobility particle sizer (SMPS) in the Antarctic (Koponen et al.,
2003; Virkkula et al., 2007; Asmi et al., 2010; Hara et al., 2011b; Kyrö
et al., 2013; Järvinen et al., 2013; Weller et al., 2015; Jokinen et
al., 2018; Kim et al., 2019; Jang et al., 2019; Lachlan-Cope et al., 2020)
and a similar instrument (Ito, 1993). Seasonal features of aerosol number
concentrations are associated with primary emissions of sea-salt aerosols,
NPF, emissions of aerosol precursors from oceanic bioactivity, and
photochemical processes (e.g., Koponen et al., 2003; Virkkula et al., 2007;
Asmi et al., 2010; Hara et al., 2011a, b, 2020; Kyrö et al., 2013;
Järvinen et al., 2013; Fiebig et al., 2014; Weller et al., 2015;
Humphries et al., 2016; Jang et al., 2019; Frey et al., 2020). Aerosol
volatility measurements revealed high abundance of less volatile particles because of dominance of sea-salt particles, even in ultrafine mode (smaller
than 100 nm diameter) during winter–early spring, whereas volatile
particles such as
Knowledge and discussion of condensable vapors (i.e., aerosol precursors) is
fundamentally important to elucidate microphysical processes such as NPF and
growth and locations at which NPF occurs. Earlier works have emphasized examination of the following condensable vapors (i.e., aerosol precursors)
for NPF and particle growth:
Some condensable vapors for NPF and growth are formed through photochemical
reactions with atmospheric oxidants such as OH,
Aerosol measurements were taken during the 45th–47th Japanese Antarctic
Research Expedition (2004–2006) at Syowa Station, Antarctica (69.0
Nano-size aerosol particles are removed rapidly through coagulation. To
elucidate the speed of removal by coagulation, coagulation sink (Coag.S) was
calculated using the following equation (Kulmala et al., 2001).
Condensation sink (Cond.S) was calculated to elucidate the rate of condensable
vapor removal by condensation on aerosol particles using the following
equation (Kulmala et al., 2001).
The nucleation rate of aerosol particles (
Schematic figure showing procedures used for the nucleation rate
of aerosol particles with a size of
Multi-modal fitting analysis of aerosol size distributions is used commonly
to understand and discuss microphysical processes in the atmosphere. In this
study, daily mean aerosol size distributions were approximated by a
lognormal function, which is given by the following equation.
The 120 h (5 d) backward trajectory was computed using the NOAA-HYSPLIT
model (Stein et al., 2015;
Figure 2 presents examples of number–size distributions of aerosol particles observed at Syowa Station. Our measurements show size distributions of ultrafine aerosol particles with mono-, bi-, tri-, and quad-modal structures. In earlier studies (Järvinen et al., 2013; Weller et al., 2015), multi-modal aerosol size distributions with mono-modal, bi-modal, and tri-modal structures were identified in the Antarctic. Although quad-modal structures were observed clearly for this study, they were not described in reports of earlier works.
Examples of aerosol size distributions with
To characterize the aerosol size distributions, we compare the modal size in each mode (Fig. 3). In mono-modal structures, the modal size ranged mostly from 40 to 105 nm. In the bi-modal structure, the modal sizes in the first and second modes were in the ranges of 20–40 nm and 60–135 nm. In the tri-modal structure, the modal sizes in the first to third modes were, respectively, in the ranges of 8–20, 20–63, and 65–135 nm. In the quad-modal structure, the modal sizes in the first to fourth modes were, respectively, in the ranges of 7–13, 14–30, 30–65, and 70–140 nm. The mono-modal structure was observed often under storm and strong wind conditions with blowing snow during winter–early spring (Hara et al., 2011b, 2020). In these conditions, sea-salt particles in ultrafine to coarse modes were released from snow and sea-ice surface in polar regions (Hara et al., 2011b, 2014; 2017, 2020; Frey et al., 2020). In the bi-modal structure, modal sizes in the modes of the smallest modal size were greater than those in tri-modal and quad-modal structures, so that the bi-modal structure was well aged relative to tri-modal and quad-modal structures. In tri-modal and quad-modal structures, modal sizes in the smallest mode appeared mostly for diameters smaller than 20 nm. As demonstrated by Asmi et al. (2010), Kyrö et al. (2013), Järvinen et al. (2013), Weller et al. (2015), Jokinen et al. (2018), and Kim et al. (2019), aerosol particles were grown to a few tens of nanometers after NPF, even in the Antarctic troposphere during summer. Because the smallest mode appeared with a diameter smaller than 20 nm, occasionally smaller than 10 nm, in tri-modal and quad-modal structures, aerosol size distributions with tri-modal and quad-modal structures might be associated with NPF and growth by vapor condensation. In bi-, tri-, and quad-modal structures, the modal sizes with the modes with the largest modal sizes had a similar diameter larger than 50 nm, which corresponded to a critical diameter for CCN activation in the Antarctic (Asmi et al., 2010).
Histogram of modal sizes in each modal structure. Symbols and lines of black, red, blue, and magenta show histograms of the first, second, third, and fourth modes in each modal structure.
Figure 4 shows the seasonal variation of abundance of modal structures at Syowa Station during our measurements. Mono-modal structure was identified mostly during May–August. Abundance of mono-modal structures was found for 16 %–60 % of days (mean, 37 %) during winter. As described above, mono-modal structures during winter–early spring were associated with sea-salt aerosol emissions. Indeed, sea-salt aerosols released from sea-ice areas were dominant during winter–early spring at the Antarctic coasts (e.g., Hara et al., 2012, 2013, 2020; Frey et al., 2020). Although bi-modal structures were observed throughout the year, abundance of bi-modal structures occurred as 23 %–76 % (mean, 56 %) in April–September and 9 %–52 % (mean, 26 %) in December–March. Particularly, abundance of mono-modal and bi-modal structures was dominant (more than 90 %) during May–August. High abundance of tri-modal structures (14 %–75 %, mean 45 %) was observed in September–April. Particularly, abundance of tri-modal structures exceeded 50 % in January–March. Surprisingly, tri-modal structures were identified even under dusk and polar night conditions during May–August. Modal sizes in the smallest mode of tri-modal structure were greater in winter than in spring–summer (details are presented in a later section). It is noteworthy that the quad-modal structure was found not only in December–February, but also in August–November and March–April. Considering the modal size in the smallest mode of quad-modal distributions, NPF might proceed in August–April in the Antarctic. Indeed, CN concentrations started to increase in August, with high concentrations in October–February at coastal stations (e.g., Hara et al., 2011a, Weller et al., 2011; Asmi et al., 2013).
Seasonal features of abundance of modal structures observed at Syowa Station, Antarctica.
Annual cycles of air mass origins in each modal structure using 120 h backward trajectory analysis are shown in Fig. 5 for comparison between the modal structure and air mass history. Regarding general features of air mass origins in February 2004–December 2006, coastal BL was dominant in summer (Fig. 5a), whereas abundance of air masses from continental FT increased during winter. Seasonal cycles of air mass origins in 2004–2006 showed good agreement with results of long-term analysis of air mass origins at Syowa during 2005–2016 (Hara et al., 2019).
Seasonal feature of air mass origins of
Seasonal features of air mass origins in mono-modal structures (Fig. 5b) resembled the general features (Fig. 5a), although the abundance of air masses from coastal FT was slightly higher in August and October. Considering that mono-modal structures corresponded mostly to storm conditions and strong winds during winter–spring (Hara et al., 2010, 2011b, 2020), the appearance of mono-modal structures was associated with primary emissions of sea-salt aerosols from the snow surface on sea ice by strong winds rather than air mass history (i.e., transport pathway), as presented by Hara et al. (2012, 2013, 2020).
Similarly, seasonal features of air mass origins in bi-modal structure (Fig. 5c) resembled the general features. In general, bi-modal structures were recognized as well-aged distributions by condensation growth, coagulation, and cloud processes. Therefore, the appearance of bi-modal structures might be compared only slightly to air mass origins classified by 120 h backward trajectory analysis.
Regarding tri-modal structures (Fig. 5d), the abundance of air masses from
continental FT and coastal FT increased in spring, compared to general
features (Fig. 5a). Similarly to bi-modal structures, the appearance of some
tri-modal structures, particularly with larger modal sizes (e.g.,
Unlike the features in mono-modal, bi-modal, and tri-modal structures, continental FT and coastal FT were the most abundant air mass origins in quad-modal structures during spring and fall (Fig. 5e). In general, features of air mass origins (Fig. 5a), MBL, and coastal BL showed an important contribution during spring and fall. Nevertheless, quad-modal structures in spring and fall were identified only in the air masses from continental FT and coastal FT. This feature implies strongly that NPF proceeded in FT during spring and fall in the Antarctic. In contrast to the high contributions of air masses from continental FT and coastal FT during spring and fall, quad-modal structures were observed also in air masses from MBL and coastal BL during summer. Therefore, NPF might occur also in MBL and coastal BL during summer, as reported from results of earlier works (Weller et al., 2011; Asmi et al., 2013; Lachlan-Cope et al., 2020).
Based on aspects of location where NPFs occur in the Antarctic troposphere, seasonal features of abundance of tri-modal and quad-modal structures and their air mass origins are presented for comparison in Fig. 6. The abundance of tri-modal and quad-modal structures reflects the frequency of NPF in the Antarctic troposphere, although the appearance of tri-modal and quad-modal structures does not necessarily mean fresh or local NPF events near Syowa in this study. The abundance of tri-modal and quad-modal structures was less than 10 % of days during May–August. In September–January, the abundance of tri-modal and quad-modal structures was 40 %–48 % (mean, 44 %). By contrast, abundance reached 60 % and 84 % in February and March. Seasonal features of NPF occurrence with highs in spring–summer and minima in winter were observed at Concordia (Järvinen et al., 2013) and King Sejong (Kim et al., 2019). The monthly occurrence (frequency) of the NPF, however, varied greatly at Syowa, Concordia and King Sejong. Differences in the abundance of NPF occurrence among Syowa, Concordia, and King Sejong might derive from different atmospheric conditions such as the concentrations of aerosols and precursors and different criteria for identification of NPF-growth events.
Seasonal features of abundance of tri-modal and quad-modal structures and air mass origins at Syowa Station, Antarctica, in 2004–2006.
Abundance of tri-modal and quad-modal distributions with air mass origins of coastal FT and continental FT (Fig. 6) ranged from 14 % to 27 % of days (mean, 22 %) during September–November, from 8 % to 16 % (mean, 11 %) during December–February, and from 11 % to 32 % (mean, 22 %) during March–April. Particularly, fresh NPF (quad-modal structures) was identified in FT (12 % of days) during September–October and March, in contrast to 1 % in BL. Abundance of quad-modal structures decreased to 3 % in FT during December–February. Considering high abundance of the quad-modal structures in the air masses from FT in September–October (Fig. 5e), spring NPF might occur dominantly in FT. However, abundance of tri-modal and quad-modal structures with air masses of MBL and coastal BL increased (27 %–52 %, mean 41 %) during December–March. Abundance of quad-modal structures in BL increased to 7 % in BL (3 % in FT) during December–February. In addition to NPF in FT, the high abundance in BL during December–March implies that more NPF proceeded in the BL during summer, as reported from earlier works (Koponen et al., 2003; Virkkula et al., 2007; Asmi et al., 2010; Kyrö et al., 2013; Weller et al., 2015; Jokinen et al., 2018; Kim et al., 2019; Jang et al., 2019; Lachlan-Cope et al., 2020). It is noteworthy that the abundance of air masses from continental FT and coastal FT during summer decreased remarkably, even in general features (Fig. 5a). Furthermore, the quad-modal structure was observed in air masses from continental FT and coastal FT in December (summer). CN enhancement by NPF and growth was observed in the lower FT over Syowa Station during summer (Hara et al., 2011a). Therefore, the difference of contributable air mass origins in quad-modal structures between spring–fall and summer might reflect not only the locations of NPF occurrence, but also seasonal features of general air mass origins (Fig. 5a). Consequently, NPF might occur dominantly in FT and partly in BL during spring and fall and in BL and FT during summer.
Figure 7 depicts seasonal variations of all of the following: (a)
concentrations of major aerosol constituents in
Seasonal variations of
Variations of CN concentrations (Fig. 7b) exhibited clear seasonal features with a minimum in winter and a maximum in summer. During winter, CN concentrations increased occasionally under storm and strong wind conditions. Increase in CN concentration from the winter minimum started at the end of August–early September at Syowa. CN concentrations and seasonal variations were similar to those measured at other coastal stations (e.g., Weller et al., 2011; Fiebig et al., 2014).
Our measurements show size distributions of ultrafine aerosol particles with
mono-, bi-, tri-, and quad-modal structures (Fig. 2). The presence of
tri-modal and quad-modal structures during spring–fall suggests a frequent occurrence of NPF and growth in the atmosphere. Indeed,
fresh-nucleation and aged-nucleation modes (
Aerosol number concentrations of
The high values of
Generally speaking, NPF is more likely to occur under conditions with (1)
lower number concentrations of preexisting particles, (2) higher
concentrations of condensable vapors, and (3) presence of sufficient
photochemical oxidants such as OH. Because of low aerosol number
concentrations in the Antarctic FT (Hara et al., 2011a), NPF can proceed
preferentially in the FT if condensable vapor is present. Earlier studies
presented that the following condensable vapors participate in tropospheric
NPF:
Atmospheric iodine cycles are related closely to snowpack chemistry in the
Antarctic (e.g., Atkinson et al., 2012; Roscoe et al., 2015; Saiz-Lopez et
al., 2015; Hara et al., 2020). Because of fast reactions of reactive iodine
species (
DMS measurements taken at Concordia suggest that DMS was transported from
coastal areas (or oceans) to the inland station via FT (Preunkert et al.,
2008). Therefore, photochemical reactions with DMS,
Polar sunrise in the upper FT occurs earlier than it does near the surface.
Additionally, the existence of the ozone hole enhances UV radiation, even in
the troposphere, during September–November (Fig. 7g–h). More noteworthy is
the higher UV of wavelengths shorter than 305 nm in October–November than in December near the surface at Syowa. For example, the monthly mean amounts of
UV radiation of 300–305 nm at Syowa were, respectively, 0.080, 0.098, and
0.068
After NPF, aerosol particles in fresh-nucleation mode are grown mainly by condensation of condensable vapors. They are coagulated efficiently onto
pre-existing particles. For a better understanding of the aerosol life cycle
including microphysical processes in the Antarctic troposphere, aerosol
lifetime must be discussed before discussion of the relation between NPF and
CCN ability. Williams et al. (2002) reported that aerosol particles of
Figure 8 depicts vertical variations of the
Vertical variations of aerosol
In contrast to the longer
Gradual change in modal sizes in fresh-nucleation mode and first Aitken mode were observed during spring–summer (Fig. 7c). When aerosol particles derived from NPF during periods with the existence of an
Schematic figure showing procedures used for
Comparison of
Trends of cloud amounts at Syowa Station were examined to assess the effects
of spring NPF enhanced by the existence of the
Our hypothesis can be summarized as shown in the schematic figure (Fig. 12).
In polar sunrise, solar radiation recovers earlier in the stratosphere and
the upper troposphere. Then, ozone depletion by catalytic reactions of
chlorine cycle starts in the Antarctic stratosphere. The ozone hole appears
in the Antarctic stratosphere from the end of August until the end of
November. Appearance of the ozone hole engenders UV enhancement and then
production of atmospheric oxidants such as OH in the troposphere.
Atmospheric oxidants such as OH are likely to be formed in the upper
troposphere by UV enhancement. Condensable vapors (i.e., aerosol precursors
with lower vapor pressure) are producible by photochemical oxidation.
Schematic figure presenting our hypothesis.
Aerosol measurements were taken using SMPS and OPC at Syowa Station,
Antarctica, in 2004–2006. Aerosol size distributions were found to have mono-, bi-, tri-, and quad-modal distributions during our measurement
period. The mono-modal distribution was dominant under strong wind
conditions during May–August. The bi-modal distribution was identified
through the year. Tri-modal and quad-modal distributions were observed
mostly during September–April. Seasonal features of
We obtained direct evidence indicating that spring UV enhancement by the ozone hole engendered spring NPF and growth in the Antarctic FT. With ozone hole recovery (Solomon et al., 2016; Kuttippurath et al., 2017), aerosol properties and populations might be modified for the next several decades. Consequently, indirect effects on atmospheric radiation budgets and climate change in the Antarctic regions during the summer can revert to levels that prevailed before the ozone hole existence. More aerosol measurements must be taken in Antarctic regions to monitor these trends and future effects.
Data are available by contacting the corresponding author (Keiichiro Hara: harakei@fukuoka-u.ac.jp).
The supplement related to this article is available online at:
KH, KO, and TY designed aerosol measurements at Syowa Station. KH, KO, and MY conducted wintering aerosol measurements at Syowa Station in 2004–2006. KH and CNH contributed to data analysis including SMPS/OPC data and meteorological data. KH prepared the manuscript and led data interpretation. All the co-authors contributed to discussions about data interpretation and the manuscript.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank the members of the 45th–47th Japanese Antarctic Research Expedition for assistance with aerosol measurements taken at Syowa Station. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for providing the HYSPLIT Transport and Dispersion Model and READY website used for this research (
This study was supported financially by the “Observation project of global atmospheric change in the Antarctic” for JARE 43–47. This research has been supported by the Japan Society for the Promotion of Science (grant nos. 16253001, 15310012, and 22310013). This study was supported by the National Institute of Polar Research (NIPR) through project research no. KP-302.
This paper was edited by Veli-Matti Kerminen and reviewed by three anonymous referees.