Airborne bacteria are widespread as a major proportion of
bioaerosols, and their coexistence with dust particles enables both bacteria
and dust particles to be more active in ice cloud
formation and to be harmful to public health. However, the abundance and
viability of particle-attached and free-floating bacteria in dusty air have
not been quantitatively investigated. We researched this subject based on
the fact that airborne bacterial cells are approximately 1 µm or
smaller in aerodynamic diameter; therefore, particle-attached bacteria
should occur in aerosol samples of particles larger than 1 µm, and
free-floating bacteria should occur among particles smaller than 1 µm.
Our observations at a coastal site in Japan in spring, when the westerlies
frequently transported dust from the Asian continent, revealed that
particle-attached bacteria in dust episodes, at the concentration of 3.2±2.1×105 cells m-3 on average, occupied 72±9 % of the total bacteria. In contrast, the fraction was 56±17 % during nondusty periods, and the concentration was 1.1±0.7×105 cells m-3. The viability, defined as the ratio
of viable cells to total cells, of particle-attached bacteria was 69±19 % in dust episodes and 60±22 % during nondusty periods on
average, both of which were considerably lower than the viabilities of
free-floating bacteria (about 87 %) under either dusty or nondusty
conditions. The presented cases suggest that dust particles carried
substantial amounts of bacteria on their surfaces, more than half of which
were viable, and spread these bacteria through the atmosphere. This implies
that dust and bacteria have important roles as internally mixed assemblages
in cloud formation and in linking geographically isolated microbial
communities, as well as possibly having a synergistic impact on human health.
Introduction
Biological particles in the atmosphere have a potentially significant effect
on climate change (Ariya et al., 2009; Delort et al., 2010; Möhler et
al., 2007; Zhang et al., 2017); efficiently link microbial communities
between continents, islands, and oceans (Fröhlich-Nowoisky et al.,
2016; Morris et al., 2011; Caliz et al., 2018); and pose risks to human health (Polymenakou et al., 2008; Reinmuth-Selzle et al., 2017). Representing a
high fraction of primary biological particles, airborne bacteria are emitted
into the atmosphere from various sources, among which desert dust is a major
source (Morris et al., 2011; Pöschl and Shiraiwa,
2015; Pöschl et al., 2010). The co-occurrence of dust and high
concentrations of bacteria has been observed frequently in different
locations, indicating the widespread nature and dissemination of bacteria
with dust at local, regional, and even global scales (Griffin, 2007; Hara
and Zhang, 2012; Iwasaka et al., 2009). Limited available observations have
revealed the coexistence of mineral and biological contents in ice crystals
(Creamean et al., 2013; Pratt et al., 2009), and laboratory experiments
have demonstrated that the ice nucleation ability of dust particles is
enhanced by biological components, including bacteria in the particles
(Boose et al., 2019; Tobo et al., 2019; Conen et al., 2011). Recent
toxicological studies with mouse exposure found that the internal mixture of
dust and pathogenic bacteria exacerbated pneumonia (He et al., 2012). In
addition, the attachment of bacteria to dust particles is expected to
largely alter the fate of bacterial cells in the air due to protection by
the dust particles from harsh environmental conditions (Bowers et al., 2013) and enhanced gravitational
settling (Zhang, 2008). All these results reflect that the
adherence of bacterial cells to dust particles, i.e., the particle-attached
state, and the viability or metabolic capability of bacterial cells are key
factors affecting the roles and fate of airborne bacteria in the evolution,
development, and conservation of the natural environment.
Quantitative data on the mutual state of airborne bacteria and dust
particles in dusty air are without any doubt scientifically very interesting
(Schuerger et al., 2018) but are rare because of a lack of
available and confident methods, leaving unidentifiable uncertainties in
both field observations and model simulations exploring the activities and
roles of bacterial cells in atmospheric processes. The cell size
distributions for bacteria separated from soils have previously been
investigated (Portillo et al., 2013). The survival
strategies, dispersal processes, and size distribution of airborne bacteria
should be different from those of bacteria in soils. The possible causes are
that the aerosolization efficiency of soil bacteria from Earth surfaces
varies according to bacterial species and soil types
(Joung et al., 2017), and airborne bacteria suffer
air turbulence and harsh atmospheric stressors (Hara and
Zhang, 2012). Bacteria-associated particles in the air have an aerodynamic
diameter significantly larger than the typical size (approximately 1 µm) of individual bacterial cells (Burrows et al., 2009). This
is because airborne bacterial cells are favorably attached to coarse
particles, such as dust particles and plant debris, or are sometimes found
as assemblages of many cells (Després et al., 2012; Iwasaka et al.,
2009; Maki et al., 2013; Lighthart, 1997). We quantified the fractions of
particle-attached and free-floating bacterial cells in dusty and nondusty air
based on the fact that airborne bacterial cells are usually ∼1µm or smaller than 1 µm (Delort et al., 2010; Després et
al., 2012; Pósfai et al., 2003; Burrows et al., 2009; Hara et al., 2011);
thus, particle-attached bacteria should be trapped in aerosol samples of
particles larger than 1 µm, and free-floating bacteria should be
located among particles smaller than 1 µm.
By utilizing eight-stage Andersen cascade impactors (Andersen samplers),
size-segregated aerosol samples were collected at a southwestern coastal
site of Japan in the spring of 2013–2016, when the middle-latitude
westerly wind in the Northern Hemisphere frequently brought dust from the
Asian continent to the observation site. Viable and nonviable bacteria in
each sample were counted using the LIVE/DEAD BacLight bacterial viability
assay to estimate bacterial concentrations (Murata and Zhang, 2013,
2016). Bacteria detected in samples of particles larger than 1.1 µm
(the cutoff size of the sampler stages) were considered particle-attached
bacteria, and those in the stages of particles smaller than 1.1 µm were
considered free-floating bacteria. An analysis of method confidence showed
that uncertainties due to the sample collection were small (Figs. S4 and S5
in the Supplement). In this study, we focus on comparisons of the
quantitative results of particle-attached and free-floating bacteria in the
air and the viability of these bacteria under dusty and nondusty conditions.
MethodsSample collection and cell enumeration
Aerosol samples were collected on the platform of a building
(32.324∘ N, 129.993∘ E, 15 m above ground level, and 23 m
above sea level) on the seaside of Amakusa, southwestern Japan (Fig. S1), during several observational campaigns in the spring of 2013 to 2016.
Dust plumes from the Asian continent, called Asian dust, frequently pass
this area in spring. There are limited fishery and agriculture activities
and few anthropogenic sources of air pollutants around the area, making the
site suitable for investigating airborne bacteria in the Asian continental
outflow (Murata and Zhang, 2016).
Aerosol samples were collected onto 0.2 µm pore polycarbonate filters
(47 mm; Merck Millipore Ltd., Cork, Ireland) with eight-stage Andersen samplers
(Model AN-200; Tokyo Dylec Corp., Japan). The flow rate of the samplers was
28.3 L min-1. Aerosol particles were collected onto eight filters according
to the particle aerodynamic diameter ranges of >11, 7.0–11,
4.7–7.0, 3.3–4.7, 2.1–3.3, 1.1–2.1, 0.65–1.1, and 0.43–0.65 µm. The collection time of one set of samples was from approximately 3 to 24 h. Details on the sample collection are given in Table 1, Table S1, and
Fig. S2 in the Supplement.
Concentration and viability of total, free-floating, and
particle-attached bacteria. The concentration of coarse particles
(>1µm) and the ratio of particle-attached bacteria to
coarse particles are also listed. The percentages of free-floating and
particle-attached bacteria are given in the parentheses. The sample ID
indicates the sequence number (1 to 27) of the sample and dust condition
(D, dusty; ND, nondusty) and synoptic weather (Pr, prefront; Po, postfront;
AA, approaching anticyclone; A, anticyclone) during the sampling period.
Before the collection of each sample set, all stages of the sampler were
cleaned carefully, and the plates for the filters were rinsed and wiped with
70 % ethanol in a clean hood to avoid contamination. A blank control for
each set of samples was prepared; i.e., a blank filter was set in the
sampler without sample collection. After sample collection, the filters were
sealed in Petri dishes and stored at -20∘C until analysis.
The viable and nonviable bacterial cells (Fig. S3) on the filters were
enumerated using the LIVE/DEAD BacLight bacterial viability assay with an
epifluorescence microscope (EFM; Eclipse 80i, Nikon Corp., Tokyo, Japan) as
described previously (Murata and Zhang, 2016, 2013; Hu et al., 2017).
Bacterial cells and other particles were detached from the aerosol-loaded
polycarbonate membranes (47 mm in diameter) in a phosphate-buffered saline
solution (PBS, pH 7.4) by vortex shaking and ultrasonic vibration in ice
bath. Then the suspension was treated with glutaraldehyde fixation and
stained with the LIVE/DEAD BacLight bacterial viability kit (L13152,
Invitrogen™, Molecular Probes Inc., Eugene, Oregon, US),
followed by filtration on a 25 mm diameter and 0.2 µm pore black
polycarbonate membrane for bacterial enumeration. An excitation wavelength
range between 450 and 490 nm (blue) was utilized, and the microscope was
operated at 1000× magnification. Fluorescent green and
red/orange/yellow cells with spherical shape and size close to or smaller
than 1 µm in diameter were counted as viable and nonviable bacteria,
respectively. There are uncertainties in the bacterial cell counting caused
by the LIVE/DEAD BacLight bacterial viability kit because the kit could not
distinguish archaea and small eukaryotes including fungi from bacteria (Berney et al., 2007). Since the abundance of archaea and fungi in air
could be several (1–6) orders of magnitude less than that of bacteria
(Fröhlich-Nowoisky et al., 2014, 2016; Delort et al., 2010) and the dominant size range of fungal spores is
2–10 µm (Bauer et al., 2008), the overestimation of
bacteria caused by the kit we used should be less than 10 %, although the
uncertainties could not be quantitatively evaluated. The cell concentrations
in the size-segregated particles in the air were estimated based on cell
counts and the sampling of air volumes following the subtraction of the
blank controls. The viability of a group of bacterial cells was defined as
the ratio of the viable bacterial cells to total bacterial cells. The
procedure for the experimental operation and the formulations for the
estimation of cell concentrations are given in the Supplement (Sect. S1 in
the Supplement).
The collection efficiency of airborne bacterial cells with Andersen samplers
was evaluated by comparing the results to those obtained by using
BioSamplers (SKC Inc., Eighty Four, PA, US) and in-line filter holders (47 mm, Millipore Corp., Billerica, MA, US). The comparison shows that the total
bacterial concentration results of the Andersen sampler were generally
consistent with those of the BioSamplers and the in-line filter holders
(Fig. S4).
Separation of particle-attached and free-floating bacteria
In this study, bacteria in the samples of stages with particles larger than
1.1 µm were considered particle-attached, and bacteria in the samples
of stages with particles ranging from 0.43 to 1.1 µm were considered
free-floating. The resuspension of bacteria trapped by upper stages and
falling onto lower stages during sample collection may cause uncertainties
in the size distribution of bacteria-associated particles and the separation
of particle-attached and free-floating bacteria.
The uncertainties in the estimation of particle-attached and free-floating
bacteria were investigated in the laboratory (Sect. S2 in the Supplement).
The fractions and concentrations of particle-attached bacteria obtained by
the presented method were potentially underestimated. But the
underestimation did not significantly affect the size distributions of
particle-attached bacteria, and, in particular, the underestimation of the
concentrations of particle-attached bacterial cells was less than 10 % on
average (Fig. S5). The total bacterial concentration results of the Andersen
sampler were generally consistent with those of the in-line filter holders
collecting total particles (Fig. S4). This result indicates that bacteria
smaller than 0.43 µm, which are not available by the Andersen samplers
in this study, were a minor fraction of the free-floating bacteria.
Atmospheric conditions
During the observation periods, the number concentrations of size-segregated
airborne particles (>0.3, >0.5, >1.0,
>2.0, and >5.0µm in diameter) were monitored
with optical particle counters (OPC, KC-01D in 2013 and KC-01E in
2014–2016, Rion Co., Ltd., Tokyo, Japan). In this study, fine particles are
in the range of 0.3–1.0 µm, and those larger than 1.0 µm are
referred to as coarse particles. Meteorological conditions, including
temperature, pressure, relative humidity, precipitation, and wind speed and
direction, were monitored with a weather transmitter (WXT520, Vaisala Inc.,
Helsinki, Finland). Airborne particle number concentrations and
meteorological data during the observation periods are summarized in Fig. S2
and Table 1.
On the basis of surface pressure and weather charts in the days before and
after sample collection (Figs. S2 and S6), the air parcels on the synoptic
scales from which samples were collected were categorized into four groups:
prefront, postfront, approaching anticyclone, and anticyclone (Tables 1 and
S1). Details of the categorization are available in Murata and
Zhang (2016).
Dust episodes were identified by significant increases in coarse particle
concentrations (>1µm), the forecast for Asian dust
distributions in the east Asian region
(http://www-cfors.nies.go.jp/~cfors/, last access: 8 September 2020; Fig. S7), and the
backward trajectory of air masses calculated with the NOAA hybrid
single-particle Lagrangian integrated trajectory (HYSPLIT) model
(http://ready.arl.noaa.gov/HYSPLIT_traj.php, last access: 8 September 2020). During dust
events, the coarse particle concentration largely increased at the study
site (Zhang et al., 2003). Dust particles were present in the
postfront air and sometimes in the approaching anticyclone air. The results
of backward trajectory analysis during dusty and nondusty episodes are shown
in Fig. S8.
ResultsConcentrations of airborne bacteria in segregated size ranges
The concentrations of bacterial cells, including viable and nonviable cells,
generally showed a bimodal number size distribution during dust episodes
(e.g., Fig. 1a, b, d, f). Most of the bacteria were present in particle fractions
with aerodynamic size (Dp) ranges larger than 2 µm (i.e.,
2.1–3.3, 3.3–4.7 and 4.7–7.0 µm; Fig. S9). These sizes are
larger than the size of individual airborne bacterial cells (approximately 1 µm or smaller), indicating that the bacteria did not float
individually in the air but were combined with other particles or were
agglomerates of bacterial cells; i.e., the bacteria were particle-attached.
The agglomerates of bacterial cells usually appear near emission sources,
e.g., sea spray and leaf water (Lighthart, 1997) and probably
contributed a limited portion to particle-attached bacteria in this study.
There were also many bacterial cells in the size ranges smaller than 1.1 µm, i.e., free-floating bacterial cells. Their concentration was
comparable to or lower than the concentrations of bacteria in the larger
size ranges (Figs. 1 and S9).
Concentrations of viable and nonviable bacteria
(CB) and mineral dust-like particles (CM) in size-segregated
airborne particles. Selected samples are shown as examples: (a) 1D-Pr, (b)
4D-Pr+Po, (c) 11ND-AA, (d) 17D-AA, (e) 22ND-A, (f) 27D-AA. The results of
all sampling periods are depicted in Fig. S9 in the Supplement.
In contrast to dust episodes, during nondusty periods, the number size
distribution of bacteria largely varied and did not show any trend with
respect to weather conditions. In six cases during nondusty periods (9ND-Pr,
11ND-AA, 12ND-A, 13ND-A, 14ND-A, and 21ND-A; Fig. S9), the bacteria appeared
mainly in size ranges smaller than 1.1 µm and accumulated the most in
the size range of 0.43–0.65 µm (e.g., Fig. 1c), indicating the
predominance of free-floating bacteria. During most of the other nondusty
periods (6ND-AA, 7ND-A, 8ND-A+Pr, 16ND-Po, 19ND-A, 20ND-A, 22ND-A,
23ND-Pr+Po, 24ND-Po+A, and 25ND-A), the distributions of bacteria were
similar to those during the dust periods, although the concentrations were
much lower than or comparable to those in the dust episodes (e.g., Fig. 1e). There were two exceptional cases in nondusty periods that had a
mono-modal distribution, with peaks at 3.3–4.7 µm (15ND-AA) or
larger than 11 µm (18ND-AA) (Fig. S9). Multiple processes including
advection, deposition, local emission, and local convective mixing could
influence the size distributions. Unfortunately, we do not have enough case
data to investigate statistically meaningful connections between the size
distribution and those processes.
Concentration of particle-attached and free-floating bacteria
The report of results when data are non-normal distribution should be viewed
with caution, since many statistical analyses (e.g., the average and
standard deviation) are only applicable to random samples from populations
with a normal distribution. Aerobiological data possibly do not have a
normal distribution (Kasprzyk and Walanus, 2014; Limpert et al., 2008).
Conversely, in this study, to make the comparisons among the values easily
understood and avoid misunderstanding, we assume the data are normally
distributed.
On average, the concentration of total bacterial cells, 4.4±2.6×105 cells m-3, during dust episodes was more than twice
that during nondusty periods, 2.0±1.0×105 cells m-3 (Table 1). This large difference (independent sample t test,
p<0.05) in concentration is consistent with the results of previous
studies (Hara and Zhang, 2012; Yamaguchi et al.,
2014). The concentrations of particle-attached bacterial cells during dust
episodes and nondusty periods were 3.2±2.1×105 and
1.1±0.7×105 cells m-3, respectively. During
dust periods particle-attached bacteria accounted for 72±9 % of
total bacterial counts, while during nondusty periods particle-attached
bacteria occupied a much lower proportion of 56±17 %
(independent sample t test, p<0.05). These results suggest that dust
particles carry a substantial amount of bacterial cells on their surfaces
from dust source areas to remote downstream areas.
On the other hand, the percentage of free-floating bacterial cells was in
some cases higher than 70 % during nondusty periods (Table 1). In
particular, the percentage ranged from 35 % to 73 % (49±15 %
on average) under anticyclone weather conditions, when the air mass moved
sluggishly and was mainly influenced by marine and local emissions and less
by continental emissions (Fig. S8). Therefore, a substantial fraction of
airborne bacteria were free-floating, and they were frequently the common
bacteria in nondusty air.
The number ratio of particle-attached bacteria to particles in the size
range larger than 1.1 µm was 12±11 % on average (Table 1).
Except for two periods when the ratios were 35 % and 59 %,
the ratio was approximately stable (9±5 % on average for the other
periods), regardless of dust episodes and nondusty periods (Table 1). That
is, assuming that a bacteria-attached coarse particle harbors at least one
bacterial cell, coarse particles including mineral dust particles with
attached bacteria usually made up less than 9 % of the total coarse
particles. Maki et al. (2008) reported that the mineral particles
with attached bacteria made up approximately 10 % of the total mineral
particles, with the remaining mineral particles possessing few or no
bacterial cells at 800 m height above the ground in an Asian dust source
region, Dunhuang, China.
The number size distributions of bacterial cells and mineral-dust-like
particles (insoluble and with irregular shapes; Fig. S3) in the microscope
fields of some samples were compared. In most cases, the size distributions
(mode sizes) of mineral-dust-like particles and bacteria in the size ranges
larger than 1.1 µm showed very good consistency (Figs. 1 and S9). In
some cases, the concentration of bacteria in the size ranges larger than 1.1 µm, especially nonviable bacteria, was closely correlated with the
mineral dust-like particles in the size-segregated samples (Fig. 2). These
results further confirm that the bacteria observed in the large size ranges
were closely associated with airborne coarse particles; i.e., they were
particle attached. In some cases, the mode size ranges of the bacterial
cells and the dust-like particles were inconsistent (Fig. S9), likely
because the number of bacteria on the surface of each coarse particle
largely varied or there were less dust-like particles in the coarse size
ranges (e.g., 26D-Po). Dust-like particles were rarely observed in the size
ranges smaller than 1.1 µm (Fig. S9), further indicating that the
bacteria observed in those size ranges were predominantly free-floating.
Relationship between bacteria and mineral-dust-like
particles in size-segregated aerosols. (a) Total bacteria, (b) viable
bacteria, and (c) nonviable bacteria. Solid and open markers represent
particles in the size ranges larger and smaller than 1.1 µm,
respectively. The Pearson correlation coefficients (r) between bacteria and
mineral-dust-like particles for particles larger than 1.1 µm are shown.
Viabilities of particle-attached and free-floating bacteria
The viability of particle-attached bacteria varied over a wide range from
18 % to 98 % (63±21 % on average), and the viability of
free-floating bacteria was between 56 % and 99 % (87±12 % on
average) (Table 1), much higher than the viability of particle-attached
bacteria (paired-sample t test, p=0.00). The attachment of airborne
bacteria to larger particles is expected to be favorable for retaining the
viability or cultivability of cells and may indirectly increase the
diversity of bacterial communities because of the possible protection of
bacterial cells from harsh atmospheric conditions (Bowers et al.,
2013; Prospero et al., 2005; Lighthart, 2000).
However, we found that the viability of particle-attached bacteria tended to
be lower than that of free-floating bacteria, regardless of weather
conditions (Table 1). This result indicates that a fraction of the
particle-attached bacterial cells were either nonviable when they were blown
into the air with the dust or had experienced atmospheric stressors for
several days during long-distance transport and changed from a viable to a
nonviable state. This is also likely the reason for the poor correlation
(Pearson correlation r=0.35, p=0.075) between the viability of
particle-attached bacteria and the ratio of particle-attached bacteria to
coarse particles (Table 1). In contrast, a large fraction of free-floating
bacteria were viable. A fraction of these bacteria were likely from local
areas, with a residence time (usually less than 1 d) shorter than that
(2–3 d) of the particle-attached bacteria transported from the Asian
continent (Fig. S8). The proportion of free-floating bacteria was higher
under nondusty conditions when the air masses moved slowly above the marine
area. However, for special cases, such as the one of 20ND-A when the air was
from the north due to the specific weather of west high pressure versus
east low pressure in the westerly, a substantial fraction of the bacteria
could be from the local and close areas due to the extremely strong wind. In
terms of concentration, viable particle-attached bacteria were usually more
abundant than viable free-floating bacteria in dust episodes (Figs. 1 and
S9).
On average, the viability (74±17 %) of total bacteria in dusty
episodes was close to the viability (75±13 %) of total bacteria
during nondusty periods (Table 1). The viability of particle-attached
bacteria (69±19 %) during dust periods was slightly higher than
that (60±22 %) during nondusty periods. The majority of
particle-attached bacteria were viable.
Free-floating bacteria exhibited a quite high viability, and the viabilities
of the bacteria in dusty (87±14 % on average) and nondusty (87±12 %) air were similar. The concentration of viable free-floating
bacteria was 3.8×104–1.5×105 cells m-3, which was lower than that of particle-attached bacteria (6.2×104–5.1×105 cells m-3). An increase in
viable free-floating bacteria on the order of 105 cells m-3 (1.1–2.2×105 cells m-3) was
observed when the weather was fine and the air masses moved slowly from
marine areas (e.g., 9ND-Pr, 12ND-A, and 13ND-A),
favoring the accumulation of bacteria emitted from local areas (Fig. S8).
DiscussionImplication from the comparison with literature data
There are few data on airborne bacterial cells available for comparison with
the present study. Observations in the multiphase atmosphere with
culture-dependent methods revealed that approximately 60 %–90 % or even
more culturable airborne bacteria were present in the size range of
particles larger than 1.1 µm (Agarwal, 2017; Burrows et al.,
2009; Montero et al., 2016; Raisi et al., 2013), and the median aerodynamic
diameter of particles containing culturable bacteria was approximately 2–4 µm at diverse sites (Lighthart, 2000; Raisi et al., 2013; Shaffer and
Lighthart, 1997; Tong and Lighthart, 2000). These results indicate the
predominance of culturable particle-attached bacteria in the air, which is
approximately in line with the results under dusty and nondusty conditions of
this study.
Early studies with single-particle analysis frequently encountered the mode
size of biological aerosol particles in the size range smaller than 1 µm (Matthias-Maser et al., 1999; Matthias-Maser and Jaenicke, 1995, 2000).
In contrast, recent real-time measurements using ultraviolet aerodynamic
particle sizer spectrometers and wideband integrated bioaerosol sensor
techniques revealed the mode size of fluorescent biological aerosol
particles (FBAPs) to be approximately 2–6 µm, and the particles were
mainly attributed to fungal spores (Pöschl et al., 2010; Savage et
al., 2017; Yue et al., 2017; Huffman et al., 2010). However, the abundant
particle-attached bacteria identified in this study in size ranges larger
than 2 µm indicate dust-particle-attached bacteria should not compose
small fractions of real-time FBAP results in the relevant size ranges. In
addition, the mode at or smaller than 1 µm observed in real-time FBAP
studies is likely consistent with the presence of free-floating bacterial
cells in the present study, but the comparison and discussion on the data
are not confident because of the large uncertainties caused by the low
counting efficiency and accuracy in submicron size ranges of the instruments
used in the studies (Yue et al., 2017; Huffman et al., 2010).
Since other equivalent data for comparison are rare, we discuss the
influences of airborne bacteria according to the results obtained in this
study and relevant general understandings in the following subsections.
Ice cloud formation
Dust particles from desert areas are constantly spread at local, regional,
and global scales in the atmosphere. These particles transport
microorganisms across continents and oceans to remote downstream areas
(Griffin, 2007; Schuerger et al., 2018). It has been shown that bacteria
in the air are more effective ice nuclei at temperatures up to
-2∘C than abiotic particles (Ariya et al., 2009; Burrows et
al., 2013; Fröhlich-Nowoisky et al., 2016; Möhler et al., 2007).
Biological particles coexisting with dust particles have been detected in
ice residues sampled from clouds (Creamean et al., 2013; Pratt et al.,
2009), and the coexistence of dust and bacterial cells increases the ability
of particles to act as ice nuclei for ice crystal formation
(Tobo et al., 2019). Proteins in bacteria are ice nucleation
active sites and are well protected when bacteria adhere to mineral dust
surfaces (Conen et al., 2011). The attachment of bacteria
to dust particles possibly increases the number of sites for ice nucleation
and consequently the ice nucleation ability of dust particles (Boose et
al., 2019; Conen et al., 2011; Augustin-Bauditz et al., 2016). The present
results show that up to 1/10 or more dust particles could be bacteria
carriers, and the concentration of particle-attached bacteria, i.e., the
number of bacteria–dust contact sites in dust episodes, was on average 3
times larger than that during nondusty periods (Table 1). The occurrence of
dust in remote downstream areas will significantly increase not only the
concentration of bacterial cells but also the concentration of dust–bacteria
mixture particles and the number of ice nucleation active sites. This
phenomenon could provide important sources of nuclei for ice cloud formation
under saturated meteorological conditions for icing, particularly in remote
elevated air, where the concentrations of aerosol particles able to act as
nuclei are usually very low (Creamean et al., 2013).
Ecosystem conservation and development
More than 60 % of particle-attached bacteria and approximately 87 % of
free-floating bacteria in the dusty air remained viable. Airborne bacteria
can multiply more easily after they settle into water (lakes, rivers, and
oceans) and soil surfaces than in the atmosphere. As a consequence, their
dissemination via the atmosphere has the potential to alter the microbial
biogeography, biogeochemistry, and ecosystem services of downstream areas.
Moreover, a recent study on phosphorus in aerosol particles in Asian
continental outflow revealed that natural dust particles supplied higher
ratios of bioavailable phosphorus than other types of particles as nutrients
for the primary production in marine ecosystems, and the phosphorus was
presumed to be from the biological particles in dust plumes (Shi
et al., 2019). The dissemination of bacteria with dust in the air is much
more efficient than that via other routes, such as rivers, because dust in
the atmosphere can travel globally within 2 weeks (Uno et al.,
2009). Therefore, the wide dispersal of atmospheric dust is an efficient
link between bacterial communities in geographically isolated ecosystems.
This linking function is likely the key process that constantly blurs the
distinctions between closely related microbial species in distant areas.
Thus, the diversities of microorganisms have a geographically weak gradient
at the global scale and are functions of habitat properties but not of
historical/evolutionary factors (Fenchel and Finlay, 2004).
Health effects
Allergenic and toxic bacteria inhaled and deposited on the surface of upper
respiratory tracts and lungs are suggested to provoke severe adverse health
effects, regardless of whether the bacteria are viable, dead, or cell
fragments (Fröhlich-Nowoisky et al., 2016; Després et al., 2007).
Dust particles carrying biological materials, including bacteria with
pathogenic, allergenic, and adjuvant activity, can cause and aggravate
respiratory disorders (Reinmuth-Selzle et al., 2017). The size
distribution of bacteria-related particles in the air is particularly
meaningful because the movement and deposition of the particles in the
airways are size-dependent. Particles larger than 0.5 µm are deposited
by sedimentation and impaction mainly in the head airways, and particles
smaller than 0.5 µm can reach the lower airways by diffusion
(Fröhlich-Nowoisky et al., 2016). According to
the size distribution of the airborne bacteria-related particles in this
study (Figs. 1 and S9), the deposition fraction and abundance of
particle-attached bacteria are much higher than those of individual cells in
both the upper and the lower airways. Polymenakou et al. (2008) reported
that a large fraction of airborne bacteria at respiratory particle sizes
(<3.3µm) during an intense dust event were phylogenetic
neighbors to human pathogens. He et al. (2012) suggested that Asian dust
caused the exacerbation of pneumonia induced by Klebsiella pneumoniae due to the enhanced
production of pro-inflammatory mediators in alveolar macrophages. Therefore,
free-floating bacterial cells are likely to more easily influence the deep
parts than the upper parts of respiratory airways, while the negative
influence of particle-attached bacteria, particularly under dust conditions,
is expected to be more serious in the upper parts than in the deep parts of
respiratory airways.
Conclusions
In this study, we aimed to quantify the particle-attached and free-floating
bacteria in dusty and nondusty air in southwestern Japan using the
fluorescent enumeration of bacterial cells in size-segregated aerosol
samples. The bacteria showed bimodal number size distributions during dust
episodes, while the distributions largely varied during nondusty periods.
Particle-attached bacteria in dust episodes, with a concentration of 3.2±2.1×105 cells m-3 on average, occupied 72±9 % of the total bacteria. In contrast, this percentage was 56±17 % during nondusty periods, with a concentration of 1.1±0.7×105 cells m-3. The results indicate that dust
particles conveyed substantial numbers of bacterial cells on their surfaces.
Viable particle-attached bacteria were more abundant than viable
free-floating bacteria in dusty air, which is compatible with the previous
results that larger particles harbor more viable and/or culturable bacteria
than smaller particles.
The viability (approximately 63±21 %) of particle-attached
bacteria was much lower than that (87±12 %) of free-floating
bacteria, likely because atmospheric stressors along with long-distance
transport inhibited the survival of particle-attached bacteria and the
entrainment of locally originating free-floating bacteria. High
concentrations and viabilities of free-floating bacteria were observed in
stagnant air, mostly under anticyclone conditions, suggesting that locally
emitted bacteria accounted for the major fractions.
The present results, quantitatively showing the state of airborne bacteria
in association with particles, i.e., particle-attached and free-floating
bacteria, could have broad implications in the disciplines of atmospheric
sciences, ecology, public health, and climate. In addition, the methods used
in this study are low cost and easily available but are time- and
labor-intensive. Verification of the status of airborne bacteria using
efficient techniques, such as in situ electron microscopy, and the exploration of
the compositions, functions, and activities of particle-attached and
free-floating bacteria in the atmosphere are necessary to deepen our
understanding of the related fields.
Data availability
All data are available from the corresponding
author upon request. Datasets for Figs. 1 and 2 are given in Tables S2 and S3
in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/bg-17-4477-2020-supplement.
Author contributions
DZ and WH designed the research. WH, KM, CF, and
SH performed the research. WH, KM, and DZ analyzed data and wrote the paper. HM
and PF reviewed and commented on the paper.
Competing interests
The authors declare that they have no conflict
of interest.
Acknowledgements
We thank Yuka Horikawa, Megumi Mukogawa, and Miki
Miyamoto for their assistance with sampling and analysis.
Financial support
This research has been supported by the Japan Society for the Promotion of Science KAKENHI (grant nos. JP16H02492 and 17K18811) and the National Natural Science Foundation of China (grant nos. 41805118 and 41977183).
Review statement
This paper was edited by Carolin Löscher and reviewed by three anonymous referees.
ReferencesAgarwal, S.: Seasonal variability in size-segregated airborne bacterial
particles and their characterization at different source-sites, Environ.
Sci. Pollut. Res., 24, 13519–13527, 10.1007/s11356-017-8705-2, 2017.Ariya, P. A., Sun, J., Eltouny, N. A., Hudson, E. D., Hayes, C. T., and Kos,
G.: Physical and chemical characterization of bioaerosols – Implications
for nucleation processes, Int. Rev. Phys. Chem., 28,
1–32, 10.1080/01442350802597438, 2009.Augustin-Bauditz, S., Wex, H., Denjean, C., Hartmann, S., Schneider, J., Schmidt, S., Ebert, M., and Stratmann, F.: Laboratory-generated mixtures of mineral dust particles with biological substances: characterization of the particle mixing state and immersion freezing behavior, Atmos. Chem. Phys., 16, 5531–5543, 10.5194/acp-16-5531-2016, 2016.Bauer, H., Claeys, M., Vermeylen, R., Schueller, E., Weinke, G., Berger, A.,
and Puxbaum, H.: Arabitol and mannitol as tracers for the quantification of
airborne fungal spores, Atmos. Environ., 42, 588–593,
10.1016/j.atmosenv.2007.10.013, 2008.Berney, M., Hammes, F., Bosshard, F., Weilenmann, H. U., and Egli, T.:
Assessment and interpretation of bacterial viability by using the LIVE/DEAD
BacLight Kit in combination with flow cytometry, Appl. Environ. Microbiol.,
73, 3283–3290, 10.1128/AEM.02750-06, 2007.Boose, Y., Baloh, P., Plötze, M., Ofner, J., Grothe, H., Sierau, B., Lohmann, U., and Kanji, Z. A.: Heterogeneous ice nucleation on dust particles sourced from nine deserts worldwide – Part 2: Deposition nucleation and condensation freezing, Atmos. Chem. Phys., 19, 1059–1076, 10.5194/acp-19-1059-2019, 2019.Bowers, R. M., Clements, N., Emerson, J. B., Wiedinmyer, C., Hannigan, M.
P., and Fierer, N.: Seasonal variability in bacterial and fungal diversity
of the near-surface atmosphere, Environ. Sci. Technol., 47, 12097–12106,
10.1021/es402970s, 2013.Burrows, S. M., Elbert, W., Lawrence, M. G., and Pöschl, U.: Bacteria in the global atmosphere – Part 1: Review and synthesis of literature data for different ecosystems, Atmos. Chem. Phys., 9, 9263–9280, 10.5194/acp-9-9263-2009, 2009.Burrows, S. M., Hoose, C., Pöschl, U., and Lawrence, M. G.: Ice nuclei in marine air: biogenic particles or dust?, Atmos. Chem. Phys., 13, 245–267, 10.5194/acp-13-245-2013, 2013.Caliz, J., Triado-Margarit, X., Camarero, L., and Casamayor, E. O.: A
long-term survey unveils strong seasonal patterns in the airborne microbiome
coupled to general and regional atmospheric circulations, Proc. Natl. Acad. Sci.
USA, 115, 12229–12234, 10.1073/pnas.1812826115, 2018.Conen, F., Morris, C. E., Leifeld, J., Yakutin, M. V., and Alewell, C.: Biological residues define the ice nucleation properties of soil dust, Atmos. Chem. Phys., 11, 9643–9648, 10.5194/acp-11-9643-2011, 2011.Creamean, J. M., Suski, K. J., Rosenfeld, D., Cazorla, A., DeMott, P. J.,
Sullivan, R. C., White, A. B., Ralph, F. M., Minnis, P., Comstock, J. M.,
Tomlinson, J. M., and Prather, K. A.: Dust and biological aerosols from the
Sahara and Asia influence precipitation in the western U.S., Science, 339,
1572–1578, 10.1126/science.1227279, 2013.Delort, A.-M., Vaïtilingom, M., Amato, P., Sancelme, M., Parazols, M.,
Mailhot, G., Laj, P., and Deguillaume, L.: A short overview of the microbial
population in clouds: potential roles in atmospheric chemistry and
nucleation processes, Atmos. Res., 98, 249–260,
10.1016/j.atmosres.2010.07.004, 2010.Després, V. R., Nowoisky, J. F., Klose, M., Conrad, R., Andreae, M. O., and Pöschl, U.: Characterization of primary biogenic aerosol particles in urban, rural, and high-alpine air by DNA sequence and restriction fragment analysis of ribosomal RNA genes, Biogeosciences, 4, 1127–1141, 10.5194/bg-4-1127-2007, 2007.Després, V. R., Huffman, J. A., Burrows, S. M., Hoose, C., Safatov, A.
S., Buryak, G., Fröhlich-Nowoisky, J., Elbert, W., Andreae, M. O., and
Pöschl, U.: Primary biological aerosol particles in the atmosphere: a
review, Tellus B, 64, 15598,
10.3402/tellusb.v64i0.15598, 2012.Fenchel, T. and Finlay, B. J.: The ubiquity of small species: patterns of
local and global diversity, BioScience, 54, 777–784,
10.1641/0006-3568(2004)054[0777:TUOSSP]2.0.CO;2, 2004.Fröhlich-Nowoisky, J., Ruzene Nespoli, C., Pickersgill, D. A., Galand, P. E., Müller-Germann, I., Nunes, T., Gomes Cardoso, J., Almeida, S. M., Pio, C., Andreae, M. O., Conrad, R., Pöschl, U., and Després, V. R.: Diversity and seasonal dynamics of airborne archaea, Biogeosciences, 11, 6067–6079, 10.5194/bg-11-6067-2014, 2014.Fröhlich-Nowoisky, J., Kampf, C. J., Weber, B., Huffman, J. A.,
Pöhlker, C., Andreae, M. O., Lang-Yona, N., Burrows, S. M., Gunthe, S.
S., Elbert, W., Su, H., Hoor, P., Thines, E., Hoffmann, T., Després, V.
R., and Pöschl, U.: Bioaerosols in the Earth system: climate, health,
and ecosystem interactions, Atmos. Res., 182, 346–376,
10.1016/j.atmosres.2016.07.018, 2016.Griffin, D. W.: Atmospheric movement of microorganisms in clouds of desert
dust and implications for human health, Clin. Microbiol. Rev., 20, 459–477,
10.1128/CMR.00039-06, 2007.Hara, K. and Zhang, D.: Bacterial abundance and viability in long-range
transported dust, Atmos. Environ., 47, 20–25,
10.1016/j.atmosenv.2011.11.050, 2012.Hara, K., Zhang, D., Yamada, M., Matsusaki, H., and Arizono, K.: A detection
of airborne particles carrying viable bacteria in an urban atmosphere of
Japan, Asian J. Atmos. Environ., 5, 152–156, 10.5572/ajae.2011.5.3.152,
2011.He, M., Ichinose, T., Yoshida, S., Yamamoto, S., Inoue, K., Takano, H.,
Yanagisawa, R., Nishikawa, M., Mori, I., Sun, G., and Shibamoto, T.: Asian
sand dust enhances murine lung inflammation caused by Klebsiella pneumoniae,
Toxicol. Appl. Pharmacol., 258, 237–247, 10.1016/j.taap.2011.11.003, 2012.Hu, W., Murata, K., Fukuyama, S., Kawai, Y., Oka, E., Uematsu, M., and
Zhang, D.: Concentration and Viability of Airborne Bacteria Over the
Kuroshio Extension Region in the Northwestern Pacific Ocean: Data From Three
Cruises, J. Geophys. Res.-Atmos., 122, 12892–12905, 10.1002/2017jd027287,
2017.Huffman, J. A., Treutlein, B., and Pöschl, U.: Fluorescent biological aerosol particle concentrations and size distributions measured with an Ultraviolet Aerodynamic Particle Sizer (UV-APS) in Central Europe, Atmos. Chem. Phys., 10, 3215–3233, 10.5194/acp-10-3215-2010, 2010Iwasaka, Y., Shi, G.-Y., Yamada, M., Kobayashi, F., Kakikawa, M., Maki, T.,
Naganuma, T., Chen, B., Tobo, Y., and Hong, C.: Mixture of Kosa (Asian dust)
and bioaerosols detected in the atmosphere over the Kosa particles source
regions with balloon-borne measurements: possibility of long-range
transport, Air Qual. Atmos. Health, 2, 29–38, 10.1007/s11869-009-0031-5,
2009.Joung, Y. S., Ge, Z., and Buie, C. R.: Bioaerosol generation by raindrops on
soil, Nat. Commun., 8, 14668, 10.1038/ncomms14668, 2017.Kasprzyk, I. and Walanus, A.: Gamma, Gaussian and logistic distribution
models for airborne pollen grains and fungal spore season dynamics,
Aerobiologia (Bologna), 30, 369–383, 10.1007/s10453-014-9332-8, 2014.Lighthart, B.: The ecology of bacteria in the alfresco atmosphere, FEMS
Microbiol. Ecol., 23, 263–274, 10.1111/j.1574-6941.1997.tb00408.x, 1997.Lighthart, B.: Mini-review of the concentration variations found in the
alfresco atmospheric bacterial populations, Aerobiologia, 16, 7–16,
10.1023/A:1007694618888, 2000.Limpert, E., Burke, J., Galan, C., del Mar Trigo, M., West, J. S., and Stahel,
W. A.: Data, not only in aerobiology: how normal is the normal
distribution?, Aerobiologia, 24, 121–124, 10.1007/s10453-008-9092-4, 2008.Maki, T., Susuki, S., Kobayashi, F., Kakikawa, M., Yamada, M., Higashi, T.,
Chen, B., Shi, G., Hong, C., and Tobo, Y.: Phylogenetic diversity and
vertical distribution of a halobacterial community in the atmosphere of an
Asian dust (KOSA) source region, Dunhuang City, Air Qual. Atmos. Health, 1,
81–89, 10.1007/s11869-008-0016-9, 2008.Maki, T., Kakikawa, M., Kobayashi, F., Yamada, M., Matsuki, A., Hasegawa,
H., and Iwasaka, Y.: Assessment of composition and origin of airborne
bacteria in the free troposphere over Japan, Atmos. Environ., 74, 73–82,
10.1016/j.atmosenv.2013.03.029, 2013.Matthias-Maser, S. and Jaenicke, R.: The size distribution of primary
biological aerosol particles with radii>0.2µm in an
urban/rural influenced region, Atmos. Res., 39, 279–286,
10.1016/0169-8095(95)00017-8, 1995.Matthias-Maser, S. and Jaenicke, R.: The size distribution of primary
biological aerosol particles in the multiphase atmosphere, Aerobiologia, 16,
207–210, 10.1023/A:1007607614544 2000.Matthias-Maser, S., Brinkmann, J., and Schneider, W.: The size distribution
of marine atmospheric aerosol with regard to primary biological aerosol
particles over the South Atlantic Ocean, Atmos. Environ., 33, 3569–3575,
10.1016/S1352-2310(98)00121-6, 1999.Möhler, O., DeMott, P. J., Vali, G., and Levin, Z.: Microbiology and atmospheric processes: the role of biological particles in cloud physics, Biogeosciences, 4, 1059–1071, 10.5194/bg-4-1059-2007, 2007.Montero, A., Dueker, M. E., and O'Mullan, G. D.: Culturable bioaerosols
along an urban waterfront are primarily associated with coarse particles,
PeerJ, 4, e2827, 10.7717/peerj.2827, 2016.Morris, C. E., Sands, D. C., Bardin, M., Jaenicke, R., Vogel, B., Leyronas, C., Ariya, P. A., and Psenner, R.: Microbiology and atmospheric processes: research challenges concerning the impact of airborne micro-organisms on the atmosphere and climate, Biogeosciences, 8, 17–25, 10.5194/bg-8-17-2011, 2011.Murata, K. and Zhang, D.: Applicability of LIVE/DEAD BacLight stain with
glutaraldehyde fixation for the measurement of bacterial cell concentration
and viability in the air, Aerosol Air Qual. Res., 13, 1755–1767,
10.4209/aaqr.2012.10.0293, 2013.Murata, K. and Zhang, D.: Concentration of bacterial aerosols in response
to synoptic weather and land-sea breeze at a seaside site downwind of the
Asian continent, J. Geophys. Res.-Atmos., 121, 11636–11647,
10.1002/2016jd025028, 2016.Polymenakou, P. N., Mandalakis, M., Stephanou, E. G., and Tselepides, A.:
Particle size distribution of airborne microorganisms and pathogens during
an Intense African dust event in the eastern Mediterranean, Environ. Health
Perspect., 116, 292–296, 10.1289/ehp.10684, 2008.
Portillo, M. C., Leff, J. W., Lauber, C. L., and Fierer, N.: Cell size
distributions of soil bacterial and archaeal taxa, Appl. Environ.
Microbiol., 79, 7610–7617, 2013.Pöschl, U. and Shiraiwa, M.: Multiphase chemistry at the
atmosphere-biosphere interface influencing climate and public health in the
anthropocene, Chem. Rev., 115, 4440–4475, 10.1021/cr500487s, 2015.Pöschl, U., Martin, S. T., Sinha, B., Chen, Q., Gunthe, S. S., Huffman,
J. A., Borrmann, S., Farmer, D. K., Garland, R. M., Helas, G., Jimenez, J.
L., King, S. M., Manzi, A., Mikhailov, E., Pauliquevis, T., Petters, M. D.,
Prenni, A. J., Roldin, P., Rose, D., Schneider, J., Su, H., Zorn, S. R.,
Artaxo, P., and Andreae, M. O.: Rainforest aerosols as biogenic nuclei of
clouds and precipitation in the Amazon, Science, 329, 1513–1516,
10.1126/science.1191056, 2010.Pósfai, M., Li, J., Anderson, J. R., and Buseck, P. R.: Aerosol bacteria
over the Southern Ocean during ACE-1, Atmos. Res., 66, 231–240,
10.1016/s0169-8095(03)00039-5, 2003.Pratt, K. A., DeMott, P. J., French, J. R., Wang, Z., Westphal, D. L.,
Heymsfield, A. J., Twohy, C. H., Prenni, A. J., and Prather, K. A.: In situ
detection of biological particles in cloud ice-crystals, Nat. Geosci., 2, 398–401,
10.1038/ngeo521, 2009.Prospero, J. M., Blades, E., Mathison, G., and Naidu, R.: Interhemispheric
transport of viable fungi and bacteria from Africa to the Caribbean with
soil dust, Aerobiologia, 21, 1–19, 10.1007/s10453-004-5872-7, 2005.Raisi, L., Aleksandropoulou, V., Lazaridis, M., and Katsivela, E.: Size
distribution of viable, cultivable, airborne microbes and their relationship
to particulate matter concentrations and meteorological conditions in a
Mediterranean site, Aerobiologia, 29, 233–248, 10.1007/s10453-012-9276-9,
2013.Reinmuth-Selzle, K., Kampf, C. J., Lucas, K., Lang-Yona, N.,
Frohlich-Nowoisky, J., Shiraiwa, M., Lakey, P. S. J., Lai, S., Liu, F.,
Kunert, A. T., Ziegler, K., Shen, F., Sgarbanti, R., Weber, B.,
Bellinghausen, I., Saloga, J., Weller, M. G., Duschl, A., Schuppan, D., and
Pöschl, U.: Air Pollution and Climate Change Effects on Allergies in the
Anthropocene: Abundance, Interaction, and Modification of Allergens and
Adjuvants, Environ. Sci. Technol., 51, 4119–4141, 10.1021/acs.est.6b04908,
2017.Savage, N. J., Krentz, C. E., Könemann, T., Han, T. T., Mainelis, G., Pöhlker, C., and Huffman, J. A.: Systematic characterization and fluorescence threshold strategies for the wideband integrated bioaerosol sensor (WIBS) using size-resolved biological and interfering particles, Atmos. Meas. Tech., 10, 4279–4302, 10.5194/amt-10-4279-2017, 2017.Schuerger, A. C., Smith, D. J., Griffin, D. W., Jaffe, D. A., Wawrik, B.,
Burrows, S. M., Christner, B. C., Gonzalez-Martin, C., Lipp, E. K., Schmale
Iii, D. G., and Yu, H.: Science questions and knowledge gaps to study
microbial transport and survival in Asian and African dust plumes reaching
North America, Aerobiologia, 34, 425–435, 10.1007/s10453-018-9541-7, 2018.Shaffer, B. T. and Lighthart, B.: Survey of culturable airborne bacteria at
four diverse locations in Oregon: urban, rural, forest, and coastal, Microb.
Ecol., 34, 167–177, 10.1007/s002489900046, 1997.Shi, J., Wang, N., Gao, H., Baker, A. R., Yao, X., and Zhang, D.: Phosphorus solubility in aerosol particles related to particle sources and atmospheric acidification in Asian continental outflow, Atmos. Chem. Phys., 19, 847–860, 10.5194/acp-19-847-2019, 2019.
Tobo, Y., Adachi, K., DeMott, P. J., Hill, T. C. J., Hamilton, D. S.,
Mahowald, N. M., Nagatsuka, N., Ohata, S., Uetake, J., Kondo, Y., and Koike,
M.: Glacially sourced dust as a potentially significant source of ice
nucleating particles, Nat. Geosci., 12, 253–258, 10.1038/s41561-019-0314-x,
2019.Tong, Y. and Lighthart, B.: The annual bacterial particle concentration and
size distribution in the ambient atmosphere in a rural area of the
Willamette Valley, Oregon, Aerosol Sci. Technol., 32, 393–403,
10.1080/027868200303533, 2000.Uno, I., Eguchi, K., Yumimoto, K., Takemura, T., Shimizu, A., Uematsu, M.,
Liu, Z., Wang, Z., Hara, Y., and Sugimoto, N.: Asian dust transported one
full circuit around the globe, Nat. Geosci., 2, 557–560, 10.1038/ngeo583,
2009.Yamaguchi, N., Ichijo, T., Baba, T., and Nasu, M.: Long-range transportation
of bacterial cells by Asian dust, Genes and Environment, 36, 145–151,
10.3123/jemsge.2014.015, 2014.Yue, S., Ren, H., Fan, S., Wei, L., Zhao, J., Bao, M., Hou, S., Zhan, J.,
Zhao, W., Ren, L., Kang, M., Li, L., Zhang, Y., Sun, Y., Wang, Z., and Fu,
P.: High abundance of fluorescent biological aerosol particles in winter in
Beijing, China, ACS Earth Space Chem., 1, 493–502,
10.1021/acsearthspacechem.7b00062, 2017.Zhang, D.: Effect of sea salt on dust settling to the ocean, Tellus B Chem.
Phys. Meteorol., 60B, 641–646, 10.1111/j.1600-0889.2008.00358.x, 2008.Zhang, D., Iwasaka, Y., Shi, G., Zang, J., Matsuki, A., and Trochkine, D.:
Mixture state and size of Asian dust particles collected at southwestern
Japan in spring 2000, J. Geophys. Res.-Atmos., 108, 4760,
10.1029/2003jd003869, 2003.Zhang, D., Murata, K., Hu, W., Yuan, H., Li, W., Matsusaki, H., and
Kakikawa, M.: Concentration and viability of bacterial aerosols associated
with weather in Asian continental outflow: current understanding, Aerosol
Sci. Eng., 1, 66–77, 10.1007/s41810-017-0008-y, 2017.