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Aerosol and Air Quality Research, 15: 572–581, 2015 Copyright ©
Taiwan Association for Aerosol Research ISSN: 1680-8584 print /
2071-1409 online doi: 10.4209/aaqr.2014.10.0258 Seasonal and
Diurnal Variations of Fluorescent Bioaerosol Concentration and Size
Distribution in the Urban Environment Sampo Saari1*, JarkkoV.
Niemi2,3, Topi Rönkkö1, Heino Kuuluvainen1, Anssi Järvinen1, Liisa
Pirjola4, Minna Aurela5, Risto Hillamo5, Jorma Keskinen1 1
Department of Physics, Tampere University of Technology, Tampere,
Finland 2 Helsinki Region Environmental Services Authority (HSY),
P.O. Box 100, FI-00066 HSY, Finland 3 Department of Environmental
Sciences, University of Helsinki, P.O. Box 65, FI-00014 Helsinki,
Finland 4 Department of Technology, Metropolia University of
Applied Science, Kalevankatu 43, FI-00180 Helsinki, Finland 5
Atmospheric Composition Research, Finnish Meteorological Institute,
Erik Palménin aukio 1, FI-00560 Helsinki, Finland ABSTRACT
A recently introduced fluorescence based real-time bioaerosol
instrument, BioScout, and an ultraviolet aerodynamic particle sizer
(UVAPS) were used to study fluorescent bioaerosol particles (FBAP)
in the Helsinki metropolitan area, Finland, during winter and
summer. Two FBAP modes at 0.5–1.5 µm (fine) and 1.5–5 µm (coarse)
were detected during the summer, whereas the fine mode dominated in
the winter. The concentration and proportion of the coarse FBAP was
high in summer (0.028 #/cm3, 23%) and low in winter (0.010 #/cm3,
6%). Snow cover and low biological activity were assumed to be the
main reasons for the low coarse FBAP concentration in the
wintertime. Both the fine and the coarse FBAP fraction typically
increased at nighttime during the summer. Correlations between the
BioScout and the UVAPS were high with the coarse (R = 0.83) and
fine (R = 0.92) FBAP. The BioScout showed 2.6 and 9.7 times higher
detection efficiencies for the coarse and fine FBAP, respectively,
compared to the UVAPS. A long-range transport episode of particles
from Eastern Europe increased the fine FBAP concentration by over
two orders of magnitude compared to the clean period in the winter,
but these FBAP probably also included fluorescent non-biological
particles. Correlation analysis indicates that local combustion
sources did not generate fluorescent non-biological particles that
can disturb fine FBAP counting. The results provide information
that can be used to estimate health risks and climatic relevance of
bioaerosols in the urban environment. Keywords: Fluorescence;
Fungal spores; Bacteria; UVAPS; BioScout. INTRODUCTION
Bioaerosols such as bacteria and fungal spores can cause adverse
health effects for people and animals both in indoor and outdoor
environments (Burge and Rogers, 2000; Peccia et al., 2008; Mendell
et al., 2011). Atmospheric bioaerosols, usually called primary
biogenic aerosol particles (PBAP), consist mainly of bacteria,
fungal spores and fragments, pollens, algae and plant debris
(Després et al., 2012). PBAPs have been recognized to have
important influence on climate, acting as cloud condensation nuclei
(CCN) and ice nuclei (IN) and thus contributing cloud formation and
precipitation processes (Vali et al., 1976; Bauer et al., 2002,
2003; Jaenicke, 2005; Sun and Ariya, 2006; Andreae and *
Corresponding author.
Tel.: +358 3 311 511; Fax: +358 3 3115 2640 E-mail address:
[email protected]
Rosenfeld, 2008; Pöschl et al., 2010; Després et al., 2012). It
seems clear that nature is the most important source of PBAP on a
global scale, but anthropogenic sources, such as agriculture, waste
treatment, buildings and cooking, may also be significant in some
regions. Potential health risks of bioaerosols are especially high
in urban environments due to their dense populations. Ryan et al.
(2009) reported that a combination of bioaerosols and
traffic-related emission exposure was associated with wheezing at
age 3 years. Bowers et al. (2011) reported that the major bacteria
sources in the urban environments are soil, leaf surfaces and dog
feces. Information on concentrations, particle size distributions
and sources of bioaerosols is needed to estimate their health risks
and climatic relevance.
Various techniques have been used to collect ambient
bioaerosols, such as impactor and filter sampling (Reponen et al.,
2011). These off-line analysis methods require separate steps
before concentration can be analyzed, resulting in relatively low
time resolution. Real-time measurement would aid in understanding
regional and global emissions,
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Saari et al., Aerosol and Air Quality Research, 15: 572–581,
2015 573
transport and abundance of PBAP (Burrows et al., 2009; Heald and
Spracklen, 2009; Huffman et al., 2010). Laser induced fluorescence
(LIF) based instruments are modern, easy-to-use tools for real-time
bioaerosol detection (e.g., Hill et al., 1995; Pinnick et al.,
1995; Ho, 2002; Jeys et al., 2007; Pöhlker et al., 2012). The LIF
technique is an effective method for detecting biological molecules
such as tryptophan, NADH and flavins that are present in microbe
cells (Lakowicz, 2006; Hill et al., 2009). Saari et al. (2013)
demonstrated that bacterial and fungal spores may be distinguished
from each other through their dissimilar fluorescence spectra. The
most well-known real-time LIF instrument is the ultraviolet
aerodynamic particle sizer (UVAPS; Hairston et al., 1997,
manufactured by TSI Inc., St. Paul, Minnesota) that is able to
measure both the aerodynamic particle size and the autofluorescence
of a single particle. Compared to the UVAPS, our recent study
showed that performance of a simple, violet diode laser based
bioaerosol detector, BioScout (Environics Oy, Finland), may be even
more sensitive in measuring common bioaerosols (Saari et al.,
2014).
Recently, LIF based real-time instruments have been used for
detection of coarse (> 1.5 µm) PBAP in urban, suburban, desert,
tropical rainforest, high-altitude and boreal forest environments
(Pan et al. 2008; Gabey et al., 2010; Huffman et al., 2010; Gabey
et al., 2011; Huffman et al., 2012; Toprak et al., 2012; Gabey et
al., 2013, Huffman et al., 2013; Schumacher et al., 2013). Huffman
et al. (2012) applied a UVAPS and a filter sampling technique to
measure PBAP size distributions and concentrations in the Amazon
rainforest region. They found a good correlation between
fluorescent particles (UVAPS) and filter analysis (scanning
electron microscopy as well as staining and light microscopy).
However, they concluded that only the lower limit of PBAP
concentration can be estimated with the UVAPS because some PBAPs
cannot be detected due to weak fluorescence. This fluorescent
fraction of bioaerosols is usually called fluorescent biological
aerosol particles (FBAP).
Note that the fluorescent particle data can also include
non-biological particles that have similar fluorescence
characteristics, such as cigarette smoke or other
combustion-generated aerosols (e.g., Pinnick et al., 1998; Huffman
et al., 2010; Gabey et al., 2011). For the FBAP measurement, these
particles form an artifact that needs to be considered when
analyzing the data. Huffman et al. (2010) found a high correlation
of the number of fine fluorescent particles with the total number
of particles, concluding that a large percentage of submicron
particles exhibiting fluorescence may have anthropogenic sources.
Generally, the combustion generated fluorescent particles are
expected to be more abundant in the fine mode. Consequently, the
fluorescent fine particles have not been much analyzed. When
observed, they have been treated as non-biological particles
(Huffman et al., 2010; Gabey et al., 2011).
In this study, we used two LIF based real-time bioaerosol
instruments, a recently introduced BioScout and a UVAPS, to study
FBAP concentrations and size distributions at urban and suburban
residential sites in the Helsinki metropolitan
area during winter and summer. To our knowledge, this is the
first study wherein the BioScout was used in an outdoor environment
and compared to another LIF based instrument. We also demonstrate
that comparison between the real-time LIF data and particle mass
(PM2.5), black carbon (BC) and nitrogen oxides (NOx), as well as
meteorological data, enables estimation of bioaerosol sources and
transportation. Contrary to the accepted custom, we especially
extend the analysis also to the fluorescent fine particles. METHODS
BioScout
The BioScout (Environics Oy, Finland) uses a 405 nm continuous
wave laser diode with 200 mW optical power to excite
autofluorescence from individual bioaerosol particles. Both the
autofluorescence and the scattering light are collected by an
elliptical mirror and focused onto two photomultiplier tubes
(PMTs). The autofluorescence is separated from the scattered light
using a beam splitter and a long-pass filter with a cut point at
442 nm, and the fluorescence intensity is recorded by a PMT and
sorted into 16 channels. The scattering light intensity is used to
analyze the optical particle size. The time resolution of the
instrument is 1 second. The particle size calibration of the
BioScout was conducted against di-octyl sebacate (DOS) and NaCl
particles using the UVAPS (TSI Model 3314) and the SCAR (Single
Charged Aerosol Reference; Yli-Ojanperä et al., 2010). The
operating particle size range was optimized between 0.3 and 5 µm. A
laboratory study by Saari et al. (2014) showed that detection
efficiency of the BioScout was high for fine bacterial particles
(< 1 µm) that may also have a role in the atmospheric fine FBAP
mode. UVAPS
Hairston et al. (1997) introduced the first prototype of the
UVAPS. The current version of the UVAPS (TSI Model 3014) measures
aerodynamic diameter of particles between 0.5 µm and 15 µm with 52
channels. The UVAPS also measures the autofluorescence emission of
individual particles using a pulsed 355 nm UV laser with 80 mW
maximum power as an excitation source. The autofluorescence is
recorded between 430 and 580 nm by a PMT and sorted into 64
channels. In this study, the UV pulse energy and fluorescence PMT
gain were set into their default values. The time resolution was
adjusted to 5 min in this study. The performance of the UVAPS
against common bioaerosols has been reported in several studies
(e.g., Agranovski et al., 2003; Kanaani et al., 2007; Jung et al.,
2012; Saari et al., 2014). Measurements
Two measurement campaigns were conducted at urban and suburban
residential sites in the Helsinki metropolitan area. The
measurement campaigns took place in winter and summer during the
time periods 2.2.–25.2.2012 and 16.6.–22.8.2012, respectively. The
winter campaign was performed in a suburban residential area in
Kattilalaakso, in Espoo. The summer measurements were placed in an
urban
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Saari et al., Aerosol and Air Quality Research, 15: 572–581,
2015 574
residential area, Kallio, beside a green sporting field in
Helsinki’s downtown area. Both the measurement locations were
influenced by local vegetation and human activity. Helsinki is
located on the southern coast of Finland and thus the marine
climate is strongly present there. The BioScout was used in both
campaigns, but the UVAPS was present only during the summer
campaign. The instruments were placed in the measurement station,
and the aerosol sample was brought via total suspended particle
inlet (TSP) on the top of the station during the summer period. In
the winter, the BioScout was placed on the roof of the station and
the sample was brought through its own inlet, which had a cut point
at 10 µm. The sampling height was about 4 meters from the street
level in both campaigns.
The supporting PM2.5, NOx and BC data was monitored at the
measurement stations by the Helsinki Region Environmental Services
Authority and Finnish Meteorological Institute. NOx was measured
using a chemiluminescence analyzer (model APNA 360, Horiba). PM2.5
was monitored with a tapered element oscillating system
microbalance (TEOM model 1400 AB, Thermo Scientific; summer) or
with a Synchronized Hybrid Ambient Real-time Particulate Monitor
(SHARP model 5030, Thermo Scientific; winter). BC was measured
using a Multiangle Absorption Photometer (MAAP model 5012, Thermo
Scientific) with PM1 cut-off. Meteorological data was observed by
the Finnish Meteorological Institute at the Kumpula measurement
site in Helsinki. Data Analysis
The BioScout data were analyzed using MATLAB software. All
particles in fluorescence channels 2–16 were classified as
fluorescent and particles in channel 1 as non-fluorescent.
Non-fluorescent NaCl particles were used to determine the detection
limit for fluorescence. The detection limit was adjusted so that
less than 1% of NaCl particles reached fluorescence channel 2 or
above. The BioScout settings were similar as in the study by Saari
et al. (2014). When the fluorescence data are combined with the
optical particle size data, fluorescent particle and total particle
size distributions and number concentrations can be calculated. The
UVAPS data were analyzed using a similar procedure,
and all particles in fluorescence channels 3–64 were classified
as fluorescent. The UVAPS fluorescence channel 3 was chosen as a
limit for fluorescent particles, similarly as in previous
atmospheric studies by Huffman et al. (2010, 2012, 2013), whereas
fluorescence channel 2 was used in laboratory studies by Agranovski
et al. (2003), Kanaani et al. (2007) and Saari et al. (2014).
Pearson’s linear correlation coefficients between the different
instruments were calculated with one-hour time resolution.
RESULTS AND DISCUSSION Size Distributions
Averaged FBAP and total particle size distributions over the
campaigns are shown in Fig. 1. There were typically two FBAP modes
at 0.5–1.5 µm (fine) and 1.5–5 µm (coarse) during the summer. In
winter, the fine mode was dominant, and no clear coarse mode was
observed. The weather during the winter campaign was typically
cold, the average temperature was −3°C, and the ground was covered
by snow. We assumed that snow cover and low biological activity
were the main reasons for the vanished coarse FBAP in the
wintertime. Low FBAP concentrations measured by the UVAPS in
wintertime have been reported also at Hyytiälä boreal forest site
in Finland in the recent study by Schumacher et al. (2013). The
coarse FBAP has been strongly linked to fungal spores that are
abundant in this size range as reported also in the previous
atmospheric FBAP studies by Huffman et al. (2010, 2012, 2013) and
by Schumacher et al. (2013).
Origin of the fine FBAP is not clear, but the size range is
typical for bacterial spores (Saari et al., 2014). The hypothesis
of bacterial presence is also supported in a recent study by
DeLeon-Rodriguez et al. (2013), wherein viable bacterial cells were
shown to represent, on average, around 20% of the total particles
in the 0.25–1 μm size range in the middle-to-upper troposphere.
Similar observations were made during three measurement campaigns
at Lake Baikal, at an urban site in Mainz and over the South
Atlantic Ocean, where around 20% of total particles (Dp > 0.2
µm) were detected as PBAPs (Matthias-Maser et al., 1995, 1999,
2000). They suggested that bacterial particle concentration
Fig. 1. Averaged fluorescent (left) and total (right) particle
size distributions during the winter and summer periods measured by
the BioScout and the UVAPS. The cut-off point between the modes is
shown at 1.5 µm.
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Saari et al., Aerosol and Air Quality Research, 15: 572–581,
2015 575
was higher in the urban environment due to the anthropogenic
sources. The fine FBAP has also been supposed to come from
combustion sources in urban environments (Huffman et al., 2010).
This possibility is discussed more below. However, our laboratory
studies showed that single bacterial spores are detectable with
both the instruments (Saari et al., 2014), so we assume that at
least a fraction of the observed fine FBAP are bacteria.
The size distributions measured by the BioScout and the UVAPS
were similar but not fully consistent because the UVAPS measures
particle aerodynamic size and the BioScout measures particle
optical size that depends on a refractive index and absorption of
particles. There were also differences in fluorescence sensitivity
between the instruments. The fine mode was clearly weaker and
barely visible with the UVAPS. The modes have been separated at a
cut-off point of 1.5 µm, and concentrations of the fine mode (0.5
< Dp < 1.5 µm) and the coarse mode (Dp > 1.5 µm) will be
analyzed separately below. Fluorescent Particle Concentrations and
Fractions
To compare the response of the instruments, correlation diagrams
and averaged values of the total particle and FBAP concentrations
measured by the BioScout and UVAPS during the summer period are
shown in Fig. 2 and Table 1. Pearson’s linear correlation
coefficient (R) showed high correlation with both total particles
(0.88 and 0.89 for the coarse and fine mode, respectively) and FBAP
(0.83 for the coarse mode and 0.92 for the fine mode) between the
instruments. Linear fit functions showed slope values close to one
for total particles (0.76 and 1.02 for coarse and fine mode,
respectively), indicating that concentrations are consistent
between the instruments despite the different particle size
measurements However, remarkably higher slope values were observed
for FBAP (2.6 and 9.7 for coarse and fine mode, respectively). This
suggests that the BioScout had 2.6 and 9.7 times higher detection
efficiencies for the coarse and fine FBAP, respectively, compared
to the UVAPS. Similar results were found also in our laboratory
studies (Saari et al., 2014). The reason for the higher
detection efficiency of the BioScout is assumed to be high
excitation laser intensity and the good fluorescence signal to
noise ratio of the instrument.
Averaged coarse FBAP concentrations were 0.010 #/cm3 in the
winter and 0.028 #/cm3 in the summer as measured by the BioScout.
In comparison, the UVAPS coarse FBAP concentration was 0.013 #/cm3
in the summer period. The BioScout coarse FBAP fraction (FPF) was
high in the summer (23%) and low in the winter (6%). The UVAPS
coarse mode FPF was 8% in the summer. Surprisingly, the BioScout
fine FBAP concentration was higher in the winter (0.13 #/cm3) than
in the summer (0.018 #/cm3), whereas the fine FPF were lower in the
winter (0.9%) than in the summer (2.9%). The UVAPS fine FBAP
concentration was 0.0025 #/cm3, and the FPF value was 0.31% in the
summer. The BioScout showed higher FPF value for both the coarse
and the fine FBAP compared to the UVAPS, which is in line with the
laboratory studies of the common fungal spores and bacteria (Saari
et al., 2014).
A comprehensive comparison of the averaged PBAP concentrations
and fractions between this study and the previous studies is
represented in Table 1. All the fluorescence based studies of PBAP
coarse mode (measured by BioScout, UVAPS and WIBS) in urban
environments are well in line: Concentrations vary from 0.01 #/cm3
to 0.10 #/cm3, and fractions were between 3% and 23% (this study;
Huffman et al., 2010; Gabey et al., 2011; Toprak et al., 2012).
Interestingly, the results of the off-line staining method showed
similar values for coarse PBAP concentration, ranging from 0.026
#/cm3 to 0.8 #/cm3 (Chi and Li, 2007; Bauer et al., 2008).
Seasonal variations of the concentrations and FPF values of
coarse FBAP are well in line with the previous studies.
Matthias-Maser et al. (2000), Toprak et al. (2012) and Schumacher
et al. (2013) reported the highest coarse PBAP concentration in
summer and the lowest values in winter, which was observed also in
this study. Seasonal behavior of our fine mode results was also
similar with the previous study by Matthias-Maser et al. (2000),
except our winter period, when a long-range transport episode
(LRT)
Fig. 2. Correlation diagrams of the total (NTOT; left) and
fluorescent particle (NFL; right) concentrations measured by the
BioScout and UVAPS during the summer period. Black circles
represent the coarse mode (Dp > 1.5 µm), and grey triangles
represent the fine mode (Dp < 1.5 µm). Pearson’s linear
correlation coefficients (R) and linear fit functions (y = kx) are
shown in diagrams.
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Saari et al., Aerosol and Air Quality Research, 15: 572–581,
2015 576
Table 1. The comparison between the results of this study and
the previous studies. Coarse and fine PBAP represent
supermicrometer and submicrometer bioaerosol particles,
respectively. PBAP concentrations are shown as (#/cm3) and
fractions as (%).
Environment (Location) Season
Measurementmethod
CoarsePBAP(#/cm3)
Fine PBAP(#/cm3)
CoarsePBAP(%)
Fine PBAP
(%) Comments
Urban (Helsinki)1 winter BioScout 0.010 0.13 6 0.9 suburban
summer BioScout 0.028 0.018 23 2.9 urban summer UVAPS 0.013 0.002 8
0.3 urban Urban (Mainz)2 autumn UVAPS 0.03 - 4 - Urban
(Manchester)5 winter WIBS-3 0.1 - 7 - Urban winter WIBS-4 0.03 - 4
- (Karlsruhe)8 spring WIBS-4 0.029 - 7 - summer WIBS-4 0.046 - 11 -
autumn WIBS-4 0.029 - 7 - Urban (Mainz)9 spring EDX + staining -
3.1 - 24 fine + coarse Urban (Vienna)12 summer staining 0.026 - 40
- fungal spores Urban (Taipei)13 summer staining 0.8 - - - bacteria
and fungal sporesMarine (Atlantic)10 winter EDX + staining - 0.6 -
17 fine + coarse Rural (Baikal)11 winter EDX + staining 0.0005 0.03
15 15 spring EDX + staining 0.0015 1 30 20 summer EDX + staining
0.01 1 60 20 autumn EDX + staining 0.001 0.03 20 20 Tropical
(Amazonia)3 winter UVAPS 0.07 - 24 - High-altitude (Colorado)4
summer UVAPS 0.03 - 10 - Tropical (Borneo)6 summer WIBS-3 1.5 - 28
- under the canopy WIBS-3 0.2 - 55 - above the canopy High-altitude
(France)7 summer WIBS-3
staining 0.1
0.017- -
35 -
- -
bacteria and fungal spores
Boreal forest (Finland)14 Rocky Mountains (Colorado)14
winter spring summer autumn winter spring summer autumn
UVAPS UVAPS UVAPS UVAPS UVAPS UVAPS UVAPS UVAPS
0.0040.0150.0460.027
0.00530.0150.0300.017
- - - - - - - -
1.1 4.4 13 9.8 3.0 2.5 8.8 5.7
- - - - - - - -
1 This study; 2,3,4 Huffman et al. (2010, 2012, 2013); 5,6,7
Gabey et al. (2011, 2010, 2013); 8 Toprak et al. (2012); 9,10,11
Matthias-Maser et al. (1995, 1999, 2000); 12 Bauer et al. (2008);
13 Chi and Li (2007); 14 Schumacher et al. (2013). of particles
from Eastern Europe had an influence on the results. Effects of the
LRT are discussed more below. Temporal Variations
In this section, the FBAP concentrations are analyzed as a
function of time and compared to PM2.5, NOx and BC concentrations.
Concentrations of the coarse and fine FBAP and their FPF values
measured by the BioScout as well as PM2.5, NOx and BC
concentrations in the winter are shown in Fig. 3. Based on the
PM2.5 data from other measurement stations of the Helsinki Region
Environmental Services Authority, high concentrations during
February 15–19, 2012, were caused by the long-range transportation
of aerosol particles, due to prevailing meteorological conditions
(Niemi et al., 2009). Back-trajectory analysis of air masses
(HYSPLIT Model; Draxler and Rolph, 2014; Rolph, 2014) showed that
air flows arrived from Eastern Europe during the LRT episode, and
clean air during
February 20–28 was coming from the direction of the Atlantic
Ocean and Greenland. During the LRT episode, concentration of the
coarse mode was only elevated a bit, but the fine FBAP
concentration was over two orders of magnitude higher compared to
the clean period. PM2.5 and BC concentrations during the LRT
episode were high. The correlation analysis between the variables
is discussed more below.
The fine FBAP during the LRT may have been influenced by several
anthropogenic sources, including the biomass burning in fireplaces
that is popular in Eastern Europe during the cold season. However,
bacteria emissions would be also higher in Eastern Europe than in
Finland due to temperature and snow cover differences during the
winter Smith et al. (2013) reported high concentrations of bacteria
taxa in the transpacific plumes from Asian to North. America and
showed that long-range transport is possible for bacteria
particles.
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Saari et al., Aerosol and Air Quality Research, 15: 572–581,
2015 577
Fig. 3. Concentrations of PM2.5, NOx, BC and fluorescent coarse
(solid) and fine (dashed) modes and their fractions (NFL/NTOT)
measured by the BioScout during the long-range transport episode
(LRT; February 15–19) and clean condition in the winter of
2012.
Time series of FBAP concentrations and averaged daily variations
of the FPF values for the modes measured by the BioScout and the
UVAPS during the summer are shown in Fig. 4. Both the coarse and
fine FPF were typically increased at night time, which is reported
to be a characteristic time for the natural emission of fungal
spores (e.g., Huffman et al., 2012). Sometimes, e.g., on June 26,
the concentrations were high also in the daytime, which indicates
the presence of different sources. The fine FBAP showed some short
peaks at nighttime during June 27–29, which indicates the presence
of a strong local source. The source may be either natural, such as
bacteria, or anthropogenic. Similar trends during the measurement
period were shown with both the BioScout and with the UVAPS. The
FBAP concentrations were lower with the UVAPS than with the
BioScout, and the difference was especially high for the fine mode.
This comes from the different bioaerosol detection efficiencies
between the instruments, which are also discussed above.
Correlation Analysis
Correlations between the FBAP and total particles measured by
the BioScout as well as PM2.5, NOx and BC were analyzed in Fig. 5.
To our knowledge, this is the first study wherein FBAP
concentrations were compared to the PM2.5, NOx and BC values. In
the winter, correlations were shown separately during the LRT
episode and the clean period.
During the LRT, the fine FBAP correlated well with the total
particle concentration (R = 0.93), whereas correlation was low
between the coarse particles (R = 0.35). Similar
results were reported in the previous study by Huffman et al.
(2010). High correlations, Rcoarse = 0.71 and Rfine = 0.73, were
found between the FBAP and the PM2.5 during the LRT, indicating
high FBAP content in the transported PM2.5 fraction. Correlations
between BC and FBAP were the highest during the LRT episode
(Rcoarse = 0.60 and Rfine = 0.65). This indicates that aged
BC-containing particles may have influenced the FBAP values acting
as non-biological fluorescent particles. Condensation and cloud
processes can increase the size of BC-containing particles in the
atmosphere, and therefore, BC also can present in larger particle
sizes in LRT aerosols (Niemi et al., 2006).
Correlations between FBAP and total particles were low with both
modes (Rcoarse = 0.15 and Rfine = 0.29) during the clean period in
winter, indicating random emission of FBAP. Interestingly, coarse
particles correlated better in the summer (Rcoarse = 0.61 and Rfine
= 0.10). This indicates regular local emission of coarse FBAP
(presumably fungal spores) and random fine FBAP emission in
summertime.
The results showed that correlations between the FBAP and NOx
were low during the all measurement periods (0.01 < Rcoarse <
0.54 and 0 < Rfine < 0.07). This indicates that local vehicle
traffic, the dominant NOx source in the Helsinki metropolitan area,
did not disturb FBAP counting at these measurement sites. Fine FBAP
correlations with BC were low during the clean period in the winter
(R = 0.32) and in the summer campaign (R = 0.09). This indicates
that local BC related combustion sources did not disturb the fine
FBAP analysis during these periods and that most of the fine FBAPs
counted by the BioScout were real bioaerosols.
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Saari et al., Aerosol and Air Quality Research, 15: 572–581,
2015 578
Fig. 4. Time series of fluorescent particle concentrations and
averaged daily variations of the fluorescent particle fractions
(NFL/NTOT) for the coarse mode (Dp > 1.5 µm; above) and fine
mode (Dp < 1.5 µm; below) measured by the BioScout and the UVAPS
during the summer campaign of 2012.
Fig. 5. Correlation scatter diagrams of fluorescent (NFL) coarse
(Dp > 1.5 µm) and fine (Dp < 1.5 µm) particle and total
particle concentrations (NTOT) measured by the BioScout, PM2.5, NOx
and BC during the winter and the summer periods. Pearson’s
correlation coefficients (R) are shown in diagrams.
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Saari et al., Aerosol and Air Quality Research, 15: 572–581,
2015 579
The previous studies typically treated fine FBAP as
non-biological particles (Huffman et al., 2010; Gabey et al.,
2011), but this study showed that this is not necessarily the case.
The types and sources of fine FBAP should be studied more and
compared with other analyzing techniques such as microscopy and
quantitative polymerase chain reaction (qPCR). CONCLUSIONS
Two fluorescence based real-time instruments were used to
measure bioaerosol concentrations and size distributions in the two
different case studies in urban environments during winter and
summer. Two FBAP modes at 0.5–1.5 µm (fine) and 1.5–5 µm (coarse)
were detected during the summer, whereas the fine mode dominated in
the winter. Correlations between the BioScout and the UVAPS were
high with the FBAP coarse mode (R = 0.83) and fine mode (R = 0.92).
The BioScout showed 2.6 and 9.7 times higher detection efficiencies
for the coarse and fine FBAP, respectively, compared to the UVAPS.
This is the first study, wherein the BioScout was used in outdoor
environment and compared to another LIF based instrument.
The coarse FBAP concentrations and fractions were the highest in
summer (0.028 #/cm3, 23%) and the lowest in winter (0.010 #/cm3,
6%). Snow cover and low biological activity were assumed to be the
main reasons for low coarse FBAP concentration in the wintertime.
Both the fine and the coarse FBAP fraction typically increased at
night during the summer; that is characteristic for natural fungal
spore emission. Various factors suggest that at least a fraction of
the observed fine FBAP were bacteria.
The long-range transport episode of particles from Eastern
Europe increased the fine FBAP concentration over two orders of
magnitude compared to the clean period in the winter, but these
FBAP probably also included fluorescent non-biological particles.
The potential sources of fluorescent fine particles during the
long-range transport episode might be bacteria and/or anthropogenic
emissions such as biomass burning. Low correlations between FBAP
and NOx and BC indicate that local traffic emissions and biomass
burning are not considerable sources for fluorescent particles at
the measurement sites. The previous studies typically treated fine
FBAP as non-biological particles, but this study showed that this
is not necessarily the case, and fine FBAP should be studied more
and compared with other analyzing techniques such as microscopy and
qPCR. The results provide information that can be used to estimate
health risks and climatic relevance of bioaerosols in urban
environments. ACKNOWLEDGMENTS
The study was a part of the MMEA research program of Cleen Ltd,
supported by funding of Tekes (MMEA WP 4.5.2.). The support of
Doctoral School of TUT, Jenny and Antti Wihuri Foundation and Eemil
Aaltonen Foundation are also gratefully acknowledged. Special
thanks are given to Ms. Katri Pihlava and Mr. Anders Svens for
their help during
the measurements. The authors gratefully acknowledge the NOAA
Air Resources Laboratory (ARL) for the provision of the HYSPLIT
transport and dispersion model and READY website
(http://www.ready.noaa.gov) used in this publication.
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