An Indoor Air Quality Study Regarding Outdoor Originating
Pollutants.
1
CHRISTOFOROS C. HALIOS and COSTAS G. HELMIS
Department of Applied Physics, Faculty of Physics, University of Athens, Building PHYS-5,
University Campus, 157 84 Athens, Greece
Abstract: - The objective of this work is to study indoor air quality in three typical residential apartments in
Athens with different characteristics (location, total area of the apartment, etc). Non-smokers inhabited the
apartments and pollution was penetrating indoors from the outdoor environment (outdoor originating
pollution). Measurements of the indoor and outdoor concentrations of nitrogen oxides, sulphur dioxide and
ozone were performed along with measurements of indoor and outdoor temperature and relative humidity and
ambient wind speed and direction. Also, the ventilation rates of the sites were measured in selected cases. The
measurements covered two different time periods of the year (summer and winter) in order to address questions
regarding seasonal variability. The decomposition rate of the various pollutants onto the indoor surfaces was
calculated for all the experimental periods.
It was found that indoor concentrations of all measured species closely track the outdoor values indicating the
importance of outdoor to indoor transport and the ventilation rate of the apartment. Ventilation rates varied
significantly between the different experimental periods. Furthermore, decomposition rates were found to vary
significantly not only between the different experimental sites, but between the different experimental periods
for the same site also. The differences between the ventilation and the decomposition rates were found to be the
main factors for the differences observed between the various microenvironments.
Key-Words: - Indoor air quality, outdoors originating pollution, ventilation, decomposition rate
1 Introduction
Although indoor air pollution is a very old
problem, scientific interest and identification as an
important factor for the public health was intensified
during the last 30 years. Large-scale experiments
like the National Human Exposure Assessment
Survey (NHEXAS) [1], the Harvard Six-City Study,
and the New York State ERDA Study [2], [3] were
conducted in the USA in order to experimentally
investigate the exposure of the population to various
pollutants. These studies showed that smoking was
the most important source of fine particle and other
pollutants and the second strongest source was
found to be cooking. Furthermore, fifty studies were
conducted between 1978 and 1990 in order to
examine the indoor air concentrations of VOC’s in
several countries [4], and a great number of other
studies focused on the indoor levels of particulate
matter [3], ozone and photochemical pollutants [5],
[6] and CO, CO2, SO2 (Jantunen et al, 1998). From
these works it was clear that the concentrations of
some pollutants (particulate matter, CO, VOC’s) in
indoor air can be greater than the respective outdoor
levels since the conditions in the indoor
environment may be favorable for the formation of
certain species (indoor sources).
During the last decade, a great number of studies
were conducted in Europe, in order to examine the
specific characteristics of the European countries’
indoor environments. Among them, the first largescale European experimental campaign, the
EXPOLIS study, focused mostly on the behavioural
and environmental determinants of personal
exposures to VOC’s, PM2.5 and CO concentrations
in large European cities [7], and the SAVIAH study
evaluated differences in concentration of air
pollutants inside homes in streets with different
traffic densities [8]. The previous studies, along
with other, small scale experiments as those found
in [9], [10] and [11], reported indoor and outdoor
concentrations of various pollutants in several large
European cities, revealing that different aspects and
habits at each site such as the type of ventilation
used, the traffic density, the materials of
construction used or the prevailing climatic
conditions and the behavioral characteristics of the
population can be significant factors that affects the
IAQ.
These measurements were conducted in the
frame of the Urban-Aerosol project, which aimed at
the characterization of air pollutants and PM both
indoors and outdoors, and the associated actual
human exposure in selected European urban areas
[12]. The objective of the present work is first to
evaluate the indoor air quality in three typical nonsmoking residential apartments of Athens, in terms
of gaseous pollution and secondly to estimate the
principle factors involving in the penetration of the
outdoor origin pollution indoors.
2
Experimental Sites, Set-up and
Methodology
The experimental campaign was carried out in
three residential apartments in the Greater Athens
Metropolitan area, Greece. One apartment (Site 1),
which is 28 m2, and it is located in a residential area
close to the historical centre, while the others are
located in suburbs within the metropolitan area
where the majority of the population lives. One of
the apartments (Site 2) which is 80 m2, is located
close to a major traffic route (Mesogeion), while
another apartment (Site 3) which is 50 m2 is located
in a purely residential neighborhood (Kypseli).
Indoor and Outdoor measurements of gaseous
species SO2, NOX and O3 were performed with
three 360 Series - Horiba Analyzers. The analyzers
were interfaced to a three - port valve that alternated
sampling between indoors and outdoors on a 15minute cycle.
Data of outdoor air temperature, relative
humidity and wind speed and direction were
collected with instruments placed by National
Observatory of Athens (NOA) on a 7- m
meteorological mast at the rooftop of the buildings.
Indoor temperature and relative humidity were
measured at a height of 1.2 m, using a small stand.
A series of measurements of air exchange rate was
performed at each house by means of the inert gas
(SF6) decay time method. Samples were sent to
NILU, where the analysis for the tracer was
performed.
In order to estimate the principle factors
concerning the penetration of the outdoor origin
pollution indoors the measured pollutants ( NOX,
SO2, O3) were considered as mainly originating
from outdoors. This hypothesis mainly holds for
SO2 and O3 while for NOX data that corresponded
to periods when known indoor sources were active
and during the subsequent hours also were excluded.
On the other hand in a recent publication it was
indicated that pollution in one apartment may enter
not only from outdoors but from adjacent
apartments as well [13]. Data that corresponded to
such cases were recognized by visual inspection of
the data series and were also excluded.
3 Results
3.1
Ventilation
In general the ventilation of one apartment consists
of two contributions: the first is the ventilation rate,
which in this work was measured by means of the
SF6 decay method with closed windows, and
represents the tightness of the building shell, cracks,
etc [14]. The second component of the ventilation is
due to the personal ventilation [15], which has to do
with the ventilation habits of the inhabitants, and
can be estimated by the fraction of time that the
windows in each site were kept open (hours of
windows opening or percentage of time).
Table 1: Characteristics of the experimental sites
with respect to the ventilation of the three
experimental sites
Experimental
Duration (h)
Duration of windows
opening (h, %)
Ventilation Rate
(h-1)
Winter
230
11, 4.7
0.5 - 1.1
_______________________________________________________________
Site 1
Summer
155
108, 70
1.4
_____________________________________________________________________
Winter
144
5, 3.5
0.5
_______________________________________________________________
Site 2
Summer
196
50, 25.5
0.8 – 0.9
_____________________________________________________________________
Winter
188
18, 9.5
0.3 – 0.5
_______________________________________________________________
Site 3
Summer
156
14, (9.3)
0.5
_____________________________________________________________________
In Table 1 the ventilation patterns of the three
experimental sites are presented along with the total
experimental duration. It is clear that the more
“tight” apartment of the three with respect to the
ventilation rate was experimental site 3 during both
the winter and summer experimental periods, with
apartment 1 to be the least sealed of all. The
ventilation rate is higher during the summer than the
wintertime period, probably due to the lower
temperature difference between the indoor and
outdoor environment and the different wind field
during these periods. The personal ventilation is
higher during the summer than the winter time
period, a characteristic aspect for the Mediterranean
countries. The prominent exception in site 3 during
the summer time period is due to the convective
storms referred above.
3.2
Decomposition rates
The decomposition rates of the various pollutants
were calculated following the methodology
proposed in [16]. This methodology is based on the
solution of the well-known mass balance equation
for one apartment:
dCi/dt = mλ Co – amλCo – mλCi – kτCi + S (1)
where V is volume of the space, Ci is the indoor
concentration (μgm-3), Co is the outdoor
concentration (μgm-3),a is the fraction of pollutants
filtered from air entering (unitless), m is the mixing
factor (unitless), λ is the ventilation rate (h-1), kτ is
the decomposition rate (h-1), and S is the source or
sink strength (μgm-3 h-1). Assuming that indoor
concentrations are in a steady state condition
(dCi/dt=0), that the indoor environment is
completely mixed (m=1), that the filtration of the
entering air is negligible (a=0), and the ventilation
rate is constant, the solution of (1) becomes:
Ci = λ/(λ+kτ) Co + S/(λ+kτ)
between the different experimental periods for the
same site also. This could be attributed to the
different furniture used during the different periods.
Table 2: Calculated decomposition rates (h-1) of the
various pollutants.
SO2
NO
NO2
O3
House 1
winter time
3.18
1.79
0.72
4.65
House 1
summer time
0.19
< 0.1
0.17
2.31
House 2
winter time
3.06
0.07
0.34
1.57
House 2
summer time
House 3
winter time
-
< 0.1
0.09
3.28
2.20
0.02
0.45
0.82
House 3
summer time
-
< 0.1
0.43
4.43
3.3
Average time series
In Figures 1,2 the diurnal variation of the mean
hourly of the indoor and outdoor concentrations of
NO and O3 for both the winter and summer
experimental periods for Site 1, are presented. In
Figure 3 the respective diurnal variation of O3 for
Site 3 is given.
60.0
Considering that the linear regression between the
indoor and outdoor measured concentrations gives:
one gets that
A= λ/(λ+kτ) and B=S/λ+kr.
Concentration (ppb)
Ci = A Co + B
in winter
out winter
in summer
out summer
50.0
40.0
30.0
20.0
10.0
In the time series presented there are no indoor
sources, thus B=0.
It should be noticed that the first assumption was
achieved by visual inspection of the time series. The
last assumption was established by taking into
consideration concentrations that corresponds to
time intervals when the meteorological parameters
that influence the ventilation rate (difference
between indoor and outdoor temperature, wind
speed and direction and also the differential
pressure) had the same values when the ventilation
rate was measured.
Results from the methodology presented above are
given in Table 2. It can be seen that O3 has the
largest decomposition rate values, while the lowest
values are of NO. An interesting aspect is the
difference of the decomposition rates not only
between the different experimental sites, but
0.0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)
Figure 1: Daily time series of both the winter and
summer experimental period indoor and outdoor
concentrations of NO for Site 1
As seen from Figure 1, indoor levels of NO follow
the outdoor ones with a time lag of the order of one
hour approximately. During the summer time both
indoor and outdoor levels are very low. Regarding
O3 (Figure 2 and 3), one may observe that outdoor
levels for both pollutants are significantly higher
than the indoor ones. Moreover, outdoor O3
concentrations follow an expected daily pattern, i.e.
peak O3 levels are observed approximately two
hours after rush hour traffic has started and
concentrations remain high until sunset at 17:00.
Outdoor O3 values are significantly higher during
the summer than the wintertime, and this holds for
the indoor values too.
70.0
60.0
winter in
winter out
summer in
summer out
Concentration (ppb)
50.0
40.0
outdoor pollution indoors and the importance of the
ventilation of the apartment. The decomposition
rates of the various species were calculated for
every site and each experimental period and the
values were found to vary significantly. The
significance of the different ventilation and
decomposition rates was apparent from the diurnal
variation of the mean hourly of the indoor and
outdoor concentrations
30.0
20.0
5 Acknowledgments
10.0
0.0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)
The authors would like to thank Mr. Michael
Petrakis from the National Observatory of Athens
and Otto Hermansen from NILU.
Figure 2: As in Figure 1 but for O3
Indoor O3 values seem to follow these peaks with a
small time lag but at lower levels. From the above it
may be concluded that O3 concentrations are
particularly low indoors since they react with
surfaces and are deposited. It can also be observed
that the reduction of the indoor concentrations are
greater for NO than for the O3 apparently due to the
different values of the decomposition rate.
It is interesting to notice that the indoor values are
lower than the outdoor ones for Site 3 than for Site
1, indicating the lower values of the ventilation rate.
70.0
in winter
in summer
60.0
out swinter
out summer
Concentration (ppb)
50.0
40.0
30.0
20.0
10.0
0.0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time (h)
Figure 3: As in Figure 2 but for Site 3.
Analogous are the results for SO2 and NO2 (not
shown here).
4 Conclusion
The indoor air quality in three typical residential
apartments were investigated experimentally in
relation to the outdoor environment during winter
and summer periods. Indoor concentrations closely
track the outdoor ones indicating the transport of the
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