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A balance-point method for assessing the effect of natural ventilation on indoor particle concentrations
ARTICLE IN PRESS
Atmospheric Environment 37 (2003) 4277–4285
A balance-point method for assessing the effect of natural ventilation on indoor particle concentrations Yuguo Lia,*, Zhengdong Chenb a
Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong b CSIRO Manufacturing and Infrastructure Technology, Highett, Vic. 3190, Australia Received 14 June 2002; received in revised form 9 June 2003; accepted 17 June 2003
Abstract A balance-point outdoor concentration was proposed to represent the value of outdoor concentration at which the air exchange between indoor and outdoor does not affect the indoor and outdoor pollutant concentration ratio (I/O). When the outdoor particle concentration is higher than the balance-point outdoor concentration, an increase in the indoor and outdoor air exchange results in a higher indoor particle concentration level and vice versa. Indoor particulate matter concentration and its relationship with outdoor particle concentration for a hypothetical naturally ventilated building are studied using a simple steady-state model. The effects of various factors such as air infiltration exchange rate, ambient particle concentration, indoor source intensity, and human activity on I/O, and the correlation between indoor and outdoor concentration were investigated, and the results compare reasonably well with the results reported in the literature. r 2003 Elsevier Ltd. All rights reserved. Keywords: Balance-point outdoor concentration; Indoor air quality; Particulate matter; Macroscopic model; I/O; Natural ventilation
1. Introduction Particles of concern to human health are those known as inhalable particles with a diameter less than 10 mm (PM10), especially those smaller than 2.5 mm (PM2.5). Exposure to indoor particulate matter has been recognised as a significant environmental and health problem. Indoor particulate matter concentrations are complex combinations of many factors, such as indoor pollutant sources, ambient air conditions, building materials, human behaviour and activities, ventilation and particle size distributions. In the past several decades, numerous measurements have been reported on the ratio between the indoor and outdoor particle concentration (or I/O), which to some extent indicates how well the building envelope protects us against outdoor particulate matter, *Corresponding author. Tel.: +852-2859-2625; fax: +852-2858-5415. E-mail address: [email protected] (Y. Li).
or how well the building ventilation system disperses the indoor-generated particles. Table 1 provides a summary of some measured I/Os reported in the literature, mainly for naturally ventilated buildings. A wide range of I/Os between 0.1 and 5 can be found. The intensity of the I/O correlation was also found to vary from very weak to very strong among different measurements by different research groups. When a strong correlation between indoor and outdoor particle concentrations exists, the personal particulate exposure may be readily estimated according to ambient particulate concentration level, which is now often a routine measurement in many cities. On the other hand, a weak I/O correlation makes the exposure estimation from the ambient particulate concentration questionable. Up to now, the question as to whether the personal exposure level can be appropriately assessed from the ambient particulate concentration is still not resolved, and conflicting results have been reported by different research groups (Mage et al., 1999; Lachenmyer and
1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00527-2
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Table 1 Examples of I/O ratios for particulate matters reported in the literature Reference
Location
Chao and Wong (2002)
Hong Kong
Fisher et al. (2000)
Netherlands
Particle size (mm)
I/O ratio
Conditions 34 residential homes and 30 homes were above fourth floor. For high air exchange rate (o3.5 h1)
PM2.5 PM10 PM2.5 PM10
0.92 0.88 1.09 1.04
PM10 PM2.5 PM10 PM2.5
0.68 0.86 0.58 0.8
High traffic homes
For low air exchange rate (o3.5 h1)
Low traffic homes
Funasaka et al. (2000)
Japan
PM10 PM10
0.62 0.86
Roadside houses in winter Background houses in winter
Jones et al. (2000)
UK
PM10
PM2.5
1.58 2.13 2.46 1.0
A A A A
whole whole whole whole
year, year, year, year,
roadside residential houses urban flats rural residential houses roadside residential house
Kingham et al. (2000)
UK
PM10 PM2.5
0.12–3.28 0.15–5.22
Non-smoker buildings
Koistinen et al. (2001)
Finland
PM2.5
2.2 0.86
Smoker homes Non-smoker homes
Lachenmyer and Hidy (2000)
US
PM2.5
0.61 0.92
Summer Winter
Lee and Chang (2000)
Hong Kong
PM10
0.63 0.82 0.75–1.3
Schools with window air-conditioning Schools with central air-conditioning Schools with ceiling fans
Monn et al. (1997)
Switzerland
PM10
PM2.5
0.7 >1 1.2–2.0 0.54 1.23
Absence of indoor source and activities With human activities With smoking and/or gas cooking Absence of indoor source and activities With human activities
Pellizzari et al. (1999)
US
PM10 PM2.5
0.98 1.2
Residential homes
Roorda-Knape et al. (1998)
Netherlands
PM10
2
Summer, school rooms
Hidy, 2000; Koistinen et al., 2001). It is believed that a systematic analysis of the data from various research groups may provide some insights into I/Os and their relationships for different types of buildings under various influencing factors. The building envelope does not necessarily provide us with good protection from exposure to certain types of pollutants. For pollutants with predominant indoor sources (such as formaldehyde and most other VOCs),
the building envelope restricts the dispersion of pollutants, and air exchange between indoor and outdoor should be encouraged. On the other side, for pollutants with predominant outdoor sources (such as pollens), a reduction in indoor and outdoor air exchange can reduce the indoor exposure level. For pollutants with origins both indoors and outdoors, e.g. particulate matters, nitrogen oxides and carbon monoxide, whether to encourage air exchange or not depends on the relative
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intensity of pollution between indoor and outdoor. However, so far there appears to be no simple quantitative index to measure the relative intensity of the pollution between indoor and outdoors, i.e. when should the air exchange between indoor and outdoor be encouraged when we consider a specified type of pollutant at a particular indoor source strength, building activities and filtration characteristics. In this paper, the indoor particulate matter concentration and its relationship with outdoor particle concentration are studied using a simple steady-state model for a hypothetical naturally ventilated building. A concept of balance-point outdoor concentration is proposed to represent the value of outdoor concentration at which the air exchange between indoor and outdoor does not affect the indoor and outdoor pollutant concentration ratios.
2. The concept of balance-point outdoor concentration Assuming that the particle concentration in the building is uniform, the following simple macroscopic mass balance for particulate matter can be written for a naturally ventilated building: Transient
Indoorsource zfflffl}|fflffl{ zfflfflffl}|fflfflffl{ dCi V ¼ aPVCo þ V’ source |fflfflfflffl{zfflfflfflffl} dt Penetration
Airflowremoval
þ RLfl Afl |fflfflfflffl{zfflfflfflffl}
zffl}|ffl{ aVCi
Resuspension
Othersinks
zffl}|ffl{ KVCi V’ sink ; |fflffl{zfflffl} Deposition
ð1Þ where a is the air exchange rate per hour due to infiltration and natural ventilation (1/h), Afl is the floor area (m2), Ci is the indoor particle concentration (mg/m3), Co is the outdoor particle concentration (mg/m3), K is the particle deposition rate (h1), Lfl is the mass loading of particles on accessible floor surfaces (mg/m2), P is the particle penetration coefficient, R is the particle resuspension rate (h1), t is the time (h), V is the volume of the room (m3), V’ source is the indoor particle generation rate (mg/h), and V’ sink is the removal rate of particles due to other sinks such as filtration (mg/h). Eq. (1) has been used in similar forms by a number of previous authors (Alzona et al., 1979; Dockery and Spengler, 1981; Tichenor et al., 1990; Kulmala et al., 1999; Mage et al., 1999). All the parameters except V and Afl are a function of both time and particle sizes. When the variations of these parameters are small, a steady-state indoor particle concentration can be obtained when the removal rate due to other sinks is zero: Ci ¼
aPCo RLfl Afl þ V’ source þ ; aþK ða þ KÞ V
ð2Þ
From Eq. (2), we obtain I/O as Ci aP RLfl Afl þ V’ source þ : ¼ Co a þ K Co V ða þ KÞ
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ð3Þ
The equation shows that various parameters affect I/O in naturally ventilated buildings. We propose a concept of balance-point outdoor concentration. It is obtained by simply setting I/O to be the value of the penetration coefficient P in Eq. (3). RLfl Afl þ V’ source Co;e ¼ : ð4Þ KVP A corresponding indoor particle concentration is defined as a balance-point indoor concentration: RLfl Afl þ V’ source : ð5Þ Ci;e ¼ KV Eq. (5) represents a balance between the indoor particle generation speed and particle deposition rate. I/O can also be written as Ci KPðCo;e Co Þ ¼Pþ : Co ða þ KÞ Co
ð6Þ
From Eq. (6), it is seen that when the outdoor particle concentration is Co;e ; the indoor particle concentration becomes Ci;e and is not affected by the infiltration of outdoor air. It can also easily be seen that the following relationships hold: Ci If Co;e > Co then > P; Co Ci IfCo;e oCo then oP: Co Thus, the balance-point outdoor concentration is a dividing point. If the outdoor concentration is higher than the balance-point outdoor concentration, I/O is always smaller than P: With this condition, the minimum I/O ðCi =Co Þ ¼ ðPCo;e =Co Þ is achieved when there is no air exchange, and the maximum I/O ðCi =Co Þ ¼ P is obtained for an infinite air exchange rate. When the outdoor concentration is less than the balance-point outdoor concentration, I/O is always larger than P: The minimum I/O ðCi =Co Þ ¼ P is achieved for an infinite air exchange rate, and the maximum I/O ðCi =Co Þ ¼ ðPCo;e =Co Þ is obtained when there is no air exchange. From Eqs. (4) and (5), it is seen that the indoor and outdoor balance-point concentrations are only functions of the indoor parameters such as particle sources, sinks, building geometry and filtration characteristics. For high indoor sources and human activity intensity, the indoor air is naturally dirtier in terms of particulate matter and is characterised by high balance-point indoor and outdoor concentrations. For buildings with low activity and no indoor sources, the indoor air is clean and the balance-point indoor and outdoor concentrations are close to zero. Consequently, Co;e and Ci;e
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Table 2 Parameters used in the model study for I/O and I/O concentration relationship Parameter
Value
References
Afl Co K
15 8 m2 30 mg/m3 for PM2.5, 50 mg/m3 for PM10 0.4 h1 for PM2.5, 1.0 h1 for PM10
Lfl PM2.5/PM10 P
100 mg/cm2 0.6 0.9
R
1.0 105 h1 for PM2.5, 2.5 105 h1 for PM10 15 8 2.4 m3 14 mg/cigarette for PM2.5, 22 mg/cigarette for PM10 1.7 mg/min for PM2.5, 4.1 mg/min for PM10 2.8 mg/h for PM10, 0.95 mg/h for PM2.5
— Chan et al. (2001) Thatcher and Layton (1995), Byrne et al. (1995), Fogh et al. (1997) Thatcher and Layton (1995) — Thatcher and Layton (1995), Wallace (1996), Tung et al. (1999) Thatcher and Layton (1995)
V V’ source (cigarette smoking) V’ source (cooking) V’ source (others)
effectively quantify the intrinsic cleanness of the indoor environment by combining the effects of indoor activities, particulate sources and sinks, as well as the building envelop filtration characteristics. The effectiveness of ventilation and infiltration on indoor particle concentration simply depends on whether the outdoor air is cleaner or dirtier than the air at the balance-point outdoor particle concentration, Co;e : High air exchange rates should be encouraged when outdoor particle concentrations are lower than Co;e ; and vice versa. Consequently, the outdoor balance-point concentration for a building can be used as an index to quantify the cleanness of the indoor environment and determine whether ventilation should be encouraged or not to reduce the indoor particulate exposure level. Seen in Eqs. (4) and (5), Co;e and Ci;e are both particle size dependent. Further, our analyses here focus entirely on particulate matter alone. Natural ventilation needs to be controlled by many other physical and environmental parameters, such as air temperature and other ambient pollutants. From the derivations of Co;e and Ci;e ; the concept of balance-point indoor and outdoor concentrations may be applied to other pollutant components. A decision to increase ventilation to reduce indoor PM levels might lead to an increase of indoor concentration of other pollutants, e.g. ozone, SO2, etc. For a complete analysis of ventilation needs, other pollutants also need to be considered. The concept of the balance-point outdoor concentration is very similar to the concept of balance-point outdoor temperature used in steady-state methods for energy consumption analysis in buildings based on degree-days (Kreider and Rabl, 1994). The balancepoint temperature is defined as the value of the outdoor temperature where, for a specified indoor air temperature, the total heat loss is equal to other indoor heat
— Wallace (1996) Wallace (1996) Wallace (1996)
gains. With this concept, heating is needed only when the outdoor air temperature drops below the balancepoint temperature. Similar to the concept of balancepoint temperature, the balance-point concentration is a steady-state concept. A short-term balance-point concentration may also be used for certain short time period within which all the affecting parameters may be considered as a constant. Although computers can now calculate the energy consumption of a building at the touch of a key with a transient analysis method, the concepts of balance-point temperature and degree-days still remain a valuable engineering tool (Kreider and Rabl, 1994). Similarly, the concept of balance-point concentration also appears to be a useful index for assessing the cleanness of the indoor environment, although the indoor sources, outdoor concentrations or ventilation rates are not constant. In the definition of balance-point indoor and outdoor concentrations, seven parameters have been involved, i.e. Afl ; K; Lfl ; P; R; V and V’ source ; within which the floor area, floor particle loading and building volume can be relatively easily estimated or measured. Many studies have contributed to the understanding of the particle penetration coefficient, resuspension rate, deposition rate and various indoor particle sources (e.g. Thatcher and Layton, 1995; Byrne et al., 1995; Wallace, 1996; Fogh et al., 1997; Tung et al., 1999). A full discussion of these parameters is out of the scope of this paper. Table 2 lists the reasonable values for these affecting parameters obtained from literature. Particle deposition rates reported by previous researchers are generally in agreement with the deposition rates of around 0.4 and 1.0 h–1 for PM2.5 and PM10, respectively (Thatcher and Layton, 1995; Byrne et al., 1995; Wallace, 1996; Fogh et al., 1997). The contribution of particle resuspension to the indoor particle
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3. Application of the balance-point concept As an attempt to provide a systematic analysis of the data from various research groups for different types of buildings under various influencing factors, we performed analyses using the simple macroscopic model and the concept of the balance-point outdoor concentration proposed in Section 2. In the following discussions, unless specified, the parameters listed in Table 2 will be used as the default values. Our analyses will be compared with the results and phenomena reported in the literature, and the comparisons should not be considered as strictly quantitative. 3.1. Buildings with no activity and without indoor sources For buildings with no human activity and without indoor sources, Eq. (3) becomes Ci aP P ¼ ¼ : Co a þ K 1 þ ðK=aÞ
ð7Þ
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1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
PM 2.5 PM 10
0 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 (b)
1
2
3
4
5
-1 Air exchange rate, a, (h )
(a)
I/O ratio
concentration can be significant, e.g. during periods of vacuum cleaner use or intense human activities (Thatcher and Layton, 1995; Jones et al., 2000). Indoor particle sources can be due to combustion, smoking, cleaning process and particles from building material, furnishings, HVAC, office equipment, and various human activities. Sources such as secondary organic aerosol formation can be very difficult to quantify (e.g. Turpin and Huntzicker, 1995). Studies showed that indoor combustion such as range-top burners, ovens, fireplaces, kerosene heaters, cigarette smoke and cooking can be significant, while other indoor sources are relatively small (Wallace, 1996, Jones et al., 2000; Chao and Wong, 2002). For simplicity, indoor particle sources are assumed to be composed of cigarette smoking, cooking and other sources listed in Table 2. The penetration coefficient is a very difficult parameter to determine in practice and is still not well documented. Various parameters can affect the penetration coefficient, including the geometry and size of various openings and cracks in the building envelope, the airflow through these openings, the particle sizes (which affect the particle deposition or trap along the flow paths in the building envelope), etc. In general, a penetration coefficient range of 0.7–1.0 was reported in previous research (Thatcher and Layton, 1995; Wallace, 1996; Tung et al., 1999). With the increasing understanding of these affecting parameters, it is expected that the balance-point indoor and outdoor concentration will be easily estimated and will provide an index of the cleanness of the indoor environment, and will be used to determine the ventilation strategy for buildings, even before construction.
I/O ratio
Y. Li, Z. Chen / Atmospheric Environment 37 (2003) 4277–4285
PM2.5, Co = 15 PM2.5, Co = 22 PM2.5, Co = 50
0
1
2
PM10, Co = 15 PM10, Co = 22 PM2.5,Co = 50
3
4
5
-1 Air exchange rate, a, (h )
Fig. 1. I/Os for buildings (a) with low activity and without indoor sources and (b) with normal human activity and without major indoor sources.
According to Eq. (4), the balance-point outdoor concentrations for both PM2.5 and PM10 are zero. Fig. 1a shows the estimated I/Os for PM2.5 and PM10 at different air exchange rates from 0.1 to 5 h1. The I/Os are less than or close to 0.9 (which is the value of the assumed penetration coefficient) and increase with an increase in the air exchange rate, approaching the penetration coefficient. Consequently, for buildings with no human activities and indoor sources, the most effective way of reducing indoor particulate matter concentration is to minimise the natural ventilation and air infiltration. This is in accordance with the fact that the outdoor concentration is always higher than the balance-point outdoor concentration, which is zero. It is noted that infiltration rates can vary in the range 0.2–1.0 h1 for relatively airtight buildings (ASHRAE, 2001). For some old or less airtight buildings, the air exchange rate may be high. In natural ventilation with open windows, the ventilation flow rate can be more than 20 h1 (e.g. Heiselberg, 2002). According to Eq. (7), the I/Os for PM2.5 and PM10 were found to increase by 114% and 200% respectively with an increase in the air exchange rate from 0.2 to 1.0 h1, but to increase 55% and 114% respectively with an increase in the air exchange rate from 0.5–3.0 h1. This suggests that good correlations between indoor and outdoor concentrations
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may be more frequently found during moderate weather when windows or doors are frequently open in old or less airtight buildings, especially for fine particles. However, for more airtight buildings, e.g. most new office buildings, the I/O concentration correlation may be relatively weak, especially for coarse particles. Eq. (7) did not consider the transient effect when the outdoor concentrations and indoor sources are not constant. At higher air change rates, the apparent lag between outdoor concentration change and indoor concentration response is reduced, which causes the I/O ratio to appear more stable. These conclusions appear to be in agreement with the observations in a number of studies. Measurements in several residential and office buildings by Baek et al. (1997) in Korea showed that I/O correlated well for residential homes in both summer and winter, but correlated weakly for more airtight offices being studied. Jones et al. (2000) reported better I/O correlation in well-ventilated houses than more airtight houses. Lachenmyer and Hidy (2000) reported a stronger correlation in summer than in winter, since windows and doors are more frequently opened during summer. Better I/O correlations for fine particles than coarse particles were observed by Sinclair et al. (1992); Funasaka et al. (2000) and Jones et al. (2000). 3.2. Buildings with normal activity and without major indoor sources Here, major indoor sources such as cooking and smoking are excluded and only the indoor sources, such as particles resuspended from loose construction materials and furnishings and other sources, are considered. The balance-point outdoor PM2.5 and PM10 concentrations defined by Eq. (4) are estimated to be 21 and 22 mg/m3 respectively for this case, as listed in Table 3. Eq. (6) suggests that I/O is a function of the outdoor concentration. Fig. 1b shows the I/Os for different air exchange rates at PM10 outdoor concentrations of 15, 22 and 50 mg/m3 respectively. It is seen that I/O decreases with an increase in the ambient particle concentration under otherwise the same conditions. This trend can be
easily seen from Eq. (6) and has been observed by a number of authors. For example, Funasaka et al. (2000) reported that the I/O for roadside houses is generally lower compared with those away from the roadside, due to an increase in the outdoor concentration for roadside houses. The I/O for fine particles is generally higher than that for coarse particles due to the high deposition rate of the latter. Relatively high I/Os for fine particles have been reported by Jones et al. (2000) and Thatcher and Layton (1995) for houses with no major indoor sources. At an outdoor PM10 concentration of 15 mg/m3, the PM10 I/O increases with a decrease in the air exchange rate, while for outdoor PM10 concentrations of 50 mg/ m3, the PM10 I/O decreases with a decrease in the air exchange rate. The outdoor PM10 concentration, which divides these two situations, is the balance-point outdoor PM10, which is 22 mg/m3 for the current case. As shown in Fig. 1b, at an outdoor PM10 concentration of 22 mg/m3, the air exchange rate has no effect on the PM10 I/O. Similarly, the dividing point for PM2.5 is the balance-point outdoor PM2.5, which is 21 mg/m3 (the corresponding PM10 concentration is around 35 mg/m3 if a PM2.5/PM10 of 0.6 is assumed). The ambient particle concentration is generally in the range of 10–100 mg/m3 in most of the cities (Ando et al., 1996; Wallace, 1996; Chan et al., 2001; Baek et al., 1997; Hooper and Hooper, 1986; Koistinen et al., 2001). Consequently, the current simple modelling suggests that for buildings with normal activity and without major indoor sources as listed in Table 2, a high air exchange rate should not be encouraged in terms of particulate exposure in some moderately and highly polluted regions with a PM10 concentration higher than 35 mg/m3. Conversely, in less polluted ambient regions with a PM10 concentration lower than 20 mg/m3, a high ventilation rate is beneficial to reduce indoor PM2.5 and PM10 concentrations. Again, Fig. 1b shows that the I/Os change rapidly with air infiltration rate and outdoor particle concentration for small air exchange rates of ao1:0; when the outdoor concentration is away from the balance-point concentration value. Consequently, poor I/O concentration correlation may be more frequently observed for relatively airtight buildings, when the outdoor
Table 3 Balance-point outdoor particle concentration, Co;c ; for various conditions Indoor sources Normal activity, no major source Normal activity, cooking alone Normal activity, smoking alone Normal activity, cooking and smoking
Daily mean Co;e for PM10 (mg/m3)
Daily mean Co;e for PM2.5 (mg/m3)
Short-term Co;e for PM10 (mg/m3)
Short-term Co;e for PM2.5 (mg/m3)
22
21
22
21
42 93 154
41 133 154
1005 831 1815
971 532 925
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dealing with highly transient sources. As shown in Table 3, the short-term balance-point outdoor concentrations (obtained from Eq. (4) during periods of smoking or cooking) are above 500 and 800 mg/m3 for PM10 and PM2.5 respectively, which are much higher than the outdoor particle concentration range of 10–100 mg/m3 normally observed. Consequently, high air exchange rates during cooking and smoking can substantially reduce indoor particulate concentration levels. Fig. 2 shows the effect of smoking and cooking on the daily averaged I/Os for outdoor particle concentrations of 15, 25, 50 and 100 mg/m3 respectively. For low outdoor particulate matter concentrations, I/O is substantially increased with cigarette smoking, while the contribution of cooking is relatively smaller. For buildings with heavy smokers (one pack of 20 per day), I/O is generally around 1.2–5 for an outdoor PM10 concentration range of 25–100 mg/m3 and an air exchange rate of 0.2–1.0 h1. The relative effect of smoking and cooking on I/O decreases with an increase in the outdoor particulate matter concentration as well as the air exchange rate. This is in accordance with the 18–30% increase in I/O reported by Baek et al. (1997) in Korea and Chao and Wong (2002) in Hong Kong with
concentration is significantly different (either lower or higher) from the balance-point concentration value. On the other hand, a relatively strong I/O correlation can be observed when the outdoor concentration is close to the balance-point concentration value or if the building is relatively leaky. 3.3. Buildings with normal activity and major indoor sources Cooking and cigarette smoking are the two major indoor particle sources. As a hypothetical example, it is assumed that (1) the intensities of particle generation due to cooking and cigarette smoking are the mean values as listed in Table 2; (2) normal cooking duration is 30 min per day; and (3) smoking intensity is one pack of 20 per day, with each cigarette taking 10 min to burn. From Eq. (4), the balance-point outdoor particle concentrations for different conditions are listed in Table 3 for PM10 and PM2.5, respectively. It is seen that cooking and smoking significantly increase the daily mean balance-point outdoor concentrations due to the high indoor particle generation rate. It should be mentioned that the concept of the daily mean balancepoint outdoor concentrations is not very useful when
12
8 6 4
4 3 1
0
0 0
1
2
3
4
5
-1
Air exchange rate, a, (h )
(a)
3.5
2 1.5
I/O ratio
2.5
0 (b)
PM 2.5, Normal activity, Co = 50 PM 10, Normal activity, Co = 50 PM 2.5, Cooking, Co = 50 PM 10, Cooking, Co = 50 PM 2.5, Smoking, Co = 50 PM 10, Smoking, Co = 50
3 I/O ratio
5
2
2
1 0.5 0 0
(c)
PM 2.5, Normal activity, Co = 25 PM 10, Normal activity, Co = 25 PM 2.5, Cooking, Co = 25 PM 10, Cooking, Co = 25 PM 2.5, Smoking, Co = 25 PM 10, Smoking, Co = 25
6 I/O ratio
I/O ratio
7
PM 2.5, Normal activity, Co = 15 PM 10, Normal activity, Co = 15 PM 2.5, Cooking, Co = 15 PM 10, Cooking, Co = 15 PM 2.5, Smoking, Co = 15 PM 10, Smoking, Co = 15
10
4283
1
2
3
-1 Air exchange rate, a, (h )
4
5
4 -1 Air exchange rate, a, (h )
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
2
3
5
PM 2.5, Normal activity, Co = 100 PM 10, Normal activity, Co = 100 PM 2.5, Cooking, Co = 100 PM 10, Cooking, Co = 100 PM 2.5, Smoking, Co = 100 PM 10, Smoking, Co = 100
0 (d)
1
1
2
3
4 -1 Air exchange rate, a, (h )
5
Fig. 2. The effect of smoking and cooking on the daily average I/O ratios at an outdoor PM10 concentration of: (a) 15 mg/m3; (b) 25 mg/m3; (c) 50 mg/m3 and (d) 100 mg/m3.
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relatively high outdoor concentrations, compared to a 100% or more increase in I/O due to cigarette smoking reported by most researchers in Europe and North America (Yocom, 1982; Wallace, 1996) where the outdoor concentrations are relatively low compared to those in Korea and Hong Kong. From Fig. 2, it can be seen that with cigarette smoking, I/Os change significantly in the air exchange rate range of 0.2–2.0 h1 and in the outdoor concentration range of 15–100 mg/m3. The I/O correlation is expected to be worse in comparison to buildings without smoking under otherwise identical conditions. Consequently, a poor I/O correlation is expected for most buildings with heavy smokers. The effect of cooking appears to be not as strong as indoor smoking and good I/O concentration correlation may be observed in old or less airtight buildings. However, for restaurants, high cooking intensity can result in significantly high indoor particle sources and poor I/O concentrations may be found, as reported by Baek et al. (1997). From the above model studies, it is seen that the short-term balance-point outdoor concentration for buildings ranged from 0 mg/m3 for no indoor sources, 20 mg/m3 for no major indoor sources and 2000 mg/m3 for major indoor sources, and ventilation strategies can be readily decided based on the balance-point outdoor concentration. It has also shown that I/O correlation is significantly affected by the indoor and outdoor air exchange rate, as well as the balance-point outdoor concentration. A number of investigations have shown that a relatively strong I/O correlation can be achieved at a high air exchange rate. From the current model study, it was found that relatively strong I/O correlation may also be observed if the outdoor concentration is close to the balance-point outdoor concentration, even at low air exchange rates.
4. Conclusions Balance-point indoor and outdoor concentrations for naturally ventilated buildings are defined, at which I/O equals the value of the penetration coefficient. Model studies showed that the balance-point outdoor concentration is a simple steady state index to quantify the cleanness of the building in terms of the outdoor particle concentration. When the outdoor particle concentration is less than the balance-point concentration, I/O is always smaller than the penetration coefficient, and the minimum indoor concentration can be achieved at high indoor and outdoor air exchange rates. Thus, introducing fresh air into the building should be encouraged. On the other hand, when the ambient particle concentration is higher than the balance-point outdoor concentration, I/O is always higher than the penetration
coefficient and ventilation and/or infiltration should be minimised. Indoor and outdoor particulate matter concentration relationships were also investigated systematically with model studies. It was found that a good I/O correlation is achieved when the outdoor particle concentration is close to the balance-point outdoor concentration or when the building is less airtight. However, poor I/O correlation may be more frequently observed for relatively airtight buildings when the outdoor particle concentration is much lower or much higher than the balance-point outdoor particle concentration.
Acknowledgements This research was supported by a grant from the Seed Funding Programme for Basic Research (2000–2002) at the University of Hong Kong.
References Alzona, J., Cohen, B.L., Rudolph, H., Jow, H.N., Frohliger, J.O., 1979. Indoor–outdoor relationships for airborne particulate matter of outdoor origin. Atmospheric Environment 13, 55–60. Ando, M., Katagiri, K., Tamura, K., Yamamoto, S., Matsumoto, M., 1996. Indoor and outdoor air pollution in Tokyo and Beijing supercities. Atmospheric Environment 30 (5), 695–702. ASHRAE, 2001. Fundamentals, SI Edition. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, US. Baek, S.-O., Kim, Y.-S., Perry, R., 1997. Indoor air quality in homes, offices and restaurants in Korean urban areas— indoor/outdoor relationships. Atmospheric Environment 31 (4), 529–544. Byrne, M.A., Goddard, A.J.H., Lange, C., Roed, J., 1995. Stable tracer aerosol deposition measurements in a test chamber. Journal of Aerosol Science 26 (4), 645–653. Chao, C.Y.H., Wong, K.K., 2002. Residential indoor PM10 and PM2.5 in Hong Kong and the elemental composition. Atmospheric Environment 36, 265–277. Chan, L.Y., Kwok, W.S., Lee, S.C., Chan, C.Y., 2001. Spatial variation of mass concentration of roadside suspended particulate matter in metropolitan Hong Kong. Atmospheric Environment 35, 3167–3176. Dockery, D.W., Spengler, J.D., 1981. Indoor–outdoor relationships of respirable sulfates and particles. Atmospheric Environment 15, 335–343. Fisher, P.H., Hoek, G., van Reeuwijk, H., Briggs, D.J., Lebret, E., van Wijnen, J.H., Kingham, S., et al., 2000. Trafficrelated differences in outdoor and indoor concentrations of particles and volatile organic compounds in Amsterdam. Atmospheric Environment 34, 3713–3722. Fogh, C.L., Byrne, M.A., Rod, J., Goddard, J.H., 1997. Size specific indoor aerosol deposition measurements and
ARTICLE IN PRESS Y. Li, Z. Chen / Atmospheric Environment 37 (2003) 4277–4285 derived I/O concentrations ratios. Atmospheric Environment 31, 2193–2203. Funasaka, K., Miyazaki, T., Tsuruho, K., Tamur, K., Mizuno, T., Kuroda, K., 2000. Relationship between indoor and outdoor carbonaceous particulates in roadside households. Environmental Pollution 110, 127–134. Heiselberg, P. (Ed.), 2002. Principles of Hybrid Ventilation. International Energy Agency Annex 35 Project Final Report, Aalborg University, Denmark. Hooper, B.M., Hooper, M.A., 1986. Indoor air pollution: a short review. Clear Air 20 (2), 47–51. Jones, N.C., Thornton, C.A., Mark, D., Harrison, R.M., 2000. Indoor/outdoor relationships of particulate matter in domestic homes with roadside, urban and rural locations. Atmospheric Environment 34 (16), 2603–2612. Kingham, S., Briggs, D., Elliott, P., Fischer, P., Erik, L., 2000. Spatial variations in the concentrations of traffic-related pollutants in indoor and outdoor air in Huddersfield, England. Atmospheric Environment 34 (6), 905–916. Koistinen, K.J., H.anninen, O., Rotko, T., Edwards, R.D., Moschandreas, D., et al., 2001. Behavioral and environmental determinants of personal exposures to PM2.5 in EXPOLIS—Helsinki, Finland. Atmospheric Environment 35, 2473–2481. Kreider, J.F., Rabl, A., 1994. Heating and Cooling of Buildings. McGraw–Hill, New York, USA (Chapter 8). Kulmala, M., Asmi, A., Pirjola, L., 1999. Indoor air aerosol model: the effect of outdoor air, filtration and ventilation on indoor concentrations. Atmospheric Environment 33, 2133–2144. Lachenmyer, C., Hidy, G.M., 2000. Urban measurements of outdoor–indoor PM2.5 concentrations and personal exposure in the deep south: Part I—pilot study of mass concentration for non-smoking subjects. Aerosol Science and Technology 32, 34–51. Lee, S.C., Chang, M., 2000. Indoor and outdoor air quality investigation at schools in Hong Kong. Chemosphere 41 (1–2), 109–113. Mage, D., Wilson, W., Hasselblad, V., Grant, L., 1999. Assessment of human exposure to ambient particulate
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matter. Journal of Air and Waste Management Association 49, 1280–1291. . Monn, C., Fuchs, A., Hogger, D., Junker, M., Kogelschatz, D., Roth, N., Wanner, H.-U., 1997. Particulate matter less than 10 mm(PM10) and fine particles less than 2.5 mm (PM2.5): relationships between indoor, outdoor and personal concentrations. Science of the Total Environment 208 (1–2), 15–21. Pellizzari, E.D., Clayton, C.A., Rodes, C.E., Mason, R.E., Piper, L.L., Fort, B., Pfeifer, G., et al., 1999. Particulate matter and manganese exposures in Toronto, Canada. Atmospheric Environment 33, 721–734. Roorda-Knape, M.C., Janssen, N.A.H., De Hartog, J.J., van Vliet, P.H.N., et al., 1998. Air pollution from traffic in city districts near major motorways. Atmospheric Environment 32, 1921–1930. Sinclair, J.D., Psota-Kelty, L.A., Peins, G.A., Ibidunni, A.O., 1992. Indoor/outdoor relationships of airborne ionic substances: comparison of electronic equipment room and factory environments. Atmospheric Environment 26 (5), 871–882. Thatcher, T.L., Layton, D.W., 1995. Deposition, resuspension, and penetration of particles within a residence. Atmospheric Environment 29 (13), 1485–1497. Tichenor, B.A., Sparks, L.A., White, J.B., Jackson, M.D., 1990. Evaluating sources of indoor air pollution. Journal of Air and Waste Management Association 40, 487–492. Tung, T.C.W., Chao, C.Y.H., Burnett, J., 1999. A methodology to investigate the particulate penetration coefficient through building shell. Atmospheric Environment 33, 881–893. Turpin, B.J., Huntzicker, J.J., 1995. Identification of secondary organic aerosol episodes and quantitation of primary and secondary organic aerosol concentrations during SCAQS. Atmospheric Environment 29 (23), 3527–3544. Wallace, L., 1996. Indoor particles: a review. Journal of Air and Waste Management Association 46, 98–126. Yocom, J.E., 1982. Indoor–outdoor air quality relationships: a critical review. Journal of Air Pollution Control Associations 32 (5), 500–520.
Atmospheric Environment 37 (2003) 4277–4285
A balance-point method for assessing the effect of natural ventilation on indoor particle concentrations Yuguo Lia,*, Zhengdong Chenb a
Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong b CSIRO Manufacturing and Infrastructure Technology, Highett, Vic. 3190, Australia Received 14 June 2002; received in revised form 9 June 2003; accepted 17 June 2003
Abstract A balance-point outdoor concentration was proposed to represent the value of outdoor concentration at which the air exchange between indoor and outdoor does not affect the indoor and outdoor pollutant concentration ratio (I/O). When the outdoor particle concentration is higher than the balance-point outdoor concentration, an increase in the indoor and outdoor air exchange results in a higher indoor particle concentration level and vice versa. Indoor particulate matter concentration and its relationship with outdoor particle concentration for a hypothetical naturally ventilated building are studied using a simple steady-state model. The effects of various factors such as air infiltration exchange rate, ambient particle concentration, indoor source intensity, and human activity on I/O, and the correlation between indoor and outdoor concentration were investigated, and the results compare reasonably well with the results reported in the literature. r 2003 Elsevier Ltd. All rights reserved. Keywords: Balance-point outdoor concentration; Indoor air quality; Particulate matter; Macroscopic model; I/O; Natural ventilation
1. Introduction Particles of concern to human health are those known as inhalable particles with a diameter less than 10 mm (PM10), especially those smaller than 2.5 mm (PM2.5). Exposure to indoor particulate matter has been recognised as a significant environmental and health problem. Indoor particulate matter concentrations are complex combinations of many factors, such as indoor pollutant sources, ambient air conditions, building materials, human behaviour and activities, ventilation and particle size distributions. In the past several decades, numerous measurements have been reported on the ratio between the indoor and outdoor particle concentration (or I/O), which to some extent indicates how well the building envelope protects us against outdoor particulate matter, *Corresponding author. Tel.: +852-2859-2625; fax: +852-2858-5415. E-mail address: [email protected] (Y. Li).
or how well the building ventilation system disperses the indoor-generated particles. Table 1 provides a summary of some measured I/Os reported in the literature, mainly for naturally ventilated buildings. A wide range of I/Os between 0.1 and 5 can be found. The intensity of the I/O correlation was also found to vary from very weak to very strong among different measurements by different research groups. When a strong correlation between indoor and outdoor particle concentrations exists, the personal particulate exposure may be readily estimated according to ambient particulate concentration level, which is now often a routine measurement in many cities. On the other hand, a weak I/O correlation makes the exposure estimation from the ambient particulate concentration questionable. Up to now, the question as to whether the personal exposure level can be appropriately assessed from the ambient particulate concentration is still not resolved, and conflicting results have been reported by different research groups (Mage et al., 1999; Lachenmyer and
1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00527-2
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Table 1 Examples of I/O ratios for particulate matters reported in the literature Reference
Location
Chao and Wong (2002)
Hong Kong
Fisher et al. (2000)
Netherlands
Particle size (mm)
I/O ratio
Conditions 34 residential homes and 30 homes were above fourth floor. For high air exchange rate (o3.5 h1)
PM2.5 PM10 PM2.5 PM10
0.92 0.88 1.09 1.04
PM10 PM2.5 PM10 PM2.5
0.68 0.86 0.58 0.8
High traffic homes
For low air exchange rate (o3.5 h1)
Low traffic homes
Funasaka et al. (2000)
Japan
PM10 PM10
0.62 0.86
Roadside houses in winter Background houses in winter
Jones et al. (2000)
UK
PM10
PM2.5
1.58 2.13 2.46 1.0
A A A A
whole whole whole whole
year, year, year, year,
roadside residential houses urban flats rural residential houses roadside residential house
Kingham et al. (2000)
UK
PM10 PM2.5
0.12–3.28 0.15–5.22
Non-smoker buildings
Koistinen et al. (2001)
Finland
PM2.5
2.2 0.86
Smoker homes Non-smoker homes
Lachenmyer and Hidy (2000)
US
PM2.5
0.61 0.92
Summer Winter
Lee and Chang (2000)
Hong Kong
PM10
0.63 0.82 0.75–1.3
Schools with window air-conditioning Schools with central air-conditioning Schools with ceiling fans
Monn et al. (1997)
Switzerland
PM10
PM2.5
0.7 >1 1.2–2.0 0.54 1.23
Absence of indoor source and activities With human activities With smoking and/or gas cooking Absence of indoor source and activities With human activities
Pellizzari et al. (1999)
US
PM10 PM2.5
0.98 1.2
Residential homes
Roorda-Knape et al. (1998)
Netherlands
PM10
2
Summer, school rooms
Hidy, 2000; Koistinen et al., 2001). It is believed that a systematic analysis of the data from various research groups may provide some insights into I/Os and their relationships for different types of buildings under various influencing factors. The building envelope does not necessarily provide us with good protection from exposure to certain types of pollutants. For pollutants with predominant indoor sources (such as formaldehyde and most other VOCs),
the building envelope restricts the dispersion of pollutants, and air exchange between indoor and outdoor should be encouraged. On the other side, for pollutants with predominant outdoor sources (such as pollens), a reduction in indoor and outdoor air exchange can reduce the indoor exposure level. For pollutants with origins both indoors and outdoors, e.g. particulate matters, nitrogen oxides and carbon monoxide, whether to encourage air exchange or not depends on the relative
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intensity of pollution between indoor and outdoor. However, so far there appears to be no simple quantitative index to measure the relative intensity of the pollution between indoor and outdoors, i.e. when should the air exchange between indoor and outdoor be encouraged when we consider a specified type of pollutant at a particular indoor source strength, building activities and filtration characteristics. In this paper, the indoor particulate matter concentration and its relationship with outdoor particle concentration are studied using a simple steady-state model for a hypothetical naturally ventilated building. A concept of balance-point outdoor concentration is proposed to represent the value of outdoor concentration at which the air exchange between indoor and outdoor does not affect the indoor and outdoor pollutant concentration ratios.
2. The concept of balance-point outdoor concentration Assuming that the particle concentration in the building is uniform, the following simple macroscopic mass balance for particulate matter can be written for a naturally ventilated building: Transient
Indoorsource zfflffl}|fflffl{ zfflfflffl}|fflfflffl{ dCi V ¼ aPVCo þ V’ source |fflfflfflffl{zfflfflfflffl} dt Penetration
Airflowremoval
þ RLfl Afl |fflfflfflffl{zfflfflfflffl}
zffl}|ffl{ aVCi
Resuspension
Othersinks
zffl}|ffl{ KVCi V’ sink ; |fflffl{zfflffl} Deposition
ð1Þ where a is the air exchange rate per hour due to infiltration and natural ventilation (1/h), Afl is the floor area (m2), Ci is the indoor particle concentration (mg/m3), Co is the outdoor particle concentration (mg/m3), K is the particle deposition rate (h1), Lfl is the mass loading of particles on accessible floor surfaces (mg/m2), P is the particle penetration coefficient, R is the particle resuspension rate (h1), t is the time (h), V is the volume of the room (m3), V’ source is the indoor particle generation rate (mg/h), and V’ sink is the removal rate of particles due to other sinks such as filtration (mg/h). Eq. (1) has been used in similar forms by a number of previous authors (Alzona et al., 1979; Dockery and Spengler, 1981; Tichenor et al., 1990; Kulmala et al., 1999; Mage et al., 1999). All the parameters except V and Afl are a function of both time and particle sizes. When the variations of these parameters are small, a steady-state indoor particle concentration can be obtained when the removal rate due to other sinks is zero: Ci ¼
aPCo RLfl Afl þ V’ source þ ; aþK ða þ KÞ V
ð2Þ
From Eq. (2), we obtain I/O as Ci aP RLfl Afl þ V’ source þ : ¼ Co a þ K Co V ða þ KÞ
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ð3Þ
The equation shows that various parameters affect I/O in naturally ventilated buildings. We propose a concept of balance-point outdoor concentration. It is obtained by simply setting I/O to be the value of the penetration coefficient P in Eq. (3). RLfl Afl þ V’ source Co;e ¼ : ð4Þ KVP A corresponding indoor particle concentration is defined as a balance-point indoor concentration: RLfl Afl þ V’ source : ð5Þ Ci;e ¼ KV Eq. (5) represents a balance between the indoor particle generation speed and particle deposition rate. I/O can also be written as Ci KPðCo;e Co Þ ¼Pþ : Co ða þ KÞ Co
ð6Þ
From Eq. (6), it is seen that when the outdoor particle concentration is Co;e ; the indoor particle concentration becomes Ci;e and is not affected by the infiltration of outdoor air. It can also easily be seen that the following relationships hold: Ci If Co;e > Co then > P; Co Ci IfCo;e oCo then oP: Co Thus, the balance-point outdoor concentration is a dividing point. If the outdoor concentration is higher than the balance-point outdoor concentration, I/O is always smaller than P: With this condition, the minimum I/O ðCi =Co Þ ¼ ðPCo;e =Co Þ is achieved when there is no air exchange, and the maximum I/O ðCi =Co Þ ¼ P is obtained for an infinite air exchange rate. When the outdoor concentration is less than the balance-point outdoor concentration, I/O is always larger than P: The minimum I/O ðCi =Co Þ ¼ P is achieved for an infinite air exchange rate, and the maximum I/O ðCi =Co Þ ¼ ðPCo;e =Co Þ is obtained when there is no air exchange. From Eqs. (4) and (5), it is seen that the indoor and outdoor balance-point concentrations are only functions of the indoor parameters such as particle sources, sinks, building geometry and filtration characteristics. For high indoor sources and human activity intensity, the indoor air is naturally dirtier in terms of particulate matter and is characterised by high balance-point indoor and outdoor concentrations. For buildings with low activity and no indoor sources, the indoor air is clean and the balance-point indoor and outdoor concentrations are close to zero. Consequently, Co;e and Ci;e
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Table 2 Parameters used in the model study for I/O and I/O concentration relationship Parameter
Value
References
Afl Co K
15 8 m2 30 mg/m3 for PM2.5, 50 mg/m3 for PM10 0.4 h1 for PM2.5, 1.0 h1 for PM10
Lfl PM2.5/PM10 P
100 mg/cm2 0.6 0.9
R
1.0 105 h1 for PM2.5, 2.5 105 h1 for PM10 15 8 2.4 m3 14 mg/cigarette for PM2.5, 22 mg/cigarette for PM10 1.7 mg/min for PM2.5, 4.1 mg/min for PM10 2.8 mg/h for PM10, 0.95 mg/h for PM2.5
— Chan et al. (2001) Thatcher and Layton (1995), Byrne et al. (1995), Fogh et al. (1997) Thatcher and Layton (1995) — Thatcher and Layton (1995), Wallace (1996), Tung et al. (1999) Thatcher and Layton (1995)
V V’ source (cigarette smoking) V’ source (cooking) V’ source (others)
effectively quantify the intrinsic cleanness of the indoor environment by combining the effects of indoor activities, particulate sources and sinks, as well as the building envelop filtration characteristics. The effectiveness of ventilation and infiltration on indoor particle concentration simply depends on whether the outdoor air is cleaner or dirtier than the air at the balance-point outdoor particle concentration, Co;e : High air exchange rates should be encouraged when outdoor particle concentrations are lower than Co;e ; and vice versa. Consequently, the outdoor balance-point concentration for a building can be used as an index to quantify the cleanness of the indoor environment and determine whether ventilation should be encouraged or not to reduce the indoor particulate exposure level. Seen in Eqs. (4) and (5), Co;e and Ci;e are both particle size dependent. Further, our analyses here focus entirely on particulate matter alone. Natural ventilation needs to be controlled by many other physical and environmental parameters, such as air temperature and other ambient pollutants. From the derivations of Co;e and Ci;e ; the concept of balance-point indoor and outdoor concentrations may be applied to other pollutant components. A decision to increase ventilation to reduce indoor PM levels might lead to an increase of indoor concentration of other pollutants, e.g. ozone, SO2, etc. For a complete analysis of ventilation needs, other pollutants also need to be considered. The concept of the balance-point outdoor concentration is very similar to the concept of balance-point outdoor temperature used in steady-state methods for energy consumption analysis in buildings based on degree-days (Kreider and Rabl, 1994). The balancepoint temperature is defined as the value of the outdoor temperature where, for a specified indoor air temperature, the total heat loss is equal to other indoor heat
— Wallace (1996) Wallace (1996) Wallace (1996)
gains. With this concept, heating is needed only when the outdoor air temperature drops below the balancepoint temperature. Similar to the concept of balancepoint temperature, the balance-point concentration is a steady-state concept. A short-term balance-point concentration may also be used for certain short time period within which all the affecting parameters may be considered as a constant. Although computers can now calculate the energy consumption of a building at the touch of a key with a transient analysis method, the concepts of balance-point temperature and degree-days still remain a valuable engineering tool (Kreider and Rabl, 1994). Similarly, the concept of balance-point concentration also appears to be a useful index for assessing the cleanness of the indoor environment, although the indoor sources, outdoor concentrations or ventilation rates are not constant. In the definition of balance-point indoor and outdoor concentrations, seven parameters have been involved, i.e. Afl ; K; Lfl ; P; R; V and V’ source ; within which the floor area, floor particle loading and building volume can be relatively easily estimated or measured. Many studies have contributed to the understanding of the particle penetration coefficient, resuspension rate, deposition rate and various indoor particle sources (e.g. Thatcher and Layton, 1995; Byrne et al., 1995; Wallace, 1996; Fogh et al., 1997; Tung et al., 1999). A full discussion of these parameters is out of the scope of this paper. Table 2 lists the reasonable values for these affecting parameters obtained from literature. Particle deposition rates reported by previous researchers are generally in agreement with the deposition rates of around 0.4 and 1.0 h–1 for PM2.5 and PM10, respectively (Thatcher and Layton, 1995; Byrne et al., 1995; Wallace, 1996; Fogh et al., 1997). The contribution of particle resuspension to the indoor particle
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3. Application of the balance-point concept As an attempt to provide a systematic analysis of the data from various research groups for different types of buildings under various influencing factors, we performed analyses using the simple macroscopic model and the concept of the balance-point outdoor concentration proposed in Section 2. In the following discussions, unless specified, the parameters listed in Table 2 will be used as the default values. Our analyses will be compared with the results and phenomena reported in the literature, and the comparisons should not be considered as strictly quantitative. 3.1. Buildings with no activity and without indoor sources For buildings with no human activity and without indoor sources, Eq. (3) becomes Ci aP P ¼ ¼ : Co a þ K 1 þ ðK=aÞ
ð7Þ
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1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
PM 2.5 PM 10
0 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 (b)
1
2
3
4
5
-1 Air exchange rate, a, (h )
(a)
I/O ratio
concentration can be significant, e.g. during periods of vacuum cleaner use or intense human activities (Thatcher and Layton, 1995; Jones et al., 2000). Indoor particle sources can be due to combustion, smoking, cleaning process and particles from building material, furnishings, HVAC, office equipment, and various human activities. Sources such as secondary organic aerosol formation can be very difficult to quantify (e.g. Turpin and Huntzicker, 1995). Studies showed that indoor combustion such as range-top burners, ovens, fireplaces, kerosene heaters, cigarette smoke and cooking can be significant, while other indoor sources are relatively small (Wallace, 1996, Jones et al., 2000; Chao and Wong, 2002). For simplicity, indoor particle sources are assumed to be composed of cigarette smoking, cooking and other sources listed in Table 2. The penetration coefficient is a very difficult parameter to determine in practice and is still not well documented. Various parameters can affect the penetration coefficient, including the geometry and size of various openings and cracks in the building envelope, the airflow through these openings, the particle sizes (which affect the particle deposition or trap along the flow paths in the building envelope), etc. In general, a penetration coefficient range of 0.7–1.0 was reported in previous research (Thatcher and Layton, 1995; Wallace, 1996; Tung et al., 1999). With the increasing understanding of these affecting parameters, it is expected that the balance-point indoor and outdoor concentration will be easily estimated and will provide an index of the cleanness of the indoor environment, and will be used to determine the ventilation strategy for buildings, even before construction.
I/O ratio
Y. Li, Z. Chen / Atmospheric Environment 37 (2003) 4277–4285
PM2.5, Co = 15 PM2.5, Co = 22 PM2.5, Co = 50
0
1
2
PM10, Co = 15 PM10, Co = 22 PM2.5,Co = 50
3
4
5
-1 Air exchange rate, a, (h )
Fig. 1. I/Os for buildings (a) with low activity and without indoor sources and (b) with normal human activity and without major indoor sources.
According to Eq. (4), the balance-point outdoor concentrations for both PM2.5 and PM10 are zero. Fig. 1a shows the estimated I/Os for PM2.5 and PM10 at different air exchange rates from 0.1 to 5 h1. The I/Os are less than or close to 0.9 (which is the value of the assumed penetration coefficient) and increase with an increase in the air exchange rate, approaching the penetration coefficient. Consequently, for buildings with no human activities and indoor sources, the most effective way of reducing indoor particulate matter concentration is to minimise the natural ventilation and air infiltration. This is in accordance with the fact that the outdoor concentration is always higher than the balance-point outdoor concentration, which is zero. It is noted that infiltration rates can vary in the range 0.2–1.0 h1 for relatively airtight buildings (ASHRAE, 2001). For some old or less airtight buildings, the air exchange rate may be high. In natural ventilation with open windows, the ventilation flow rate can be more than 20 h1 (e.g. Heiselberg, 2002). According to Eq. (7), the I/Os for PM2.5 and PM10 were found to increase by 114% and 200% respectively with an increase in the air exchange rate from 0.2 to 1.0 h1, but to increase 55% and 114% respectively with an increase in the air exchange rate from 0.5–3.0 h1. This suggests that good correlations between indoor and outdoor concentrations
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may be more frequently found during moderate weather when windows or doors are frequently open in old or less airtight buildings, especially for fine particles. However, for more airtight buildings, e.g. most new office buildings, the I/O concentration correlation may be relatively weak, especially for coarse particles. Eq. (7) did not consider the transient effect when the outdoor concentrations and indoor sources are not constant. At higher air change rates, the apparent lag between outdoor concentration change and indoor concentration response is reduced, which causes the I/O ratio to appear more stable. These conclusions appear to be in agreement with the observations in a number of studies. Measurements in several residential and office buildings by Baek et al. (1997) in Korea showed that I/O correlated well for residential homes in both summer and winter, but correlated weakly for more airtight offices being studied. Jones et al. (2000) reported better I/O correlation in well-ventilated houses than more airtight houses. Lachenmyer and Hidy (2000) reported a stronger correlation in summer than in winter, since windows and doors are more frequently opened during summer. Better I/O correlations for fine particles than coarse particles were observed by Sinclair et al. (1992); Funasaka et al. (2000) and Jones et al. (2000). 3.2. Buildings with normal activity and without major indoor sources Here, major indoor sources such as cooking and smoking are excluded and only the indoor sources, such as particles resuspended from loose construction materials and furnishings and other sources, are considered. The balance-point outdoor PM2.5 and PM10 concentrations defined by Eq. (4) are estimated to be 21 and 22 mg/m3 respectively for this case, as listed in Table 3. Eq. (6) suggests that I/O is a function of the outdoor concentration. Fig. 1b shows the I/Os for different air exchange rates at PM10 outdoor concentrations of 15, 22 and 50 mg/m3 respectively. It is seen that I/O decreases with an increase in the ambient particle concentration under otherwise the same conditions. This trend can be
easily seen from Eq. (6) and has been observed by a number of authors. For example, Funasaka et al. (2000) reported that the I/O for roadside houses is generally lower compared with those away from the roadside, due to an increase in the outdoor concentration for roadside houses. The I/O for fine particles is generally higher than that for coarse particles due to the high deposition rate of the latter. Relatively high I/Os for fine particles have been reported by Jones et al. (2000) and Thatcher and Layton (1995) for houses with no major indoor sources. At an outdoor PM10 concentration of 15 mg/m3, the PM10 I/O increases with a decrease in the air exchange rate, while for outdoor PM10 concentrations of 50 mg/ m3, the PM10 I/O decreases with a decrease in the air exchange rate. The outdoor PM10 concentration, which divides these two situations, is the balance-point outdoor PM10, which is 22 mg/m3 for the current case. As shown in Fig. 1b, at an outdoor PM10 concentration of 22 mg/m3, the air exchange rate has no effect on the PM10 I/O. Similarly, the dividing point for PM2.5 is the balance-point outdoor PM2.5, which is 21 mg/m3 (the corresponding PM10 concentration is around 35 mg/m3 if a PM2.5/PM10 of 0.6 is assumed). The ambient particle concentration is generally in the range of 10–100 mg/m3 in most of the cities (Ando et al., 1996; Wallace, 1996; Chan et al., 2001; Baek et al., 1997; Hooper and Hooper, 1986; Koistinen et al., 2001). Consequently, the current simple modelling suggests that for buildings with normal activity and without major indoor sources as listed in Table 2, a high air exchange rate should not be encouraged in terms of particulate exposure in some moderately and highly polluted regions with a PM10 concentration higher than 35 mg/m3. Conversely, in less polluted ambient regions with a PM10 concentration lower than 20 mg/m3, a high ventilation rate is beneficial to reduce indoor PM2.5 and PM10 concentrations. Again, Fig. 1b shows that the I/Os change rapidly with air infiltration rate and outdoor particle concentration for small air exchange rates of ao1:0; when the outdoor concentration is away from the balance-point concentration value. Consequently, poor I/O concentration correlation may be more frequently observed for relatively airtight buildings, when the outdoor
Table 3 Balance-point outdoor particle concentration, Co;c ; for various conditions Indoor sources Normal activity, no major source Normal activity, cooking alone Normal activity, smoking alone Normal activity, cooking and smoking
Daily mean Co;e for PM10 (mg/m3)
Daily mean Co;e for PM2.5 (mg/m3)
Short-term Co;e for PM10 (mg/m3)
Short-term Co;e for PM2.5 (mg/m3)
22
21
22
21
42 93 154
41 133 154
1005 831 1815
971 532 925
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dealing with highly transient sources. As shown in Table 3, the short-term balance-point outdoor concentrations (obtained from Eq. (4) during periods of smoking or cooking) are above 500 and 800 mg/m3 for PM10 and PM2.5 respectively, which are much higher than the outdoor particle concentration range of 10–100 mg/m3 normally observed. Consequently, high air exchange rates during cooking and smoking can substantially reduce indoor particulate concentration levels. Fig. 2 shows the effect of smoking and cooking on the daily averaged I/Os for outdoor particle concentrations of 15, 25, 50 and 100 mg/m3 respectively. For low outdoor particulate matter concentrations, I/O is substantially increased with cigarette smoking, while the contribution of cooking is relatively smaller. For buildings with heavy smokers (one pack of 20 per day), I/O is generally around 1.2–5 for an outdoor PM10 concentration range of 25–100 mg/m3 and an air exchange rate of 0.2–1.0 h1. The relative effect of smoking and cooking on I/O decreases with an increase in the outdoor particulate matter concentration as well as the air exchange rate. This is in accordance with the 18–30% increase in I/O reported by Baek et al. (1997) in Korea and Chao and Wong (2002) in Hong Kong with
concentration is significantly different (either lower or higher) from the balance-point concentration value. On the other hand, a relatively strong I/O correlation can be observed when the outdoor concentration is close to the balance-point concentration value or if the building is relatively leaky. 3.3. Buildings with normal activity and major indoor sources Cooking and cigarette smoking are the two major indoor particle sources. As a hypothetical example, it is assumed that (1) the intensities of particle generation due to cooking and cigarette smoking are the mean values as listed in Table 2; (2) normal cooking duration is 30 min per day; and (3) smoking intensity is one pack of 20 per day, with each cigarette taking 10 min to burn. From Eq. (4), the balance-point outdoor particle concentrations for different conditions are listed in Table 3 for PM10 and PM2.5, respectively. It is seen that cooking and smoking significantly increase the daily mean balance-point outdoor concentrations due to the high indoor particle generation rate. It should be mentioned that the concept of the daily mean balancepoint outdoor concentrations is not very useful when
12
8 6 4
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Air exchange rate, a, (h )
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3 I/O ratio
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6 I/O ratio
I/O ratio
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PM 2.5, Normal activity, Co = 15 PM 10, Normal activity, Co = 15 PM 2.5, Cooking, Co = 15 PM 10, Cooking, Co = 15 PM 2.5, Smoking, Co = 15 PM 10, Smoking, Co = 15
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1
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-1 Air exchange rate, a, (h )
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0 (d)
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4 -1 Air exchange rate, a, (h )
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Fig. 2. The effect of smoking and cooking on the daily average I/O ratios at an outdoor PM10 concentration of: (a) 15 mg/m3; (b) 25 mg/m3; (c) 50 mg/m3 and (d) 100 mg/m3.
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relatively high outdoor concentrations, compared to a 100% or more increase in I/O due to cigarette smoking reported by most researchers in Europe and North America (Yocom, 1982; Wallace, 1996) where the outdoor concentrations are relatively low compared to those in Korea and Hong Kong. From Fig. 2, it can be seen that with cigarette smoking, I/Os change significantly in the air exchange rate range of 0.2–2.0 h1 and in the outdoor concentration range of 15–100 mg/m3. The I/O correlation is expected to be worse in comparison to buildings without smoking under otherwise identical conditions. Consequently, a poor I/O correlation is expected for most buildings with heavy smokers. The effect of cooking appears to be not as strong as indoor smoking and good I/O concentration correlation may be observed in old or less airtight buildings. However, for restaurants, high cooking intensity can result in significantly high indoor particle sources and poor I/O concentrations may be found, as reported by Baek et al. (1997). From the above model studies, it is seen that the short-term balance-point outdoor concentration for buildings ranged from 0 mg/m3 for no indoor sources, 20 mg/m3 for no major indoor sources and 2000 mg/m3 for major indoor sources, and ventilation strategies can be readily decided based on the balance-point outdoor concentration. It has also shown that I/O correlation is significantly affected by the indoor and outdoor air exchange rate, as well as the balance-point outdoor concentration. A number of investigations have shown that a relatively strong I/O correlation can be achieved at a high air exchange rate. From the current model study, it was found that relatively strong I/O correlation may also be observed if the outdoor concentration is close to the balance-point outdoor concentration, even at low air exchange rates.
4. Conclusions Balance-point indoor and outdoor concentrations for naturally ventilated buildings are defined, at which I/O equals the value of the penetration coefficient. Model studies showed that the balance-point outdoor concentration is a simple steady state index to quantify the cleanness of the building in terms of the outdoor particle concentration. When the outdoor particle concentration is less than the balance-point concentration, I/O is always smaller than the penetration coefficient, and the minimum indoor concentration can be achieved at high indoor and outdoor air exchange rates. Thus, introducing fresh air into the building should be encouraged. On the other hand, when the ambient particle concentration is higher than the balance-point outdoor concentration, I/O is always higher than the penetration
coefficient and ventilation and/or infiltration should be minimised. Indoor and outdoor particulate matter concentration relationships were also investigated systematically with model studies. It was found that a good I/O correlation is achieved when the outdoor particle concentration is close to the balance-point outdoor concentration or when the building is less airtight. However, poor I/O correlation may be more frequently observed for relatively airtight buildings when the outdoor particle concentration is much lower or much higher than the balance-point outdoor particle concentration.
Acknowledgements This research was supported by a grant from the Seed Funding Programme for Basic Research (2000–2002) at the University of Hong Kong.
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