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Characteristics of particles and black carbon emitted by combustion of incenses, candles and anti-mosquito products
Building and Environment 56 (2012) 184e191
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Characteristics of particles and black carbon emitted by combustion of incenses, candles and anti-mosquito products L. Stabile*, F.C. Fuoco, G. Buonanno Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, via G. Di Biasio, 43, 03043 Cassino (FR), Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2011 Received in revised form 21 February 2012 Accepted 7 March 2012
Indoor combustion sources are of great concern when accurate evaluations of the overall human exposure to particles have to be provided. Even if cooking activity was claimed to be the most emitting source in indoor environments, other indoor activities involving incense, candle and anti-mosquito product combustions can produce high particle concentrations. To this purpose, studies evaluating PM fraction emission rates from such indoor activities were performed, even if there is still a lack of understanding in terms of particle number and carbonaceous amount (black carbon, BC) carried by particles themselves. The aim of the present study was to characterize the particle emission due to the combustion processes of incenses, candles and anti-mosquito products. Emission factors in terms of number, surface area and PM fraction concentrations were evaluated through a condensation particle counter, a scanning mobility particle sizer, and an aerodynamic particle sizer. Moreover, BC emission factors were measured through an aethalometer. Particles’ BC content distribution was also measured proposing an experimental method made up of a particle size classification device connected to an aethalometer. Emission factors due to incenses and anti-mosquito products were higher than 1014 part h1 and 48 mg h1 in terms of number and PM10 concentrations, respectively. Differently, PM fraction emissions from candle burning were well below 1 mg h1. Nonetheless, BC emission rate and distribution measurements showed that candle flaming combustion produces mainly carbonaceous particles (BC/ PM10 ratio higher than 80%). Differently, smoldering combustion processes, like incense and antimosquito product combustions, showed a negligible amount of BC. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Indoor aerosol Emission factors Black carbon distribution Particle number PM fractions
1. Introduction Aerosol exposure is of great concern for medical and air-quality experts because of the particles’ ability in: i) crossing human respiratory system, ii) depositing in its deepest and defenseless regions and iii) carrying condensed toxic compounds. Moreover, particles are produced by several indoor and outdoor sources that, in this way, are present in large doses in every kind of people’s lifestyle. For this reason, the scientific community carried out several scientific studies to deepen the health effect caused by the exposure to particles [1]. In particular, epidemiological, toxicological and environmental researchers are trying to understand which particle size, assumption rate, and chemical component is mainly related to negative effect on human health. About the size, the interest of such
experts is moving from particle mass concentrations (PM10 or PM2.5) to particle number [2] and surface area [3] concentrations, mainly characterized in terms of sub-micrometer and ultrafine particles (UFPs). About the assumption rate, a number of authors are more confident that short term fluctuations in airborne can increase human morbidity and mortality [4,5], whereas other researchers attempted to demonstrate that mortality is due to long term chronic exposure [6]. With regard to the chemical composition, several papers demonstrated the toxic effect of metals and compounds surrounding the particle [7], nonetheless, also the carbonaceous core (black carbon, BC) itself seems to affect the human health [8e10].
1.1. BC effect on human health: the state-of-art * Corresponding author. Tel.: þ39 0776 2993668, þ39 3398673239 (mobile); fax: þ39 0776 2993393. E-mail address: [email protected] (L. Stabile). 0360-1323/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2012.03.005
The United Nations report on the effects of air pollution [11] suggested that BC can be considered a better indicator of harmful particulate substances caused by combustion sources than PM10
L. Stabile et al. / Building and Environment 56 (2012) 184e191
and PM2.5, especially for short-term effects. In fact, a number of authors suggest that a more suitable traffic-related policy measure should be stated in terms of BC concentration reduction [10e12]. Indeed, BC is a typical combustion by-product, therefore, it is better related to particle number emissions than mass ones [13,14]. Even if it was difficult to separate the contribution of BC fraction to health outcomes from co-emitted organic carbon (OC) and other compounds one, numerous toxicological and epidemiological studies were performed to deepen the BC-related health effects. BC exposure is supposed to have negative effects on the respiratory system [15], nevertheless, most of the studies are focused on cardiovascular effects such as cardiac and ventricular arrhythmias, lowered heart rate variability, changes in blood pressure and increased cardiovascular mortality [9,16e18]. Such results highlight the necessity to measure BC along with mass and number concentrations in freshly emitted particles. To this purpose, an increasing number of air quality studies reporting BC spatial and temporal evolutions in urban areas were produced in the last years clearly indicating that the contribution of BC was significant in UFPs [12,14,19,20]. 1.2. Indoor source emissions An overall assessment of the personal exposure to particles, and their BC content, cannot neglect indoor microenvironments where many activities can lead to high doses received by people as these are the places where we spend most of the time of our life (80e90%). In particular, several studies were performed to characterize particles emitted from cooking activities, by measuring emission factors and size distributions [21e24] as well as particle volatility [25] as function of cooking method and several influential parameters. However, today people are also exposed to other relevant indoor combustion sources, like incenses and candles, commonly used both for esthetic and religious purposes in various indoor environments. For example, measurements carried out in a worship building showed high particle concentrations in terms of number and mass [26,27]: this is due to the high particle emission factor characterizing such indoor activities. In confirmation of this, a number of experimental analyses were performed in test chambers in order to evaluate the emission strength of particles. A wide deviation of the data was found from literature results, for example PM2.5 emission factors in the range 5e250 mg h1 and 0.02e25 mg h1 were measured during incense [28,29] and candle burning [30,31], respectively. Such variability of the results is probably due to the several types of additives and/or essential oils used in candles and incenses to improve their esthetic appearance and scent. The use of additives can also lead to emission of toxic gaseous pollutants. In particular, high emission rates were measured for dibenzo-p-dioxins/dibenzofurans [32], PAHs [33] and metals [26] during incense burning, whereas, candle burning events seem to show lower PAH and VOC emission levels compared to carbonaceous emissions (EC, BC) [31]. Also mosquito-coils smoldering activity emits a significant amount of metals, aldehydes and hydrocarbons, as well as gas-phase and particle-phase PAHs [34,35].
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terms of mass fractions (PM2.5 and PM10); actually, very few studies reported emission factor data also in terms of number [35e37] and carbonaceous amount [29,31,38]. In the present paper, the authors deepened the knowledge of particle emission due to the combustion of incenses, candles and mosquito coils. In particular, measurements of emission factors in terms of PM10, PM2.5, PM1, number and surface area were performed through a condensation particle counter, a scanning mobility particle sizer spectrometer, and an aerodynamic particle sizer spectrometer in a chamber. Moreover, BC emission factors were also performed through an aethalometer. Finally, since the health effect strongly depends on the distribution of carbonaceous matter over the particle size spectrum [39,40], an experimental apparatus to characterize the amount of BC carried by particle of different size was proposed. Actually, the few previous studies attempting to measure BC distributions were focused on outdoor microenvironment characterization by using size segregated impactor samples. In the present study, a particle classification was performed before the BC measurement through electrical mobility technique by connecting an Electrostatic Classifier in series ahead of an aethalometer leading to the BC (mass) distribution measurement in the sub-micrometer particle range. 2. Materials and methods 2.1. Emission sources and site description Three different emission sources were tested during the experimental campaign: incense sticks, candles and anti-mosquito products. Incense sticks are cored sticks coated through finely ground fragrant materials. In particular we considered three different incense fragrances: freesia (I1), citronella (I2), and church (I3). In regard to the candles, two different kinds of tapered candles were studied: a paraffin wax candle (C1) and a natural corn wax candle (C2). Finally, two different anti-mosquito products were tested: a mosquito coil (M1) and a citronella stick (M2). Measurements were carried out in the European Accredited (EA) Laboratory of Industrial Measurements (LAMI) at the University of Cassino and Southern Lazio, Italy, where thermo-hygrometric conditions were continuously monitored, in order to guarantee temperature and relative humidity values equal to 20 1 C and 50 10%, respectively. Tested sources are placed on the floor so that the smoke produced by the combustion naturally was channeled through a 1 m vertical duct in a plenum (a little chamber of 0.25 m3) where sampling points were supplied. The duct was kept about 3 cm distant from the floor (through a supporting framework) in order to guarantee a slight opening able to naturally supply the combustive air to the combustion phenomena. Candles were normally burnt, while, incenses and mosquito products are lit by a flame and fanned out so that the glowing ember on the incense will continue to smolder and burn away the rest of the materials. 2.2. Experimental apparatus In order to measure total particle number concentrations and size distributions the following instruments were used:
1.3. Aims of the work If exhaustive papers characterizing the emission of cooking activities were produced by the scientific literature, not many definitive results were reached in measuring the emission of other indoor activities like incense, candle and coil combustions. In fact, in despite of the relevance of particle number, surface area, and BC fraction concentrations in terms of health effect, emission factors due to such kind of combustions are reported almost exclusively in
- a Condensation Particle Counter, CPC 3775 (TSI Inc.), able to measure total particle number concentration down to 4 nm in diameter with 1 s time resolution; - a Scanning Mobility Particle Sizer spectrometer, SMPS 3936 (TSI Inc.), made up of an Electrostatic Classifier EC 3080 (TSI Inc.), used to classify the sampled particles in different channels according to their size, and another CPC 3775 (TSI Inc.) to count the size selected particles. The SMPS 3936 is able to
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measure particle number distribution in the range 6e800 nm with a minimum time resolution of 120 s; - an Aerodynamic Particle Sizer spectrometer, APS 3321 (TSI Inc.), able to measure particle number distribution and total concentration in the range 0.5e20 mm with 1 s time resolution. Mass and surface area distributions and total concentrations in the range 0.006e20 mm were evaluated using the SMPS/APS data and hypothesizing an ad-hoc particle density. In particular, a density of 1.1 g cm3 was used to determine PM fraction concentrations due to incense and mosquito product combustions [33,41,42], as well as a 1.5 g cm3 density value was considered in candle burning events [31]. Sphere-shaped particles were considered in the evaluation of mass and surface area concentrations and distributions for all the combustion sources. Black carbon concentrations were measured through the aethalometer AE51 (Magee Scientific); it determines the BC concentration through the BeereLambert law measuring the light’s absorption (attenuation) of optically-absorbing particles (the BC particle’s fraction). In particular, a 880 nm wavelength beam of light is produced by a LED light source reaching a photo diode detector: during its travel the light beam passes through the aerosol sample collected on a filter. The AE51 is able to measure total BC mass concentrations with a minimum time resolution of 1 s. Besides, in order to determine BC distributions, as hereinafter described (distribution measurements section), a further Electrostatic Classifier EC 3080 (TSI Inc.) was used to classify particle and to send them to the AE51. 2.3. Experimental methodology 2.3.1. Emission rate and factor measurements The emission of particles in terms of PM10, PM2.5, PM1, surface area (S), number (N), and BC was characterized by the evaluation of the emission factors and the emission rates during the experimental campaign. In order to evaluate the emission factor, the mass balance equation proposed by Chen et al. [43], and Thatcher and Layton [44] was considered:
dCin Qs ¼ P$AER$Cout þ þ ðAER þ kÞ$Cin dt V
(1)
This formula was introduced to determine indoor particles concentration levels taking into account the contributions from indoor and outdoor sources, the deposition rate of particles on indoor surfaces and the air exchange ratio. In particular, Cin and Cout stand for indoor and outdoor particle concentrations, respectively, P is the penetration efficiency, k is the deposition rate, Qs is the indoor particle generation rate, t is time and V is the efficient volume of the plenum. Anyway, the Eq. (1) can be simplified using average values instead of functions, and also making further assumptions about the experimental conditions reported in He et al. [21], (e.g. P assumed close to one; when no indoor source is in operation, the indoor particle concentration can be approximated by outdoor particle concentration and the initial indoor particle concentration could be used to replace outdoor particle concentrations). Therefore, we evaluated the emission factor (E) through such simplified equation proposed by He et al. [21], for indoor sources and already applied in our previous works [24,25]:
Cin Cin;0 þ k,C AER,C E ¼ V$ þ AER in in;0 Dt
(2)
where, Cin and Cin,0 represent the peak and initial indoor concen þ k is the trations (in terms of N, S, PM, or BC), respectively, AER
average total removal rate (taking into account the deposition rate, k, and the air exchange ratio, AER), DT is time difference between initial and peak concentration and V is the efficient volume of the plenum. This equation ignores the effects of particle dynamics such as condensation, evaporation and coagulation, since these are considered to be minor, particularly under the conditions normally encountered in indoor environments [44]. In order to avoid contradictions concerning the terms “emission factor” and “emission rate” already present in the scientific literature, we want to point out that He et al., 2004 [21] named the result of the Eq. (2) as emission rate, whereas Buonanno et al. [24,25], termed it as emission factor: actually both the papers refer to the same parameter which is “particle emission per unit time”. In the present paper we defined the emission factor, E, the “particle emission per unit time” and the emission rate, F, the “particle emission per unit mass of source burned”. Measurements of N, S, PM, or BC total concentrations (C in the Eq. (2)), aimed to determine the E, were performed through the above-mentioned apparatus by sampling aerosol from the plenum through flexible, conductive tubing able to minimize the losses due to electrostatic forces. The aerosol in the plenum can be hypothesized spatially homogeneous since the small volume of the plenum þ k) was evaluated itself. The average total removal rate (AER through the particle concentration trend analyses as reported in He et al. [21], whereas the AER was considered negligible as the plenum was kept close during the tests. Samplings were performed since 5 min before the sources lightening for about 45 min. N and BC concentration trends were measured (with a 1-s time resolution) through CPC 3775 and AE51, respectively, whereas S and PM trends were obtained through SMPS 3936 (with a resolution of 120 s) as described in the experimental apparatus section. The procedure for the combustion activity was as follows: i) the source was lit and placed just below the vertical duct so that the produced smoke naturally flows inside the chamber, ii) the source combustion was left on for 5 min, iii) after that it was moved away from the duct and put out (we do not blown out the source when it is still below the duct because it can change the emitted particle characteristics as also reported by Pagels et al. [31]). The emission rate (F) was evaluated by dividing the emission factor (E) per the burning rate (B, mass of the emission source burned during the combustion process) as reported in Eq. (3)
F ¼ E=B
(3)
To this purpose, the source emission was weighted before and after the 5-min combustion processes through a 1 mg balance resolution. Every emission factor and emission rate value reported in the results represents the mean value of three tests. In Table 1 the burning time and burning rate (B) for all the experiment performed are reported. 2.3.2. Distribution measurements In order to determine particle and BC content distributions for the selected indoor activities, emission sources were burned for Table 1 Experimental characteristics of the combustion processes during the emission factor (and rate) measurement. Source
Mass burned (g)
I1 I2 I3 C1 C2 M1 M2
0.141 0.132 0.178 3.110 1.872 0.270 0.210
0.021 0.021 0.020 0.377 0.239 0.028 0.020
Burn time (min)
Burning rate, B (g min1)
5 5 5 5 5 5 5
0.028 0.026 0.036 0.622 0.374 0.054 0.042
0.004 0.004 0.004 0.075 0.048 0.006 0.004
1013 1013 1013 1011 1011 1013 1013 7.51 2.52 7.11 1.61 2.19 3.25 3.07 1014 1014 1014 1012 1012 1014 1014 4.61 2.91 5.00 1.30 1.82 1.41 1.16 1017 1017 1018 1013 1014 1018 1018 5.62 9.05 3.08 6.47 1.18 2.31 1.04 1018 1019 1019 1014 1015 1019 1018 3.70 1.18 2.36 5.69 1.08 1.05 4.10 103 103 103 103 104 102 103 8.17 3.06 6.90 2.68 2.06 1.46 3.86 102 102 102 102 103 102 102 4.70 3.16 4.48 1.99 1.56 6.04 1.40 100 100 100 103 104 100 100 1.75 2.22 3.19 2.89 2.16 6.12 3.63 101 101 101 102 103 101 101 1.11 2.56 2.25 2.34 1.79 2.66 1.37 100 100 100 103 104 100 100 1.75 2.08 3.03 2.65 1.94 5.99 3.63 101 101 101 102 103 101 101 1.15 2.69 2.33 2.34 1.79 2.73 1.44 100 100 100 103 104 100 100 5.20 2.70 3.99 2.49 1.73 6.50 4.68 101 101 101 102 103 101 101
Standard deviation Source
3.68 3.96 3.37 2.40 1.79 3.12 1.94
Average Average
1.23 1.06 1.37 1.24 8.09 2.40 7.76 101 101 101 101 102 102 101 Emission rate (F)
I1 I2 I3 C1 C2 M1 M2
Average Average Average Average
PM2.5 (mg g1) PM10 (mg g1)
101 101 101 101 103 101 100
1.94 4.20 5.03 8.71 4.02 8.86 3.62
101 101 101 101 102 101 101
4.07 7.71 9.98 1.31 8.51 2.20 6.16
Standard deviation
100 100 100 101 103 101 100
1.81 3.99 4.85 8.71 4.02 8.60 3.47
101 101 101 101 102 101 101
PM1 (mg g1)
3.99 7.77 1.02 1.40 8.94 2.24 6.30
Standard deviation
100 100 100 101 103 101 100
7.89 4.92 9.67 7.43 3.49 1.96 3.53
102 102 102 101 102 102 102
BC (mg g1)
1.83 1.01 2.13 1.28 8.16 5.31 6.28
Standard deviation
102 102 102 101 103 102 103
6.21 1.83 5.11 2.12 2.43 3.41 1.03
1018 1019 1019 1016 1016 1019 1019
S (nm2 g1)
1.30 3.36 1.01 3.18 5.16 8.46 1.76
Standard deviation
1018 1018 1019 1015 1015 1018 1018
7.74 4.54 1.08 4.85 4.07 4.57 2.92
1014 1014 1015 1013 1013 1014 1014
N (part g1)
1.71 8.85 2.26 7.80 9.07 1.19 5.23
Standard deviation
1014 1013 1014 1012 1012 1014 1013
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6.19 6.18 7.28 8.95 4.02 1.01 4.88 I1 I2 I3 C1 C2 M1 M2
Standard deviation Average Average Standard deviation Average
Standard deviation
Average
Standard deviation
Average
Standard deviation
Average
Standard deviation
N (part h1) S (nm2 h1) BC (mg h1) PM1 (mg h1) PM2.5 (mg h1)
In Table 2 the emission factors (Es) and the emission rates (Fs) in terms of PM10, PM2.5, PM1, BC, S and N for all the analyzed sources are reported. Every measurement represents the average of three tests. Emission factors (as well as emission rates) in terms of particle number are higher for incenses and anti-mosquito products compared to candles. In particular, such emission factors are similar to the measured ones during cooking activities which are claimed as the main indoor particle source [24,25]. Amongst incenses and anti-mosquito products, the most emitting source in terms of particle number and surface area is the I3 (incense stick with a church fragrance), whereas M1 shows the highest PM fraction emission factors. The difference between incenses/anti-mosquito products and candles is also significant in terms of mass concentrations. For example, PM2.5 emission factors for incenses and anti-mosquito products are in the range 19e89 mg h1, whereas 0.871 and 0.0402 mg h1 were measured for C1 (paraffin wax candle) and C2 (natural wax candle), respectively. Therefore, amongst the candles, the natural wax one emits the lowest amount of particle mass, whereas surface area and number emission factors are comparable.
PM10 (mg h1)
3.1. Emission factors and emission rates
Source
3. Results
Emission factor (E)
about 1 h. Also in this experimental test the produced smoke was channeled in the plenum, but an opening of about 20 cm2 was made on one of the plenum walls allowing the smoke to flow out the plenum naturally: in fact, unlike the emission factor test, in this test it was necessary a quasi-stationary concentration. To this purpose the plenum has a double purpose: i) to guarantee constant concentrations by hiding short unstable emissions of the source, ii) to lead to high concentrations the emitted aerosols. The latter is a not-trivial condition since the aim is to measure BC content of selected diameters in the ultrafine range having a very low volume (mass) and, hence, a very low amount of BC. BC content mass distributions for I1, I2, I3, C1, M1 burning events were measured connecting the aethalometer AE51 to the Electrostatic Classifier 3080. In particular, the EC 3080 was used to classify dimensionally monodisperse particles which are immediately flown to the AE51 to measure BC mass concentration of such selected diameter. The AE51 flow rate used in the experimental test was 0.15 L min1 which also corresponds to the aerosol sample flow used at the EC 3080. A 1.5 L min1 sheath flow rate was chosen for the EC 3080 in order to guarantee an aerosol-sheath flow rate ratio equal to 1:10 through which a best resolution (10%), in the particle classification of a selected electrical mobility diameter can be obtained [45]. Diameter logarithmically equally spaced were used to built the BC distribution in the range 20e500 nm with a resolution of 8 channels per decade (12 channels). 5 min-samplings for every selected channel diameter were performed, thus the time spent to measure the complete BC distribution was about 1 h. Finally, BC mass values of every channel were corrected by charging and selecting efficiencies characteristics of the EC 3080 [46]. Number (N), surface area (S) and mass (PM1, PM2.5 and PM10) distributions and total concentrations were measured through the SMPS 3936 and the APS 3321 with 120 s time resolution. S and PM fraction trends were obtained through SMPS/APS data as described in the experimental apparatus section. A CPC 3775 was also used during this test in order to check the particle number concentration trend. Such particle number concentration trend analysis was performed to make sure that sampled aerosol did not change during the test. Aerosols’ particle and BC content distribution results represent the mean value of three tests.
Table 2 Emission factor (E, “particle emission per unit time”) and emission rate (F, “particle emission per unit mass of source burned”) data measured during the experimental analysis for the selected indoor combustion activities.
L. Stabile et al. / Building and Environment 56 (2012) 184e191
Sb (nm2 cm3)
102 102 102 103 102
BCc (mg m3)
5.92 3.53 5.35 7.80 8.89
103 103 103 104 103
5.62 1.66 4.62 1.92 3.08
Moreover, concerning mass emission of burning candle, data reported in Table 2 clearly show that for both the candles PM10, PM2.5, and PM1 are identical. Differently, this behavior was not verified during incense and anti-mosquito product combustion events. This diverse behavior could be ascribed to the different combustion phenomena involving the analyzed sources; in particular, the process involving incenses and anti-mosquito products is merely a smoldering combustion phenomena, whereas candles are characterized by a flaming combustion whose predominant emitted mass fraction is PM1 [24,30]. The different combustion process also influences the amount of carbonaceous fraction of the particles. Data of BC emission factors and rate (reported in Table 2) clearly show that BC fraction in smoldering combustion activities (I1, I2, I3, M1, and M2) are well below 1% of PM10, PM2.5, and PM1 fractions. Otherwise, emission of BC from candle combustion phenomena are larger than 80% of the PM10, PM2.5, and PM1 fraction ones: flaming combustions are almost completely made up of BC, whilst in smoldering combustion particle composition is dominated by organic matter [31].
1010 1.18 1010 1011 3.04 1010 1011 9.17 1010 108 2.88 107 1011 7.65 1010
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2.55 1.71 2.43 4.53 3.28
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100 1.33 100 101 2.70 100 101 2.55 100 103 8.50 104 101 3.74 100
c
Measurement performed with 1-s time resolution through the CPC 3775. Evaluated from the average distributions measured through the SMPS-APS. Evaluated from the BC distribution measured during the 1-h test. a
b
PM1b (mg m3)
6.02 1.39 1.22 5.28 1.44 100 1.31 100 101 2.68 100 101 2.51 100 103 7.91 104 101 3.68 100
PM2.5b (mg m3)
6.26 1.46 1.26 5.28 1.48 101 3.97 100 101 3.70 100 101 3.43 100 103 7.52 104 101 4.01 100
PM10b (mg m3)
2.00 2.15 1.83 5.42 1.69 105 105 106 104 105 8.40 4.11 1.46 6.31 6.20 106 106 106 105 106
Na (part cm3)
7.00 4.11 9.75 4.38 4.13 I1 I2 I3 C1 M1
Source
Particle size distributions in terms of PM, S, and N as well as the BC content distributions were measured for I1, I2, I3, C1, and M1 burning activities through the experimental methodology described in the Section 2.3.2 (Distribution measurements). As above reported, aerosol samplings were performed in the plenum in order to guarantee nearly stable conditions during the test since BC content distribution measurement lasts about 1-h. To this purpose a CPC 3775 was used to monitor the number concentration trend in the chamber during every experiment. In Table 3 mean and standard deviation values in terms of number concentration are reported: standard deviations below 15% were measured thereby demonstrating quasi-stationary aerosol concentrations. In Fig. 1 particle number distributions (fitted through log-normal equations) of the sub-micrometer aerosols produced through the combustion of the analyzed sources are reported. Such distributions represent the average curves of three tests. In order to better compare the different distributions, they were normalized to the respective average particle number concentrations (reported in Table 3). As shown in the Fig. 1, all five particle number distributions measured during burning events are unimodal, even if the mode really varies according to the combustion process. In particular, the flaming combustion of the paraffin candle produces a particle distribution with a peak around 35 nm, which is comparable to the findings reported in Pagels et al. [31] for steady burning candles. Otherwise, particle distributions concerning not-flaming combustion, like incenses and mosquito coil smoldering combustion, present modes larger than 100 nm in diameter as also shown in See et al. [36], Roy et al. [34] and Ji et al. [33]. In particular, the mode of the distribution due the combustion of the three incenses (I1eI3) is close to 200 nm, whereas the one due to mosquito coil is about at 140 nm. In Fig. 2 particle mass distributions of the aerosol produced through the combustion of the analyzed sources are reported. Such distributions represent the average curves of three tests; they were normalized to the respective average PM10 concentrations (reported in Table 3). All the particle mass distributions present a main mode in the range 240e290 nm, whereas second minor modes were detected during candle burning (at about 50 nm, which is related to the peak of the number distribution in the ultrafine range), and mosquito coils and citronella incense smoldering (at about 800e900 nm). In Fig. 3 the particles’ BC content distribution obtained through the proposed experimental methodology (Section 2.3.2) connecting the aethalometer AE51 to the Electrostatic Classifier 3080 are
Table 3 Total concentration in terms of PM10, PM2.5, PM1, BC, S and N during the BC distribution measurements for I1, I2, I3, C1, and M1 burning activities.
3.2. Particle and BC content distributions
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Fig. 1. Relative particle number distributions measured in the plenum through the SMPS during the combustion of the I1, I2, I3, C1 and M1 indoor sources. Distributions reported represent the average of three tests.
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Fig. 3. BC content distribution measured in the plenum through the electrostatic classifier-aethalometer system during the combustion of the I1, I2, I3, C1 and M1 indoor sources. Distributions reported represent the average of three tests.
reported. These curves represent the average of three tests and they are normalized to the respective BC total concentration measured in the plenum during the test. First of all, Fig. 3 demonstrates that also the BC content carried by the particles was found to be lognormally distributed, even if the mode considerably changes amongst the different sources. In particular, the modes of particles’ BC content during citronella (I2) and church (I3) stick incense smoldering, as well as, paraffin candle (C1) burning are basically the same of mass distribution (Fig. 2). On the contrary, freesia stick incense (I1) and mosquito coil (M1) show modes lower than the measured ones in terms of mass. Such results seem to demonstrate that organic compound amount surrounding the carbonaceous bulk was equally distributed overall particles as a layer over their active surface area during I2 and I3 burning events, whereas I1 and M1 burning events produce volatile compounds not homogeneously distributed over the different carbonaceous bulk sizes. However, as shown in the previous paragraph (Table 2), the amount of BC in the particles originated by such four combustion processes is always lower than 1%. As example, in Fig. 4a a comparison amongst particle mass distribution and particles’ BC content
Fig. 2. Relative mass distributions measured in the plenum through the SMPS-APS system during the combustion of the I1, I2, I3, C1 and M1 indoor sources. Distributions reported represent the average of three tests.
Fig. 4. Comparison amongst particle mass distribution and particles’ BC content distribution measured in the plenum through the electrostatic classifier-aethalometer system during the combustion of I3 (a) and C1 (b). Distributions reported represent the average of three tests.
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distribution measured during the combustion of church stick incense I3 is reported. The figure clearly shows that the slopes (and the modes) of the two distributions are similar. Anyway, a comparison can only be made using a logarithmic scale as the total BC concentration is 0.2% of the PM1. On the contrary, the comparison between particle mass and BC content distribution produced by candle burning (C1) is more interesting. As shown in Table 2 the mass of the particles produced by candle flaming combustion are almost completely made up of BC: this observation can be further confirmed by the comparison between mass particle and BC content distributions measured during C1 combustion activity reported in Fig. 4b. As expected, in such particles the BC content distribution is almost the same of the particle mass distribution: here the organic compound amount is negligible compared to the carbonaceous fraction. The particles’ BC content distribution measured during candle burning is quite similar to the one evaluated in urban microenvironments (using size segregated impactor samples, Hitzenberger et al. [40]). In any case, even if carbonaceous particles are typically emitted by vehicular traffic, in such microenvironments the BC amount is only 8e12% of the total mass [12,40] since externally mixed particles emitted tend to absorb organic and inorganic compounds combining with other outdoor aerosols. In our experimental procedure, where freshly emitted aerosols were characterized, the generated particles mix themselves with ambient aerosol carrying a very low amount of particles, then, such mixing is expected to have a little effect on the number, mass and BC distributions. Therefore, a very low BC fraction was measured in the presence of an excess mass made up of organic compounds (typical of smoldering combustion, i.e. incenses and anti-mosquito products), such mass, due to an incomplete combustion, condenses on to the generated black carbon particles. 4. Conclusions Emission rate and emission factors during combustion process involving incenses, candles and anti-mosquito products were measured in terms of particle number, particle surface area, PM fraction and BC concentrations. Particle number and PM fractions emitted during incenses and anti-mosquito products burning (smoldering combustion) were comparable to typical cooking activities emission. On the contrary, candle combustion, having flaming combustion, produced less amount of particles and they were mainly carbonaceous particles. The different properties in characterizing particles emitted by smoldering and flaming combustions were evidenced by the analysis of number and PM distributions. Particle number distributions due to smoldering combustions were characterized by a mode higher than 100 nm, whereas flaming combustion activity (paraffin candle burning) existed in an ultrafine range mode. BC content in particle distributions found during smoldering combustion was negligible when compared to the respective whole particle mass distribution as the BC/PM10 ratios for such combustions were lower than 1%. Otherwise, particles’ BC content distributions due to candle combustion was nearly close to the relative mass distribution as the BC/PM10 ratios for candle flaming combustions are higher than 80%. References [1] Pope CA, Dockery DW. Health effects of fine particulate air pollution: lines that connect. J Air Waste Manag Assoc 2006;56:709e42. [2] Franck U, Herbarth O, Röder S, Schlink U, Borte M, Diez U, et al. Respiratory effects of indoor particles in young children are size dependent. Sci Total Environ 2011;409(9):1621e31. [3] Giechaskiel B, Alföldy B, Drossinos Y. A metric for health effects studies of diesel exhaust particles. J Aerosol Sci 2009;40:639e51.
[4] Brugge D, Durant JL, Rioux C. Near-highway pollutants in motor vehicle exhaust: a review of epidemiologic evidence of cardiac and pulmonary health risks. Environ Health 2007;6:23. [5] Strak M, Boogaard H, Meliefste K, Oldenwening M, Zuurbier M, Brunekreef B, et al. Respiratory health effects of ultrafine and fine particle exposure in cyclists. Occup Environ Med 2010;67(2):76e7. [6] Miller KA, Siscovick DS, Sheppard L, Shepherd K, Sullivan JH, Anderson GL, et al. Long-term exposure to air pollution and incidence of cardiovascular events in women. N Engl J Med 2007;365:447e58. [7] Eiguren-Fernandez A, Shinyashiki M, Schmitz DA, Di Stefano E, Hinds W, Kumagai Y, et al. Redox and electrophilic properties of vapor- and particlephase components of ambient aerosols. Environ Res 2010;10:207e12. [8] Soto KF, Garza KM, Shi Y, Murr LE. Direct contact cytotoxicity assays for filtercollected, carbonaceous (soot) nanoparticulate material and observations of lung cell response. Atmos Environ 2008;42:1970e82. [9] Wilker EH, Baccarelli A, Suh H, Vokonas P, Wright RO, Schwartz. Black carbon exposures, blood pressure, and interactions with single nucleotide polymorphisms in microRNA processing genes. J Environ Health Perspect 2010; 118(7):943e8. [10] Janssen N, Simic-Lawson M, Hoek G, Fischer P, Brunekreef B, Atkinson R, et al. Black smoke as an additional indicator to evaluate the health benefits of traffic-related policy measures: a systematic review of the health effects of black smoke compared to PM mass. Epidemiology 2011;22(1):S199e200. [11] United Nations. Effects of air pollution on health report by the joint task force on the health aspects of air pollution. United Nations: Economic and Social Council; 2011. [12] Vanderstraeten P, Forton M, Brasseur O, Offer ZY. Black carbon instead of particle mass concentration as an indicator for the traffic related particles in the Brussels capital region. J Environ Prot 2011;2:525e32. [13] Morawska L, Keogh DU, Thomas SB, Mengersen KL. Modality in ambient particle size distributions and its potential as a basis for developing air quality regulation. Atmos Environ 2008;42(7):1617e28. [14] Schneider J, Kirchner U, Borrmann S, Vogt R, Scheer V. In situ measurements of particle number concentration, chemically resolved size distributions and black carbon content of traffic-related emissions on German motorways, rural roads and in city traffic. Atmos Environ 2008;42(18):4257e68. [15] Suglia SF, Gryparis A, Schwartz J, Wright RJ. Association between trafficrelated black carbon exposure and lung function among urban women. Environ Health Perspect 2008;116:1333e7. [16] Rich DQ, Schwartz J, Mittleman MA, Link M, Luttmann-Gibson H, Catalano PJ, et al. Association of short-term ambient air pollution concentrations and ventricular arrhythmias. Am J Epidemiol 2005;161:1123e32. [17] Delfino RJ, Rjoa T, Gillen DL, Staimer N, Polidor A, Arhami M, et al. Trafficrelated air pollution and blood pressure in elderly subjects with coronary artery disease. Epidemiology 2010;21:396e404. [18] Zanobetti A, Baccarelli A, Schwartz J. Gene-air pollution interaction and cardiovascular disease: a review. Prog Cardiovasc Dis 2011;53:344e52. [19] Kim KH, Sekiguchi K, Kudo S, Sakamoto K. Characteristics of atmospheric elemental carbon (char and soot) in ultrafine and fine particles in a roadside environment, Japan. Aerosol Air Qual Res 2011;11(1):1e12. [20] Zhou X, Gao J, Wang T, Wu W, Wang W. Measurement of black carbon aerosols near two Chinese megacities and the implications for improving emission inventories. Atmos Environ 2009;43(25):3918e24. [21] He C, Morawska L, Hitchins J, Gilbert D. Contribution from indoor sources to particle number and mass concentrations in residential houses. Atmos Environ 2004;38(21):3405e15. [22] Wallace LA, Emmerich SJ, Howard-Reed C. Source strengths of ultrafine and fine particles due to cooking with a gas stove. Environ Sci Technol 2004;38:2304e11. [23] See SW, Balasubramanian R. Physical characteristics of ultrafine particles emitted from different gas cooking methods. Aerosol Air Qual Res 2006;6: 82e92. [24] Buonanno G, Stabile L, Morawska L. Particle emission factors during cooking activities. Atmos Environ 2009;43(20):3235e42. [25] Buonanno G, Johnson G, Morawska L, Stabile L. Volatility characterization of cooking-generated aerosol particles. Aerosol Sci Technol 2011;45(9): 1069e77. [26] Fang GC, Chang CN, Chu CC, Wu YS, Pi-Cheng Fu P, Chang SC, et al. Fine (PM2.5), coarse (PM2.5-10), and metallic elements of suspended particulates for incense burning at Tzu Yun Yen temple in central Taiwan. Chemosphere 2003;51(9):983e91. [27] Loupa G, Karageorgos E, Rapsomanikis S. Potential effects of particulate matter from combustion during services on human health and on works of art in medieval churches in Cyprus. Environ Pollut 2010;158(9):2946e53. [28] Lee SC, Wang B. Characteristics of emissions of air pollutants from burning of incense in a large environmental chamber. Atmos Environ 2004;38:941e51. [29] See SW, Balasubramanian R. Characterization of fine particle emissions from incense burning. Build Environ 2011;46(5):1074e80. [30] Afshari A, Matson U, Ekberg LE. Characterization of indoor sources of fine and ultrafine particles: a study conducted in a full-scale chamber. Indoor Air 2005; 15:141e50. [31] Pagels J, Wierzbicka A, Nilsson E, Isaxon C, Dahl A, Gudmundsson A, et al. Chemical composition and mass emission factors of candle smoke particles. J Aerosol Sci 2009;40(3):193e208. [32] Hu MT, Chen SJ, Huang KL, Lin YC, Lee WJ, Chang-Chien GP, et al. Characterization of, and health risks from, polychlorinated dibenzo-p-dioxins/
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[39] Viidanoja J, Kerminen VM, Hillamo R. Measuring the size distribution of atmospheric organic and black carbon using impactor sampling coupled with thermal carbon analysis: method development and uncertainties. Aerosol Sci Technol 2002;36:607e16. [40] Hitzenberger R, Ctyroky P, Berner A, Tursic J, Podkrajsek B, Grgic I. Size distribution of black (BC) and total carbon (TC) in Vienna and Ljubljana. Chemosphere 2006;65:2106e13. [41] Cheng YS, Bechtold WE, Yu CC, Hung IF. Incense smoke: characterization and dynamics in indoor environments. Aerosol Sci Technol 1995;23(3):271e81. [42] Nazaroff WW. Indoor particle dynamics. Indoor Air 2004;14(S7):175e83. [43] Chen YC, Zhang YH, Barber EM. A dynamic method to estimate indoor dust sink and source. Build Environ 2000;35(3):215e21. [44] Thatcher TL, Layton DW. Deposition, resuspension, and penetration of particles within a residence. Atmos Environ 1995;29:1487e97. [45] Knutson EO, Whitby KT. Aerosol classification by electric mobility: apparatus, theory, and applications. J Aerosol Sci 1975;6:443e51. [46] Buonanno G, Dell’Isola M, Stabile L, Viola A. Uncertainty budget of the SMPSAPS system in the measurement of PM1, PM2.5, and PM10. Aerosol Sci Technol 2009;43(11):1130e41.
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Characteristics of particles and black carbon emitted by combustion of incenses, candles and anti-mosquito products L. Stabile*, F.C. Fuoco, G. Buonanno Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, via G. Di Biasio, 43, 03043 Cassino (FR), Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2011 Received in revised form 21 February 2012 Accepted 7 March 2012
Indoor combustion sources are of great concern when accurate evaluations of the overall human exposure to particles have to be provided. Even if cooking activity was claimed to be the most emitting source in indoor environments, other indoor activities involving incense, candle and anti-mosquito product combustions can produce high particle concentrations. To this purpose, studies evaluating PM fraction emission rates from such indoor activities were performed, even if there is still a lack of understanding in terms of particle number and carbonaceous amount (black carbon, BC) carried by particles themselves. The aim of the present study was to characterize the particle emission due to the combustion processes of incenses, candles and anti-mosquito products. Emission factors in terms of number, surface area and PM fraction concentrations were evaluated through a condensation particle counter, a scanning mobility particle sizer, and an aerodynamic particle sizer. Moreover, BC emission factors were measured through an aethalometer. Particles’ BC content distribution was also measured proposing an experimental method made up of a particle size classification device connected to an aethalometer. Emission factors due to incenses and anti-mosquito products were higher than 1014 part h1 and 48 mg h1 in terms of number and PM10 concentrations, respectively. Differently, PM fraction emissions from candle burning were well below 1 mg h1. Nonetheless, BC emission rate and distribution measurements showed that candle flaming combustion produces mainly carbonaceous particles (BC/ PM10 ratio higher than 80%). Differently, smoldering combustion processes, like incense and antimosquito product combustions, showed a negligible amount of BC. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Indoor aerosol Emission factors Black carbon distribution Particle number PM fractions
1. Introduction Aerosol exposure is of great concern for medical and air-quality experts because of the particles’ ability in: i) crossing human respiratory system, ii) depositing in its deepest and defenseless regions and iii) carrying condensed toxic compounds. Moreover, particles are produced by several indoor and outdoor sources that, in this way, are present in large doses in every kind of people’s lifestyle. For this reason, the scientific community carried out several scientific studies to deepen the health effect caused by the exposure to particles [1]. In particular, epidemiological, toxicological and environmental researchers are trying to understand which particle size, assumption rate, and chemical component is mainly related to negative effect on human health. About the size, the interest of such
experts is moving from particle mass concentrations (PM10 or PM2.5) to particle number [2] and surface area [3] concentrations, mainly characterized in terms of sub-micrometer and ultrafine particles (UFPs). About the assumption rate, a number of authors are more confident that short term fluctuations in airborne can increase human morbidity and mortality [4,5], whereas other researchers attempted to demonstrate that mortality is due to long term chronic exposure [6]. With regard to the chemical composition, several papers demonstrated the toxic effect of metals and compounds surrounding the particle [7], nonetheless, also the carbonaceous core (black carbon, BC) itself seems to affect the human health [8e10].
1.1. BC effect on human health: the state-of-art * Corresponding author. Tel.: þ39 0776 2993668, þ39 3398673239 (mobile); fax: þ39 0776 2993393. E-mail address: [email protected] (L. Stabile). 0360-1323/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2012.03.005
The United Nations report on the effects of air pollution [11] suggested that BC can be considered a better indicator of harmful particulate substances caused by combustion sources than PM10
L. Stabile et al. / Building and Environment 56 (2012) 184e191
and PM2.5, especially for short-term effects. In fact, a number of authors suggest that a more suitable traffic-related policy measure should be stated in terms of BC concentration reduction [10e12]. Indeed, BC is a typical combustion by-product, therefore, it is better related to particle number emissions than mass ones [13,14]. Even if it was difficult to separate the contribution of BC fraction to health outcomes from co-emitted organic carbon (OC) and other compounds one, numerous toxicological and epidemiological studies were performed to deepen the BC-related health effects. BC exposure is supposed to have negative effects on the respiratory system [15], nevertheless, most of the studies are focused on cardiovascular effects such as cardiac and ventricular arrhythmias, lowered heart rate variability, changes in blood pressure and increased cardiovascular mortality [9,16e18]. Such results highlight the necessity to measure BC along with mass and number concentrations in freshly emitted particles. To this purpose, an increasing number of air quality studies reporting BC spatial and temporal evolutions in urban areas were produced in the last years clearly indicating that the contribution of BC was significant in UFPs [12,14,19,20]. 1.2. Indoor source emissions An overall assessment of the personal exposure to particles, and their BC content, cannot neglect indoor microenvironments where many activities can lead to high doses received by people as these are the places where we spend most of the time of our life (80e90%). In particular, several studies were performed to characterize particles emitted from cooking activities, by measuring emission factors and size distributions [21e24] as well as particle volatility [25] as function of cooking method and several influential parameters. However, today people are also exposed to other relevant indoor combustion sources, like incenses and candles, commonly used both for esthetic and religious purposes in various indoor environments. For example, measurements carried out in a worship building showed high particle concentrations in terms of number and mass [26,27]: this is due to the high particle emission factor characterizing such indoor activities. In confirmation of this, a number of experimental analyses were performed in test chambers in order to evaluate the emission strength of particles. A wide deviation of the data was found from literature results, for example PM2.5 emission factors in the range 5e250 mg h1 and 0.02e25 mg h1 were measured during incense [28,29] and candle burning [30,31], respectively. Such variability of the results is probably due to the several types of additives and/or essential oils used in candles and incenses to improve their esthetic appearance and scent. The use of additives can also lead to emission of toxic gaseous pollutants. In particular, high emission rates were measured for dibenzo-p-dioxins/dibenzofurans [32], PAHs [33] and metals [26] during incense burning, whereas, candle burning events seem to show lower PAH and VOC emission levels compared to carbonaceous emissions (EC, BC) [31]. Also mosquito-coils smoldering activity emits a significant amount of metals, aldehydes and hydrocarbons, as well as gas-phase and particle-phase PAHs [34,35].
185
terms of mass fractions (PM2.5 and PM10); actually, very few studies reported emission factor data also in terms of number [35e37] and carbonaceous amount [29,31,38]. In the present paper, the authors deepened the knowledge of particle emission due to the combustion of incenses, candles and mosquito coils. In particular, measurements of emission factors in terms of PM10, PM2.5, PM1, number and surface area were performed through a condensation particle counter, a scanning mobility particle sizer spectrometer, and an aerodynamic particle sizer spectrometer in a chamber. Moreover, BC emission factors were also performed through an aethalometer. Finally, since the health effect strongly depends on the distribution of carbonaceous matter over the particle size spectrum [39,40], an experimental apparatus to characterize the amount of BC carried by particle of different size was proposed. Actually, the few previous studies attempting to measure BC distributions were focused on outdoor microenvironment characterization by using size segregated impactor samples. In the present study, a particle classification was performed before the BC measurement through electrical mobility technique by connecting an Electrostatic Classifier in series ahead of an aethalometer leading to the BC (mass) distribution measurement in the sub-micrometer particle range. 2. Materials and methods 2.1. Emission sources and site description Three different emission sources were tested during the experimental campaign: incense sticks, candles and anti-mosquito products. Incense sticks are cored sticks coated through finely ground fragrant materials. In particular we considered three different incense fragrances: freesia (I1), citronella (I2), and church (I3). In regard to the candles, two different kinds of tapered candles were studied: a paraffin wax candle (C1) and a natural corn wax candle (C2). Finally, two different anti-mosquito products were tested: a mosquito coil (M1) and a citronella stick (M2). Measurements were carried out in the European Accredited (EA) Laboratory of Industrial Measurements (LAMI) at the University of Cassino and Southern Lazio, Italy, where thermo-hygrometric conditions were continuously monitored, in order to guarantee temperature and relative humidity values equal to 20 1 C and 50 10%, respectively. Tested sources are placed on the floor so that the smoke produced by the combustion naturally was channeled through a 1 m vertical duct in a plenum (a little chamber of 0.25 m3) where sampling points were supplied. The duct was kept about 3 cm distant from the floor (through a supporting framework) in order to guarantee a slight opening able to naturally supply the combustive air to the combustion phenomena. Candles were normally burnt, while, incenses and mosquito products are lit by a flame and fanned out so that the glowing ember on the incense will continue to smolder and burn away the rest of the materials. 2.2. Experimental apparatus In order to measure total particle number concentrations and size distributions the following instruments were used:
1.3. Aims of the work If exhaustive papers characterizing the emission of cooking activities were produced by the scientific literature, not many definitive results were reached in measuring the emission of other indoor activities like incense, candle and coil combustions. In fact, in despite of the relevance of particle number, surface area, and BC fraction concentrations in terms of health effect, emission factors due to such kind of combustions are reported almost exclusively in
- a Condensation Particle Counter, CPC 3775 (TSI Inc.), able to measure total particle number concentration down to 4 nm in diameter with 1 s time resolution; - a Scanning Mobility Particle Sizer spectrometer, SMPS 3936 (TSI Inc.), made up of an Electrostatic Classifier EC 3080 (TSI Inc.), used to classify the sampled particles in different channels according to their size, and another CPC 3775 (TSI Inc.) to count the size selected particles. The SMPS 3936 is able to
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measure particle number distribution in the range 6e800 nm with a minimum time resolution of 120 s; - an Aerodynamic Particle Sizer spectrometer, APS 3321 (TSI Inc.), able to measure particle number distribution and total concentration in the range 0.5e20 mm with 1 s time resolution. Mass and surface area distributions and total concentrations in the range 0.006e20 mm were evaluated using the SMPS/APS data and hypothesizing an ad-hoc particle density. In particular, a density of 1.1 g cm3 was used to determine PM fraction concentrations due to incense and mosquito product combustions [33,41,42], as well as a 1.5 g cm3 density value was considered in candle burning events [31]. Sphere-shaped particles were considered in the evaluation of mass and surface area concentrations and distributions for all the combustion sources. Black carbon concentrations were measured through the aethalometer AE51 (Magee Scientific); it determines the BC concentration through the BeereLambert law measuring the light’s absorption (attenuation) of optically-absorbing particles (the BC particle’s fraction). In particular, a 880 nm wavelength beam of light is produced by a LED light source reaching a photo diode detector: during its travel the light beam passes through the aerosol sample collected on a filter. The AE51 is able to measure total BC mass concentrations with a minimum time resolution of 1 s. Besides, in order to determine BC distributions, as hereinafter described (distribution measurements section), a further Electrostatic Classifier EC 3080 (TSI Inc.) was used to classify particle and to send them to the AE51. 2.3. Experimental methodology 2.3.1. Emission rate and factor measurements The emission of particles in terms of PM10, PM2.5, PM1, surface area (S), number (N), and BC was characterized by the evaluation of the emission factors and the emission rates during the experimental campaign. In order to evaluate the emission factor, the mass balance equation proposed by Chen et al. [43], and Thatcher and Layton [44] was considered:
dCin Qs ¼ P$AER$Cout þ þ ðAER þ kÞ$Cin dt V
(1)
This formula was introduced to determine indoor particles concentration levels taking into account the contributions from indoor and outdoor sources, the deposition rate of particles on indoor surfaces and the air exchange ratio. In particular, Cin and Cout stand for indoor and outdoor particle concentrations, respectively, P is the penetration efficiency, k is the deposition rate, Qs is the indoor particle generation rate, t is time and V is the efficient volume of the plenum. Anyway, the Eq. (1) can be simplified using average values instead of functions, and also making further assumptions about the experimental conditions reported in He et al. [21], (e.g. P assumed close to one; when no indoor source is in operation, the indoor particle concentration can be approximated by outdoor particle concentration and the initial indoor particle concentration could be used to replace outdoor particle concentrations). Therefore, we evaluated the emission factor (E) through such simplified equation proposed by He et al. [21], for indoor sources and already applied in our previous works [24,25]:
Cin Cin;0 þ k,C AER,C E ¼ V$ þ AER in in;0 Dt
(2)
where, Cin and Cin,0 represent the peak and initial indoor concen þ k is the trations (in terms of N, S, PM, or BC), respectively, AER
average total removal rate (taking into account the deposition rate, k, and the air exchange ratio, AER), DT is time difference between initial and peak concentration and V is the efficient volume of the plenum. This equation ignores the effects of particle dynamics such as condensation, evaporation and coagulation, since these are considered to be minor, particularly under the conditions normally encountered in indoor environments [44]. In order to avoid contradictions concerning the terms “emission factor” and “emission rate” already present in the scientific literature, we want to point out that He et al., 2004 [21] named the result of the Eq. (2) as emission rate, whereas Buonanno et al. [24,25], termed it as emission factor: actually both the papers refer to the same parameter which is “particle emission per unit time”. In the present paper we defined the emission factor, E, the “particle emission per unit time” and the emission rate, F, the “particle emission per unit mass of source burned”. Measurements of N, S, PM, or BC total concentrations (C in the Eq. (2)), aimed to determine the E, were performed through the above-mentioned apparatus by sampling aerosol from the plenum through flexible, conductive tubing able to minimize the losses due to electrostatic forces. The aerosol in the plenum can be hypothesized spatially homogeneous since the small volume of the plenum þ k) was evaluated itself. The average total removal rate (AER through the particle concentration trend analyses as reported in He et al. [21], whereas the AER was considered negligible as the plenum was kept close during the tests. Samplings were performed since 5 min before the sources lightening for about 45 min. N and BC concentration trends were measured (with a 1-s time resolution) through CPC 3775 and AE51, respectively, whereas S and PM trends were obtained through SMPS 3936 (with a resolution of 120 s) as described in the experimental apparatus section. The procedure for the combustion activity was as follows: i) the source was lit and placed just below the vertical duct so that the produced smoke naturally flows inside the chamber, ii) the source combustion was left on for 5 min, iii) after that it was moved away from the duct and put out (we do not blown out the source when it is still below the duct because it can change the emitted particle characteristics as also reported by Pagels et al. [31]). The emission rate (F) was evaluated by dividing the emission factor (E) per the burning rate (B, mass of the emission source burned during the combustion process) as reported in Eq. (3)
F ¼ E=B
(3)
To this purpose, the source emission was weighted before and after the 5-min combustion processes through a 1 mg balance resolution. Every emission factor and emission rate value reported in the results represents the mean value of three tests. In Table 1 the burning time and burning rate (B) for all the experiment performed are reported. 2.3.2. Distribution measurements In order to determine particle and BC content distributions for the selected indoor activities, emission sources were burned for Table 1 Experimental characteristics of the combustion processes during the emission factor (and rate) measurement. Source
Mass burned (g)
I1 I2 I3 C1 C2 M1 M2
0.141 0.132 0.178 3.110 1.872 0.270 0.210
0.021 0.021 0.020 0.377 0.239 0.028 0.020
Burn time (min)
Burning rate, B (g min1)
5 5 5 5 5 5 5
0.028 0.026 0.036 0.622 0.374 0.054 0.042
0.004 0.004 0.004 0.075 0.048 0.006 0.004
1013 1013 1013 1011 1011 1013 1013 7.51 2.52 7.11 1.61 2.19 3.25 3.07 1014 1014 1014 1012 1012 1014 1014 4.61 2.91 5.00 1.30 1.82 1.41 1.16 1017 1017 1018 1013 1014 1018 1018 5.62 9.05 3.08 6.47 1.18 2.31 1.04 1018 1019 1019 1014 1015 1019 1018 3.70 1.18 2.36 5.69 1.08 1.05 4.10 103 103 103 103 104 102 103 8.17 3.06 6.90 2.68 2.06 1.46 3.86 102 102 102 102 103 102 102 4.70 3.16 4.48 1.99 1.56 6.04 1.40 100 100 100 103 104 100 100 1.75 2.22 3.19 2.89 2.16 6.12 3.63 101 101 101 102 103 101 101 1.11 2.56 2.25 2.34 1.79 2.66 1.37 100 100 100 103 104 100 100 1.75 2.08 3.03 2.65 1.94 5.99 3.63 101 101 101 102 103 101 101 1.15 2.69 2.33 2.34 1.79 2.73 1.44 100 100 100 103 104 100 100 5.20 2.70 3.99 2.49 1.73 6.50 4.68 101 101 101 102 103 101 101
Standard deviation Source
3.68 3.96 3.37 2.40 1.79 3.12 1.94
Average Average
1.23 1.06 1.37 1.24 8.09 2.40 7.76 101 101 101 101 102 102 101 Emission rate (F)
I1 I2 I3 C1 C2 M1 M2
Average Average Average Average
PM2.5 (mg g1) PM10 (mg g1)
101 101 101 101 103 101 100
1.94 4.20 5.03 8.71 4.02 8.86 3.62
101 101 101 101 102 101 101
4.07 7.71 9.98 1.31 8.51 2.20 6.16
Standard deviation
100 100 100 101 103 101 100
1.81 3.99 4.85 8.71 4.02 8.60 3.47
101 101 101 101 102 101 101
PM1 (mg g1)
3.99 7.77 1.02 1.40 8.94 2.24 6.30
Standard deviation
100 100 100 101 103 101 100
7.89 4.92 9.67 7.43 3.49 1.96 3.53
102 102 102 101 102 102 102
BC (mg g1)
1.83 1.01 2.13 1.28 8.16 5.31 6.28
Standard deviation
102 102 102 101 103 102 103
6.21 1.83 5.11 2.12 2.43 3.41 1.03
1018 1019 1019 1016 1016 1019 1019
S (nm2 g1)
1.30 3.36 1.01 3.18 5.16 8.46 1.76
Standard deviation
1018 1018 1019 1015 1015 1018 1018
7.74 4.54 1.08 4.85 4.07 4.57 2.92
1014 1014 1015 1013 1013 1014 1014
N (part g1)
1.71 8.85 2.26 7.80 9.07 1.19 5.23
Standard deviation
1014 1013 1014 1012 1012 1014 1013
187
6.19 6.18 7.28 8.95 4.02 1.01 4.88 I1 I2 I3 C1 C2 M1 M2
Standard deviation Average Average Standard deviation Average
Standard deviation
Average
Standard deviation
Average
Standard deviation
Average
Standard deviation
N (part h1) S (nm2 h1) BC (mg h1) PM1 (mg h1) PM2.5 (mg h1)
In Table 2 the emission factors (Es) and the emission rates (Fs) in terms of PM10, PM2.5, PM1, BC, S and N for all the analyzed sources are reported. Every measurement represents the average of three tests. Emission factors (as well as emission rates) in terms of particle number are higher for incenses and anti-mosquito products compared to candles. In particular, such emission factors are similar to the measured ones during cooking activities which are claimed as the main indoor particle source [24,25]. Amongst incenses and anti-mosquito products, the most emitting source in terms of particle number and surface area is the I3 (incense stick with a church fragrance), whereas M1 shows the highest PM fraction emission factors. The difference between incenses/anti-mosquito products and candles is also significant in terms of mass concentrations. For example, PM2.5 emission factors for incenses and anti-mosquito products are in the range 19e89 mg h1, whereas 0.871 and 0.0402 mg h1 were measured for C1 (paraffin wax candle) and C2 (natural wax candle), respectively. Therefore, amongst the candles, the natural wax one emits the lowest amount of particle mass, whereas surface area and number emission factors are comparable.
PM10 (mg h1)
3.1. Emission factors and emission rates
Source
3. Results
Emission factor (E)
about 1 h. Also in this experimental test the produced smoke was channeled in the plenum, but an opening of about 20 cm2 was made on one of the plenum walls allowing the smoke to flow out the plenum naturally: in fact, unlike the emission factor test, in this test it was necessary a quasi-stationary concentration. To this purpose the plenum has a double purpose: i) to guarantee constant concentrations by hiding short unstable emissions of the source, ii) to lead to high concentrations the emitted aerosols. The latter is a not-trivial condition since the aim is to measure BC content of selected diameters in the ultrafine range having a very low volume (mass) and, hence, a very low amount of BC. BC content mass distributions for I1, I2, I3, C1, M1 burning events were measured connecting the aethalometer AE51 to the Electrostatic Classifier 3080. In particular, the EC 3080 was used to classify dimensionally monodisperse particles which are immediately flown to the AE51 to measure BC mass concentration of such selected diameter. The AE51 flow rate used in the experimental test was 0.15 L min1 which also corresponds to the aerosol sample flow used at the EC 3080. A 1.5 L min1 sheath flow rate was chosen for the EC 3080 in order to guarantee an aerosol-sheath flow rate ratio equal to 1:10 through which a best resolution (10%), in the particle classification of a selected electrical mobility diameter can be obtained [45]. Diameter logarithmically equally spaced were used to built the BC distribution in the range 20e500 nm with a resolution of 8 channels per decade (12 channels). 5 min-samplings for every selected channel diameter were performed, thus the time spent to measure the complete BC distribution was about 1 h. Finally, BC mass values of every channel were corrected by charging and selecting efficiencies characteristics of the EC 3080 [46]. Number (N), surface area (S) and mass (PM1, PM2.5 and PM10) distributions and total concentrations were measured through the SMPS 3936 and the APS 3321 with 120 s time resolution. S and PM fraction trends were obtained through SMPS/APS data as described in the experimental apparatus section. A CPC 3775 was also used during this test in order to check the particle number concentration trend. Such particle number concentration trend analysis was performed to make sure that sampled aerosol did not change during the test. Aerosols’ particle and BC content distribution results represent the mean value of three tests.
Table 2 Emission factor (E, “particle emission per unit time”) and emission rate (F, “particle emission per unit mass of source burned”) data measured during the experimental analysis for the selected indoor combustion activities.
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Sb (nm2 cm3)
102 102 102 103 102
BCc (mg m3)
5.92 3.53 5.35 7.80 8.89
103 103 103 104 103
5.62 1.66 4.62 1.92 3.08
Moreover, concerning mass emission of burning candle, data reported in Table 2 clearly show that for both the candles PM10, PM2.5, and PM1 are identical. Differently, this behavior was not verified during incense and anti-mosquito product combustion events. This diverse behavior could be ascribed to the different combustion phenomena involving the analyzed sources; in particular, the process involving incenses and anti-mosquito products is merely a smoldering combustion phenomena, whereas candles are characterized by a flaming combustion whose predominant emitted mass fraction is PM1 [24,30]. The different combustion process also influences the amount of carbonaceous fraction of the particles. Data of BC emission factors and rate (reported in Table 2) clearly show that BC fraction in smoldering combustion activities (I1, I2, I3, M1, and M2) are well below 1% of PM10, PM2.5, and PM1 fractions. Otherwise, emission of BC from candle combustion phenomena are larger than 80% of the PM10, PM2.5, and PM1 fraction ones: flaming combustions are almost completely made up of BC, whilst in smoldering combustion particle composition is dominated by organic matter [31].
1010 1.18 1010 1011 3.04 1010 1011 9.17 1010 108 2.88 107 1011 7.65 1010
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2.55 1.71 2.43 4.53 3.28
188
100 1.33 100 101 2.70 100 101 2.55 100 103 8.50 104 101 3.74 100
c
Measurement performed with 1-s time resolution through the CPC 3775. Evaluated from the average distributions measured through the SMPS-APS. Evaluated from the BC distribution measured during the 1-h test. a
b
PM1b (mg m3)
6.02 1.39 1.22 5.28 1.44 100 1.31 100 101 2.68 100 101 2.51 100 103 7.91 104 101 3.68 100
PM2.5b (mg m3)
6.26 1.46 1.26 5.28 1.48 101 3.97 100 101 3.70 100 101 3.43 100 103 7.52 104 101 4.01 100
PM10b (mg m3)
2.00 2.15 1.83 5.42 1.69 105 105 106 104 105 8.40 4.11 1.46 6.31 6.20 106 106 106 105 106
Na (part cm3)
7.00 4.11 9.75 4.38 4.13 I1 I2 I3 C1 M1
Source
Particle size distributions in terms of PM, S, and N as well as the BC content distributions were measured for I1, I2, I3, C1, and M1 burning activities through the experimental methodology described in the Section 2.3.2 (Distribution measurements). As above reported, aerosol samplings were performed in the plenum in order to guarantee nearly stable conditions during the test since BC content distribution measurement lasts about 1-h. To this purpose a CPC 3775 was used to monitor the number concentration trend in the chamber during every experiment. In Table 3 mean and standard deviation values in terms of number concentration are reported: standard deviations below 15% were measured thereby demonstrating quasi-stationary aerosol concentrations. In Fig. 1 particle number distributions (fitted through log-normal equations) of the sub-micrometer aerosols produced through the combustion of the analyzed sources are reported. Such distributions represent the average curves of three tests. In order to better compare the different distributions, they were normalized to the respective average particle number concentrations (reported in Table 3). As shown in the Fig. 1, all five particle number distributions measured during burning events are unimodal, even if the mode really varies according to the combustion process. In particular, the flaming combustion of the paraffin candle produces a particle distribution with a peak around 35 nm, which is comparable to the findings reported in Pagels et al. [31] for steady burning candles. Otherwise, particle distributions concerning not-flaming combustion, like incenses and mosquito coil smoldering combustion, present modes larger than 100 nm in diameter as also shown in See et al. [36], Roy et al. [34] and Ji et al. [33]. In particular, the mode of the distribution due the combustion of the three incenses (I1eI3) is close to 200 nm, whereas the one due to mosquito coil is about at 140 nm. In Fig. 2 particle mass distributions of the aerosol produced through the combustion of the analyzed sources are reported. Such distributions represent the average curves of three tests; they were normalized to the respective average PM10 concentrations (reported in Table 3). All the particle mass distributions present a main mode in the range 240e290 nm, whereas second minor modes were detected during candle burning (at about 50 nm, which is related to the peak of the number distribution in the ultrafine range), and mosquito coils and citronella incense smoldering (at about 800e900 nm). In Fig. 3 the particles’ BC content distribution obtained through the proposed experimental methodology (Section 2.3.2) connecting the aethalometer AE51 to the Electrostatic Classifier 3080 are
Table 3 Total concentration in terms of PM10, PM2.5, PM1, BC, S and N during the BC distribution measurements for I1, I2, I3, C1, and M1 burning activities.
3.2. Particle and BC content distributions
L. Stabile et al. / Building and Environment 56 (2012) 184e191
Fig. 1. Relative particle number distributions measured in the plenum through the SMPS during the combustion of the I1, I2, I3, C1 and M1 indoor sources. Distributions reported represent the average of three tests.
189
Fig. 3. BC content distribution measured in the plenum through the electrostatic classifier-aethalometer system during the combustion of the I1, I2, I3, C1 and M1 indoor sources. Distributions reported represent the average of three tests.
reported. These curves represent the average of three tests and they are normalized to the respective BC total concentration measured in the plenum during the test. First of all, Fig. 3 demonstrates that also the BC content carried by the particles was found to be lognormally distributed, even if the mode considerably changes amongst the different sources. In particular, the modes of particles’ BC content during citronella (I2) and church (I3) stick incense smoldering, as well as, paraffin candle (C1) burning are basically the same of mass distribution (Fig. 2). On the contrary, freesia stick incense (I1) and mosquito coil (M1) show modes lower than the measured ones in terms of mass. Such results seem to demonstrate that organic compound amount surrounding the carbonaceous bulk was equally distributed overall particles as a layer over their active surface area during I2 and I3 burning events, whereas I1 and M1 burning events produce volatile compounds not homogeneously distributed over the different carbonaceous bulk sizes. However, as shown in the previous paragraph (Table 2), the amount of BC in the particles originated by such four combustion processes is always lower than 1%. As example, in Fig. 4a a comparison amongst particle mass distribution and particles’ BC content
Fig. 2. Relative mass distributions measured in the plenum through the SMPS-APS system during the combustion of the I1, I2, I3, C1 and M1 indoor sources. Distributions reported represent the average of three tests.
Fig. 4. Comparison amongst particle mass distribution and particles’ BC content distribution measured in the plenum through the electrostatic classifier-aethalometer system during the combustion of I3 (a) and C1 (b). Distributions reported represent the average of three tests.
190
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distribution measured during the combustion of church stick incense I3 is reported. The figure clearly shows that the slopes (and the modes) of the two distributions are similar. Anyway, a comparison can only be made using a logarithmic scale as the total BC concentration is 0.2% of the PM1. On the contrary, the comparison between particle mass and BC content distribution produced by candle burning (C1) is more interesting. As shown in Table 2 the mass of the particles produced by candle flaming combustion are almost completely made up of BC: this observation can be further confirmed by the comparison between mass particle and BC content distributions measured during C1 combustion activity reported in Fig. 4b. As expected, in such particles the BC content distribution is almost the same of the particle mass distribution: here the organic compound amount is negligible compared to the carbonaceous fraction. The particles’ BC content distribution measured during candle burning is quite similar to the one evaluated in urban microenvironments (using size segregated impactor samples, Hitzenberger et al. [40]). In any case, even if carbonaceous particles are typically emitted by vehicular traffic, in such microenvironments the BC amount is only 8e12% of the total mass [12,40] since externally mixed particles emitted tend to absorb organic and inorganic compounds combining with other outdoor aerosols. In our experimental procedure, where freshly emitted aerosols were characterized, the generated particles mix themselves with ambient aerosol carrying a very low amount of particles, then, such mixing is expected to have a little effect on the number, mass and BC distributions. Therefore, a very low BC fraction was measured in the presence of an excess mass made up of organic compounds (typical of smoldering combustion, i.e. incenses and anti-mosquito products), such mass, due to an incomplete combustion, condenses on to the generated black carbon particles. 4. Conclusions Emission rate and emission factors during combustion process involving incenses, candles and anti-mosquito products were measured in terms of particle number, particle surface area, PM fraction and BC concentrations. Particle number and PM fractions emitted during incenses and anti-mosquito products burning (smoldering combustion) were comparable to typical cooking activities emission. On the contrary, candle combustion, having flaming combustion, produced less amount of particles and they were mainly carbonaceous particles. The different properties in characterizing particles emitted by smoldering and flaming combustions were evidenced by the analysis of number and PM distributions. Particle number distributions due to smoldering combustions were characterized by a mode higher than 100 nm, whereas flaming combustion activity (paraffin candle burning) existed in an ultrafine range mode. BC content in particle distributions found during smoldering combustion was negligible when compared to the respective whole particle mass distribution as the BC/PM10 ratios for such combustions were lower than 1%. Otherwise, particles’ BC content distributions due to candle combustion was nearly close to the relative mass distribution as the BC/PM10 ratios for candle flaming combustions are higher than 80%. References [1] Pope CA, Dockery DW. Health effects of fine particulate air pollution: lines that connect. J Air Waste Manag Assoc 2006;56:709e42. [2] Franck U, Herbarth O, Röder S, Schlink U, Borte M, Diez U, et al. Respiratory effects of indoor particles in young children are size dependent. Sci Total Environ 2011;409(9):1621e31. [3] Giechaskiel B, Alföldy B, Drossinos Y. A metric for health effects studies of diesel exhaust particles. J Aerosol Sci 2009;40:639e51.
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