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Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea
Chemosphere xxx (2015) xxx–xxx
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Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea Jeonghoon Lee ⇑, Jeongseok Yun, Kyeong Jun Kim School of Mechanical Engineering, Korea University of Technology and Education, 1600 Choong Jeol ro, Byeong Cheon myeon, Cheonan si 330-708, Republic of Korea
h i g h l i g h t s BC concentration monitoring at a rural area including fixed sources. Diurnal variation of BC concentration on weekdays and weekends. BC concentration measured at night was higher than that measured during the day. Characteristics of a rural area with an urban-typed fixed source and agricultural burning.
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Article history: Received 29 December 2014 Received in revised form 1 April 2015 Accepted 2 April 2015 Available online xxxx Keywords: Black carbon Elemental carbon Multi angle absorption photometer Diurnal variation
a b s t r a c t We monitored black carbon (BC) concentration for 6 months to understand the characteristics of atmospheric aerosols of an inland rural area in Korea. A multi-angle absorption photometer was used to continuously monitor the BC concentration, which was compared with elemental carbon (EC) concentration measured by an OC/EC Analyzer. For the atmospheric aerosols less than 10 lm, size distributions were measured using both an optical particle counter and a scanning mobility particle sizer. The diurnal variations for BC concentration show that the average BC concentration was 1.43 lg m 3 and exhibited peaks in the morning rush hours. However, the BC concentration measured at night from 20:00 to 08:00 was higher than that measured during the day. The reason why the BC concentration at night was higher would be partly due to the regional characteristics influenced by the combination of local fixed sources and traffic condition. It is suggested that the traffic and transporting of pollutants from the west influenced the increase in the BC concentration at inland rural area including fixed sources. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Black carbon (BC) has been studied for many years in the atmospheric sciences community since it was known to act as a positive radiative forcing (Andrea, 2000). Not only the environmental implication but also the effects of particles on illness (Nel, 2005) creates necessity for the research of the BC, which is prevalent in the ambient aerosol due to the soot generated in sources such as diesel engines. Korea has been suffering from Yellow Dust coming from the west during the spring season (Kwon et al., 2013). Recently, Korea has been suffering from PM2.5 transported from the west even during the winter season, so that the air pollution is ubiquitous at any time. In this situation, measurement of BC in a rural area is relevant in order to estimate the generation, transporting and fate of BC. In order to understand how the rural area is polluted, it is important to know the level of BC of pristine areas. The BC measured
at Southern Himalayas was not higher than 0.6 lg m 3 during the pre-monsoon season and less than 0.1 lg m 3 during the monsoon season (Marinoni et al., 2010). BC measured at Harwell known as a rural background in the UK was reported to be around 0.5 lg m 3 (Harrison et al., 2013). However, it is difficult to find BC data measured at inland rural area in Korea. Stimulated by this, we used a Multiangle Absorption Photometer (MAAP) to evaluate the BC concentration of a rural area, more specifically Byeongcheon area, which is located nearly at the center of South Korea. In this study, the ambient BC concentration of the inland rural area has been monitored for approximately 6 months. In addition, the BC concentrations were compared to the ambient EC concentrations. As far as the authors know, this study is the first attempt to measure BC concentrations and compare to the EC concentrations measured at a rural area in Korea. 2. Method
⇑ Corresponding author. Tel.: +82 41 560 1151; fax: +82 41 560 1253. E-mail address: [email protected] (J. Lee).
The BC concentration was measured by a MAAP (Multiangle Absorption Photometer, Thermo Scientific, 5012). MAAP is one of
http://dx.doi.org/10.1016/j.chemosphere.2015.04.003 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
J. Lee et al. / Chemosphere xxx (2015) xxx–xxx
the filter-based instruments, which measures the BC concentration using the difference of transmitted signals from a clean filter (reference) and a particle-laden filter (probe). The filter-based technique intrinsically suffers from the issues regarding light scattering from the filter itself. Thus, most of the filter-based instruments should make a data correction for the scattering effect. However, MAAP does not require a data correction for scattering effects which is required for the analysis of PSAP or Aethalometer data (Petzold and Schönlinner, 2004). Data was recorded every 1 min and ordered from 00:00 to 23:59 each day. Then, we separated the weekday (from Monday to Friday) data and weekend (Saturday and Sunday) data for each month. After separation, the data ordered from 00:00 to 23:59 was arithmetically averaged every min for 16–20 d for weekdays and 8–10 d for weekends depending on the number of weeks included in the month, respectively. Finally, the monthly data was smoothed for better display using a ‘‘box averaging algorithm’’ supported by IGOR Pro (WaveMetrics). The size of the box was set to 60, meaning that the averaged data was equivalent to the data measured every 60 min. For maintaining the data quality assurance, we did not include the data collected on Korean national holidays into the weekday data. The elemental carbon (EC) concentration was measured by a commercially available EC analyzer (Sunset Laboratory, Lab OC/ EC Aerosol Analyzer) following a NIOSH 5040 protocol. The samples were collected for 8 h to compare BC and EC. To ascertain the effect of sampling time, the samples were collected for 12 h and compared with the samples collected for 8 h. The size distribution of atmospheric aerosols smaller than 0.1 lm was determined by the combination system of a DMA (Differential Mobility Analyzer, homemade) and a CPC (Condensation Particle Counter, TSI, 3775). The size distribution of the aerosols between 0.3 lm and 10 lm was measured by the OPC (Optical Particle Counter, Labco, MDL-125). The size distributions were scanned 5 times at typical time both in the afternoon and at night. Averaged values were displayed in Section 3. Windrose diagrams were obtained from the data openly accessed website of KMA (Korea Meteorology Administration). The diurnal pattern of monthly average wind speeds were also obtained from the KMA website which announces meteorological data on hourly basis.
Traffic densities were measured from the numbers of vehicles which passed through the main gate of the campus. Therefore, the traffic density is an important indicator of moving sources that directly affect the BC concentration. We separated the traffic data into weekdays and weekends. Then, we accumulated the numbers of vehicles every hour and averaged to display the traffic density per hour. BC concentrations were monitored at the Korea University of Technology and Education (KOREATECH) located in Byeongcheon, Cheonan. Fig. 1 shows the sky view of the monitoring site. As can be seen in Fig. 1, the site is surrounded by rice fields, mountains and farming fields. At the left side of the monitoring site, there is a factory where automotive brake pads are mass produced. A stream flows near the southern part of campus and there is a 2lane road near the northern part of campus. Thus, the monitoring site represents the combination of rural characteristics and urban characteristics. 3. Results and discussion 3.1. Diurnal variation of black carbon We measured the BC concentrations for 6 months from April to September, 2012. Diurnal patterns of the BC concentration of both weekdays and weekends are shown in Fig. 2. The variation of BC concentrations for hourly average lies between 1.12 lg m 3 and
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Fig. 1. Geographical condition near the monitoring site.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
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J. Lee et al. / Chemosphere xxx (2015) xxx–xxx
4 April 3 2 1
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contribute to the increase in BC concentration. The morning rush hour peaks of BC were observed for every month with varying concentrations. Surprisingly, April to July also shows morning rush hour peaks of BC even on weekends. The monthly evolution of the daily variation for BC concentration shows that the BC concentration peaked not only in the rush hour but also at night during the measurement period. On weekends, we observed interesting peak positions in the diurnal variations. In April, the peak in BC concentration was found around midnight. In May and June, the peaks in BC concentration were found around 02:00, 07:00 and 23:00. In July, the peaks in BC concentration were monitored around 05:30, 07:40 and 21:00. In August, the peaks in BC concentration were observed around 09:00 and 01:00. In September, peaks in BC concentration were observed around 02:00, 10:00, 20:00 and 23:00. The increase in the BC concentration during the morning rush hour and the evening rush hour ought to be results from road traffic emissions. Surprisingly, the maximum level of BC concentration was measured to be 29.85 lg m 3 when it spiked at precisely 06:43, May, 23rd, just before the morning rush hour. But the cause of the increase in the BC concentration around midnight is still vague. Though it is very unusual that the BC concentration at night is relatively higher than that measured during the day, some research groups suggested that the higher BC concentration could be monitored. Saha and Despiau (2009) suggested that the cause of the increase of the BC concentration at night is due to the increase in the mid-night traffic density. Not only the night traffic population but also low mixing height was suggested to cause the increase in the BC concentration at night (Järvi et al., 2007). The monthly average wind speeds at midnight
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2.20 lg m 3 on weekdays, and 1.21 lg m 3 and 2.58 lg m 3 on weekends. These BC concentration values are comparable to those measured at metropolitan cities such as London and Barcelona (Reche et al., 2011). The diurnal patterns of the BC concentration is very similar to those measured at other big cities such as São Paulo, Mexico City and Beijing (Baumgardner et al., 2007; Paredes-Miranda, 2009; Garland et al., 2009; Backman et al., 2012). The decrease between midnight and 6:00 seems probably due to the reduced traffic and the fact that the mixing height is constantly maintained. The increase from 6:00 to 8:30 is thought to be influenced by the morning rush hour, and the increase after 18:00 to 21:00 is also affected by the evening rush hour. The decrease after morning rush hour appears to be primarily caused by the atmospheric turbulent mixing (Sahu et al., 2011; Tiwari et al., 2013). Even though our measurement site is located at a rural area, the diurnal pattern of BC is very similar to those measured in urban areas. Even more interestingly, the BC concentration measured on weekends is nominally higher than that monitored on weekdays. The high BC concentration measured at night from 18:00 to 21:00 on weekdays appears to be presumably due to the road traffic emissions. The diurnal variations of the BC concentration monitored month by month are shown in Fig. 3. The high BC concentration after midnight on weekends is specifically evident in April, May and June. No distinct difference between weekdays and weekends was found in August, which is due to the typical summer rainy and windy season. The BC concentrations measured around midnight are usually high except in July. On weekdays, the morning rush hour seemed to
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Fig. 3. Diurnal variation of BC concentration averaged from hourly means for each month on weekday and weekend.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
J. Lee et al. / Chemosphere xxx (2015) xxx–xxx
3.2. Influence of traffic conditions, fixed sources and wind direction
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3.3. Size distribution of ambient aerosols We measured size distributions of ambient aerosol. The size distribution measured at night (approximately in 23:10) was compared to that measured during the day (approximately in 13:20) as shown in Fig. 6A for the ambient aerosols smaller than 0.1 lm. Particle number concentration at night was observed to be higher than that measured during the day, which could partly explain why the BC concentration was high at night. It is noted that nanoparticles smaller than 0.1 lm can be generated from the brake pad factory near the campus. For the ambient aerosols between 0.3 lm and 10 lm, however, the size distribution measured at night is similar to that measured during the day within the uncertainty
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Fig. 4 shows BC concentrations as well as the traffic densities during weekdays and weekends. As can be seen, the pattern of the traffic density on weekdays is different from that on weekends. On weekdays, peaks of traffic density were observed in the morning, at noon and in the evening. The morning peak and evening peak are definitely due to the commuting vehicles. The peak at noon, however, is due to the vehicles that went out for lunch. It is interesting to note that the BC peak in the morning coincides with the traffic density peak in the morning. However, at noon, the BC concentration decreases even though the traffic density increases. The decrease in BC concentration shows that the atmospheric turbulent mixing after the morning rush hour is more dominant than the emission from the traffic, so that the BC concentration is gradually reduced. On weekends, the traffic density did not show a morning peak but did show an evening peak as well as a small late-lunch peak. However, the total amount of traffic for the weekend is less than 20% of that for the weekdays. On weekends, the BC concentration peak was observed in the morning even though no traffic peak was observed. Essentially, the diurnal pattern of weekdays is similar to that of weekends. In fact, the BC concentration of weekends is slightly higher than that of weekdays even though the traffic density in the weekdays is higher. Thus, it seems plausible that BC concentration showed a peak in the morning due to non-traffic sources. Despite the high traffic density and the low mixing height, they do not seem to be the only cause of the increase in the BC concentration at night in the present study. The variation of the BC at night is clearly influenced by some other sources than road traffic emissions since the traffic at night in this area was rare. The increase in BC concentration at night seems to be due to pollutants from a fixed source surrounding the campus rather than the moving sources like the night road traffic. There is one potential fixed source, a factory, adjacent to the university campus. As shown in Fig. 1, the factory is located to the west of the campus and produces automotive brake pads. The factory claims that it installed some facilities for removing hazardous materials. The cleaning facilities consist of several scrubbers, regenerative thermal oxidizers (RTO) and dust collectors. The BC concentration has sometimes suddenly increased just after 18:00 in the evening and dramatically decreased after 08:00 in the morning. Taking into account the fact that the workers start to work at 08:00 and finish working at 18:00, we question whether the cleaning facilities were working or not during the night while the factory was producing the brake
pads for 24 h. We already showed that the BC concentration on weekends is higher than that on weekdays. In fact, air in the campus is sometimes impregnated with an offensive odor. The cause of the bad order was analyzed to be aldehyde, sulfide, benzene and ammonia (Taesung Environment Research Laboratory, 2011), which seems to contribute to the increase in BC concentrations at night. In addition, the burning of haystacks, excreta from cows, ducks and poultry emitted offensive odors too. This type of biomass burning is also believed to contribute to the increase in BC concentration. The diurnal variation of monthly averaged wind speed from April to September in 2012 was obtained from the KMA (http:// www.kma.go.kr/) data. The minima of monthly average wind speeds at night in April, May, June, July, August and September are calculated to be 1.29, 1.08, 1.20, 1.27, 1.80 and 1.03 m s 1, respectively. In contrast, the maxima of monthly average wind speeds in the afternoon in April, May, June, July, August and September are 4.58, 3.10, 3.26, 2.43, 3.10 and 2.68 m s 1, respectively. Thus, relatively tranquil air at night creates a lower mixing height and this is one of the causes of the increase in BC concentrations at night. Fig. 5 shows windrose diagrams of Cheonan from April to September, 2012 measured by KMA. In April and May, main winds from the west are dominant, so that the pollutant emitted from the factory could directly affect the diurnal pattern of the BC concentration. Relatively slow winds allow BC to be stagnant from the source and the wind from the west affects the increase in BC monitored at our site. In June, July and August, however, the direction of the wind was changed to the south east, where mountains and agricultural areas are located. Thus, it seems that the BC concentration was maintained at a low level through the day and night. The wind speed also shows a small difference between night and afternoon during this summer season. In September, however, the wind was multi-directional and the period that the wind was slower than 0.5 m s 1 was the longest, that is, calm wind condition was established.
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and at 15:00 from April to September are 1.50 m s 1 and 3.01 m s 1, respectively. Relatively slow wind speed at midnight caused a lower mixing height, contributing to the increase in BC concentration at night. The details are to be mentioned later.
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Fig. 4. Traffic density and BC concentration (A) weekday; (B) weekend.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
J. Lee et al. / Chemosphere xxx (2015) xxx–xxx
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Fig. 5. Windrose diagrams (A) April; (B) May; (C) June; (D) July; (E) August; (F) September (Korea Meteorological Administration, http://www.kma.go.kr/).
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
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3.4. Comparison between black carbon and elemental carbon 6
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Finally, we compared the BC concentration with the EC concentration for ambient aerosols after collecting aerosols for 8 h (09:00–17:00 for the day and 20:00–04:00 for the night) as shown in Fig. 7A. The comparison between BC and EC is important in that the difference of the correlation between BC and EC could indicate the characteristic of monitoring site. The relationship between the BC concentration and the EC concentrations measured at night was observed to be quite similar to that measured during the day. The proportionality coefficients between BC concentration and EC concentration are 1.59 at night and 1.64 during the day, respectively. It is interesting to note that the average BC and EC concentrations measured at night are higher than those measured during the day even though the difference between them is small. To ascertain the effect of sampling time, we collected aerosols for 12 h (08:00– 20:00 for the day). Fig. 7B shows that the proportionality coefficient between the BC concentration and the EC concentration was 1.54 for the samples collected during the day for 12 h. The results show that the correlation between the BC and the EC has nothing to do with the sampling time. Thus, it is not unreasonable to think that the proportionality coefficient measured in the present study lies between 1.5 and 1.7 without regard to how long and whether the sample is collected during the day or at night. It is worthwhile to compare our results to others. For example, BC was measured to be 2.7 times higher than EC in Philadelphia but lower than EC by a factor of 0.35 during a Canadian forest fire, which resulted from the fact that the BC/EC ratio showed different values depending on the physical and chemical characteristics of carbonaceous particles (Jeong et al., 2004). In the present study, BC was measured to be larger than EC by a factor of 1.5–1.7, which is approximately between 0.35 and 2.7 representing a forest fire in rural and urban conditions, respectively. This result rationalizes the fact that our monitoring site lies in a rural area with an urban-type fixed source. The BC/EC ratio acquired in the present study is also consistent with the values measured in Hyytiälä, Finland and Aspvreten, Sweden (Genberg et al., 2013).
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(B) Fig. 7. BC concentration vs. EC concentration for ambient aerosols (A) 8 h sampling at night and during the day (B) 12 h sampling during the day only.
as shown in Fig. 6B. This implies that nanoparticles smaller than 0.1 lm affected the measurement of BC concentration rather than particles such as PM2.5 and PM10.
BC concentration in a rural area was monitored for 6 months and the pattern of diurnal variation was found to be similar to those measured in urban areas. Our monitoring site is not rural exactly nor urban because it is surrounded by farming fields, mountains and a stream but there is a factory in the vicinity of the campus. Thus, our monitoring site can be redefined as a rural area with an urban-type fixed source. The BC concentrations averaged for 6 months show similar patterns between weekdays and weekends. On weekdays, BC concentrations were definitely influenced by traffic density since the daily pattern of traffic density
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
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during the morning and evening rush hours was similar to that of BC concentration. On weekends, however, the diurnal patterns of BC concentration were presumably influenced by several causes including pollutants from the adjacent factory, biomass burning, burning of straw pile at farming fields, and so on. In the spring season, the BC concentration was relatively high because of various factors. First, the wind is blown from the west where a factory is located. Second, the burning of haystacks is frequent in the spring season, increasing the BC concentration in April. Third, traffic density increased because of the start of a new school semester. But the BC concentration was reduced in the summer season because of the precipitation of rainfall and turbulent mixing by the wind. In September, however, the BC concentration increased again because of the wind from the west. Traffic density was monitored and it partly affected the increase in BC concentration in the morning and evening. Interestingly, the BC concentration measured at night was higher than that measured during the day. The high concentration at night appears to be partly caused by the chemicals such as aldehyde, sulfide, benzene and ammonia which failed to be separated by the cleaning facility. The factory is believed to be a second contributor during weekdays but becomes a main contributor on weekends. It is believed that the local influence from neighbor contributed to the increase in the BC concentration at night. The burning of haystack and biomass burning of excreta from livestock such as cows, ducks and poultry seem to be minor contributors. The BC/EC ratios obtained in the present study lay in between the value measured at an urban site and the value measured at a rural site. They also showed similar trends to those measured elsewhere in Europe. Acknowledgements This work was supported by the Korea Ministry of Environment as Converging Technology Project (2013001650004). The authors give thanks to Mr. S. Han for collecting EC and BC data and Mr. H. Choi for collecting meteorological data from KMA website. This study was partly supported by Education and Research Promotion Program of KOREATECH. References Andreae, M., 2001. The dark side of aerosols. Nature 409, 671–672. Backman, J., Rizzo, L.V., Hakala, J., Nieminen, T., Manninen, H.E., Morais, F., Aalto, P.P., Siivola, E., Carbone, S., Hillamo, R., Artaxo, P., Virkkula, A., Petäjä, T., Kulmala, M., 2012. On the diurnal cycle of urban aerosols, black carbon and the
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Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea Jeonghoon Lee ⇑, Jeongseok Yun, Kyeong Jun Kim School of Mechanical Engineering, Korea University of Technology and Education, 1600 Choong Jeol ro, Byeong Cheon myeon, Cheonan si 330-708, Republic of Korea
h i g h l i g h t s BC concentration monitoring at a rural area including fixed sources. Diurnal variation of BC concentration on weekdays and weekends. BC concentration measured at night was higher than that measured during the day. Characteristics of a rural area with an urban-typed fixed source and agricultural burning.
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Article history: Received 29 December 2014 Received in revised form 1 April 2015 Accepted 2 April 2015 Available online xxxx Keywords: Black carbon Elemental carbon Multi angle absorption photometer Diurnal variation
a b s t r a c t We monitored black carbon (BC) concentration for 6 months to understand the characteristics of atmospheric aerosols of an inland rural area in Korea. A multi-angle absorption photometer was used to continuously monitor the BC concentration, which was compared with elemental carbon (EC) concentration measured by an OC/EC Analyzer. For the atmospheric aerosols less than 10 lm, size distributions were measured using both an optical particle counter and a scanning mobility particle sizer. The diurnal variations for BC concentration show that the average BC concentration was 1.43 lg m 3 and exhibited peaks in the morning rush hours. However, the BC concentration measured at night from 20:00 to 08:00 was higher than that measured during the day. The reason why the BC concentration at night was higher would be partly due to the regional characteristics influenced by the combination of local fixed sources and traffic condition. It is suggested that the traffic and transporting of pollutants from the west influenced the increase in the BC concentration at inland rural area including fixed sources. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Black carbon (BC) has been studied for many years in the atmospheric sciences community since it was known to act as a positive radiative forcing (Andrea, 2000). Not only the environmental implication but also the effects of particles on illness (Nel, 2005) creates necessity for the research of the BC, which is prevalent in the ambient aerosol due to the soot generated in sources such as diesel engines. Korea has been suffering from Yellow Dust coming from the west during the spring season (Kwon et al., 2013). Recently, Korea has been suffering from PM2.5 transported from the west even during the winter season, so that the air pollution is ubiquitous at any time. In this situation, measurement of BC in a rural area is relevant in order to estimate the generation, transporting and fate of BC. In order to understand how the rural area is polluted, it is important to know the level of BC of pristine areas. The BC measured
at Southern Himalayas was not higher than 0.6 lg m 3 during the pre-monsoon season and less than 0.1 lg m 3 during the monsoon season (Marinoni et al., 2010). BC measured at Harwell known as a rural background in the UK was reported to be around 0.5 lg m 3 (Harrison et al., 2013). However, it is difficult to find BC data measured at inland rural area in Korea. Stimulated by this, we used a Multiangle Absorption Photometer (MAAP) to evaluate the BC concentration of a rural area, more specifically Byeongcheon area, which is located nearly at the center of South Korea. In this study, the ambient BC concentration of the inland rural area has been monitored for approximately 6 months. In addition, the BC concentrations were compared to the ambient EC concentrations. As far as the authors know, this study is the first attempt to measure BC concentrations and compare to the EC concentrations measured at a rural area in Korea. 2. Method
⇑ Corresponding author. Tel.: +82 41 560 1151; fax: +82 41 560 1253. E-mail address: [email protected] (J. Lee).
The BC concentration was measured by a MAAP (Multiangle Absorption Photometer, Thermo Scientific, 5012). MAAP is one of
http://dx.doi.org/10.1016/j.chemosphere.2015.04.003 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
J. Lee et al. / Chemosphere xxx (2015) xxx–xxx
the filter-based instruments, which measures the BC concentration using the difference of transmitted signals from a clean filter (reference) and a particle-laden filter (probe). The filter-based technique intrinsically suffers from the issues regarding light scattering from the filter itself. Thus, most of the filter-based instruments should make a data correction for the scattering effect. However, MAAP does not require a data correction for scattering effects which is required for the analysis of PSAP or Aethalometer data (Petzold and Schönlinner, 2004). Data was recorded every 1 min and ordered from 00:00 to 23:59 each day. Then, we separated the weekday (from Monday to Friday) data and weekend (Saturday and Sunday) data for each month. After separation, the data ordered from 00:00 to 23:59 was arithmetically averaged every min for 16–20 d for weekdays and 8–10 d for weekends depending on the number of weeks included in the month, respectively. Finally, the monthly data was smoothed for better display using a ‘‘box averaging algorithm’’ supported by IGOR Pro (WaveMetrics). The size of the box was set to 60, meaning that the averaged data was equivalent to the data measured every 60 min. For maintaining the data quality assurance, we did not include the data collected on Korean national holidays into the weekday data. The elemental carbon (EC) concentration was measured by a commercially available EC analyzer (Sunset Laboratory, Lab OC/ EC Aerosol Analyzer) following a NIOSH 5040 protocol. The samples were collected for 8 h to compare BC and EC. To ascertain the effect of sampling time, the samples were collected for 12 h and compared with the samples collected for 8 h. The size distribution of atmospheric aerosols smaller than 0.1 lm was determined by the combination system of a DMA (Differential Mobility Analyzer, homemade) and a CPC (Condensation Particle Counter, TSI, 3775). The size distribution of the aerosols between 0.3 lm and 10 lm was measured by the OPC (Optical Particle Counter, Labco, MDL-125). The size distributions were scanned 5 times at typical time both in the afternoon and at night. Averaged values were displayed in Section 3. Windrose diagrams were obtained from the data openly accessed website of KMA (Korea Meteorology Administration). The diurnal pattern of monthly average wind speeds were also obtained from the KMA website which announces meteorological data on hourly basis.
Traffic densities were measured from the numbers of vehicles which passed through the main gate of the campus. Therefore, the traffic density is an important indicator of moving sources that directly affect the BC concentration. We separated the traffic data into weekdays and weekends. Then, we accumulated the numbers of vehicles every hour and averaged to display the traffic density per hour. BC concentrations were monitored at the Korea University of Technology and Education (KOREATECH) located in Byeongcheon, Cheonan. Fig. 1 shows the sky view of the monitoring site. As can be seen in Fig. 1, the site is surrounded by rice fields, mountains and farming fields. At the left side of the monitoring site, there is a factory where automotive brake pads are mass produced. A stream flows near the southern part of campus and there is a 2lane road near the northern part of campus. Thus, the monitoring site represents the combination of rural characteristics and urban characteristics. 3. Results and discussion 3.1. Diurnal variation of black carbon We measured the BC concentrations for 6 months from April to September, 2012. Diurnal patterns of the BC concentration of both weekdays and weekends are shown in Fig. 2. The variation of BC concentrations for hourly average lies between 1.12 lg m 3 and
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Fig. 1. Geographical condition near the monitoring site.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
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J. Lee et al. / Chemosphere xxx (2015) xxx–xxx
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contribute to the increase in BC concentration. The morning rush hour peaks of BC were observed for every month with varying concentrations. Surprisingly, April to July also shows morning rush hour peaks of BC even on weekends. The monthly evolution of the daily variation for BC concentration shows that the BC concentration peaked not only in the rush hour but also at night during the measurement period. On weekends, we observed interesting peak positions in the diurnal variations. In April, the peak in BC concentration was found around midnight. In May and June, the peaks in BC concentration were found around 02:00, 07:00 and 23:00. In July, the peaks in BC concentration were monitored around 05:30, 07:40 and 21:00. In August, the peaks in BC concentration were observed around 09:00 and 01:00. In September, peaks in BC concentration were observed around 02:00, 10:00, 20:00 and 23:00. The increase in the BC concentration during the morning rush hour and the evening rush hour ought to be results from road traffic emissions. Surprisingly, the maximum level of BC concentration was measured to be 29.85 lg m 3 when it spiked at precisely 06:43, May, 23rd, just before the morning rush hour. But the cause of the increase in the BC concentration around midnight is still vague. Though it is very unusual that the BC concentration at night is relatively higher than that measured during the day, some research groups suggested that the higher BC concentration could be monitored. Saha and Despiau (2009) suggested that the cause of the increase of the BC concentration at night is due to the increase in the mid-night traffic density. Not only the night traffic population but also low mixing height was suggested to cause the increase in the BC concentration at night (Järvi et al., 2007). The monthly average wind speeds at midnight
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2.20 lg m 3 on weekdays, and 1.21 lg m 3 and 2.58 lg m 3 on weekends. These BC concentration values are comparable to those measured at metropolitan cities such as London and Barcelona (Reche et al., 2011). The diurnal patterns of the BC concentration is very similar to those measured at other big cities such as São Paulo, Mexico City and Beijing (Baumgardner et al., 2007; Paredes-Miranda, 2009; Garland et al., 2009; Backman et al., 2012). The decrease between midnight and 6:00 seems probably due to the reduced traffic and the fact that the mixing height is constantly maintained. The increase from 6:00 to 8:30 is thought to be influenced by the morning rush hour, and the increase after 18:00 to 21:00 is also affected by the evening rush hour. The decrease after morning rush hour appears to be primarily caused by the atmospheric turbulent mixing (Sahu et al., 2011; Tiwari et al., 2013). Even though our measurement site is located at a rural area, the diurnal pattern of BC is very similar to those measured in urban areas. Even more interestingly, the BC concentration measured on weekends is nominally higher than that monitored on weekdays. The high BC concentration measured at night from 18:00 to 21:00 on weekdays appears to be presumably due to the road traffic emissions. The diurnal variations of the BC concentration monitored month by month are shown in Fig. 3. The high BC concentration after midnight on weekends is specifically evident in April, May and June. No distinct difference between weekdays and weekends was found in August, which is due to the typical summer rainy and windy season. The BC concentrations measured around midnight are usually high except in July. On weekdays, the morning rush hour seemed to
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Fig. 3. Diurnal variation of BC concentration averaged from hourly means for each month on weekday and weekend.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
J. Lee et al. / Chemosphere xxx (2015) xxx–xxx
3.2. Influence of traffic conditions, fixed sources and wind direction
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3.3. Size distribution of ambient aerosols We measured size distributions of ambient aerosol. The size distribution measured at night (approximately in 23:10) was compared to that measured during the day (approximately in 13:20) as shown in Fig. 6A for the ambient aerosols smaller than 0.1 lm. Particle number concentration at night was observed to be higher than that measured during the day, which could partly explain why the BC concentration was high at night. It is noted that nanoparticles smaller than 0.1 lm can be generated from the brake pad factory near the campus. For the ambient aerosols between 0.3 lm and 10 lm, however, the size distribution measured at night is similar to that measured during the day within the uncertainty
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Fig. 4 shows BC concentrations as well as the traffic densities during weekdays and weekends. As can be seen, the pattern of the traffic density on weekdays is different from that on weekends. On weekdays, peaks of traffic density were observed in the morning, at noon and in the evening. The morning peak and evening peak are definitely due to the commuting vehicles. The peak at noon, however, is due to the vehicles that went out for lunch. It is interesting to note that the BC peak in the morning coincides with the traffic density peak in the morning. However, at noon, the BC concentration decreases even though the traffic density increases. The decrease in BC concentration shows that the atmospheric turbulent mixing after the morning rush hour is more dominant than the emission from the traffic, so that the BC concentration is gradually reduced. On weekends, the traffic density did not show a morning peak but did show an evening peak as well as a small late-lunch peak. However, the total amount of traffic for the weekend is less than 20% of that for the weekdays. On weekends, the BC concentration peak was observed in the morning even though no traffic peak was observed. Essentially, the diurnal pattern of weekdays is similar to that of weekends. In fact, the BC concentration of weekends is slightly higher than that of weekdays even though the traffic density in the weekdays is higher. Thus, it seems plausible that BC concentration showed a peak in the morning due to non-traffic sources. Despite the high traffic density and the low mixing height, they do not seem to be the only cause of the increase in the BC concentration at night in the present study. The variation of the BC at night is clearly influenced by some other sources than road traffic emissions since the traffic at night in this area was rare. The increase in BC concentration at night seems to be due to pollutants from a fixed source surrounding the campus rather than the moving sources like the night road traffic. There is one potential fixed source, a factory, adjacent to the university campus. As shown in Fig. 1, the factory is located to the west of the campus and produces automotive brake pads. The factory claims that it installed some facilities for removing hazardous materials. The cleaning facilities consist of several scrubbers, regenerative thermal oxidizers (RTO) and dust collectors. The BC concentration has sometimes suddenly increased just after 18:00 in the evening and dramatically decreased after 08:00 in the morning. Taking into account the fact that the workers start to work at 08:00 and finish working at 18:00, we question whether the cleaning facilities were working or not during the night while the factory was producing the brake
pads for 24 h. We already showed that the BC concentration on weekends is higher than that on weekdays. In fact, air in the campus is sometimes impregnated with an offensive odor. The cause of the bad order was analyzed to be aldehyde, sulfide, benzene and ammonia (Taesung Environment Research Laboratory, 2011), which seems to contribute to the increase in BC concentrations at night. In addition, the burning of haystacks, excreta from cows, ducks and poultry emitted offensive odors too. This type of biomass burning is also believed to contribute to the increase in BC concentration. The diurnal variation of monthly averaged wind speed from April to September in 2012 was obtained from the KMA (http:// www.kma.go.kr/) data. The minima of monthly average wind speeds at night in April, May, June, July, August and September are calculated to be 1.29, 1.08, 1.20, 1.27, 1.80 and 1.03 m s 1, respectively. In contrast, the maxima of monthly average wind speeds in the afternoon in April, May, June, July, August and September are 4.58, 3.10, 3.26, 2.43, 3.10 and 2.68 m s 1, respectively. Thus, relatively tranquil air at night creates a lower mixing height and this is one of the causes of the increase in BC concentrations at night. Fig. 5 shows windrose diagrams of Cheonan from April to September, 2012 measured by KMA. In April and May, main winds from the west are dominant, so that the pollutant emitted from the factory could directly affect the diurnal pattern of the BC concentration. Relatively slow winds allow BC to be stagnant from the source and the wind from the west affects the increase in BC monitored at our site. In June, July and August, however, the direction of the wind was changed to the south east, where mountains and agricultural areas are located. Thus, it seems that the BC concentration was maintained at a low level through the day and night. The wind speed also shows a small difference between night and afternoon during this summer season. In September, however, the wind was multi-directional and the period that the wind was slower than 0.5 m s 1 was the longest, that is, calm wind condition was established.
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and at 15:00 from April to September are 1.50 m s 1 and 3.01 m s 1, respectively. Relatively slow wind speed at midnight caused a lower mixing height, contributing to the increase in BC concentration at night. The details are to be mentioned later.
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Fig. 4. Traffic density and BC concentration (A) weekday; (B) weekend.
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
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Fig. 5. Windrose diagrams (A) April; (B) May; (C) June; (D) July; (E) August; (F) September (Korea Meteorological Administration, http://www.kma.go.kr/).
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
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3.4. Comparison between black carbon and elemental carbon 6
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Finally, we compared the BC concentration with the EC concentration for ambient aerosols after collecting aerosols for 8 h (09:00–17:00 for the day and 20:00–04:00 for the night) as shown in Fig. 7A. The comparison between BC and EC is important in that the difference of the correlation between BC and EC could indicate the characteristic of monitoring site. The relationship between the BC concentration and the EC concentrations measured at night was observed to be quite similar to that measured during the day. The proportionality coefficients between BC concentration and EC concentration are 1.59 at night and 1.64 during the day, respectively. It is interesting to note that the average BC and EC concentrations measured at night are higher than those measured during the day even though the difference between them is small. To ascertain the effect of sampling time, we collected aerosols for 12 h (08:00– 20:00 for the day). Fig. 7B shows that the proportionality coefficient between the BC concentration and the EC concentration was 1.54 for the samples collected during the day for 12 h. The results show that the correlation between the BC and the EC has nothing to do with the sampling time. Thus, it is not unreasonable to think that the proportionality coefficient measured in the present study lies between 1.5 and 1.7 without regard to how long and whether the sample is collected during the day or at night. It is worthwhile to compare our results to others. For example, BC was measured to be 2.7 times higher than EC in Philadelphia but lower than EC by a factor of 0.35 during a Canadian forest fire, which resulted from the fact that the BC/EC ratio showed different values depending on the physical and chemical characteristics of carbonaceous particles (Jeong et al., 2004). In the present study, BC was measured to be larger than EC by a factor of 1.5–1.7, which is approximately between 0.35 and 2.7 representing a forest fire in rural and urban conditions, respectively. This result rationalizes the fact that our monitoring site lies in a rural area with an urban-type fixed source. The BC/EC ratio acquired in the present study is also consistent with the values measured in Hyytiälä, Finland and Aspvreten, Sweden (Genberg et al., 2013).
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(B) Fig. 7. BC concentration vs. EC concentration for ambient aerosols (A) 8 h sampling at night and during the day (B) 12 h sampling during the day only.
as shown in Fig. 6B. This implies that nanoparticles smaller than 0.1 lm affected the measurement of BC concentration rather than particles such as PM2.5 and PM10.
BC concentration in a rural area was monitored for 6 months and the pattern of diurnal variation was found to be similar to those measured in urban areas. Our monitoring site is not rural exactly nor urban because it is surrounded by farming fields, mountains and a stream but there is a factory in the vicinity of the campus. Thus, our monitoring site can be redefined as a rural area with an urban-type fixed source. The BC concentrations averaged for 6 months show similar patterns between weekdays and weekends. On weekdays, BC concentrations were definitely influenced by traffic density since the daily pattern of traffic density
Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003
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during the morning and evening rush hours was similar to that of BC concentration. On weekends, however, the diurnal patterns of BC concentration were presumably influenced by several causes including pollutants from the adjacent factory, biomass burning, burning of straw pile at farming fields, and so on. In the spring season, the BC concentration was relatively high because of various factors. First, the wind is blown from the west where a factory is located. Second, the burning of haystacks is frequent in the spring season, increasing the BC concentration in April. Third, traffic density increased because of the start of a new school semester. But the BC concentration was reduced in the summer season because of the precipitation of rainfall and turbulent mixing by the wind. In September, however, the BC concentration increased again because of the wind from the west. Traffic density was monitored and it partly affected the increase in BC concentration in the morning and evening. Interestingly, the BC concentration measured at night was higher than that measured during the day. The high concentration at night appears to be partly caused by the chemicals such as aldehyde, sulfide, benzene and ammonia which failed to be separated by the cleaning facility. The factory is believed to be a second contributor during weekdays but becomes a main contributor on weekends. It is believed that the local influence from neighbor contributed to the increase in the BC concentration at night. The burning of haystack and biomass burning of excreta from livestock such as cows, ducks and poultry seem to be minor contributors. The BC/EC ratios obtained in the present study lay in between the value measured at an urban site and the value measured at a rural site. They also showed similar trends to those measured elsewhere in Europe. Acknowledgements This work was supported by the Korea Ministry of Environment as Converging Technology Project (2013001650004). The authors give thanks to Mr. S. Han for collecting EC and BC data and Mr. H. Choi for collecting meteorological data from KMA website. This study was partly supported by Education and Research Promotion Program of KOREATECH. References Andreae, M., 2001. The dark side of aerosols. Nature 409, 671–672. Backman, J., Rizzo, L.V., Hakala, J., Nieminen, T., Manninen, H.E., Morais, F., Aalto, P.P., Siivola, E., Carbone, S., Hillamo, R., Artaxo, P., Virkkula, A., Petäjä, T., Kulmala, M., 2012. On the diurnal cycle of urban aerosols, black carbon and the
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Please cite this article in press as: Lee, J., et al. Monitoring of black carbon concentration at an inland rural area including fixed sources in Korea. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2015.04.003