Measurement of Black Carbon Concentration and Comparison with PM10 and PM2.5 Concentrations Monitored in Chungcheong Province, Korea

Black carbon concentrations are closely related to global warming. To characterize the atmospheric aerosols in Chungcheong Province, Korea, we measured the concentrations of black carbon for about eight months (September 2015– April 2016) and compared them with PM10 and PM2.5 concentrations as well as various meteorological parameters (e.g., wind velocity and wind direction). We used a multi-angle absorption photometer to measure the black carbon; the PM10 and PM2.5 concentrations, wind velocity, and wind direction were obtained from local monitoring stations. The highest and lowest PM10, PM2.5, and BC concentrations were observed in spring and fall, respectively. The high concentrations in spring and winter were likely due to the dominance of westerly winds, which transported pollutants, whereas the low concentrations in fall were likely due to increased wind variations, which drove turbulent mixing. Overall, although BC concentrations exhibited directly proportional correlations with PM10 and PM2.5, the correlations were relatively low, probably because of differences between the sources of these three atmospheric pollutants. These results help clarify the characteristics of BC concentrations over the Korean Peninsula.


INTRODUCTION
Elemental carbon refers to carbonaceous aerosols in particulate matter (PM).As a type of elemental carbon, black carbon (BC) encompasses carbonaceous aerosols in PM defined by their optical properties and BC is usually produced by the incomplete combustion of fossil fuels.BC is important in climate change research because it can alter radiative forcing via light absorption (McMurry et al., 2004), making BC one of the most notorious substances among air pollutants for its influence on global warming.Although global warming is considered to be mainly caused by greenhouse gases, research has suggested the possibility that warming of the earth's atmosphere may be caused by BC (Andreae, 2001).Unlike greenhouse gases, BC is composed of solid particles that heat the atmosphere via This article is an English version of "Measurement of Black Carbon Concentration and Comparison with PM 10 and PM 2.5 Concentrations monitored in the Chungcheong Province, Korea [Korean]" published in the Particle and Aerosol Research in June 2017.direct absorption of solar radiation.Because it is only present in the atmosphere for short durations, BC is referred to as a short-lived climate forcer, and has been reported to influence local climate change due to the high variations in BC concentrations among industrialized cities and remote suburbs (Chameides and Bergin, 2002).
Several preliminary studies have assessed the factors affecting BC concentrations.Before industrialization, largescale forest fires or volcanic activity were the main causes of increased BC concentrations.However, after the Industrial Revolution, particulate emissions from direct combustion of hydrocarbon fuels (e.g., coal and petroleum) increased drastically (McConnell et al., 2007).In particular, PM is generated not only via anthropogenic processes (e.g., fuel combustion, vehicle emissions, and chemical production processes), but also by condensation of SO 2 and volatile organic compounds via secondary processes (Volkamer et al., 2006).As such, studies on the secondary generation of aerosols have been actively conducted in recent years.Overall, a great attention has been paid not only to primary (i.e., direct) production, but also to the secondary (i.e., indirect) production of atmospheric PM.
As typical products of primary atmospheric aerosols, we measured and monitored BC concentrations for eight months from September 2015 to April 2016 in Chungcheong Province, Korea, to clarify regional BC emissions.Chungcheong Province is located in central South Korea far from the Seoul Metropolitan Area; therefore, it is recognized as being less affected by pollutants from the Seoul Metropolitan Area and other urban regions.Furthermore, we assessed the variations in BC concentrations in atmospheric aerosols in relation to PM 10 and PM 2.5 concentrations, as well as wind direction and wind velocity.Using these data, we identified the main meteorological factors influencing the behavior of PM 10 , PM 2.5 , and BC.

EXPERIMENTAL METHODS
We used a multi-angle absorption photometer (MAAP) for the measurement of BC.MAAPs employ a filter-based technique to measure the light absorption of BC, where atmospheric aerosols are deposited onto a filter substrate and a laser beam with a wavelength of 637 nm is shot toward the deposited filter.Two detectors located on the same side as the laser source measure the light back-scattered by BC deposited on the filter.In addition, a detector located on the opposite side of the laser source measures the light transmitted through the filter.Conventional BC measurement instruments only have a detector on the opposite side of the laser source and do not measure the scattering of light, which means that only transmitted light can be detected.As a result of the detection of signal from back-scattering in MAAPs, the scattering effect can be compensated for by correcting the signal from transmitted light with the signals of the scattered light (Petzold and Schönlinner, 2004).The BC concentration derived from filter-based techniques (e.g., aethalometers and MAAPs) is referred to as equivalent BC (eBC).As such, "BC concentration" hereafter represents the equivalent BC concentration.
In the present study, the BC concentration was measured in 1-min increments at the Korea University of Technology and Education (KOREATECH) located in Byeongcheonmyeon, Cheonan city, Chungcheongnam-do (Fig. 1), using a commercially available instrument (MAAP 5012; Thermo Scientific).BC concentrations were monitored from September 2015 to April 2016.The PM 10 and PM 2.5 concentrations were obtained from the Air Korea database (http://www.airkorea.or.kr) operated by the Korea Environment Corporation.According to Air Korea, PM 10 and PM 2.5 concentrations are measured with unmanned automatic equipment using the β-ray absorption method, where PM is collected on a filter for 1 h and β-rays pass through the PM on the filter.Then, changes in absorption or extinction before and after the deposition of PM for 1 h are measured and converted into mass concentration.The principle of the β-ray absorption method is similar to the quantification of BC by filter-based instruments (e.g., aethalometers).In this study, we used PM data from Ochang-eup, Cheongju city, Chungcheongbuk-do (Fig. 1), from September 2015 to April 2016.
The seasonal mean BC, PM 10 , and PM 2.  Wind direction and wind speed displayed hourly were collected from the Korea Meteorology Administration (KMA) database (http://www.kma.go.kr).Wind direction and wind speed were measured at Shibang-dong, Cheonan, Chungcheongnam-do, which is located 14 km from KOREATECH (Fig. 1).Wind rose diagrams of wind direction and wind speed obtained from September 2015 to April 2016 are shown in Fig. 2. It should be noted that these wind rose diagrams show only wind intensity and direction, not BC concentrations.In fall 2015, southeasterly winds were equally dominant as westerly winds.However, in winter 2015, westerly winds were dominant.
In spring 2016, westerly winds were more dominant than easterly winds.Overall, westerly winds were dominant during the measurement period from fall 2015 to spring 2016.

Concentrations of BC, PM 10 , and PM 2.5
Table 1 presents the monthly average BC, PM 10 , and PM 2.5 concentrations from September 2015 to April 2016.
In fall 2015, winter 2015, and spring 2016, the average BC concentrations were 1.39 µg m -3 , 1.57 µg m -3 , and 2.30 µg m -3 , the average PM 10 concentrations were 36.7 µg m -3 , 48.8 µg m -3 , and 66.4 µg m -3 , and the average PM 2.5 concentrations were 30.9 µg m -3 , 37.7 µg m -3 , and 41.9 µg m -3 , respectively.Upon initial inspection, the trends in BC concentrations appeared to be similar to those of PM 10 and PM 2.5 concentrations.For instance, the highest and lowest average concentrations of BC, PM 10 , and PM 2.5 were observed in spring and fall, respectively.As shown in Fig. 2, westerly winds were dominant in spring.Thus, the high concentrations in spring were likely related to the influence of Asian Dust transported by westerly winds.In addition, the higher concentrations of BC, PM 10 , and PM 2.5 in winter and spring were possibly related to the transport of primary air pollutants produced via combustion processes from surrounding areas via westerly winds.Meanwhile, BC, PM 10 , and PM 2.5 concentrations in fall were slightly lower than those in winter.The wind direction  pattern in fall 2015 differed from that in winter 2015 (Fig. 2), with greater wind direction variations in fall 2015.These conditions could support turbulent mixing, reducing overall air pollutant concentrations.By contrast, the relatively consistent wind direction in winter 2015 could support the formation of atmospheric laminar flow mainly from the west, enabling the formation of a stable and stagnant air mass over the Korean Peninsula.In addition, the low BC, PM 10 , and PM 2.5 concentrations in fall may have been caused by decreased production of fine PM, although further studies are necessary to confirm these trends and mechanisms.Fig. 3 shows the 24-h running average of the data collected during the eight-month measurement period.The discontinuity of BC concentrations in Fig. 3 reflects the fact that the measurement instrument was temporarily stopped for maintenance.Although the seasonal BC concentrations initially appeared to follow the same trends as PM 10 and PM 2.5 concentrations, a closer examination of the data in Fig. 3 revealed that BC concentrations did not follow the same trends as PM 10 and PM 2.5 concentrations.This can be explained by the fact that PM consists not only of BC, but also other substances (e.g., sulfates, nitrates, or minerals).For example, in December 2015, BC concentrations decreased, whereas PM 10 and PM 2.5 concentrations increased (Fig. 3).Meanwhile, from January 2016 to mid-February 2016, BC concentrations increased markedly while PM 10 and PM 2.5 concentrations decreased slightly.Overall, the relationship between BC concentrations and PM concentrations may vary spatially or temporally depending on their sources or transport patterns.
Fig. 4 shows a bar chart of the mass concentration composition of BC, PM 10 , and PM 2.5 .Notably, the proportion of PM 2.5 was high throughout the study period, indicating that coarser particles (i.e., PM 10 ) were relatively rare.We speculated that PM 2.5 newly formed through secondary processes was dominant in this area, although it was beyond the scope of this study determine the mechanism driving the high proportions of PM 2.5 .

Linear Regression Analysis of BC, PM 10 , and PM 2.5 Concentrations
We analyzed the correlations among BC, PM 10 , and PM 2.5 with linear regression, where the coefficient of determination (R 2 ) was obtained from the linear regression of BC versus PM 2.5 or PM 10 concentrations and PM 2.5 versus PM 10 concentrations.
Table 2 shows the R 2 values of the linear regressions among BC, PM 10 , and PM 2.5 concentrations.Except for the R 2 between PM 10 and PM 2.5 in winter 2015, the R 2 values were generally low, indicating that BC was not strongly correlated with PM 10 and PM 2.5 .Although the results in Fig. 3 suggested that the BC concentrations appeared to generally follow PM 10 and PM 2.5 concentrations on a seasonal scale, the seasonal average BC concentrations were less correlated with PM 10 and PM 2.5 concentrations based on linear regression.

Correlation between BC and PM 10
We performed a linear regression analysis of BC and PM 10 concentrations during the three seasons in the study period.Although BC concentrations appeared to be directly proportional to PM 10 concentrations, the calculated R 2 was low (Fig. 5).PM 10 mostly consists of soil-derived dust particles, which are generated in large amounts from various   types of emission sources and has a short atmospheric residence time due to their larger size, thereby contributing less to increases in BC concentrations.By contrast, BC consists of carbonaceous particles smaller than 1 µm generated from incomplete combustion of hydrocarbon fuel.
In particular, in the study area, there are some unpaved roads in small cities with relatively few mobile pollution sources.Therefore, the low correlation between BC and PM 10 could be explained by the differences in PM 10 and BC sources.The R 2 between BC and PM 10 was lower in winter 2015 than in the other seasons (Table 2), indicating that fine dust in winter was weakly related to BC.In other words, the regression analysis indicated that BC and PM 10 had different compositions and emission sources.

Correlation between BC and PM 2.5
Next, we performed a linear regression analysis of BC and PM 2.5 concentrations during the three seasons in the study period.The correlations between BC with PM 2.5 concentrations were similar in fall 2015 (R 2 = 0.47), winter 2015 (R 2 = 0.45), and spring 2016 (R 2 = 0.58) (Fig. 5).The R 2 values were higher than those between BC and PM 10 , but still relatively low, suggesting that the PM 2.5 did not contain a large proportion of BC, and that BC concentrations were not highly correlated with PM 2.5 concentrations in the study area.Mobile pollution (e.g., diesel vehicle exhaust) is the main source of BC, whereas PM 2.5 typically originates from secondary formation from atmospheric industrial plant emissions.Therefore, the contribution of BC to PM 2.5 concentrations was likely low given that the measurements were performed in a small city, where mobile sources did not appear to influence the concentrations.

Correlation between PM 10 and PM 2.5
Finally, we performed a linear regression analysis of PM 10 and PM 2.5 concentrations during the three seasons in the study period.PM 10 and PM 2.5 concentrations were positively correlated in fall 2015 (R 2 = 0.53), winter 2015 (R 2 = 0.90), and spring 2016 (R 2 = 0.61) (Fig. 5).The particularly high correlation in winter 2015 may have been driven by the transport of primary pollutants generated from increased fossil fuel use for heating in continental Asia to the Korean Peninsula via westerly winds (Choi, 2008).In contrast, the lowest correlation was observed in fall 2015.Fossil fuel use for heating purposes was likely lower in fall than in winter.In addition, south-easterly and easterly winds were equally dominant as westerly winds in fall 2015, whereas westerly winds were dominant in winter 2015 (Fig. 2).Therefore, PM 2.5 was retardedly entrained by the Korean Peninsula, resulting in the relatively low correlation between PM 2.5 and PM 10 in fall.

Analysis on Event Days
Periods with particularly high PM 10 , PM 2.5 , and BC concentrations can be observed in Fig. 3.We attempted to examine the cause of such events by referring to monthly reports generated by the KMA.In particular, PM 10 , PM 2.5 , and BC concentrations increased from mid-October to early November in 2015 (Fig. 6).Based on the monthly KMA reports, Asian Dust may have been one of the reasons for these increases.From the KMA report, Asian Dust originating from Inner Mongolia on October 26 passed over the Yellow Sea, and was observed over the Korean Peninsula on October 26.Asian Dust events are rare in October, although other fall events have been observed in 2009 and 2014, suggesting that Asian Dust transport may not occur exclusively in spring, but also occasionally in fall.As mentioned previously, the discontinuity in the Fig. 6.BC, PM 2.5 , and PM 10 concentrations on two event days.
BC measurements in Fig. 6 was due to regular maintenance of the measurement instrument.
High PM 10 and PM 2.5 concentrations were observed in March 2016 (Fig. 6).From the monthly KMA report, an Asian Dust event even occurred at the end of March, when Asian Dust originating from Mongolia, the Inner Mongolian Plateau, and northern China was transported over the Korean Peninsula via north-westerly winds, increasing PM concentrations.

CONCLUSIONS
We analyzed the concentrations of BC, PM 10 , and PM 2.5 from September 2015 to April 2016 in central Korea and compared them with various meteorological parameters.The highest and lowest PM 10 , PM 2.5 , and BC concentrations were observed in spring and fall, respectively.PM 10 concentrations were high in spring due to Asian Dust transported via westerly winds.In winter, incomplete combustion of fossil fuels used for heating in continental Asia resulted in the transport of emissions over the Korean Peninsula via westerly winds, augmenting PM concentrations.Finally, the low PM concentrations in fall were likely due to increased wind variations driving turbulent mixing.
The BC concentrations generally showed low correlations with PM 10 and PM 2.5 concentrations in the three studied seasons.Sometimes, BC exhibited directly proportional relationships with both the PM 10 and PM 2.5 concentrations, albeit with low R 2 values.The low correlations were possibly due to differences between the sources of BC (i.e., vehicles), PM 2.5 (i.e., secondary pollution from industrial plant emissions), and PM 10 (i.e., soil-derived dust).In particular, the low contribution of BC to PM 2.5 may have been attributable to the study site being located in a small city with little influence from mobile pollution sources.
The PM 10 and PM 2.5 concentrations showed a directly proportional relationship, with R 2 values as high as 0.90 in winter 2015.This correlation was likely due to the use of fossil fuels for heating in continental Asia-where the primary pollutants generated via incomplete combustion of heating fuel were introduced to the Korean Peninsula via westerly winds from the western coast of Korea-in addition to high concentrations of pollutants from industrial plants.By contrast, the relatively low correlation (R 2 = 0.53) in fall was likely caused by south-easterly winds, which blocked the influx of westerly-wind-driven Asian Dust over the Korean Peninsula.
The BC concentrations were high from late October to early November in 2015.According to the monthly KMA report, an Asian Dust event originating in Inner Mongolia occurred during this period.Additionally, in March 2016, Asian Dust from Mongolia was transported over the Korean Peninsula.As such, it is necessary to study the correlation between BC and Asian Dust in greater detail to clarify the contribution of BC to PM concentrations.Given these results, the potentially inaccurate belief held by some researchers that BC is only correlated with PM 2.5 and PM 10 warrants further investigation.Overall, this study helps clarify the characteristics of BC over the Korean Peninsula.We believe that the BC concentrations obtained in this research can be used as urban background values for this region.

Table 2 .
Correlation coefficients (R 2 ) between PM 10 and BC; PM 2.5 and BC; and PM 10 and PM 2.5 concentrations.