3.1. Source Profile of Biomass Burning
Table 2 and
Figure 2 show the emission in chemical classes of PM
2.5 from burning of woods and agricultural byproducts. The burning materials appeared according to characteristics thereof; approximately 48%, 7% and 6% of chemical components that consist of PM
2.5 appeared as OC, ionic compounds and EC, respectively. Based on results of previously conducted studies, approximately 49% of chemical components except for OC, EC and ionic chemical components, are estimated to be comprised of heavy metals, tiny amount of moisture, H, N, S and O, etc. that consist of organic substances other than carbon components [
7,
8,
9].
To estimate the contents of H, N, S and O except for carbon components, the ratios of OM/OC, based on molecular weight of 114 individual OC compounds which were analyzed in the present study, were calculated. From calculations of OM/OC, the WIOC appeared as 1.1 while the WSOC appeared as 1.5. Approximately 66% of chemical components consisting of PM
2.5 appeared as OC based organic matters from the application of the ratio of OM/OC to calculations of WIOC and WSOC, while the occupancy of components of OC, EC and ionic chemical components in PM
2.5 appeared as 79%. The correlation of the ratio of OM/OC with PM
2.5 was identified wherein the correlation coefficient more than 0.85 was found thereby the estimation of chemical components consisting of PM
2.5 through employing the ratio of OM/OC was identified reliable as shown in
Figure 2.
As a means to appraise the source of emission of PM
2.5, the ratio of OC/EC is used [
34]. The ratio of OC/EC of PM
2.5 resulted from the burning of coal has been known to be distributing in the range 1.6–3 [
35,
36] while the ratio of OC/EC of PM
2.5 resulted from combustion of engine has been known to be distributing in the range 0.5–1.3 [
16,
37]. The ratio of OC/EC of PM
2.5 emitted from the biomass burning has been known over 3 which is higher than those of other sources; according to part of previously conducted studies, the ratio of OC/EC appeared higher than 12 of rice straw and 24 of wheat straw [
37,
38]. The ratio of OC/EC resulted from the biomass burning appeared distributing in the range 2.46–29.85 wherein the mean ratio thereof was 10.98. The ratio of OC/EC of 8 specimens among 10 specimens of analysis appeared over 3.0 and corresponded to results of previous studies however the ratios of OC/EC of stems of red-pepper and green perilla appeared below 3.0 suggesting different consequences from results of previous studies. To identify the causes behind the consequences, the specimens were distinguished into the hard ones of higher density (pine trees, cherry tree, red-pepper stems and stems of green perilla) and soft ones of lower density (pine needles, gingko leaves, maple leaves, cherry leaves, rice straws and stems of beans). The resulting ratio of 15.99 of OC/EC of soft specimens appeared relatively high while the ratio of 3.47 of OC/EC of hard specimens appeared lower than that of soft specimens.
The four chemical components of K
+, SO
42−, NO
3− and NH
4+, constituting PM
2.5, were analyzed as ionic components. Contents of respective components of PM
2.5 appeared as approximately 2.36% of K
+, 1.99% of SO
42−, 1.43% of NO
3− and 0.8% of NH
4+. Among them, K
+ has been known as a major indicator ingredient of biomass burning; the level of content of K
+ contained in dried woods has been known approximately over 0.1%, over 0.2% for dried herbaceous plant and over 3% for crops such as olive, etc. [
39]. The K
+, contained in crops, is emitted as KCl, KOH or K
+ at temperature over 1000 K [
40] and according to previously conducted studies, the K
+ in PM
2.5, discharged from biomass burning, has been known to be contained 1%–10% in wheat straw and stems of maize and over 10% in rice straws [
41]. The content of K
+ analyzed in the present study appeared with lower levels of average 1.82% in the six woods and average 3.33% in herbaceous plants comparing to results reported from previous studies. In particular, the specimens of rice straw, analyzed in the present study, contained approximately 0.81% of K
+ showing significant difference from results of previous studies.
Generally, in the case of using K
+ as an indicator material of the biomass burning, the ratio of K
+/EC is used [
41].
Table 3 shows the ratio of K
+/EC derived from the previously conducted studies and from the present study. As presented in the table, the ratio of K
+/EC of herbaceous plant, employed for the present study, appeared distributing in the lower range 0.25–0.73 comparing to the ratio of K
+/EC of 1.12–3.45 of herbaceous plant employed for the previous studies. In particular, the ratio of K
+/EC of rice straw, which was predicted as pseudo-crop, was 3.45 in the previous studies exhibiting significant difference from 0.45 of the present study. On the contrary, the ratio of K
+/EC of woods of the present study appeared distributing in the range 0.1–0.75 which were similar to those of 0.19 and 0.76 of previously conducted studies. The similarity (of woods) and difference (of herbaceous plant) in the ratio of K
+/EC of the present study from those of previously conducted studies were attributed to the differences in components of specimens, species and corresponding cultivation environment. In the present study, the leaves and branches of the part of specimens of woods were distinguished wherein the ratio of K
+/EC in branches of pine tree and cherry tree appeared approximately 50% higher than those in the leaves thereof. This suggests the ratio of K
+/EC can be varied according to the ratio of composition of leaves and branches to be burnt, though they belong to the same kind of biomass of identical species. In addition, the content of K
+ in leaves and branches of cherry tree appeared higher than other woods with respective values of 3.80% and 4.18%, while the content of K
+ in stems of green perilla and red-pepper appeared 6.34% and 8.39%, respectively, suggesting the contents of K
+ appeared distributing in the variable range of 0.43%–8.39% according to species of crops. Additionally, the K
+, contained in plants, is affected by microorganisms and amount of potassium in soil. Potassium is the one of major nutrients for the growth of plants, the representative element of fertilizer. Water soluble potassium among fertilizer elements spread over soils are absorbed by crops, whereas the solidified potassium are absorbed by crops via microorganisms enabling the solubilization of potassium [
42]. Therefore, the amount of potassium, contained in plants, is significantly dependent on the cultivation environment of plants. In the meantime, the red-pepper in Korea is regarded as one of the crops creating the highest value added as well as essential seasoning agent for which the area of cultivation of 32,865 ha in 2018 for red-pepper appeared higher than that of other flavor vegetables [
43]. In addition, since the red-peppers are cultivated in an open field, it is included as the representative one of burning of agricultural byproducts in the registry of national atmospheric pollutants in Korea. Based on these facts, the kinds of species and cultivation environment of crops in each country, and the emission of K
+ from respective crops need to be identified preemptively for the employment of K
+ as an indicator material of the biomass burning. This is because the crops to be cultivated in countries are different according to respective dietary habits and the emission of K
+ varies significantly according to types of species of crops cultivated.
The OC, occupying the highest portion among the components of PM
2.5, was classified into WIOC and WSOC, wherein the ratio of WIOC to WSOC appeared as approximately 1.2; the occupancies of WIOC and WSOC in PM
2.5 were approximately 16% and 32%, respectively. Further, for the specimens of woods, the weight percentage of WIOC and WSOC to total weight of PM
2.5 appeared approximately 20.0% and 29.2%, respectively, whereas the weight percentage of WIOC and WSOC to total weight of PM
2.5 in herbaceous plant appeared approximately 8.5% and 37.6%, respectively. That is, the WIOC appeared higher in woods than in herbaceous plant, whereas the WSOC appeared higher in herbaceous plant than in woods. To determine the concentration of components in WIOC and WSOC, the 114 organic compounds, comprising the 23 compounds of PAHs and 33 compounds of alkanes were analyzed for the analysis of WIOC, as well as the 27 compounds of alkanoic acids, 8 compounds of benzene carboxylic acid, 7 compounds of di-carboxylic acid, 15 compounds of amino acids and levoglucosan, were analyzed for the analysis of WSOC. The results of the analysis are presented in
Figure 3 and
Figure 4. A total of 56 compounds of analysis of the PAHs and alkanes occupied approximately 7% of entire WIOC, wherein the weight percentage of PAHs and alkanes were 0.47% and 0.64%, respectively, to the weight of PM
2.5. PAHs appeared as in the order of phenanthrene > fluoranthene > pyrene; the emission of PAHs from woods appeared higher comparing to that from the herbaceous plant. In particular, retene was detected from the 3 ones among the 4 herbaceous plants with corresponding average concentration of 0.07 mg/g-OC, whereas the wood was detected from all 8 crops with corresponding average concentration of 0.92 mg/g-OC, which was higher than that of the herbaceous plant. In particular, retene, which is emitted from woods, was discharged highly from specimens of pine tree wherein the concentration in pine needle and in stalk exhibited 1.07 mg/g-OC and 6.11 mg/g-OC, respectively. For the case of alkanes, the detected ratios from woods and herbaceous plants appeared varying according to compounds of analysis. In the analyses from C
11 to C
29, the concentrations of wood exhibited higher emission than concentrations of herbaceous plant, whereas in the analyses from C
30 to C
40, the concentrations of herbaceous plant manifested characteristics of higher emission than that of concentrations of wood. In addition, by the chemical classification into hard- and soft ones, the 5 compounds of analysis among alkanes (n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane and norpristane) appeared only from the soft ones.
The compounds employed for the analysis of WSOC occupied approximately 22% of entire compounds, which were equivalent to 7% of the weight of PM2.5. In regard to each item employed for the analysis, the alkanoic acid appeared as 4.94% of the weight of PM2.5, while the levoglucosan, di-carboxylic acid, benzene carboxylic acid and amino acids appeared with 2.13%, 0.16%, 0.02% and 0.02% of the weight of PM2.5, respectively. Major chemical components contained in the alkanoic acid which manifested the highest content in WSOC appeared in the order of hexadecanoic acid > triacontanoic acid > oleic acid > tetradecanoic acid > linoleic acid > dehydroabietic acid. hexadecanoic acid exhibited higher content in alkanoic acid group and it occupied approximately 25.25% among entire alkanoic acid, while triacontanoic acid, oleic acid, tetradecanoic acid, linoleic acid and dehydroabietic acid appeared with respective occupancies of 8.20%, 7.16%, 6.95%, 6.36% and 6.25%; the six compounds occupied more than 60% of the entire 27 compounds. In regard to the comparison of specimens of woods with herbaceous plant, the content of octanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid and pentadecanoic acid appeared higher in specimens of woods than in herbaceous plant; the other compounds appeared with higher content in specimen of herbaceous plant. With regard to the classification of compounds according to respective characteristics, the average concentration of alkanoic acid in the soft specimens appeared as 110.60 mg/g-OC while the concentration of alkanoic acid in the hard specimens appeared as 57.88 mg/g-OC, signifying the concentration of alkanoic acid appeared increasing in accordance with decreasing density of specimen.
Levoglucosan is created solely by the decomposition of cellulose and hemicellulose to be burnt at temperature over 300 °C [
44,
45]. Therefore, the levoglucosan is employed as one of organic molecular markers of PM
2.5 created from the biomass burning. To trace the biomass materials of burning by using the acceptance model, the ratio of levoglucosan to OC (levoglucosan/OC, mg/g-OC) is generally used [
46,
47,
48] In the present study, the content of levoglucosan in WSOC appeared as the second largest one, which was corresponded to 2.11% of the weight of PM
2.5; the levoglucosan/OC appeared distributing in the range 26.99–157.29 mg/g-OC. With regard to the classification according to characteristics of compounds employed for the analysis, the average levoglucosan/OC of hard specimen appeared as approximately 100.24 mg/g, while it was in the range 32.67 mg/g-OC for the soft specimen. This agrees with the results of previous studies reported the yield of levoglucosan/OC of hard specimen (109–168 mg/g-OC) appeared higher than that of soft specimen (52–95 mg/g-OC) [
49].
Di-carboxylic acid occupied approximately 0.16% of the weight of PM2.5 for both specimens of woods and herbaceous plants. The percentage of contents of analyzed di-carboxylic acid appeared in the following order of succinic acid > glutaric acid > azelaic acid, wherein the suberic acid was detected only from specimens of woods while the adipic acid was detected only from the stalk of woods. pimelic acid was not detected from all specimens. Benzene carboxylic acid and amino acids appeared as occupying 0.02% and 0.04% of the weight of PM2.5, respectively. From the analyzed 8 compounds of benzene carboxylic acid, both the phthalic acid and methylphthalic acid were commonly detected however, the rest of 6 compounds were not detected. The analyzed amino acid was detected with the average 0.48 mg/g-OC emitted from specimens of woods and 1.25 mg/g-OC from specimens of herbaceous plant. In regard to the classification of specimens into the hard- and soft ones, it appeared as 0.52 mg/g-OC from the hard specimens, while it was 0.81 mg/g-OC from the soft specimens, suggesting lower emission from hard specimens than soft specimens. In particular, the emission of amino acid from rice straw appeared as 1.96 mg/g-OC, which was approximately 3.5 times higher than that from hard specimens and twice as much as that from soft specimens.
The characteristics of chemical components in PM2.5, which is created from the burning of woods and agricultural byproducts, were examined together with characteristics of emitted materials which were varied according to inherent characteristics of woods (hard) and herbaceous plant (soft). In short, the ratio of OC/EC of soft specimens appeared higher than that of hard specimens, while the PAHs of woods appeared higher than that of herbaceous plant. In addition, for the case of alkane, the compounds of analysis of C11~C29 exhibited higher level in specimens of woods than that in specimens of herbaceous plant, whereas the compounds of analysis in the range C30~C40, they manifested characteristics of higher level in specimens of herbaceous plant than in specimens of woods. Hard specimen of alkanoic acid appeared higher than that of soft specimen, while the yield of levoglucosan/OC appeared higher in hard specimens than that of soft specimens. The above characteristics of emission represent the detailed organic molecular markers of the biomass burning. The results obtained from the present study are expected to be presenting organic molecular markers of PM2.5, wherein the artificial burning of agricultural byproducts and spontaneously generated forest fire, etc. are distinguished.
3.2. Ambient Concentrations
The seasonal PM
2.5 was collected to determine the contribution of causes to resulting PM
2.5 in the subject area of the present study. A total of 160 chemical components, comprising PM
2.5, OC, EC, 6 ionic components, 23 PAHs, 16 kinds of hopanes and steranes, 33 kinds of alkanes, 6 kinds of cyclo-alkanes, 33 kinds of alkanoic acids, 8 kinds of benzene carboxylic acids, 8 kinds of alkanoic diacids, levoglucosan, cholesterol and 20 kinds of the other chemical components, were analyzed (
Table 4).
Annual average concentration of PM2.5 in the subject area of the present study was 25.44 μg/m3, whereas those in residential area and on roads were 19.07 μg/m3 and 31.81 μg/m3, respectively. Seasonal PM2.5 in the subject area appeared as in the following order of summer (22.71 μg/m3) > autumn (18.59 μg/m3) ≥ winter (18.50 μg/m3) > spring (16.47 μg/m3), whereas it appeared on roads as in the following order of winter (62.79 μg/m3) > spring (29.27 μg/m3) > summer (23.17 μg/m3) > autumn (12.02 μg/m3).
Annual average concentration of OC and EC in the subject area of the present study appeared as 6.37 μg/m
3 and 1.50 μg/m
3, respectively. The annual average concentration of OC and EC in the residential area were 4.99 μg/m
3 and 1.10 μg/m
3, respectively, whereas those on roads appeared as 7.75 μg/m
3 and 1.91 μg/m
3. In general, EC refers to the primary particles emitted from the biomass burning, coal and diesel oil, etc. [
7,
9]. On the contrary, OC is classified into the primary organic carbon (POC) and secondary organic carbon (SOC) according to respective processes of creation [
8]. Thereby, the ratio of OC to EC as well as EC tracer method can be employed for the prediction of the ratio of SOC in OC [
50]. According to the EC tracer method, the ratio of OC/EC over 2.5 is generally known that it contributes largely to the creation of the secondary OC. The ratio of annual average OC/EC appeared in residential area as 5.35, while it appeared in roads as 4.24; this suggests comparatively higher content of SOC therein.
The compounds, analyzed as an organic indicator of WIOC, were 56 compounds of PAHs and alkanes; the percentage of PAHs and alkanes to weight of PM
2.5 were 0.15% and 0.97%, respectively. PAHs are emitted through incomplete combustion of fossil fuels and biomass. In the present study, the annual average concentration of PAHs in residential area appeared as approximately 84.82 ng/m
3, while it appeared on roads as 229.16 ng/m
3 showing higher level of annual average concentration than that appeared in the residential area. This was estimated that it would be attributable to PAHs created from the combustion of fuels of motor vehicles that affected the area of roads. The detected seasonal concentration of PAHs commonly marked the highest level in both residential area and on roads in wintertime (residential area 43.22 ng/m
3, roads 170.72 ng/m
3), whereas it marked the lowest level in summertime (residential area 5.32 ng/m
3, roads 5.33 ng/m
3). According to previously conducted studies, the higher concentration of PAHs in wintertime was reported to be associated with the effect of phenomenon of cold ignition of vehicles; while the lower concentration of PAHs has been reported that it would be attributable to the effect of photochemical decomposition [
51,
52]. The varied seasonal concentration of PAHs, identified in the present study, was also estimated to be affected by effects of cold ignition of vehicles in wintertime and photochemical decomposition in summertime, as it was reported in previously conducted studies.
The sum of annual average concentration of alkanes appeared as 538 ng/m
3 in residential area and 1428 ng/m
3 on roads; the seasonal concentration thereof tended to show behaviors similar to seasonal variations of PAHs however the concentration in residential area appeared higher in summertime than that in wintertime. The sum of concentration of alkanes, observed in wintertime, appeared as 147 ng/m
3 in residential area and 841 ng/m
3 on roads; it appeared in residential area and on roads as 69 ng/m
3 and 66 ng/m
3, respectively, in summertime. Carbon Preference Index (CPI) signifies the ratio of concentrations of odd numbered alkanes to even numbered alkanes, wherein the C
max is defined as the number of carbons of detected peak concentration, and it represents the input of anthropogenic sources [
53]. The value of CPI close to 1 in the acceptance model implies the artificial emission of fossil fuels whereas the value over 2.0 represents the alkanes originated from biomass [
53,
54,
55].
Hopanes and steranes are the ones of organic indicators of PM
2.5 which are mainly created from fossil fuels. Therefore, the hopanes and steranes, contained in the exhaust from vehicles or thermoelectric power plants wherein fossil fuels are used, are detected comparatively in higher level [
10,
56,
57]. A total of 16 compounds of hopanes and steranes substances including 17α(H)-22,29,30-trinorhopane, 17β(H)-21α(H)-30-norhopane and 17α(H)-21β(H)-hopane were analyzed as an ingredient of organic indicators of fossil fuels. Annual average concentration hopanes and steranes appeared in the subject area as 0.66 ng/m
3 in the residential area and 2.93 ng/m
3 on roads; the roads appeared with higher level of concentration.
Levoglucosan is a substance of organic indicator resulted from the biomass burning. Annual average concentration of levoglucosan in the subject area of the present study appeared as 582.75 ng/m
3 (residential area) and 1007.25 ng/m
3 (roads), respectively. With regard to the seasonal concentrations of levoglucosan, the highest concentration of 1173 ng/m
3 appeared in residential area in autumn, while the highest concentration of 2834 ng/m
3 appeared on roads in wintertime. However, the ratio of levoglucosan /OC, employed for the acceptance model, appeared higher in wintertime regardless of the area of residence or roads; the ratio of levoglucosan /OC in wintertime appeared as 272.45 mg/g (residential area) and 247.29 mg/g (on roads), respectively. Further, the ratio of K
+/EC, the one of indicator chemical components of biomass burning, also appeared with the highest level of 0.45 (annual average 0.18) in the residential area and 0.31 (annual average 0.12) on roads in wintertime. Therefore, the biomass burning in the subject area of the present study was identified to be increasing mainly in wintertime. The cholesterol, one of WSOC, is the one of organic indicator chemical components resulted from the burning of meats [
24]. Previous study had employed the ratio of cholesterol to OC for the CMB model as an indicator material of burning of meats; the ratio of cholesterol/OC used in the previously conducted study was 0.0010 [
58]. The annual average concentration of cholesterol appeared as 0.20 ng/m
3 in residential area and 3.86 ng/m
3 on roads, respectively, in the present study. The ratio of cholesterol/OC appeared commonly below 0.0000 in all four seasons in the residential area, whereas it appeared on roads as 0.0004 in spring and 0.0010 in wintertime. In regard to the annual average concentration and seasonal concentration, the creation of PM
2.5 resulted from the burning of meats appeared higher on roads. This was attributed to the effects of restaurants placed around roads.