## 3 Results and Discussion
### Identification of the Biomass Burning Tracer Levoglucosan  
Levoglucosan, generated through the thermal decomposition of cellulose and hemicellulose at temperatures above 300 \({}^{\circ}\)C, can serve as a tracer for BB [41]. According to [42], levoglucosan, a molecular marker of organic components, can be utilized as a tracer for BB emissions. During the measurement period, the concentrations of levoglucosan and mannosan were found to be 151.3 ng m\({}^{-3}\) and 43.1 ng m\({}^{-3}\), respectively. The highest concentrations were observed on the evening of February 18 th, coinciding with a low wind speed of 0.7 m s\({}^{-1}\). According to [43], levoglucosan measured in background areas exhibited an average concentration of 43 ng m\({}^{-3}\) during winter. Furthermore, measurements \begin{table}
\begin{tabular}{c c c c c} \hline \hline BC\({}_{370}\) & BC\({}_{370}\) & BrC\({}_{370}\) & BrC Contribution & Site & Reference \\ (Mm\({}^{-1}\)) & (Mm\({}^{-1}\)) & (\%) & & \\ \hline
38 & 20 & 52 & Urban & [10] \\
26 & 10 & 38 & Urban & [27] \\
47 & 22 & 46 & Urban & [37] \\
67 & 24 & 36 & Urban & [38] \\
175 & 67 & 38 & Rural & [38] \\
56 & 14 & 24 & Urban & [39] \\
231 & 46 & 20 & Rural & [40] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of previous research on BC and BrC light absorption coefficients at 370 nm. in Shanghai reported similar concentrations of 161 ng m\({}^{-3}\) in the winter, aligning with the findings of this study. Among the saccharide components analyzed, levoglucosan contributed the most, accounting for 81% of the markers for BB emissions, followed by mannosan, which contributed 13%. Burning residential wood for heating in winter has been identified as a significant source of atmospheric PM\({}_{2.5}\)[44]. Furthermore, the levoglucosan/OC ratio widely estimates BB contribution [45]. According to the report by the Environmental Protection Agency (EPA), the levoglucosan/OC ratio for wood combustion was 0.15 [45]. In this study, the average levoglucosan/OC ratio was found to be 0.02, with a maximum value of 0.08 observed on 17 February at 0:00. The levoglucosan concentration at that time measures 679 ng m\({}^{-3}\), the second-highest concentration during the measurement period. In addition, the estimated BC\({}_{370}\) and BrC\({}_{370}\) mass absorption coefficients are 11.3 Mm\({}^{-1}\) and 15.1 Mm\({}^{-1}\), respectively, with BrC\({}_{370}\) accounting for 57% of the total BC light absorption coefficient at 370 nm, the highest contributor. These results suggest that BB activities for residential heating increased significantly as the temperature gradually decreased from 6.5 \({}^{\circ}\)C to \(-\)0.1 \({}^{\circ}\)C, from 15:00 on 16 February to 04:00 on 17 February. The BrC\({}_{370}\) light absorption coefficient's diurnal variations closely mirrored those of levoglucosan and mannosan, as shown in Figure 5a. Since no dust events occurred during the study period, it can be inferred that BB emissions influenced the majority of BrC. Figure 5 illustrates the correlations between levoglucosan, mannosan, and K\({}^{+}\), well-known tracers of BB. Levoglucosan and mannosan exhibited a high correlation with BrC\({}_{370}\) (r\({}^{2}\) = 0.84, 0.82, respectively). However, K\({}^{+}\), commonly used as a BB tracer, is not dependent on BB sources alone and may be misinterpreted due to the interference of crustal materials and other sources [46]. The levoglucosan/OC ratio was used to estimate the contribution of BB to OC mass. During the study period, the levoglucosan/OC ratio was determined to be 0.022 (Table 1). Furthermore, levoglucosan/BC\({}_{370}\) (0.010) was approximately three times higher than levoglucosan/BrC\({}_{370}\) (0.031). Ref. [47] reported that the contribution of BB was 1.7 to 2.8 times higher in winter compared to the other seasons. In addition, the levoglucosan/OC ratio was 0.019 in winter, which is similar to the findings of this study. Soluble K\({}^{+}\) is well known to be generated from BB and commonly used as a tracer for wood smoke.