# Vertical Measurement of Equivalent Black Carbon Concentration at Low Altitude
## 3 Results and Discussion
### Preliminary Remedy for Low eBC Concentration Case  
The flight time of our hexacopter is about 13-15 min with a fully charged battery. It takes 1-2 min to translocate our drone from one place to another one. In addition, it takes 1-2 min to stabilize the position of our drone when the wind is strong. Therefore, it is very difficult to continuously measure the eBC concentrations at a certain place for a long time. Therefore, instead of utilizing a hexacopter, we examined the eBC concentrations continuously collected by a multi-angle absorption photometer at the ground level for the analysis of low eBC concentrations. We selected several cases where the concentration at the ground level is low in order to examine how noisy and unrealistic the value is. Low concentration cases include several days when the eBC concentrations were below 1 \(\upmu\)g/m\({}^{3}\) which was confirmed as a nominal concentration at mountain near KOREATECH. The precipitations and  
Figure 5: The eBC concentrations measured in Eun Seok Mountain. The three black horizontal lines indicate average eBC concentrations at 400 m agl, 500 m agl, and 500 m agl after battery change. wind speed for CASE I, II, and III were shown in Table 1. The precipitation was present and the wind speed was high when eBC concentrations were very low. Figure 6a-c show eBC concentrations of CASE I, CASE II, and CASE III, respectively. The eBC concentrations were mostly less than 1 \(\mu\)g/m\({}^{3}\) and negative intermittently. For better comparison of low concentration cases to a typical case, a diurnal variation of the eBC concentration was displayed in Figure 6d. Morning rush hour and evening rush hour peaks were observed and the concentration increased to approximately 5 \(\mu\)g/m\({}^{3}\), which was higher than a typical eBC concentration near KOREATECH. CASE II was selected to investigate how to simply correct noisy and negative eBC concentrations. The simplest way to remove the noise is to do a box-averaging. We performed a box-averaging with 2 min and 5 min time window. As can be seen in Figure 7a, the 5 min averaging looks fair enough to consider that the noise is nominally reduced down to a discernable level. Consequently, a box-averaging for at least 5 min can remove noisiness. However, negative signals were still frequently observed in CASE II as shown in Figure 7b. The negative concentration seems partly due to a compensation scheme which forcedly displays zero concentration under nearly zero concentration condition [21]. In other words, negative values equivalent to positive values are alternately displayed so as to become zero at an extremely low concentration if averaged in time series. The absolute values of negative concentrations are reduced with an averaging time until it becomes zero. Therefore, it is reasonable to assume that negative concentration disappears at an ideal averaging time. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Case (Date)** & **Precipitation (mm) 1** & **Wind Speed (m/s) 1** \\ \hline CASE I (August 20) & 21 & 1.0 \\ CASE II (August 23–24) & 6 & 4.5 \\ CASE III (September 15–16) & 0 & 6.0 \\ \hline \hline \end{tabular}  
* Data were obtained from the website of Korea Meteorological Administration ([[http://www.kma.go.kr/weather/](http://www.kma.go.kr/weather/)]([http://www.kma.go.kr/weather/](http://www.kma.go.kr/weather/)) observation/aws\_table_popup.jsp).  
\end{table} 
Table 1: Precipitation and wind speed for low eBC concentration cases. Figure 6: Several cases of low eBC concentrations measured at the ground level: (**a**) CASE I; (**b**) CASE II; (**c**) CASE III; (**d**) diurnal variation of eBC concentration for a normal day. We performed the box-averaging with the time window of 2, 5, 10, 15, 20, 25, 30 and 60 min. We selected the minimum eBC concentration after averaging, then we attempted to find the time window where the negative signal completely disappears. We regarded the minimum eBC concentration as a reference value for comparison.