## 3 Results and Discussion
### Amplified Noise Relating to Power Source  
As shown earlier, the internal battery lasted only about 20 h. For this reason, Inst_1 was plugged to an AC-DC adaptor while Inst_2 was connected to a USB port on a laptop for the monitoring longer than 20 h. Figure 5 shows the comparison between Inst_1 and Inst_2 for the measurement of indoor BC concentration approximately 10 days long without changing a filter. The author did not replace the filter intentionally, in order to observe the effect of overloading on the BC measurement. Unexpectedly, noise of Inst_1 gradually increased and was significantly amplified until the end of the test. The noise of Inst_1 started to increase after approximately 18 h since the beginning of measurement. The \"18 hours\" is similar to the duration of internal battery of Inst_1. It is thought that this noise was associated with a process of AC to DC conversion. As soon as the Inst_1 consumed up the internal battery, it started to depend on the AC-DC adapter to charge the internal battery. At this moment, the noise of AC seemed to be transferred to the input circuit of Inst_1. The ATN of Inst_1 was larger than that of Inst_2 in the beginning of the comparison measurement though it was reversed at the end of the test. The difference of ATN between the start and the end of the test ought to be same if the other conditions remain same. However, as can be seen in Figure 5b, the difference of ATN between Inst_1 and Inst_2 definitely exists, implying that some factors of Inst_1 are different from those of Inst_2. The factor could be the difference in the sampling flow rate, the sensitivity of photodiode, the power of the light source, and the efficiency of optics module. The author may refer it to a \"system response factor\". If the system response factor is evaluated before manufacturing, calibration for the device will become more robust. It is intriguing that Inst_2 also becomes noisy around Mar. 24 when ATN value reaches 80. It is better for users to replace the filter before ATN reaches 80 in order to acquire reliable data.  
Figure 4: Comparison of the black carbon (BC) concentration and normalized numerical differentiation of ATN. (**a**) Inst_1, (**b**) Inst_2.  
Figure 6 displays the comparison between Inst_1 and Inst_2 for the case where the relative error is low in the 10 days long term comparison. The correlation is still good enough to be used as a BC sensor, though Inst_2 is slightly overestimated with respect to Inst_1. Inst_1 was selected as a reference in the present study because it has shown more stable performance than Inst_2 since its purchase. In addition, Inst_2 has sporadically failed in collecting data for the sampling flow rate settings except 0.1 lpm.  
In order to analyze the origin of the intensified noise in BC concentration, an analysis using a Fast Fourier Transform (FFT) was performed for the BC data measured every 1 min. Figure 7 demonstrates that Inst_2 did not pick up a signal at most frequencies but the noise was induced from all frequencies for Inst_1. FFT results show that Inst_1 suffered noise intrusion at all frequency domain of interest.  
Figure 5: Noise is intensified when instrument is connected to an AC-DC adaptor. (**a**) black carbon (BC) concentration, (**b**) time series of attenuation (ATN) values.  
Figure 6: Comparison between Inst_1 and Inst_2 for low relative error case.  
Figure 8 shows that the difference of \(\Delta\)ATN between Inst_1 and Inst_2 fluctuates gradually. Not only do the absolute values for the difference between \(\Delta\)ATNInst_1 and \(\Delta\)ATNInst_2 become larger, but the \(\Delta\)ATN values also exhibit negative values which are trivial and do not contain any physical meaning. The envelope of the difference of \(\Delta\)ATN between Inst_1 and Inst_2 increases until the end of the test. It would have increased gradually if the test had continued. It appears that not only the connection to an AC-DC adapter but also the filter-overloading effect, or the combination of both, could contribute to the amplification of noise.