## 1 Introduction  
Real-time measurements are important to research studies characterizing short-term variability such as measuring rapidly changing source emissions, quantifying the amount of black carbon (BC) including short-lived pollutants emitted by mobile sources such as vehicles, or monitoring dynamic trends in indoor or ambient air quality [1]. Previously, instruments emerged from either technology-intensive private companies or research laboratories and was used first by a small number of high-skill early adopters [2]. The usage and development process of microAeth(r) AE51 (AethLabs, San Francisco, CA, USA), however, seem slightly different from that of most other analytical devices. Many of microAeth(r) AE51 have been used for exposure measurements because of their easy handling features similar to other personal air quality sensors and deployed to measure ambient BC as well. A wide range of researchers, including university labs, private companies, and government scientific research teams have used personal air quality sensors, though there has been a demand that addresses issues around interferences, humidity, calibration, and data usability [2]. Measurement devices are upgraded through the feedback from various users who have tested them in the field and have reported their characteristics, which is a development process as well in a broad sense. Incomplete-combustion-generated BC absorbs solar radiation and influences aerosol radiative forcing by directly changing the single scattering albedo (SSA) and by altering the lifetime of a cloud droplet, so called, through \"semi-direct\" effects [3; 4]. Not only the climate effects, but health effects have also been addressed with respect to BC, because BC may cause cancer. BC concentrations have been measured by filter-based absorption instruments such as the aethalometer, the particle soot absorption photometer, the multi-angle absorption photometer, and the tri-color absorption photometer due to their ease of operation [3; 5; 6; 7]. The aethalometer measures BC by detecting the difference in attenuation by particles collected on a filter at a given sampling interval. The particle soot absorption photometer adopts the same principle as the aethalometer but uses a different wavelength of light source. The multiangle absorption photometer measures backward scattering at two different positions to correct attenuation by the backward scattering. The tri-color absorption photometer detects attenuation at three different wavelengths with a similar principle to the aethalometer. Some instruments among mentioned above display the equivalent black carbon (eBC) mass concentration others report the light absorption coefficients. It is straightforward to measure BC under high concentration condition. However, it is challenging to measure BC concentration where the BC concentration at the region of interest becomes close to the detection limit of the instrument. In this case, taking a running-average improves the data quality produced by the instrument. The running-average, however, is a simple mathematical calculation to analyze data without having a particular meaning for black carbon. An alternative method to reduce noise in aethalometers has been proposed by [PERSON] et al. [1] in the name of an optimized noise-reduction averaging (ONA) though it is currently no longer updated for newer operating systems. The principle regarding \"attenuation\" (ATN) is simple, but converting the ATN into BC concentration is complicated, because both scattering and absorption are involved in the attenuation. A well-known issue in BC measurement by aethalometer-type instruments is the sensitivity. Filter-based optical techniques always suffer from the sensitivity issue. This sensitivity is directly related to the loading rate of particles on the filter materials, the air flow rate, and the sample spot size [1].