# Vertical Measurement of Equivalent Black Carbon Concentration at Low Altitude
## 1 Introduction  
Monitoring air quality, using small sensors onboard a drone is very complicated, not only for constraints, such as power consumption, weight and propeller effect, but also because of sensor dependence on the pollution sources [1]. The aerosol light absorption of black carbon (BC) is an important measure to quantify the solar radiation toward the earth. In addition, BC is related to environmental climate change, as well as the air quality. At pristine sites, such as remote mountains, small amount of light absorbing aerosols have the potential for thawing permanent snow. The interactions of BC with the atmosphere and light strongly depend on the distribution of BC in the atmosphere. Usually, BC concentration is measured on the ground by a filter-based technique in the form of the equivalent BC (eBC) concentration, which derives mass concentrations associated with absorption coefficient. Light absorption by BC can be measured by a multiangle absorption photometer (MAAP) or an aethalometer which basically use Beer-Lambert law for quantification [2, 3]. Among them, the MAAP is effective in that it corrects scattering artifacts. However, the MAAP still suffers from high noise when the aerosol concentration is low. To overcome the shortcomings of any filter-based techniques, corrections have been made by applying various schemes [4, 5, 6, 7, 8]. In this situation, it is needed to ascertain the eBC concentration at tens of meters above ground level (agl) because the atmosphere at low altitude is highly related to the air quality on the ground level. Vertical measurement of eBC concentration, using balloons, have already been studied by other research groups [9, 10]. The elevation of a balloon, however, may be affected by air drift caused by the turbulence of wind; therefore, it would be difficult to detect BC at the exact position along the vertical axis of elevation. There are several studies for vertical profiles of air pollutants measured using a fixed-wing unmanned aerial vehicle (FW-UAV) [11, 12]. However, measurements using FW-UAVs are dangerous near the ground level where most people live. Therefore, measurements of BC using a hexacopter are conducted in this study to investigate short-range vertical profiles of BC starting from the ground level to about 100 m in altitude. Drones offer a new approach to large scale air pollution elimination that will be able to autonomously monitor and remove pollutants that are already in existence in the environment [13]. Drones can offer a high resolution spatiotemporal sampling, which is not feasible with manned aircraft. Some drone systems are more flexible than others, with the ability to carry multiple sensors and operate at different flight modes (hybrid rotary/fixed wing designs) [1]. Even more, a drone is used to measure a plume of volcanic degassing with the help of arrays for the measurement of multiple gas species such as CO\({}_{2}\), SO\({}_{2}\) and H\({}_{2}\)O [14]. [PERSON] et al. (2020) performed vertical profile measurements of PM\({}_{2.5}\) and BC by using a hexacopter unmanned aerial vehicle (UAV) for 23 days in the course of 2 months in 2018 in Macau, China and reported that higher particle concentrations, but less pronounced vertical variations were observed in February than those in March [15]. Another study shows that a multi-rotor drone, carrying a remote-controlled whole air sampling device, collects aerial samples with high sample integrity and preservation conditions [16]. A particle collection system was installed on a multicopter UAV for in situ measurement of the concentration of aerosol particles such as pollen grains and spores in the atmospheric boundary layer (ABL) [17]. The utility of a quadrotor unmanned aircraft system (UAS) was explored as a sampling platform to measure vertical and horizontal concentration gradients of trace gases and aerosol particles at high spatial resolution (1 m) within the mixed layer (0\(-\)100 m) [18].