Vertical Profiling of Fine Particulate Matter and Black Carbon by Using Unmanned Aerial Vehicle in Macau, China

Abstract

An unmanned aerial vehicle 1 (UAV) equipped with miniature monitors was used to study the vertical profiles of PM2.5 (particulate matter [PM] with a ≤2.5-μm diameter) and black carbon (BC) in Macau, China, from the surface to 500 m above ground level (AGL). Twelve- and 11-day measurements were conducted during February and March 2018, respectively. In total, 46 flights were conducted between 05:00 and 06:00 AM Local Time (LT) during the measurement days. The average concentrations of PM2.5 and BC were significantly lower in March (40.1 ± 17.9 and 2.3 ± 2.0 μg m−3 , respectively) when the easterly winds dominantly prevailed compared with the concentrations in February (69.8 ± 35.7 and 3.6 ± 2.0 μg m−3, respectively) when the northerly winds typically occurred. In general, PM2.5 concentrations decreased with height. A vertical decrement of 0.2 μg m−3 was observed per 10 m. BC concentrations exhibited diverse vertical profiles with an overall vertical decrement of 0.1 μg m−3 per 10 m. Meteorological analyses including back-trajectory analysis and atmospheric stability categorization (i.e., Pasquill stability class) revealed that both advection and convection transports may have notable influences on the vertical profiles of PM pollutants. The near-surface accumulation of PM pollutants is positively associated with atmospheric stability. The height of the planetary boundary layer influences atmospheric dispersion. In our study, the concentration of PM pollutants above the boundary layer was lower than that below the layer, thus exhibiting a sigmoid profile in some cases. The air mass origin and vertical wind influences the vertical profiles of the PM pollutants, particularly when the atmosphere was neutral. We presented that primary emissions, such as those from nearby fishing vessels, lighting of firecrackers and fireworks during the Chinese New Year (CNY), and the takeoff of civil flights from a nearby airport, may affect the vertical profiles of the PM pollutants and CO in different ways. In particular, the lighting of firecrackers and fireworks on February 16 (first day of the CNY) resulted in the elevated concentrations of PM2.5 and BC within 150 m AGL and the pronounced accumulation of CO at approximately 400 m AGL. The takeoff of a civilian flight on February 10 may have resulted in a substantial increase in the PM2.5 concentrations from 80.8 (±2.1) μg m−3 at the ground level to 119.2 (±9.3) μg m−3 at a height of 330 m. Although the results are confined to a height of 500 m AGL, the current study provides a rich dataset for PM vertical distributions along with explanations of its possible causes rather than the more commonly investigated spatiotemporal variations by conducting ground-based measurements.