Progress on the vertical observation methods of volatile organic compounds and their applications within the atmospheric boundary layer

Abstract

<p indent="0mm">Volatile organic compounds (VOCs) are important trace gases in the troposphere that play vital roles in the formation of secondary air pollutants such as ozone and organic aerosol components. An in-depth characterization of the spatiotemporal variability, particularly in three-dimensions, of VOCs in the atmospheric boundary layer is an essential prerequisite for understanding the formation mechanisms of regional pollution episodes. However, vertically resolved VOCs measurements are seldom made during field campaigns due to the limitation of available observation techniques. This review summarizes the progress in vertical observation of VOCs and their applications to the atmospheric boundary layer, focusing on the introduction of platforms for vertical measurement techniques for VOCs, including high-rise buildings, tethered balloons, aircraft, unmanned aerial vehicles (UAVs), and remote sensing. All of these platforms have been used in previous studies and have their respective advantages and disadvantages. High-rise buildings are generally characterized by low altitudes and fixed locations but could serve as long-term platforms for both online and offline VOCs measurements. Tethered balloons, generally used for offline VOCs measurements, have flexible launch locations and altitude ranges. Aircraft can provide platforms for both offline and online VOCs measurements over large spatial scales, but aircrafts are not suitable for long-term observations because of their high operating cost. Similar to tethered balloons, UAVs are used for offline VOCs measurements, while they are much more flexible in utilization and can be launched in complex environments. However, small payload, short flight duration, and availability of airspace are key factors that limit the wide application of UAVs in the field of environmental monitoring. Remote sensing platforms (including ground-based and space-based techniques) could provide vertical measurements of several VOCs species over large temporal and spatial scales. However, the vertical VOCs profiles provided by remote sensing techniques are frequently characterized by large uncertainty. With the rapid development of VOCs measurement techniques (such as UAVs and proton-transfer-reaction time-of-flight mass spectrometry), future studies are expected to obtain vertical VOCs observations with higher spatiotemporal resolutions as well as lower operating costs. Vertical observations of VOCs concentrations could provide valuable opportunities for a thorough analysis of VOCs emissions and variation of their related chemical processes within the atmospheric boundary layer. Therefore, this review also summarizes the progress in applications of vertical VOCs observations in the atmospheric boundary layer, focusing on introducing the key factors influencing VOCs vertical distribution patterns, determination of VOCs emission fluxes, evaluation and calibration of VOCs emission inventories, investigation of the vertical distribution of photochemical ozone production sensitivity, and the vertical distribution of VOCs degradation mechanisms at night. At present, vertical observations are predominantly used to identify impacts of transport and boundary-layer dynamics on the vertical distributions of different VOCs species and to estimate speciated VOCs emissions. However, the vertical variability of the oxidation and degradation mechanisms for different VOCs species and the environmental effects of this variability have been reported in only a few studies. More effort should be made to elucidate how the interactions between VOCs chemistry and vertical transport affect the formation and concentration of secondary air pollutants at ground level. In summary, there is an urgent need to obtain reliable vertical observations of key VOCs species in the atmospheric boundary layer for better investigating the formation mechanisms of regional atmospheric pollution.