Vertical Distribution Characteristics of VOCs within Beijing Base on High-Temporal-Resolution Airborne Measurements

To elucidate the vertical distribution characteristics and photochemical behavior of volatile organic compounds (VOCs) in the Beijing region, a vertical profile airborne survey was conducted on September 15, 2017, using a Y-12 aircraft equipped with a proton transfer reaction- time of flight-mass spectrometry. The flight covered altitudes from ground level to 3600?m at 5s interval, simultaneously collected meteorological data and concentrations for 16 VOC species. The results reveal markedly distinct vertical distributions among different VOCs. Methyl ethyl ketone (MEK) and the primary-emission-dominated aromatic hydrocarbons (e.g., benzene and toluene) exhibited peak concentrations near the surface, decreasing rapidly with increasing altitude. Their surface-level concentrations were further diluted in the afternoon due to lifted boundary layer. In contrast, OVOCs (e.g., formaldehyde, acetaldehyde and acetone) and oxidation products of isoprene (MVK & MACR) showed significantly higher concentrations in the lower troposphere during the afternoon compared to the morning, indicating that active photochemical production outweighed the dilution effect associated with boundary layer mixing. Isoprene, a typical biogenic VOC, displayed a unique vertical distribution pattern: its concentration in the lower level was lower in the morning than at mid-to-upper levels, likely attributable to intense oxidative condition near the surface. Monoterpenes, by comparison, exhibited no pronounced vertical gradient. Further evidence for afternoon photochemical reactions of OVOCs was provided by elevated OVOC/benzene ratios, with benzene serving as a chemically inert tracer. These findings demonstrate that the vertical structure of VOCs over Beijing results from the combined effects of source emission, atmospheric dispersion and photochemical processes.