Comment on acp-2022-116

Abstract

Volatile organic compounds (VOCs) are key precursors of ozone and particulate matter that are the two dominant air pollutants in urban environments. However, compositions and sources of VOCs in urban air aloft were rarely reported by far. To address this matter, highly time-resolved measurements of VOCs were made by proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) at a 450-m platform on the Canton Tower in Guangzhou, China. A combination of in-situ measurements and modeling techniques was used to characterize variations and sources of VOCs. Five sources were identified from positive matrix factorization (PMF) analysis, namely daytime-mixed (e.g., biogenic emissions and secondary formation), visitor-related (e.g., human breath and volatilization of ethanol-containing products), vehicular+industrial, regional transport, and volatile chemical product (VCP)-dominated (i.e., volatilization of personal care products), contributing on average to 22 %, 30 %, 28 %, 10 %, and 11 % of total VOC (TVOC) mixing ratios, respectively. We observe that contributions of the visitor-related source, mainly composed of ethanol, followed well with the variation patterns of visitor number on the tower. The VCP-dominated source only had an average contribution of ~5.7 ppb during the campaign, accounting for a small fraction (11 %) of TVOC mixing ratios. However, large fractions of some VOC species, e.g., monoterpenes (49 %), were attributed to the VCP-dominated source, indicating significant contributions of VCPs to ambient concentrations of these species in urban environments. Vertical profiles of air pollutants (including NOx, ozone, Ox, and PM2.5), measured at 5 m, 118 m, 168 m, and 488 m, exhibited more evident gradients at night than in the daytime owing to stronger stability of the nocturnal boundary layer. Mixing ratios of VOC species during the nighttime generally decreased with time when the 450-m platform was located in the nocturnal residual layer and significantly increased when impacted by emissions at ground. The results in this study demonstrated composition characteristics and sources of VOCs in urban air aloft, which could provide valuable implications in making control strategies of VOCs and secondary air pollutants.