Variations and sources of volatile organic compounds (VOCs) in urban region: insights from measurements on a tall tower

Abstract

Volatile organic compounds (VOCs) are key precursors of ozone and particulate matter, which are the two dominant air pollutants in urban environments. However, compositions and sources of VOCs in urban air aloft have rarely been reported so far. To address this matter, highly time-resolved measurements of VOCs were made by a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) at a 450gm platform on the Canton Tower in Guangzhou, China. A combination of in situ measurements and modeling techniques was used to characterize variations in 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, cooking, and volatilization of ethanol-containing products), vehicular-industrial, regional transport, and volatile chemical product (VCP) (i.e., volatilization of personal care products), contributing on average to 21g%, 30g%, 28g%, 10g%, and 11g% of total VOC (TVOC) mixing ratios, respectively. We observe that contributions of the visitor-related source, mainly composed of ethanol, followed with the variation in visitor numbers on the tower well. The VCP-dominated source only had an average contribution of g1/45.7gppb during the campaign, accounting for a small fraction (11g%) of TVOC mixing ratios but a large fraction (22g%) of the total OH reactivity. However, large fractions of reactive VOC species, e.g., monoterpenes (49g%), were attributed to the VCP-dominated source, indicating important contributions of VCPs to ambient concentrations of these species in urban environments. Vertical profiles of air pollutants (namely NOx, ozone, Ox, and PM2.5), measured at 5, 118, 168, and 488gm, exhibited more evident gradients at night than in the daytime owing to the stronger stability of the nocturnal boundary layer. Mixing ratios of VOC species during the nighttime generally decreased with time when the 450gm platform was located in the nocturnal residual layer and markedly increased when impacted by emissions at ground level. The results in this study demonstrated composition characteristics and sources of VOCs in urban air aloft, which could provide valuable implications in making strategies for control of VOCs and secondary air pollutants.