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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 450 m 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 21 %, 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 with the variation in visitor numbers on the tower well. 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 but a large fraction (22 %) of the total OH reactivity. However, large fractions of reactive VOC species, e.g., monoterpenes (49 %), 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 PM 2.5), measured at 5, 118, 168, and 488 m, 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 450 m 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. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is a key process occurring in the planetary boundary layer (PBL). Newly formed particles are an important source of aerosols and cloud condensation nuclei (CCN) that influence clouds and climate. However, the distribution of these new particles at different altitudes has rarely been studied. In-situ measurements of ultrafine particles (UFP) observed at the ground level and at the top of the Canton Tower (454 m) located in downtown Guangzhou in southern China were analyzed, along with measurements of multiple meteorological and physicochemical quantities observed during a field campaign carried out from December 2019 to March 2020 and simulated with the Weather Research and Forecasting (WRF-Chem) model. We found that turbulence and NPF characteristics vary considerably with height, with UFP concentrations diminishing by half from the surface to the tower top. This indicates that UFP are transported upward from the ground in the lower boundary layer. A consistent relationship is established between the occurrences of NPF and the evolution of turbulence. The correlation between the exchange ratio at the tower top correlates well with nucleation growth, suggesting that turbulence can play an important role in episodes of NPF growth. The growth rate is closely related to the turbulence exchange ratio, effectively dictating the UFP concentration before and during the COVID-19 lockdown period. A new mechanism is thus hypothesized: NPF happens earlier near the surface and grows faster in the upper PBL, attributed to condensable vapor being transported by turbulent vertical mixing in the boundary layer. Model simulations using the WRF-Chem model reveal that the exchange ratio changed NPF parameters, supporting the proposed mechanism that the evolution of the PBL has a significant impact on NPF. This should not be neglected in NPF research because this physical factor may be the dominant one underlying the NPF mechanism. • New particle formation (NPF) observed at the ground and the top of the Canton Tower (454 m) found the feature vary considerably at different heights. • Turbulence evolution is closely related to the growth rate. • NPF happens earlier near the surface and grows faster at the upper PBL attributed to turbulent mixing. [ABSTRACT FROM AUTHOR]

Roughness length (Z 0) is a fundamental parameter in atmospheric boundary layer theory that affects the transfer of momentum and heat. However, this parameter in urban areas in the WRF model is typically assigned a fixed value. In a previous study, we developed a Z 0 database for the heterogeneous urban underlying surface of Guangzhou and demonstrated that its application in the WRF model had significantly improved the simulations of wind speed. In this study, the impact of Z 0 update on the dynamic and thermal environment in Guangzhou urban areas is investigated through two numerical experiments: the base experiment applies the model-default Z 0 value (0.5 m) for the urban areas and the sensitivity one uses a heterogeneous Z 0 map (with higher Z 0 values in most urban areas) estimated in the previous study. The results show that the temperature and relative humidity at 2 m height are not sensitive to this Z 0 modification. For the dynamic environment, the modification of Z 0 decreases the wind speed at 10 m height and increases the friction velocity. The reduction of horizontal wind speed due to Z 0 increases reaches heights of several hundred meters, especially below a height of 400 m (950 hPa). Additionally, the updated Z 0 leads to an increase in the turbulent momentum exchange coefficient over the urban underlying surface, resulting in a more vigorous turbulent mixing and elevated planetary boundary layer height. For the thermal environment, the Z 0 increases result in a higher turbulent heat exchange coefficient near the surface layer, which contributes to the vertical heat flux transport. Moreover, the Z 0 increase enhances the sensible heat flux during daytime and reduces the land surface temperature and ground heat flux in Guangzhou central urban area. Due to the reduced heat storage during the daytime, the amount of heat released at night decreases accordingly, thus causing a reduction in the land surface temperature at night and a decrease in the sensible heat flux. This study provides a comprehensive understanding of the sensitivity of the thermodynamic environment to the urban Z 0 increase and reconfirms the significance of a realistic Z 0 dataset in atmospheric modeling. [Display omitted] • The effect of Z 0 modifications on spatiotemporal variations of meteorological factors was explored. • Horizontal wind speed reduction due to Z 0 increases reached heights of several hundred meters. • Surface energy balance was used to illustrate the impact of Z 0 increases during daytime and nighttime. [ABSTRACT FROM AUTHOR]