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Nighttime ozone in the lower boundary layer regulates atmospheric chemistry and surface ozone air quality, but our understanding of its vertical structure and impact is largely limited by the extreme sparsity of direct measurements. Here we present 3-year (2017–2019) measurements of ozone in the lower boundary layer (up to 500 m) from the Canton Tower at Guangzhou, the core megacity in South China, and interpret the measurements with a one-month high-resolution chemical simulation from the Community Multiscale Air Quality (CMAQ) model. Measurements are available at 10 m, 118 m, 168 m, and 488 m, with the highest 488 m measurement platform higher than the typical height of nighttime stable boundary layer that allows direct measurements of ozone in the nighttime residual layer (RL). We find that ozone increases with altitude in the lower boundary layer throughout the day, with nighttime (daytime) ozone at the 488 m height being 2.4–5.4 (1.5–2.4) times as that at the 10 m height. This indicates a persistent high ozone level and oxidation capacity aloft the surface. The ozone vertical gradient between the 10 m and 488 m height (∆O3/∆H10–488 m) is 3.6–6.4 ppbv/hm in nighttime and 4.4–5.8 ppbv/hm daytime. We identify a strong ozone residual capacity, defined as the ratio of the ozone concentration averaged over nighttime to that in the afternoon (14:00–17:00 LT), of 67 %–90 % in January, April and October, remarkably higher than that in the other three layers (29 %–51 %). Ozone in the afternoon convective mixing layer provides the source of ozone in the RL, and strong temperature inversion facilitates the ability of RL to store ozone from the daytime convective mixing layer, by constraining the exchange of RL ozone with ozone inside the nocturnal stable boundary layer that is subject to strong chemical destruction and deposition. The tower-based measurement also indicates that nighttime surface Ox (Ox=O3+NO2) level can be an effective indicator of RL ozone if direct measurement is not available. We further find significant influences of nocturnal RL ozone on both nighttime and the following day's daytime surface ozone air quality. During the surface nighttime ozone enhancement (NOE) event, we observe significant decrease in ozone and increase in NO2 and CO at the 488 m height, in contrast to their changes at the surface, a typical feature of enhanced vertical mixing. The enhanced vertical mixing leads to NOE event by introducing ozone-rich air in the RL to enter the nighttime stable boundary layer and weakens the titration effect by diluting NOx concentrations. The CMAQ model simulations also demonstrate enhanced positive contribution of vertical diffusion (ΔVDIF) to ozone at the 10 m and 118 m and negative contribution at the 168 m and 488 m during the NOE event. We also observe strong correlation between nighttime RL ozone and the following day's surface MDA8 ozone. This is tied to enhanced vertical mixing with the collapse of nighttime RL and the development of convective mixing layer, which is supported by the CMAQ simulated increase in positive ΔVDIF of +50 ppbv·hr−1 at the 10 m and negative ΔVDIF of -10 ppbv·hr−1 at 488 m at early morning (08:00–09:00 LT), suggesting that the mixing of ozone-rich air from nighttime RL downward to surface via the entrainment is an important mechanism to aggravate ozone pollution in the following day. We find that the bias of CMAQ simulated surface MDA8 ozone in the following day shows a strong correlation coefficient (r =0.74) with the bias in nighttime ozone in the RL, highlighting the necessity to correct air quality model bias in the nighttime RL ozone for accurate prediction of daytime ozone. Our study thus highlights the value of long-term tower-based measurements for understanding the coupling between nighttime ozone in the RL, surface ozone air quality, and boundary layer dynamics. [ABSTRACT FROM AUTHOR]

Abstract: Rock magnetic measurements were conducted on fault rocks from the Dagou Village outcrop at Shuiguan Town, Pingwu County, Sichuan Province (China), which is located on the Yingxiu–Beichuan fault, the main fault ruptured in the 2008 Wenchuan Ms 8.0 earthquake. Results revealed that magnetic behavior of fault gouge is possibly dominated by paramagnetic siderite and lepidocrocite, as well as pyrrhotite. Thermomagnetic analysis indicated that these Fe-bearing minerals are readily decomposed once temperature over 250–400°C, and form newly ferrimagnetic magnetite/maghemite that would increase the magnetization of fault rocks. Consequently, the paramagnetic nature and relatively low magnetization of fresh fault gouge suggest that the shallow fault plane has not been subjected to thermal event over 250–400°C during the Wenchuan earthquake. This inference can be confirmed by a kinetic model analysis. These findings proposed that thermal pressurization was not likely to occur during the Wenchuan earthquake, at least at the shallow part of slip zone. Onset of an elastohydrodynamic lubrication mechanism was alternatively discussed using a model in which fault zone containing viscous gouge. Results indicated that the lubrication pressure could reach up to 30.2MPa, which can widen the gap between the fault surfaces and reduce the area of asperity contact. This prediction well matches the reported injection veins within the coseismic shear zone, and the largest ground motions associated with lower predominant frequencies and large stress drop during the Wenchuan earthquake. [Copyright &y& Elsevier]