Nighttime ozone in the lower boundary layer: insights from 3-year tower-based measurements in South China and regional air quality modeling

Abstract

Abstract. 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 in Guangzhou, the core megacity in South China, and interpret the measurements with a 1-month high-resolution chemical simulation from the Community Multiscale Air Quality (CMAQ) model. Measurements are available at 10, 118, 168, and 488 m, with the highest (488 m) measurement platform higher than the typical height of the 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 a vertical ozone gradient between the 10 and 488 m heights (ΔO3/ΔH10–488 m) of 3.6–6.4 ppbv hm−1 in nighttime and 4.4–5.8 ppbv hm−1 in daytime. We identify a high ozone residual ratio, defined as the ratio of ozone concentration averaged over nighttime to that in the afternoon (14:00–17:00 LT), of 69 %–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. The tower-based measurement also indicates that the 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 the nighttime and the following day's daytime surface ozone air quality. During the surface nighttime ozone enhancement (NOE) event, we observe a significant decrease in ozone and an 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 an NOE event by introducing ozone-rich and NOx-poor air into the RL to enter the nighttime stable boundary layer. The CMAQ model simulations also demonstrate an enhanced positive contribution of vertical diffusion (ΔVDIF) to ozone at the 10 and 118 m heights and a negative contribution at the 168 and 488 m heights during the NOE event. We also observe a strong correlation between nighttime RL ozone and the following day's surface maximum daily 8 h average (MDA8) ozone. This is tied to enhanced vertical mixing with the collapse of nighttime RL and the development of a convective mixing layer, which is supported by the CMAQ diagnosis of the ozone budget, suggesting that the mixing of ozone-rich air from nighttime RL downward to the surface via the entrainment is an important mechanism for aggravating ozone pollution the following day. We find that the bias in CMAQ-simulated surface MDA8 ozone 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.

A Comparative Investigation of the Characteristics of Nocturnal Ozone Enhancement Events and Their Effects on Ground-Level Ozone and PM2.5 in the Central City of the Yellow River Delta, China, in 2022 and 2023

“…This is comparable to the results of He et al's research on the frequency of NOE events in China [14]. The NOE events may be attributed to the nocturnal transport of high ozone concentrations from upwind cities, the downward transport of ozone from the night residual layer, and some specific meteorological processes, such as low-level jet streams, convective storms, and local circulation [14,[24][25][26][27]45,46]. Overall, the frequency of NOE events in Dongying was higher in spring, summer, and autumn and lower in winter from 2022 to 2023 (Table 1 and Figure 8).…”

“…During the study period, the nocturnal ozone peak exceeded 160 µg/m 3 on six occasions, and the maximum nocturnal ozone peak was 210 µg/m 3 . This is comparable to the maximum of nocturnal ozone peaks in Jinan, China, Shenzhen, China, and Malaysia [45][46][47]49,50]. It could be seen that Dongying, a central city of the Yellow River Delta, had extremely high nocturnal ozone peaks when NOE events occurred, which might have adverse effects on human health.…”

“…The NOE events exhibited a higher frequency around midnight and a lower frequency in the early night hours. This pa ern may be a ributed to the stronger NOx titration effects shortly after sunset, making NOE events less likely to occur during the early night hours [14,24,46]. Compared with 2022, the frequency of NOE events at 22:00, 23:00, 1:00, and 3:00 in 2023 decreased, while the frequency at 0:00, 4:00, and 6:00 increased (Figure 10).…”

“…The NOE events exhibited a higher frequency around midnight and a lower frequency in the early night hours. This pattern may be attributed to the stronger NOx titration effects shortly after sunset, making NOE events less likely to occur during the early night hours [14,24,46].…”