Search

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 (Δ O 3/Δ H 10–488 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 O x (O x= O 3+ NO 2) 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 NO 2 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 NO x -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 (Δ VDIF) 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. [ABSTRACT FROM AUTHOR]

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]

In this study, localised and non-uniform urban morphology (UM) and urban fraction (UF) parameters are implemented in a single-layer urban canopy scheme in the Weather Research and Forecasting (WRF) mesoscale meteorological model. The purpose of this research is to evaluate the effect of the refined parameterisation scheme on the simulation of dynamic and thermal fields in the urban canopy of the Guangzhou metropolitan area. The results showed that, compared with the default urban canopy parameters of the WRF model, using the localised UM parameters resulted in the most significant improvement in the 10 m wind speed simulation. In urban districts, the mean bias between the observed and simulated 10 m wind speed was reduced significantly by 59% from 2.63 m/s to 1.09 m/s during the daytime. For the thermal environment simulation during the daytime, higher UF and UM values resulted in lower surface albedos and generated narrower street canyons compared with the default modelling setting, which caused more heat to be trapped in the urban canopy and ultimately led to an increase in the surface skin temperature (TSK) and a largely increased ground heat flux (GRD). As a result, at night, more heat was transferred from the ground to the surface, producing a higher TSK. The effect of the localised UF on the sensible heat flux (HFX) was closely related to the near-surface temperature gradient. The UM caused the HFX to increase during the daytime, which was related to the near-surface heat exchange coefficient in the lower model layers. As the high-resolution UM significantly altered the urban geometry, the dynamic environment simulation resulted in a large increase in friction velocity and a decrease in wind speed. [ABSTRACT FROM AUTHOR]

The Chinese government launched the Air Pollution Prevention and Control Action Plan in 2013, and various stringent measures have since been implemented, which have resulted in significant decreases in emissions and ambient concentrations of primary pollutants such as SO2 , NOx , and particulate matter (PM). However, surface ozone (O3) concentrations have still been increasing in urban areas across the country. In a previous analysis, we examined in detail the roles of meteorological variation during 2013–2017 in the summertime surface O3 trend in various regions of China. In this study, we evaluated the effect of changes in multi-pollutant emissions from anthropogenic activities on O3 levels during the same period by using an up-to-date regional chemical transport model (WRF-CMAQ) driven by an interannual anthropogenic emission inventory. The Community Multiscale Air Quality (CMAQ) model was improved with regard to heterogeneous reactions of reactive gases on aerosol surfaces, which led to better model performance in reproducing the ambient concentrations of those gases. The model simulations showed that the maximum daily 8 h average (MDA8) O3 mixing ratio in urban areas increased by 0.46 ppbv per year (ppbva-1) (p=0.001) from 2013 to 2017. In contrast, a slight decrease in MDA8 O3 by 0.17 ppbva-1 (p=0.005) in rural areas was predicted, mainly attributable to the NOx emission reduction. The effects of changes in individual pollutant emissions on O3 were also simulated. The reduction of NOx emission increased the O3 levels in urban areas due to the nonlinear NOx and volatile organic compound (VOC) chemistry and decreasing aerosol effects; the slight increase in VOC emissions enhanced the O3 levels; the reduction of PM emissions increased the O3 levels by enhancing the photolysis rates and reducing the loss of reactive gases on aerosol surfaces; and the reduction of SO2 emissions resulted in a drastic decrease in sulfate concentrations, which increased O3 through aerosol effects. In contrast to the unfavorable effect of the above changes in pollutant emissions on efforts to reduce surface O3 , the reduction of CO emissions did help to decrease the O3 level in recent years. The dominant cause of increasing O3 due to changes in anthropogenic emissions varied geographically. In Beijing, NOx and PM emission reductions were the two largest causes of the O3 increase; in Shanghai, the reduction of NOx and increase in VOC emissions were the two major causes; in Guangzhou, NOx reduction was the primary cause; in Chengdu, the PM and SO2 emission decreases contributed most to the O3 increase. Regarding the effects of decreasing concentrations of aerosols, the drop in heterogeneous uptake of reactive gases – mainly HO2 and O3 – was found to be more important than the increase in photolysis rates. The adverse effect of the reductions of NOx , SO2 , and PM emissions on O3 abatement in Beijing, Shanghai, Guangzhou, and Chengdu would have been avoided if the anthropogenic VOCs emission had been reduced by 24 %, 23 %, 20 %, and 16 %, respectively, from 2013 to 2017. Our analysis revealed that the NOx reduction in recent years has helped to contain the total O3 production in China. However, to reduce O3 levels in major urban and industrial areas, VOC emission controls should be added to the current NOx - SO2 -PM policy. [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]

Ozone plays a significant role in the troposphere due to its influence on human health, air pollution, and global warming. Despite the decline of NO x and VOCs in China in recent years, obvious increases in O 3 concentration took place, while the attributions to it remained unclear. Previous studies have revealed that the contribution of meteorological conditions is getting increasingly significant to O 3 concentration. Given that O 3 concentration continued to climb in China, the influence of meteorology on O 3 concentration should not be neglected. In this study, the WRF-CMAQ model was utilized to study an O 3 episode in the Pearl River Delta (PRD) region, China. In this short-term simulation, anthropogenic emissions remained unchanged during the Polluted Week (PW) compared with the Clean Week (CW), which allowed us to investigate the effects of changes in meteorological conditions (ΔMET) on O 3 variations via an efficient method of sensitivity experiment. And the contribution of changes in anthropogenic emissions (ΔEMIS) was evaluated similarly. It turns out that ΔMET explained 85.8% of the elevated O 3 concentration in urban Guangzhou (GZ), which resulted from an increase in temperature and solar radiation, and a decrease in wind speed and humidity, while ΔEMIS explained 13.3%. We also analyzed the 3-dimensional development process of the O 3 pollution and quantitatively assessed it by process analysis in the CMAQ model. Meteorology affected O 3 concentration through both chemical reactions and physical processes, with the former contributing 36.9% and the latter 63.1%. Most of the elevated O 3 was found near the surface at a height less than 50 m above the ground, which accounted for 56.8%. This study emphasized the contribution of meteorological conditions to O 3 concentration and demonstrated a streamlined method of sensitivity experiment to quantitatively identify the impact of meteorology, which could be useful for O 3 prevention and emissions reduction strategies. [Display omitted] • Meteorologically- and Anthropogenically-driven O 3 variations in Guangzhou were investigated. • The method is applicable for assessing contribution from meteorology on a short-time scale. • Contributions of physical and chemical processes, and of different heights to O 3 were estimated. [ABSTRACT FROM AUTHOR]