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Traditional tropospheric ozone (O3) climatology uses a simple average substantially smoothed stratification structure in individual O3 profiles, limiting our ability to properly describe and understand how O3 is vertically distributed at the interface between the mixing layer (ML) and free troposphere (FT). In this study, we collected 1897 ozonesonde profiles from two Chinese megacities (Beijing and Hong Kong) over the period 2000–2022 to investigate the climatological vertical heterogeneity of the lower-tropospheric O3 distribution with a mixing-layer-height-referenced (h -referenced) vertical coordinate system. The mixing-layer height (h) was first estimated following an integral method that integrates the information of temperature, humidity, and cloud. After that, a so-called h -referenced vertical distribution of O3 was determined by averaging all individual profiles expressed as a function of z/h rather than z (where z is altitude). We found that the vertical stratification of O3 is distributed heterogeneously in the lower troposphere, with stronger vertical gradients at the surface layer and ML–FT interface. There are low vertical autocorrelations of O3 between the ML and FT but high autocorrelations within each of the two atmospheric compartments. These results suggest that the ML–FT interface acts as a geophysical "barrier" separating air masses of distinct O3 loadings. This barrier effect varies with season and city, with an ML–FT detrainment barrier in summer (autumn) and an FT–ML entrainment barrier in other seasons in Beijing (Hong Kong). Based on a Student's t test, daily h -referenced O3 profiles were further classified into three typical patterns: MLO 3 -dominated, FTO 3 -dominated, and uniform distribution. Although the FTO 3 -dominated pattern occurs most frequently during the whole study period (69 % and 54 % of days in Beijing and Hong Kong, respectively), the MLO 3 -dominated pattern prevails in the photochemically active season, accounting for 47 % of summer days in Beijing and 54 % of autumn days in Hong Kong. These occurrences of the MLO 3 -dominated pattern are significantly more frequent than in previously reported results at northern mid-latitudes, indicating intensive photochemical MLO 3 production under the high-emission background of a Chinese megacity. From a FTO 3 -dominated to MLO 3 -dominated pattern, the O3 precursor CH2O (NO2) experiences a substantial increase (decrease) in Beijing but a slight change in Hong Kong. Vertically, the increment of CH2O is larger in the upper ML, and the decrement of NO2 is larger in the lower ML. Such vertical changes in O3 precursors push O3 production sensitivity away from the VOC-limited regime and facilitate high-efficiency production of O3 via photochemical reactions, particularly in the upper ML of Beijing. [ABSTRACT FROM AUTHOR]

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is an important pathway in nitrate formation; however, its formation rate and relative contribution to total particulate nitrate (pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$) are highly variable. Here we report that nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3. Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime. The results have crucial implications for effective control of nitrate pollution in the future. An exclusive decrease in NO2 will decrease nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in a NO2‐limited regime but may be less effective or even increase nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in an O3‐limited regime. Plain Language Summary: Our observations show that nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via dinitrogen pentoxide (N2O5) hydrolysis in the residual layer over megacity Beijing is more efficient than at ground level. Further investigations suggest nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3. Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime. As a result, an exclusive decrease in NO2 will decrease nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in a NO2‐limited regime but may be less effective or even increase nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in an O3‐limited regime. The above result is also substantiated by observations during the COVID‐19. Key Points: Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis in the residual layer over megacity Beijing is more efficient than at ground levelNocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime [ABSTRACT FROM AUTHOR]

Ozone (O3) in the upper troposphere and lower stratosphere (UTLS) is strongly regulated by upper-level circulation dynamics. Understanding the coupling between UTLS O3 distribution and upper-level circulation dynamics is important not only to understand synoptic processes governing O3 distribution and variability, but also to test the fidelity of chemistry transport models in simulating the stratosphere–troposphere exchange (STE) processes. This study presents the first systematic assessment of observationally constrained UTLS O3 variability associated with upper-level circulation patterns over the Northeast Asia region. By applying the self-organized mapping (SOM) technique to 500, 250, and 100 hPa geopotential height (GPH) data, 12 circulation patterns are quantified and then used to characterize the UTLS O3 distribution in the period 2000–2020 in both four-site (Beijing, Pohang, Tateno, and Sapporo) ozonesonde data and regional-scale satellite products. The underlying dynamic transport mechanism responsible for UTLS O3 responses to different circulation patterns are further explored through correlation analysis between O3 anomalies and transport indicators. The results indicate that although O3 at almost all altitudes shows statistically significant sensitivity to circulation patterns, lower-stratospheric O3 exhibits a far stronger sensitivity when compared with upper-tropospheric O3. Circulation patterns featuring the East Asian Trough (EAT) show clear enhancement of O3 southwest of the trough, and the enhancement zone moves with the eastward propagation of the EAT. Circulation patterns featuring eastward-shedding vortices of the Asia Summer Monsoon Anticyclone (ASMA) show the opposite signal, in which O3 concentrations are decreased, especially at Sapporo, and the negative O3 anomaly zone stretches from South Japan to Sakhalin Island. Each circulation pattern is characterized by distinct transport pathways, which play a determining role in the pattern-specific UTLS O3 response. Positive O3 anomalies are usually associated with post-trough downward and southward transport, whereas negative O3 anomalies are commonly associated with fore-trough upward and northward transport. In the lower stratosphere, the correlation between O3 anomalies and transport indicators is significantly stronger than that in the upper troposphere, and the strongest correlation occurs in the lower stratosphere of Beijing. [ABSTRACT FROM AUTHOR]

Aerosol–cloud interaction (ACI) in fog and planetary boundary layer (PBL) conditions plays critical roles in the fog life cycle. However, it is not clear how ACI in the first fog (Fog1) affects the PBL and subsequently affects ACI in the second fog (Fog2), which is important information for understanding the interaction between ACI and the PBL, as well as their effects on fog properties. To fill this knowledge gap, we simulate two successive radiation fog events in the Yangtze River Delta, China, using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Our simulations indicate that the PBL conditions conducive to Fog2 formation are affected by ACI with high aerosol loading in Fog1; subsequently, the PBL promotes ACI in Fog2, resulting in a higher liquid water content, higher droplet number concentration, smaller droplet size, larger fog optical depth, wider fog distribution, and longer fog lifetime in Fog2 than in Fog1. This phenomenon is related to the following physical factors. The first factor involves meteorological conditions conducive to Fog2 formation, including low temperature, high humidity, and high stability. The second factor is the feedbacks between microphysics and radiative cooling. A higher fog droplet number concentration increases the liquid water path and fog optical depth, thereby enhancing long-wave radiative cooling and condensation near the fog top. The third factor is the feedbacks between macrophysics, radiation, and turbulence. A higher fog top presents stronger long-wave radiative cooling near the fog top than near the fog base, which weakens temperature inversion and strengthens turbulence, ultimately increasing the fog-top height and fog area. In summary, under polluted conditions, ACI postpones the dissipation of Fog1 owing to these two feedbacks and generates PBL meteorological conditions that are more conducive to the formation of Fog2 than those prior to Fog1. These conditions promote the earlier formation of Fog2, further enhancing the two feedbacks and strengthening the ACI in Fog2. Our findings are critical for studying the interaction between aerosols, fog, and the PBL; moreover, they shed new light on ACI. [ABSTRACT FROM AUTHOR]

A field campaign in Liupan Mountains was carried out by the Weather Modification Center of the China Meteorological Administration to study the impact of terrain on precipitation in Northwest China. The vertical structures and microphysical characteristics of a mixed cloud and precipitation process, which means stratiform clouds with embedded convection, over three topographic positions of the Liupan Mountains, namely, the Longde (LD, located on the windward slope), Liupan (LP, located on the mountain top), and Dawan sites (DW, located on the leeward slope), are compared using measurements from ground-based cloud radar (CR), micro rain radar (MRR), and disdrometer (OTT). The 17 h process is classified into cumulus mixed (1149 min), shallow (528 min), and stratiform (570 min) cloud and precipitation stages. Among them, the vertical structures over the three sites are relatively similar in the third stage, while the differences, mainly in cloud-top heights (CTHs) and rain rates (Rs), are significant in the second stage due to the strong instability. Overall, the characteristics of higher concentrations and smaller diameters of raindrops are found in this study, especially at the LP site. Topographic forcing makes the microphysical and dynamic processes of mountaintop clouds and precipitation more intense. The updrafts are the strongest at the LP, caused by orographic uplifting, and the DW is dominated by the downdrafts due to the topography impact on the dynamic structure. Meanwhile, particle falling velocities (Vts) and downdrafts rapidly increase within 0.6 km near the ground over the LP, forming positive feedback, and the collision–coalescence process is dominant. [ABSTRACT FROM AUTHOR]

Increases anthropogenic emission poses a severe environmental problem in the North China Plain (NCP), China. The WRF/Chem model coupled with the local anthropogenic emissions is used to simulate a severe fog event occurring in NCP. Sensitivity studies with five different levels of anthropogenic emission are performed to examine the comprehensive impacts of aerosols on fog microphysical, macrophysical, radiative, and dynamical properties. Fog droplet number concentration, and liquid water content all increase nonlinearly, but effective radius decreases with increasing aerosol concentrations. Aerosols also extent the fog region, height and prolong the fog duration. Aerosol reveals distinct effects on thermodynamic and dynamical conditions during different stages of fog evolution: invigorate fog formation and development by radiation- cooling-induced condensation, enhancing latent heat, longwave-induced instability, and thus promoting turbulence, but delay fog dissipation by reducing surface solar radiation, surface sensible and latent heat fluxes and thus suppressing turbulence during the dissipation stage. [ABSTRACT FROM AUTHOR]