With the rapid development of economy and urbanization acceleration, ozone (O₃) pollution has become the main factor of urban air pollution in China after particulate matter. In this study, 90th percentile of maximum daily average (MDA) 8 h O₃ (O₃-8h-90per) and PM_2.5 data from the Tracking Air Pollution in China (TAP) dataset were used to determine the mean annual, seasonal, monthly, and interannual distribution of O₃-8h-90per and PM_2.5 concentrations in Northeast China (NEC). The O₃-8h-90per concentration was highest in Liaoning (>100 μg/m³), whereas the highest PM_2.5 concentration was observed mainly in urban areas of central Liaoning and the Harbin–Changchun urban agglomeration (approximately 60 μg/m³). The O₃-8h-90per concentrations were highest in spring and summer due to more intense solar radiation. On the contrary, the PM_2.5 concentration increased considerably in winter influenced by anthropogenic activities. In May and June, the highest monthly mean O₃-8h-90per concentrations were observed in central and western Liaoning, about 170–180 μg/m³, while the PM_2.5 concentrations were the highest in January, February, and December, approximately 100 μg/m³. The annual mean O₃-8h-90per concentration in NEC showed an increasing trend, while the PM_2.5 concentration exhibited an annual decline. By 2020, the annual mean O₃-8h-90per concentration in southern Liaoning had increased considerably, reaching 120–130 μg/m³. From the perspective of city levels, PM_2.5 and O₃-8h-90per also showed an opposite variation trend in the 35 cities of NEC. The reduced tropospheric NO₂ column is consistent with the decreasing trend of the interannual PM_2.5, while the increased surface temperature could be the main meteorological factor affecting the O₃-8h-90per concentration in NEC. The results of this study enable a comprehensive understanding of the regional and climatological O₃-8h-90per and PM_2.5 distribution at distinct spatial and temporal scales in NEC.

The characteristics of turbulence in the planetary boundary layer (PBL) and the aerosol optical and radiative properties during haze and haze–fog mixed episodes on 22–27 January 2021, in Shenyang, a provincial city in Northeast China, were analyzed using meteorological and aerosol observations. During the haze episode, the hourly mean PM_2.5 concentration reached a maximum of 337 µg m⁻³ and visibility decreased to 1.6 km. The PM_2.5 concentration decreased gradually during the haze–fog mixed episode as a result of the scavenging effects of fog, but visibility mostly remained below 1 km owing to high ambient relative humidity (>90%). During the haze–fog mixed episode, an increasing proportion of PM_2.5 led to a higher ratio of the backward to the total scattering coefficient. As fog occurred, downward shortwave radiation arriving at the surface was significantly reduced, and upward longwave radiation increased and almost equaled the downward longwave radiation, which can be used as a good indicator for distinguishing haze and fog. Mechanical turbulence was weak during both episodes, and latent heat flux varied within a wider range during the haze–fog mixed episode. The PBL dynamic structure affected the vertical distribution of aerosols/fog droplets. Aerosol-rich layers appeared at altitudes below 0.5 km and above 0.6 km during the haze episode. The elevated aerosol layer was related to the aerosol transport from upstream polluted areas caused by strong upper-level turbulence, and it began to mix vertically after sunrise because of convective turbulence. Aerosols and fog droplets were mostly trapped in a shallower PBL with a height of 0.2–0.4 km during the haze–fog mixed episode because of weaker turbulence.

The influence of urbanization on the frequent winter aerosol pollution events in Northeast China is not fully understood. The Weather Research and Forecasting Model with Chemistry (WRF–Chem) coupled with urban canopy (UC) models was used to simulate the impact of urbanization on an aerosol pollution process in the Central Liaoning city cluster (CLCC), China. To investigate the main mechanisms of urban expansion and UC on the winter atmospheric environment and the atmospheric diffusion capacity (ADC) in the CLCC, three simulation cases were designed using land-use datasets from different periods and different UC schemes. A comparative analysis of the simulation results showed that the land-use change (LU) and both LU and UC (LUUC) effects lead to higher surface temperature and lower relative humidity and wind speed in the CLCC by decreasing surface albedo, increasing sensible heat flux, and increasing surface roughness, with a spatial distribution similar to the distribution of LU. The thermal effect leads to an increase in atmospheric instability, an increase in boundary layer height and diffusion coefficient, and an increase in the ADC. The LU and LUUC effects lead to a significant decrease in near-surface PM_2.5 concentrations in the CLCC due to changes in meteorological conditions and ADC within the boundary layer. The reduction in surface PM_2.5 concentrations due to the LU effect is stronger at night than during daytime, while the LUUC effect leads to a greater reduction in surface PM_2.5 concentrations during the day, mainly due to stronger diffusion and dilution caused by the effect of urban turbulence within different levels caused by the more complex UC scheme. In this study, the LU and LUUC effects result in greater thermal than dynamic effects, and both have a negative impact on surface PM_2.5 concentrations, but redistribute pollutants from the lower urban troposphere to higher altitudes.