[Spatio-temporal Distribution Characteristics of Secondary Aerosol in Beijing-Tianjin-Hebei Urban Agglomeration in Winter].

The secondary component is an important factor causing PM2.5 pollution in the Beijing-Tianjin-Hebei urban agglomeration in winter. In this study, the CO tracer method was used to estimate the secondary PM2.5 concentration of the Beijing-Tianjin-Hebei urban agglomeration in the winter of 2017-2021. The temporal and spatial distribution characteristics were analyzed, and the influencing factors of regional secondary PM2.5 were discussed. The results showed that the decreasing trend of PM2.5 concentration in the Beijing-Tianjin-Hebei Region in the winter of 2017-2021 was obvious, and the cities with the largest decline were located in the central and southern part of Hebei Province, mainly contributed by primary PM2.5. There was a good correlation between secondary PM2.5 and PM2.5 in all cities of the Beijing-Tianjin-Hebei urban agglomeration, and the proportion of secondary PM2.5 in Beijing and Tianjin was significantly higher than that in other cities. With the aggravation of pollution degree, the mass concentration of primary PM2.5 and secondary PM2.5 increased in varying degrees, and the proportion of secondary PM2.5 increased significantly. Compared with the direct measurement results, the estimated value obtained by this method was lower as a whole. The selection of appropriate primary aerosol reference value was the key to improving this method and estimating the secondary PM2.5 concentration.

Land-sea difference of the planetary boundary layer structure and its influence on PM2.5 - Observation and numerical simulation.

A comprehensive set of observations were collected along a sea-coast-inland area. According to these observations, the planetary boundary layer heights (PBLH) during winter and summer for sampling locations in sea, coast, and inland areas were 737 m, 483 m and 372 m, and 450 m, 645 m and 646 m, respectively. Little seasonal difference was observed for the daily variation of sensible heat flux over the sea, with the maximum of 55 W/m2 at 12:00 in winter and 27 W/m2 at 13:00 in summer. The duration of sea breeze was ∼10 h in summer and only 3 h in winter, extended to inland area ∼ 50 km and upward 920 m vertically. PM2.5 at coastal area was about 5 µg/m3 higher than inland during summer afternoon. Over the sea, PM2.5 mainly concentrated below 200 m in winter, increased with height at night and decreased with height in the morning below 300 m in summer. A typical land-sea breeze episode was analyzed with observed and simulated result. According to the observed data, the sea breeze extended to inland ∼50 km and upward 300 m vertically. From the simulating result, there was a clear temperature gradient between sea and land from surface to 400 m, when influenced by the sea breeze, the wind and temperature profiles showed different characteristics, and forming a high concentration center of PM2.5 at 300 m. These results provide insights into the structure of land-sea planetary boundary layer, and provide support for the prediction of heavy pollution episode.

[Impacts of Changes in Meteorological Conditions During COVID-19 Lockdown on PM2.5 Concentrations over the Jing-Jin-Ji Region].

The Chinese government triggered the immediate implementation of a lockdown policy in China following the outbreak of the COVID-19 pandemic, leading to drastic decreases in air pollutant emissions. However, concentrations of PM2.5 and other pollutants increased during the COVID-19 lockdown over the Jing-Jin-Ji region compared with those averaged over 2015-2019, and two PM2.5 pollution events occurred during the lockdown. Using the ERA5 reanalysis data, we found that the Jing-Jin-Ji region during the COVID-19 lockdown was characterized by higher relative humidity, lower planetary boundary layer height, and anomalous updraft. These conditions were favorable for condensation and the secondary formation of aerosols and prevented turbulent diffusion of pollutants. Furthermore, we conducted sensitivity tests using the WRF-Chem model and found that ρ(PM2.5) increased by 20-55 µg·m-3(60%-170%) over the middle region of Jing-Jin-Ji during the COVID-19 lockdown due to changes in meteorological conditions. Furthermore, the enhanced aerosol chemistry and unfavorable diffusion conditions were identified as the key factors driving increases in PM2.5 concentrations during the lockdown. Planetary boundary layer height and relative humidity may become the important factors in forecasting PM2.5 pollution events over the Jing-Jin-Ji region under the background of emission reduction.

[Exploring Formation of Ozone in Typical Cities in Beijing-Tianjin-Hebei Region Using Process Analysis].

As the problem of O3 pollution in the Beijing-Tianjin-Hebei region becomes increasingly prominent, it is of great significance to explore and analyze the ozone variation characteristics and causes of the pollution process in the Beijing-Tianjin-Hebei region for regional air pollution prevention and control. The observations in this study showed that high O3 concentration in spring and summer of the Beijing-Tianjin-Hebei region was higher in the south and lower in the north. The high O3 concentration in Beijing, Tianjin, and Shijiazhuang was often accompanied by the influence of southern wind. Based on WRF-Chem model simulation and process analysis technology, the variation characteristics and causes of O3 in The Beijing-Tianjin-Hebei region in 2019 were deeply analyzed. The diurnal variations in chemical processes, vertical mixing, and transportation in typical cities showed distinct seasonal variations. In summer afternoons, chemical processes were the main source of O3 concentration increase in each city. Vertical mixing resulted in an increase in O3 concentration in Tianjin and Shijiazhuang but a decrease in Beijing. Tianjin and Shijiazhuang had a net output, whereas Beijing had a net inflow. In the polluted O3 process, the chemical process dominated the afternoon O3 concentration increasing in Beijing and Shijiazhuang, whereas vertical mixing dominated in Tianjin. In addition, there was a net input of O3 in Beijing and Shijiazhuang and a net output of O3 in Tianjin. In the clean O3 process, vertical mixing dominated the increase in O3 concentration in Beijing and Shijiazhuang in the afternoon, whereas in Tianjin it was chemical processes. At the same time, the net output of O3 existed in all three cities.

[Analysis of Change and Driving Factors of PM2.5 Mass Concentration in Tianjin from 2000 to 2020].

Based on real-time tracking data, PM2.5 mass concentration, and meteorological observations of the Tianjin Meteorological Bureau and the Ecological Environment Bureau, combined with the fine particle meteorological condition diffusion index constructed using the environmental model, the change and driving factors of the PM2.5 mass concentration in Tianjin from 2000 to 2020 were studied to analyze the impact of meteorology on the atmospheric environment. The study showed that change in PM2.5 mass concentration in Tianjin took place in three stages from 2000 to 2020; the first stage showed a continuous increase from 2000 to 2007. The rapid increase in emissions in this stage was the dominant factor, and its effect was four times that of the annual fluctuation in meteorological conditions. The second stage was from 2007 to 2013, in which the PM2.5 mass concentration fluctuated, with two peak years (2007 and 2013). The emissions were stable in this stage. The annual fluctuation of meteorological conditions had an important influence on the annual fluctuation in PM2.5 mass concentration. The third stage was from 2013 to 2020; the PM2.5 mass concentration decreased rapidly, and the decline in emissions was decisive, which reduced the PM2.5 mass concentration by 40% to 50%. The improvement in the meteorological diffusion conditions also provided a positive contribution, which reduced the PM2.5 mass concentration by approximately 10%. Based on the analysis of the data over the past 20 years, the annual variation in atmospheric diffusion conditions caused by the annual variation in meteorological conditions was periodic, with trough values from 2003 to 2004 and 2013 to 2015 and peaks from 2008 to 2010 and 2018 to 2020; the distance between peaks and valleys was approximately 11 years. It was estimated that the next atmospheric diffusion condition valley stage will occur circa 2025. The average intensity of the annual fluctuation in atmospheric diffusion conditions caused by the annual variation in meteorological conditions was 4%, which can explain 25%-50% of the annual variation in PM2.5 mass concentration over the past 20 years, with a difference between peaks and valleys of 16%. The periodic fluctuations in meteorological diffusion conditions have an important impact on the future PM2.5 target setting and corresponding measures design.

[Impact of Air Humidity on PM2.5 Mass Concentration and Visibility During Winter in Tianjin].

Air humidity is a key meteorological factor in regulating visibility changes and haze episodes. Based on multi-year historical data of PM2.5 mass concentration, visibility, relative humidity(RH), and specific humidity(q) during winter in Tianjin, the impact of air humidity on PM2.5 mass concentration and visibility was investigated. Between 2015 and 2020, the PM2.5 mass concentration showed an overall decline of 28.0%. The frequency of visibility above 10 km significantly increased between 2015 and 2018, indicating an improvement in visibility during this period. However, the visibility deteriorated again in the winter of 2019 and 2020, with a decreased frequency of visibility above 10 km. Specifically, the mean RH in January and February in 2020 of Tianjin reached 63% and 67%, respectively, which were higher than the historical 30-year average for the same period. The frequency of extremely low visibility(lower than 2 km) rebounded to a level equivalent to that during the winter of 2016. The enhanced air humidity visually obscured the reduction effect of PM2.5. For Tianjin, the external sources of water vapor are southwestern and eastern transport. Particularly, water vapor transported from eastern Bohai Bay(59%) is significantly greater than that from southwestern direction(25%). However, the eastern air mass is generally clean, hence, although the condensed water may increase the PM2.5 mass concentration in the humid air, the eastern air mass affects visibility to a greater extent. On the other hand, the haze episodes during winter frequently occurred when the southwestern wind dominated and specific humidity was greater than 2.0 g·kg-1, with a frequency of 83.6%. In a short period of time, the variation of specific humidity is less significant than RH, therefore, the relationship between specific humidity and PM2.5 mass concentration or air quality can be utilized to predict the occurrence of haze episodes and pollution during winter. When the average RH is higher than 80% or the mean specific humidity is greater than 3.0 g·kg-1, the frequency of PM2.5 mass concentration greater than 75 µg·m-3 is 78% and 80%, respectively. For the air quality forecast during winter, weather conditions with specific humidity greater than 3.0 g·kg-1 should be carefully monitored.

[Heavy Pollution Episode in Tianjin Based on UAV Meteorological Sounding and Numerical Model].

Pollution occurs in the boundary layer, and the thermal and dynamic vertical structure of the boundary layer has a significant influence on the formation of heavy pollution episodes. Based on unmanned aerial vehicle (UAV) sounding, ground-based remote sensing and numerical modeling, this paper analyzes the vertical structure of the boundary layer and the causes of pollution during the heavy pollution episode in Tianjin from January 10 to 15, 2019, with a view to strengthening the understanding of the influence law of boundary layer processes on heavy pollution in northern coastal cities and improving the accuracy of weather forecasts and heavy pollution warnings. The results show that atmospheric temperature stratification had a significant influence on the formation, persistence, and dissipation of heavy pollution episodes. During an episode, accompanied by the development and dissipation of the inversion layer, a high PM2.5 concentration area developed to the upper atmosphere with a height of over 300 m in the daytime and compressed to the ground at night with a height about 100 m. When fog appeared and continued in the daytime, the vertical structure characteristics of the boundary layer changed. A temperature inversion above the fog restrained the diffusion of pollutants to the upper air and made the contribution of turbulence vertical mixing process decrease significantly in the daytime, leading to the persistence and development of heavy pollution near the surface. Regional pollution transport accounted for 66.6% during the episode, which was closely related to regional pollution transport. Regional pollution transport mainly appeared at the top of the boundary layer and above the fog inversion layer where high wind speeds occurred. Pollutants were transported to the ground by a sinking motion as the boundary layer and fog height changed. This is how regional pollution transport occurred when Tianjin was controlled by a weak high pressure field in the north. The vertical structure of the boundary layer also affected the improvement of air quality by cold air. The strong temperature inversion at the top of the fog resulted in the failure of the cold air to transmit to the ground through turbulent shear stress in the S3 stage. There was an obvious difference in wind speed between the upper and lower air. The influence of cold air on the ground was delayed, and the effect of it was weakened. Thus, the heavy pollution episode could not be alleviated completely.

Evaluation and improvement study of the Planetary Boundary-Layer schemes during a high PM2.5 episode in a core city of BTH region, China.

Accurate depictions of planetary boundary layer (PBL) processes are important for both meteorological and air quality simulations. This study examines the sensitivity of the model performance of the Weather Research Forecasting model coupled with Chemistry (WRF-Chem) to five different PBL schemes and further to different turbulence parameters for the simulation of a winter haze episode in Tianjin, a core city of the Beijing-Tianjin-Hebei (BTH) region in China. To provide a direct and comprehensive evaluation of the PBL schemes, measurements from multiple instruments are employed, including both meteorological and air quality quantities from near-surface observations, vertical sounding measurements and ceilometer data. Moreover, the vertical distribution of the turbulent exchange coefficient is derived from sounding measurements and is utilized to evaluate the PBL schemes. The results suggest that the Mellor-Yamada-Janjic (MYJ) scheme is generally statistically superior to the other schemes when comparing observations. However, considerable model discrepancies still exist during certain stages of this haze episode, which are found to be predominantly due to the deficiency of MYJ in distinguishing the intensity of turbulent mixing between different pollution stages. To improve the model performance, this study further tests the impact of different closure parameters on the simulation of winter haze episode. In the MYJ scheme, the closure parameters play a key role in the turbulent mixing within the PBL and therefore in haze simulations. Sensitivity experiments with different MYJ parameters confirm this diagnosis and suggest that a larger Prandtl number (Pr), rather than the default value in the MYJ formulation, may be more applicable for haze simulations under stable atmospheric conditions.