Impacts of synoptic circulation on surface ozone pollution in a coastal eco-city in Southeastern China during 2014-2019.

The coastal eco-city of Fuzhou in Southeastern China has experienced severe ozone (O3) episodes at times in recent years. In this study, three typical synoptic circulations types (CTs) that influenced more than 80% of O3 polluted days in Fuzhou during 2014-2019 were identified using a subjective approach. The characteristics of meteorological conditions linked to photochemical formation and transport of O3 under the three CTs were summarized. Comprehensive Air Quality Model with extensions was applied to simulate O3 episodes and to quantify O3 sources from different regions in Fuzhou. When Fuzhou was located to the west of a high-pressure system (classified as "East-ridge"), more warm southwesterly currents flowed to Fuzhou, and the effects of cross-regional transport from Guangdong province and high local production promoted the occurrence of O3 episodes. Under a uniform pressure field with a low-pressure system occurring to the east of Fuzhou (defined as "East-low"), stagnant weather conditions caused the strongest local production of O3 in the atmospheric boundary layer. Controlled by high-pressure systems over the mainland (categorized as "Inland-high"), northerly airflows enhanced the contribution of cross-regional transport to O3 in Fuzhou. The abnormal increases of the "East-ridge" and "Inland-high" were closely related to O3 pollution in Fuzhou in April and May 2018, resulting in the annual maximum number of O3 polluted days during recent years. Furthermore, the rising number of autumn O3 episodes in 2017-2019 was mainly related to the "Inland-high", indicating the aggravation of cross-regional transport and highlighting the necessity of enhanced regional collaboration and efforts in combating O3 pollution.

Meteorological influence on persistent O3 pollution events in Wuxi in the Yangtze River Delta, China.

The number of O3 pollution days indicates an overall increasing trend over 2014-2021 in Wuxi in the Yangtze River Delta, with the pollution concentrations of MDA8-O3 between 186 and 200 µg·m-3. Specifically, a total of 62 POPEs (persistent O3 pollution events), defined as episodes with 3 or more continuous O3 pollution days, were observed for the 8 years. Using a multi-linear regression model, we find that the meteorology can explain approximately 56.5 % of the O3 variations for the 8 years in Wuxi, with temperature being the most crucial meteorological factor, followed by relative humidity (RH) and wind speeds. High temperature, low RH, low wind speeds and downward airflows significantly correlate with POPE-O3 changes. Three types of synoptic circulations are further identified during the POPEs from 2014 to 2021 by the T-mode (T-PCA) classification method. The primary circulation patterns governing the interannual changes of POPEs are characterized by the largest positive anomalies of temperature and planetary boundary layer (PBL) height; moreover, a distinct vertical mixing process is observed with uplifting airflows in the convective PBL during the afternoon and sinking airflows in the stable PBL at night, which is incredibly conducive to the downward transport of O3 after its upward delivery during daytime and substantially contributes to midnight O3 at the surface. The other two circulation types are associated with uniform descending flows in the PBL; as a result, surface O3 accumulates only near the ground and decreases significantly at night due to the titration effect. This study systematically highlights the influence of critical meteorological factors regulated by different synoptic circulations on the POPE in Wuxi, which provides a scientific basis for pollution control and prediction.

Multi-scale analysis of the impacts of meteorology and emissions on PM2.5 and O3 trends at various regions in China from 2013 to 2020 1: Synoptic circulation patterns and pollution.

A multiscale analysis of meteorological trends was carried out to investigate the impacts of the large-scale circulation types as well as the local-scale key weather elements on the complex air pollutants, i.e. PM2.5 and O3 in China. As the first paper in the series, the relationship between synoptic circulation patterns and pollution was investigated. Six types of circulation patterns are defined and clustered to correlate with the observed pollutant levels, resulting in the identification of the impact similarity and difference of circulations on PM2.5 and O3 for three regions in China, i.e., the BTH (Beijing, Tianjin and Hebei), YRD (Yangtze River Delta) and PRD (Peral River Delta), from 2013 to 2020. It is found that the six clustered circulation patterns were able to classify the circulation patterns that influence the pollutants and yield significant correlations with O3 and PM2.5 in three regions. The major circulation patterns governing the heavy PM2.5 and O3 were identified separately for each region and found to show inter-annual variabilities. Composite analysis indicated that there were some circulation patterns that caused the dual-highs of PM2.5 and O3 with about 13%, 8% and 3% occurrences during the period of 2013 to 2020 in Beijing, Shanghai and Guangzhou, respectively. The key weather elements for each type of circulation pattern were also identified. A detailed study of the impacts of key weather elements and emissions on the PM2.5 and O3 trends will accompany this paper (Gong et al., 2022).

The dynamic multi-box algorithm of atmospheric environmental capacity.

It is very important for air pollution prevention and control to accurately quantify atmospheric environment capacity (AEC) in the planetary boundary layer (PBL). This study developed a high temporal-resolution dynamic multi-box algorithm to estimate PM2.5 AEC with a PBL ceilometer and Doppler wind profile lidar in Beijing City. Compared with the traditional A-value method, two primary improvements are introducing the time coefficient and vertical multi-box assumption into the original box model. The algorithm can accurately calculate the PM2.5 AEC under different circulation patterns and predict the short-time dynamic change of AEC. The results show that the time coefficient effectively reduced the estimation errors when the initial PM2.5 concentration, horizontal wind speed and PBL heights change greatly with time, such situation is consistent with most circulation patterns. And the improvement of multi-box model is much more remarkable when the PM2.5 concentration and horizontal wind change greatly in the vertical direction, such as A, NE and W type circulations. The ideal AEC under polluted circulation patterns won't increase infinitely with wind speed and PBL height, generally less than 30 t/h. The horizontal advection has a much greater effect on expanding the capacity of PM2.5 than the vertical diffusion under clean circulation patterns, and the maximum value of ideal AEC can reach 50 t/h. The positive residual AEC under clean circulations indicates surplus capacity for PM2.5 because of vigorous turbulences, while weak diffusion and ventilation conditions under polluted circulations lead to negative residual AEC and insufficient capacity of atmosphere.

Seasonal changes in the recent decline of combined high PM2.5 and O3 pollution and associated chemical and meteorological drivers in the Beijing-Tianjin-Hebei region, China.

China suffers from combined air pollution (CAP) comprising dual high O3 and PM2.5, particularly in the Beijing-Tianjin-Hebei (BTH) region, which is an urban agglomeration in the North China Plain. To characterize the seasonal changes in regional CAP, 82 CAP days were identified during the study period from 2015 to 2019 with the co-occurring pollution of O3 and PM2.5 in the BTH. It is found that CAP seasonality has undergone distinct changes with a declining trend in the interannual variations in CAP over recent years. It is also revealed that the monthly CAP peaks have recently shifted from summer to early spring (March and April), indicating seasonal changes in CAP in the BTH. Furthermore, the of chemical and meteorological roles in CAP changes was investigated using environmental and meteorological observation data. The recent reduction in PM2.5 and O3 concentrations had enhanced O3 production and atmospheric oxidizability, thereby causing increments in secondary PM2.5 proportion. The interaction between O3 and PM2.5 was responsible for changing the CAP of dual high O3 and PM2.5 to the transition/spring season in the context of mitigation of air pollutant emissions. Furthermore, principal component analysis in the T-mode (T-PCA) was applied to identify four synoptic circulation patterns that regulate CAP occurrence. The results show that the CAP occurrence was regulated by the dominant patterns of synoptic circulation in the BTH. Warm temperature and strong downward ultraviolet radiation anomalies were observed in the BTH, indicating the importance of meteorological drivers in O3 photochemical production on the CAP. The frequency of key synoptic circulation patterns during the spring season increased annually, thereby inducing seasonal changes in the atmospheric environment with CAP in the BTH in recent years.

Black carbon in Xiamen, China: Temporal variations, transport pathways and impacts of synoptic circulation.

Black carbon (BC) plays a vital role in atmospheric environment and climate change. Temporal variations and transport pathways of BC in Xiamen, China with the impacts of synoptic circulation were investigated in 2014 with Aethalometer. Annual mean BC concentration was 4270 ng m-3. BC exhibited clear diurnal (seasonal) variations, with the maximum of 6182 (4755) ng m-3 at 6:00 (in spring) and minimum of 2847 (3774) ng m-3 at 13:00 (in summer). Conditional probability function analysis indicated that high BC concentrations were associated with northwesterly winds with low wind speed. Air masses originating from the East China Sea and passing along with East China Coast had the highest BC concentrations. Potential source contribution function and concentration weighted trajectory analysis suggested that major sources for BC included the surrounding region, southwestern Fujian and eastern Guangdong to the southwest, Hubei, Hunan and Jiangxi to the northwest, the East China Sea and the South China Sea. Of the nine synoptic circulation patterns, three cyclone-related patterns were associated with low BC concentrations and small biomass burning (BCbb) contributions. Of the six anticyclone-related patterns, the three cold-high circulations around winter were associated with moderate BC concentrations and large BCbb contributions. The two cold-high patterns in spring and autumn were associated with high BC concentrations and small BCbb contributions, while the warm-high pattern was associated with moderate BC concentration and small BCbb contribution. The findings provide insights into the transport mechanisms of BC with the impacts of synoptic pattern in China.

The impact of synoptic patterns on summertime ozone pollution in the North China Plain.

Surface ozone pollution is a challenging environmental issue in most parts of China. In particular, the North China Plain (NCP) region suffers from the severest ozone pollution throughout the country. In addition to the emission of precursors, ozone concentration is closely related to meteorological conditions resulting from regional atmospheric circulation. In this study, we investigate the relationship between synoptic patterns and summertime ozone pollution in the NCP using the objective principal component analysis in T-mode (T-PCA) classification method. Four dominant synoptic patterns are identified during the summers of 2014-2018. The heaviest ozone pollution is found to be associated with a high pressure anomaly over the Northwest Pacific and a distinct low pressure center in Northeast China. The southwesterly wind surrounding the low pressure center brings dry, warm air from inland South China, resulting in a high temperature, low humidity environment in the NCP, which favors the chemical formation of surface ozone. Locally, this type is associated with a moderate planetary boundary layer height (PBLH) of ~860 m and a stronger warm anomaly within the boundary layer than the upper level. We also notice a non-linear relationship between surface ozone concentration and the PBLH, i.e., ozone concentration first increases with PBLH till ~0.9 km, and then remains stable. This initial increase may relate to enhanced mixing with upper levels where ozone concentration is typically higher than that near the surface. However, when PBLH further increases, this downward mixing effect is balanced with the stronger upward turbulent mixing so that surface ozone shows little change. The synoptic patterns identified here, however, is unlikely responsible for the observed increasing trend in ozone concentration over the NCP region. Our study sheds light on the meteorological contribution to surface ozone pollution in North China and provides a reference for the pollution control and prediction.

The covariance of air quality conditions in six cities in Southern Germany - The role of meteorology.

This paper analyzed air quality in six cities in Southern Germany (Ulm, Augsburg, Konstanz, Freiburg, Stuttgart and Munich), in conjunction with the prevailing synoptic conditions. Air quality was estimated through the calculation of a daily Air Stress Index (ASI) constituted by five independent components, each one expressing the contribution of one of the five main pollutants (PM10, O3, SO2, NO2 and CO) to the total air stress. As it was deduced from ASI components, PM10 from combustion sources and photochemically produced tropospheric O3 are the most hazardous pollutants at the studied sites, throughout cold and warm periods respectively, yet PM10 contribute substantially to the overall air stress during both seasons. The influence of anticyclonic high pressure systems, leading to atmospheric stagnation, was associated with increased ASI values, mainly due to the entrapment of PM10. Moderate air stress was generally estimated in all cities however a cleaner atmosphere was detected principally in Freiburg when North Europe was dominated by low pressure systems. Daily events of notably escalated ASI values were further analyzed with backward air mass trajectories. Throughout cold period, ASI episodes were commonly related to eastern airflows carrying exogenous PM10 originated from eastern continental Europe. During warm period, ASI episodes were connected to the arrival of regionally circulated air parcels reflecting lack of dispersion and accumulation of pollutants in accordance with the synoptic analysis.

The impact of synoptic circulation on air quality and pollution-related human health in the Yangtze River Delta region.

PM2.5 and O3 pollution are of concern for the Yangtze River Delta (YRD) region due to their adverse impact on human health. In conjunction with a complex distribution of emission sources, the synoptic circulation conditions control the temporal and spatial variability of air pollution levels and hence the pollution-related health burdens. In this study, a long-term synoptic circulation catalogue is developed by applying the automated Lamb weather type method to the ECMWF mean sea level pressure reanalysis for the YRD region during 2013-2016. Ten typical circulation types are examined in relation to the transport pathways and diffusion conditions, and then multi-site surface observations of PM2.5 and O3 are composited for different circulation conditions. The results show that each circulation type is characterized with distinct air mass origin, diffusion condition and air quality level. The anticyclonic type (Type A) corresponds to the highest regional PM2.5 concentration (68.5µg/m3) due to the subsidence flow and long-range transport, while the westerly types (Types SW, W and NW) correspond to the higher regional maximum daily 8-h running average O3 (MDA8 O3) concentration (>100µg/m3) due to favorable local meteorological conditions. Regional transport causes an east-high and west-low PM2.5 distribution in westerly types but a west-high and east-low PM2.5 distribution in easterly types (Types SE, E and NE). In contrast, nearly all the types show an east-high and west-low O3 distribution, suggesting the predominated impacts of precursor emissions. By using established exposure-response functions, the health impact assessment (HIA) shows that Type W poses the greatest public health risk with mean daily excess mortality of 77.3 (95% CI: 61.9, 92.6) deaths and O3 pollution accounts for approximately 70% of this health burden.

What are the most fire-dangerous atmospheric circulations in the Eastern-Mediterranean? Analysis of the synoptic wildfire climatology.

Wildfire management is closely linked to robust forecasts of changes in wildfire risk related to meteorological conditions. This link can be bridged either through fire weather indices or through statistical techniques that directly relate atmospheric patterns to wildfire activity. In the present work the COST-733 classification schemes are applied in order to link wildfires in Greece with synoptic circulation patterns. The analysis reveals that the majority of wildfire events can be explained by a small number of specific synoptic circulations, hence reflecting the synoptic climatology of wildfires. All 8 classification schemes used, prove that the most fire-dangerous conditions in Greece are characterized by a combination of high atmospheric pressure systems located N to NW of Greece, coupled with lower pressures located over the very Eastern part of the Mediterranean, an atmospheric pressure pattern closely linked to the local Etesian winds over the Aegean Sea. During these events, the atmospheric pressure has been reported to be anomalously high, while anomalously low 500hPa geopotential heights and negative total water column anomalies were also observed. Among the various classification schemes used, the 2 Principal Component Analysis-based classifications, namely the PCT and the PXE, as well as the Leader Algorithm classification LND proved to be the best options, in terms of being capable to isolate the vast amount of fire events in a small number of classes with increased frequency of occurrence. It is estimated that these 3 schemes, in combination with medium-range to seasonal climate forecasts, could be used by wildfire risk managers to provide increased wildfire prediction accuracy.