Browsing by Author "Yuan, Zhengxuan"

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Boundary layer perspective assessment of air pollution status in Wuhan city from 2013 to 2017
(Springer Nature Switzerland, 2019) Mbululo, Yassin; Qin, Jun; Yuan, Zhengxuan; Nyihirani, Fatuma; Zheng, Xiang
This study used daily air pollution data (PM2.5, PM10, SO2, NO2, CO, and O3) from nine monitoring stations inWuhan city to calculate the air quality index (AQI) from 2013 to 2017. Together with this data, L-band sounding data, ground meteorological data, and air mass back trajectories were also used to describe the dynamics of atmospheric boundary layer (ABL) during pollution process. Analysis of the results shows that, even though the city is still polluted, the number of polluted days was decreasing. Ranking the years in terms of pollution status shows that the year 2013 was the most polluted year while the year 2017 was the cleanest year. Average annual limit of PM10, PM2.5, and NO2 during these 5 years were 1.3~1.8, 1.5~2.7, and 1.2~1.5 times higher than the annual average acceptable limit, respectively. The average ratio of PM2.5/ PM10 for 5 years was 0.67 which signifies that a significant portion of PM2.5 accounted for the total mass of PM10. Moreover, the condition of ABL during the pollution process shows the dominance of strong ground inversion and weak to calm winds. These conditions are not favorable for horizontal and vertical mixing of air pollutants and prevent dilution of pollutants with clean air. Mean cluster analysis of air mass back trajectory shows that pollutants of local origin were more important than the trans-boundary movement of air pollutants. This indicates that the observed pollution in Wuhan was more of local origin
Characteristics of atmospheric boundary layer structure during pm2.5 and ozone pollution events in Wuhan, China
(MDPI, 2018) Mbululo, Yassin; Qin, Jun; Hong, Jun; Yuan, Zhengxuan
In this study, we investigated six air pollutants from 21 monitoring stations scattered throughout Wuhan city by analyzing meteorological variables in the atmospheric boundary layer (ABL) and air mass backward trajectories from HYSPLIT during the pollution events. Together with this, ground meteorological variables were also used throughout the investigation period: 1 December 2015 to 30 November 2016. Analysis results during this period show that the city was polluted in winter by PM2.5 (particulate matter with aerodynamics of less than 2.5 microns) and in summer by ozone (O3). The most polluted day during the investigation period was 25 December 2015 with an air quality index (AQI) of 330 which indicates ‘severe pollution’, while the cleanest day was 26 August 2016 with an AQI of 27 indicating ‘excellent’ air quality. The average concentration of PM2.5 (O3) on the most polluted day was 265.04 (135.82) g/m3 and 9.10 (86.40) g/m3 on the cleanest day. Moreover, the percentage of days which exceeded the daily average limit of NO2, PM10, PM2.5, and O3 for the whole year was 2.46%, 14.48%, 23.50%, and 39.07%, respectively, while SO2 and CO were found to be below the set daily limit. The analysis of ABL during PM2.5 pollution events showed the existence of a strong inversion layer, low relative humidity, and calm wind. These observed conditions are not favorable for horizontal and vertical dispersion of air pollutants and therefore result in pollutant accumulation. Likewise, ozone pollution events were accompanied by extended sunshine hours, high temperature, a calm wind, a strongly suspended inversion layer, and zero recorded rainfall. These general characteristics are favorable for photochemical production of ozone and accumulation of pollutants. Apart from the conditions of ABL, the results from backward trajectories suggest trans-boundary movement of air masses to be one of the important factors which determines the air quality ofWuhan.
Chemical composition and deposition fluxes of water-soluble inorganic ions on dry and wet deposition samples in Wuhan, China
(MDPI, 2019) Qin, Jun; Mbululo, Yassin; Yang, Muyi; Yuan, Zhengxuan; Nyihirani, Fatuma; Zheng, Xiang
Measurement of PM2.5 concentration, dry and wet deposition of water-soluble inorganic ions (WSII) and their deposition flux was carried out. During sampling, a total number of 31 samples of PM2.5, five wet deposition samples and seven dry deposition samples were collected. The analyses results showed that the average concentration of PM2.5 was 122.95 g/m3 whilst that of WSII was 51.63 g/m3, equivalent to 42% of the total mass of PM2.5. The correlation coefficients between WSII in samples of PM2.5 was significant (r = 0.50 and p-value of 0.0019). Ions of SO2􀀀 4 , NO􀀀 3 , Cl􀀀, and NH+ 4 were dominant in the entire samples (PM2.5, dry and wet depositions), nevertheless, the average concentration of both SO2􀀀 4 and Cl􀀀 were below the China environmental quality standard for surface water. The ratio of dominant anions in wet deposition (SO2􀀀 4 /NO􀀀 3 ) was 1.59, whilst that for dry deposition (SO2􀀀 4 /Cl􀀀) was 1.4, indicating that acidity was mainly derived from sulphate. In the case of dominant cations, the dry and wet deposition ratios (Ca2+/NH+ 4 ) were 1.36 and 1.37, respectively, suggesting the alkaline substances were mainly dominated by calcium salts. Days with higher recorded concentrations of PM2.5 were accompanied by dry and warm boundary layer structure, weak low-level wind and strong inversion layer.
The development of boundary layer structure index (BLSI) and its relationship with ground air quality
(2018) Zheng, Xiang; Qin, Jun; Liang, Shengwen; Yuan, Zhengxuan; Mbululo, Yassin
Ambient air quality monitoring data and radar tracking sonde data were used to study the atmospheric boundary layer structure (ABLS) and its changing characteristics over Wuhan. The boundary layer structure index (BLSI), which can effectively describe the ABLS, was accordingly developed and its ability to describe the near-surface air quality was analyzed. The results can be summarized as follows. (1) An analysis of the ABLS during seriously polluted cases revealed that the ABLS was usually dry and warm with a small ventilation index (VI); meanwhile, the ABLS during clean cases was usually wet and cold with a large VI. (2) The correlation between the air quality and BLSI at 100~300 m was good and passed the confidence level limit at 99%. Moreover, the correlation coefficient increased with the altitude at 10~250 m and showed a downward trend at 250~500 m. The correlation between the BLSI at 250 m and the ground air quality was the most significant (r = 0.312), indicating that the layer ranging from 0 to 250 m is essential for determining the ground air quality. (3) The BLSI considers both the vertical diffusion capability and horizontal removal capability of the atmosphere. Therefore, it is highly capable of describing the ABLS and the ground air quality
Impact of spring AAO on summertime precipitation in the north China part: observational analysis
(Korean Meteorological Society and Springer Nature, 2020) Yuan, Zhengxuan; Qin, Jun; Li, Shuanglin; Huang, Sijing; Mbululo, Yassin
By using the 160 stations monthly precipitation data and NOAA-ESRL reanalysis data, the effect of spring Antarctic Oscillation (AAO) on Summer Precipitation in North China (SPNC) is studied. The analysis results show that the positive phase of March AAO leads the easterly-southerly and ascend anomalies in the southern part of North China by affecting zonal wind, and meridional circulation in the Northern Hemisphere (NH) late summer, results in more SPNC. Key regions of Antarctic sea-ice may store preceding March AAO signal which is cross-seasonal sustained by the heat exchanging with underlying surface, and the Summer Antarctic Sea-ice Index (SSI) is defined. The upward and poleward propagations of the planetary wave are enhanced in the SouthernHemisphere (SH) high latitude, and there are significant anomalous variabilities of PSI over SH low latitude, high latitude and polar regions in the high SSI years and vice versa. The wave-current interaction weakens the descendingmotion over the Antarctic, strengthens the circumpolar westerlies and deepens the SH polar vortex (AAO positive phase) in the high SSI years and vice versa. The results of observation and CAM5 show that SSI is positively correlated with indices of Quasi-Biennial Oscillation (QBOI). In QBO westerly phase, the SPNC anomaly in the high SSI years shows out of phase with that in the low SSI years. In the low SSI years, Rossby wave has the characteristics of anomalous southward propagation which corresponds to the abnormal weakening of ascend motion (the decrease of precipitation) over North China. Accompanied with a significant PSI anomaly, there exists a region of weak to strong wave energy over North China which is regarded as the source region of wave energy in both high and low SSI years of QBO westerly phase. But in QBO easterly phase, SPNC anomaly is not significant. QBO may be a bridge that connecting the signals of zonal wind anomalies in the stratosphere of the SH and the NH.
The relationship between atmospheric circulation, boundary layer and near-surface turbulence in severe fog-haze pollution periods
(2019) Yuan, Zhengxuan; Jun, Qin; Zheng, Xiang; Mbululo, Yassin
Wind, temperature, relative humidity and aerosol mass concentration were monitored simultaneously in Wuhan, China. Several observations were found after analyzing the physical fields of these data. It was obvious that weak pressure and saddle patterns occurred during fog-haze episodes. An inversion layer occurred before heavy fog haze events and became thicker during fog-haze events. The boundary layer structure index was relatively higher during fog-haze days and had a significant negative correlation with the planetary boundary layer height and turbulence parameters. Wind speeds were generally less than 5 m/s and rarely exceeded this speed on the selected polluted days. Turbulence variation characteristics had special representations, especially before fog haze events. Turbulence intensities always reached abnormal peak values before fog-haze processes, while the intensities remained steady before and during pollution processes with low relative humidity. Both the turbu lence kinetic energy and momentum flux decreased to near zero before heavy fog-haze processes. Momentum flux often presented abnormal disturbances before heavy fog-haze processes. These disturbances were often in an active phase before and during pollution processes with low relative humidity, a situation that is not similar to fog-haze events that maintained high relative humidity. There was a feedback mechanism between solar radi ation and aerosol mass concentration, and the occurrence of turbulence anomalies may be related to the regu lation of atmospheric circulation by wave-flow interaction. The results presented in this study suggest that the turbulence parameters, which display anomalies before the occurrence of heavy fog-haze processes under the background of inversion layers and stable atmospheric patterns, can serve as a means of predicting disastrous weather conditions such as fog-haze pollution