Mingxu Liu

Impacts of the Decadal Urbanization on Thermally Induced Circulations in Eastern China

AbstractSignificant urbanization has occurred in the Yangtze River Delta region of eastern China, which exerts important effects on the local thermally induced circulations through regulating the heat flux and thermal structure. Previous studies lack a correct representation of the seasonal vegetation phenology associated with urban expansion, and therefore it is difficult to accurately describe the land–atmosphere coupling. In this study, high-resolution Moderate Resolution Imaging Spectroradiometer (MODIS) observations are used to describe the changes in land surface characteristics, including land-cover type, green vegetation fraction, and leaf area index with the Weather Research and Forecasting Model. The use of MODIS satellite observations provides a clear improvement in model performance when compared with ground-based measurements. A typical urban heat island is generated around Shanghai, Wuxi–Suzhou–Yangzhou, and cities along the Yangtze River and Hangzhou Bay, which subsequently modifies the loc...

Impacts of decadal variations in natural emissions due to land-cover changes on ozone production in southern China

The decadal variations in emissions of high-reactivity biogenic volatile organics (BVOCs), as a result of land-cover changes, could significantly impact ozone (O3) production. In this study, the Weather Research and Forecasting/Chemistry (WRF/Chem) modelling system, coupled with dynamic vegetation data sets derived from Moderate Resolution Imaging Spectroradiometer (MODIS, 2001–2012) and Advanced Very High Resolution Radiometer (AVHRR, early 1990s) measurements, were used to investigate the impacts of land-cover changes on natural emissions, and consequently O3 production, in the Pearl River Delta (PRD) region of southern China over the past two decades. Model results indicate that BVOC emissions were highly dependent on forest area. The total BVOC emissions in the modelling domain increased by a factor of two due to afforestation since the early 1990s, declined slowly (−5.8% yr−1) until 2006 and then increased continuously (+9.1% yr−1) to 2012. The decadal variations in BVOC emissions have complex implications for summer O3 production in PRD, depending on the chemical regimes and prevailing winds. The impacts on O3 production were most sensitive in downwind areas, and it was found that the large increase in BVOC emissions during 2006–2012 tended to reduce surface O3 concentrations by 1.6–2.5 ppb in rural regions, but caused an increment of O3 peaks by up to 2.0–6.0 ppb in VOC-limited urban areas (e.g., Guangzhou, Foshan and Zhongshan). The opposite was true in the period 2001–2006, when the reduced BVOC emissions resulted in 1.3–4.0 ppb increases in daytime O3 concentrations over northern rural regions. Impact of the two-fold increase in BVOC emissions since the early 1990s to 2006 was a 0.9–4.6 ppb increment in surface O3 concentrations over the downwind areas. This study suggests that the potential impacts on ozone chemistry should be considered in long-term land-use planning and air-quality management.

Temperature inversions in severe polluted days derived from radiosonde data in North China from 2011 to 2016

Temperature inversion tends to inhibit the transfer of momentum, heat and moisture in the atmospheric boundary layer, which is often accompanied by severe air pollution. Recently, severe haze pollution has frequently occurred in North China. In this study, the characteristics of temperature inversion on severe polluted days (SPDs) in Beijing were investigated by using radiosonde data with standard pressure levels from 2011 to 2016. Both surface-based inversion (SI) and elevated inversion (EI) were analyzed. 93% of the SPDs were accompanied by temperature inversion, most of which occurred in wintertime. Annual frequency of SI (FSI) and EI (FEI) showed slight fluctuations with mean value of 0.18 and 0.67, respectively. Overall, the annual SI was stronger and deeper than annual EI. Seasonally, the SI was most frequent (0.39) in autumn, in contrast to EI that occurred most frequently (0.95) in summer. Both SI and EI were weakest in summer and strongest in winter. Average monthly SI strength was about 0.38 °C in summer and 2.40 °C in winter, average monthly EI strength was about 0.64 °C in summer and 2.20 °C in winter. The average monthly SI and EI were deepest in winter and shallowest in summer. SI depth were 778 m and 221 m in winter and summer, EI were 630 m and 336 m in winter and summer. The substantially strong liner relationship was found between seasonal inversion strength and PM2.5 concentration, and the inversion strength was found to be better compared with the inversion depth at predicting the PM2.5 concentration during SPDs. Obvious lower air outflow and turbulent kinetic energy were found in SPDs compared to non-SPDs, which indicated weaker turbulence in SPDs. Future efforts should focus on accurate model simulations of temperature inversions in SPDs.

High efficiency of livestock ammonia emission controls on alleviating particulate nitrate during a severe winter haze episode in northern China

Although nitrogen oxide (NOx) emission controls have been implemented for several years, northern China is still facing high particulate nitrate (NO−3 ) pollution during severe haze events in winter. In this study, the thermodynamic equilibrium model (ISORROPIA-II) and the Weather Research and Forecast model coupled with chemistry (WRFChem) were used to study the efficiency of NH3 emission controls on alleviating particulate NO−3 during a severe winter haze episode. We found that particulate-NO−3 formation is almost NH3-limited in extremely high pollution but HNO3limited on the other days. The improvements in manure management of livestock husbandry could reduce 40 % of total NH3 emissions (currently 100 kt month−1) in northern China in winter. Consequently, particulate NO−3 was reduced by approximately 40 % (on average from 40.8 to 25.7 μg m−3). Our results indicate that reducing livestock NH3 emissions would be highly effective in reducing particulate NO−3 during severe winter haze events.