Abstract
Abstract. Diurnal variation of surface PM2.5 concentration (diurnal PM2.5) could dramatically affect aerosol radiative and health impacts and can also well reflect the physical and chemical mechanisms of air pollution formation and evolution. So far, diurnal PM2.5 and its modeling capability over East China have not been investigated and therefore are examined in this study. Based on the observations, the normalized diurnal amplitude of surface PM2.5 concentrations averaged over East China is weakest (∼1.2) in winter and reaches ∼1.5 in other seasons. The diurnal PM2.5 shows the peak concentration during the night in spring and fall and during the daytime in summer. The simulated diurnal PM2.5 with WRF-Chem and its contributions from multiple physical and chemical processes are examined in the four seasons. The simulated diurnal PM2.5 with WRF-Chem is primarily controlled by planetary boundary layer (PBL) mixing and emission variations and is significantly overestimated against the observation during the night. This modeling bias is likely primarily due to the inefficient PBL mixing of primary PM2.5 during the night. The simulated diurnal PM2.5 is sensitive to the PBL schemes and vertical-layer configurations with WRF-Chem. Besides the PBL height, the PBL mixing coefficient is also found to be the critical factor determining the PBL mixing of pollutants in WRF-Chem. With reasonable PBL height, the increase in the lower limit of the PBL mixing coefficient during the night can significantly reduce the modeling biases in diurnal PM2.5 and also the mean concentrations, particularly in the major cities of East China. It can also reduce the modeling sensitivity to the PBL vertical-layer configurations. The diurnal variation and injection height of anthropogenic emissions also play roles in simulating diurnal PM2.5, but the impact is relatively smaller than that from the PBL mixing. This study underscores that more efforts are needed to improve the boundary mixing process of pollutants in models with observations of PBL structure and mixing fluxes in addition to PBL height, in order to simulate reasonably the diurnal PM2.5 over East China. The diurnal variation and injection height of anthropogenic emissions must also be included to simulate the diurnal PM2.5 over East China.
Highlights
The Yangtze River delta (YRD) region of East China hosts the economic engine and a major portion of the Chinese population
In order to understand the modeling mechanisms driving the diurnal variations of surface PM2.5 concentrations over East China, this study updates the University of Science and Technology of China (USTC) version of WRF-Chem to include the diagnosis of contributions to surface PM2.5 concentrations from individual processes, including transport, emission, dry and wet deposition, planetary boundary layer (PBL) mixing, and chemical production/loss, by estimating the difference of surface PM2.5 concentrations before and after individual processes during the simulation
In order to investigate the diurnal cycle of surface PM2.5 concentrations, this study defines an index to better show the diurnal variation
Summary
The Yangtze River delta (YRD) region of East China hosts the economic engine and a major portion of the Chinese population. During the past 2 decades, the rapid economic growth has resulted in significant elevated surface air pollutants over East China, especially particulate matter (PM), called aerosols. Liu et al, 2018) It has become the fourth risk factor of deaths in China, and 11.1 % of all deaths are attributable to the ambient elevated concentrations of particulate matter (GBD Risk Factors Collaborators, 2017). Atmospheric aerosols can influence the radiative energy budget of the Earth’s system by interacting with radiation, serving as cloud condensation nuclei (CCN) and ice nuclei (IN), and modifying cloud microphysics and precipitation (e.g., Ackerman, 1977; Dickerson et al, 1997; Jacobson, 1998; Li et al, 2009; Zhao et al, 2012)
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