Background, aim, and scopeFine particulate matters (PM2.5) or aerosols contribute to regional and global climate changes directly by absorbing and scattering the solar radiation and indirectly by serving as cloud condensation nuclei (CCN) and ice nuclei (IN) to modify cloud properties. Elevated aerosols also reduce the visibility of the atmosphere and exert deleterious impacts on air quality, ecosystems, and human health. The Guanzhong Basin is located in northwestern China and nestled between the Qinling Mountain in the south and the Loess Plateau in the north, with a warm-humid climate. The rapid increasing industries and city expansions, as well as the unique topography, have caused frequent occurrence of haze in the basin, which has been drawing more attention to clarify the haze sources, formation, and influences. The purpose of the present study is to investigate the formation and source apportionments of ambient PM2.5 and the mutual inlfuences of the east (Xi’an and surrounding area) and west (Baoji and surrounding area) part of the Guanzhong Basin during springtime (14 to 16 February 2014) using the WRF-CHEM model.Materials and methodsA speciifc version of the WRF-CHEM model is utilized to investigate the organic aerosol formation in Guanzhong Basin. This version employs a lfexible gas-phase chemical module and the CMAQ (version 4.6) aerosol module developed by US EPA. The dry deposition of chemical species is parameterized according to and the wet deposition follows the method in CMAQ. The FTUV module considering the impacts of aerosols and clouds on photochemistry is used to calculate the photolysis rates. The ISORROPIA Version 1.7 (http:/enes.eas.gatech.edu/ISORROPIA/) is employed to the WRF-CHEM model to simulate the inorganic aerosols. The NCEP 1°×1° reanalysis data are involved for the meteorological initial and boundary conditions. The chemical initial and boundary conditions are interpolated from MOZART output with a 6-hour interval. The anthropogenic emission inventory (EI) including agriculture, industry, power plant, residential, and transportation sources is developed by Zhang et al (2009). The MEGAN model is used to on-line calculate the biogenic emissions in the WRF-CHEM model. The Factor Separation Approach (FSA) is adopted to analyze the source apportionment of ambient PM2.5 and the mutual inlfuences of the east (Xi’an and surrounding area) and west (Baoji and surrounding area) part of the basin.Results The near-surface wind speeds and directions are reasonably replicated. The temporal variation and spatial evolution of urban PM2.5 over Xi’an and Baoji are well reproduced, which indicates a reasonable replication of the event.Discussion Sensitivity studies show that the PM2.5 formed from the east part of the Guanzhong Basin (Xi’an and surrounding area) significantly influences the air quality over the west part of the basin (Baoji and surrounding area) with a contribution of 30% to the near-surface PM2.5 levels.Conclusions During the heavy air pollution process, the prevailing westward wind that entrains a great amount of pollutants from Xi’an and surrounding areas and contaminates the air quality over Baoji. The source apportionment analysis reveals the contributions from each emission source in the Guanzhong Basin during February. The residential emission constitutes the most with a contribution of over 40%, while the transportation emission only contributes less than 10%, which indicates that vehicles play a minor role during heavily hazy days.Recommendations and perspectives Ambient gas-phase species and PM2.5 compositions should be included in future studies to give a more flexible simulation and better present the evolution of haze events.%本文应用WRF-CHEM模式模拟分析了关中地区2014年2月14日至16日的一次重污染过程。模式模拟了西安地区和宝鸡地区城市大气PM2.5的时间变化和空间分布特征，较好地再现了污染过程。敏感性试验分析表明，关中盆地东部地区（西安市及其周边地区）形成的PM2.5对盆地西部地区（宝鸡市及其周边地区）影响较大，贡献可以达到30%，其主要原因为盆地发生重污染时，盛行东风造成西安市及其周边地区形成的污染物向西输送，影响宝鸡市的空气质量。污染源分析表明，居民生活源是关中盆地在2月份最重要的PM2.5源，贡献超过40%，交通运输源的贡献小于10%。因此在重霾情况下，限行机动车的作用很小。
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