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As a particular mechanism of soil salinization, freeze±thaw action has an obvious control on soil salinization in northeast China; it essentially differs from the present-day salinization process by involving violent evaporation. A special rule of water/ salt movement is formed during the process of freezing and thawing, and the resultant soil pro®le is divided into three layers: frozen layer, semi-frozen layer, and unfrozen layer. It is evident that the salinity in the frozen layer increases, along with soil water and salt mass moves towards the frozen layer from the underlying beds, through the process of soil freezing.

Abstract:Over the past decades, regional haze episodes have frequently occurred in eastern China, especially in the Yangtze River Delta (YRD). Satellite derived Aerosol Optical Depth (AOD) has been used to retrieve the spatial coverage of PM 2.5 concentrations. To improve the retrieval accuracy of the daily AOD-PM 2.5 model, various auxiliary variables like meteorological or geographical factors have been adopted into the Geographically Weighted Regression (GWR) model. However, these variables are always arbitrarily selected without deep consideration of their potentially varying temporal or spatial contributions in the model performance. In this manuscript, we put forward an automatic procedure to select proper auxiliary variables from meteorological and geographical factors and obtain their optimal combinations to construct four seasonal GWR models. We employ two different schemes to comprehensively test the performance of our proposed GWR models: (1) comparison with other regular GWR models by varying the number of auxiliary variables; and (2) comparison with observed ground-level PM 2.5 concentrations. The result shows that our GWR models of "AOD + 3" with three common meteorological variables generally perform better than all the other GWR models involved. Our models also show powerful prediction capabilities in PM 2.5 concentrations with only slight overfitting. The determination coefficients R 2 of our seasonal models are 0.8259 in spring, 0.7818 in summer, 0.8407 in autumn, and 0.7689 in winter. Also, the seasonal models in summer and autumn behave better than those in spring and winter. The comparison between seasonal and yearly models further validates the specific seasonal pattern of auxiliary variables of the GWR model in the YRD. We also stress the importance of key variables and propose a selection process in the AOD-PM 2.5 model. Our work validates the significance of proper auxiliary variables in modelling the AOD-PM 2.5 relationships and provides a good alternative in retrieving daily PM 2.5 concentrations from remote sensing images in the YRD.

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