*4.4. Limitations*

The study results have explained the impacts of socioeconomic development on PM2.5 concentrations and the causal relationships among them to a large extent in Liaoning Province; however, there are still some limitations. For example, the surface PM2.5 data used in this study are the longest time series pollutant data available at present, but there is also a possibility that the lower spatial resolution of the data has a ffected the accuracy of the assessment results. If better data could be obtained (i.e., higher spatial resolution and longer time series), it would be beneficial to further explore the causes of regional and internal pollution di fferences in the future. On the other hand, complex coupling relationships among economic growth, urbanization, industrialization and PM2.5 concentrations were observed in this study. Determining how to decouple these relationships to further develop targeted solutions that tackle the pollution issue remains a challenging and urgen<sup>t</sup> task. In the future, it is also necessary and meaningful to study and compare the relationships between policies and environment in other regions such as agriculture- or service-oriented areas and comprehensive areas.
