*4.3. Further Work*

Our results suggest that the worsening of air quality in Xi'an is mainly caused by the combination of human activities and natural sources. Many regulators believe that local air quality can be improved by efforts to reduce anthropogenic emissions, and controlling trans-boundary pollutants from the Gobi desert through increasing vegetation cover. It is necessary to investigate the effects of different types of human activities on air quality in the urban environment, and this will form the basis of our future work. Air monitoring stations that cover different environments are also needed. Further research should involve more monitoring systems by factoring in local land use, while involving both the downtown and suburban areas.
