*2.3. Meteorological Datasets*

The meteorological datasets for Xi'an covers 5 years from 2013 to 2017, obtained from the National Meteorological Information Centre (http://data.cma.cn/, accessed on 8 June 2019). Six meteorological indices were selected to study the influence of meteorological conditions on atmospheric pollutants, including mean daily total precipitation, atmospheric pressure, relative humidity, atmospheric temperature, wind speed, and wind direction.
