Abstract: Background, aim, and scope Xi’an City is located in the central basin of the Guanzhong Plain. The topographic conditions obstruct the airflow, which is not conducive to the spread of pollutants, and the mutual transmission of pollutants between cities has a greater impact. Those reasons made the air pollution in this region more serious. In addition, due to the increasingly serious atmospheric pollution in recent years, Xi’an is going to be listed in the key areas for air pollution control. In order to improve regional air quality and provide a good environment for the second "Belt and Road" Forum for International Cooperation to be held in Xi’an in 2019, it is necessary to understand the current pollution status of the city and its surrounding areas. Materials and methods Therefore, based on the data from 13 State-controlled air quality monitoring stations and meteorological data in Xi’an from July 2013 to December 2017, we analyzed the variation feature and correlation of the mass concentration of PM2.5, which can be changed with time and space, to provide reliable scientific data for further research, governance and control of PM2.5 pollution and early warning of air quality. Results The results reveal that PM2.5 concentration has obvious temporal and spatial patterns. In the interannual trend, from 2014 to 2017, with the implementation of various environmental policies, the overall PM2.5 concentration in Xi’an has a slight downward trend, and the 1st standard annual average daily qualified rate has gradually increased from 15.71% to 27.25%. In summer, PM2.5 pollution gradually improved, with the average concentration decreasing from 64.1 μg/m3 to 33.4 μg/m3, but high-polluted weather still remained in winter. Discussion In terms of monthly and quarterly changes, due to the low atmospheric pressure and low rainfall, the inversion layer is relatively thick, the height of the atmospheric boundary layer is low, and the effect of heating, the mass concentration in winter is higher compared to summer. In the daily changes, due to the difference in emissions caused by human activities, there is a "weekend effect" that the concentration on weekends and holidays is higher than that on the working day. In hourly fluctuation, due to the temperature difference between day and night, the change of the atmospheric boundary layer, and the influence of human activities emissions, there is a bimodal pattern which the concentration is higher in the morning and midnight, while lower in the afternoon. In the spatial distribution, the areas with the highest concentration of PM2.5 pollution are the central urban areas, northern and western area. The concentration in the northern city is higher than that in the southern area. The main urban area is significantly higher than the surrounding counties. This spatial difference is caused by internal emissions and external transport. Meteorological elements are important factors that restrict the dilution, diffusion, migration and transformation of pollutants in the atmosphere, and will affect the PM2.5 mass concentration to some extent. As the temperature is low, the inversion layer is relatively thick and the pollutants are difficult to diffuse in the atmosphere and easy to have secondary reactions, which made the concentration of PM2.5 has a certain negative correlation with the temperature. In overcast and in cloudy weather of autumn and winter, PM2.5 concentrations are high. When rainfall is heavy, the pollutants will be partially removed by wet deposition under the effect of rain and snow, and the concentration of PM2.5 is relatively low. The wind direction and wind speed will affect the horizontal migration of particles. Under windy conditions, the PM2.5 polluting situation is roughly inversely related to the wind speed. With the increase of wind power, the particulate matter diffusion conditions are getting better and the concentration is lower. However, under the static wind conditions, particulate matter transported by the regional transportation has less impact, the pollution mainly comes from the interior of the city, and the high-pollution weather is less. Conclusions In a word, urban atmospheric PM2.5 pollution is the result of the combined effects of anthropogenic emissions and weather. Recommendations and perspectives Meteorological conditions can affect air pollution to some extent, but man-made emissions are the root cause of PM2.5 pollution. At present, energy conservation and emission reduction are the most suitable methods for improving atmospheric conditions.
Keywords: Xi’an; PM2.5; temporal and spatial variation; meteorological factor