*3.2. The Concentrations of PM10 and PM2.5 at Different Air Monitoring Stations in Xi'an*

As a whole, the concentrations of PM10 and PM2.5 exhibited positive correlations (*p* < 0.05) during the days that exceeded the Grade 3 concentration limits (24 h average); and negative correlations (*p* < 0.05) during the days with Grade 1 concentration limits (24 h average) or lower (Table 1 and Figure 3).

**Table 1.** Spearman correlation coefficients between concentrations of PM10 and PM2.5 and the days that PM10 > 150 μg/m3, PM10 < 50 μg/m3, PM2.5 > 75 μg/m3, PM2.5 < 35 μg/m3.


\* indicates a significance level of 0.05, \*\* indicates a significance level of 0.01.

The concentrations of PM10 and PM2.5 from all 13 of the air monitoring stations were involved to analyze the spatial differences in Xi'an. The concentrations of PM10, as well as PM2.5, among the 13 air monitoring stations were nonsignificant (*p* > 0.05), exhibiting similar characteristics of annual and seasonal concentrations (Figure 3).

**Figure 3.** Annual and seasonal concentrations of PM10 and PM2.5 and the days that the concentrations lower than Grade 1 and higher than Grade 3 at different monitoring stations in Xi'an.

The concentrations of PM10 and PM2.5 among monitoring stations varied across temporal scales (Table 2). Annually, PM10 concentrations in Caotan (CT, 156.4 μg/m3) and High-Tech District (HT, 156.4 μg/m3) exceeded the Grade 3 concentration limit (>150 μg/m3); while PM2.5 concentrations in Voltage Switchgear Factory (VS, 80.2 μg/m3) (TC, 76.0 μg/m3), People's Stadium (PS, 80.6 μg/m3), High-Tech District (HT, 76.4 μg/m3), Economic development District (ED, 79.2 μg/m3), and Caotan (CT, 76.9 μg/m3) exceeded the Grade 3 concentration limit (>75 μg/m3). Seasonally, in spring, PM10 concentrations in Voltage Switchgear Factory (VS, 157.1 μg/m3), Xiaozhai (XZ, 152.5 μg/m3), People's Stadium (PS, 161.2 μg/m3), High-Tech District (HT, 157.3 μg/m3), Economic development District (ED, 156.4 μg/m3), and Caotan (CT, 158.3 μg/m3) exceeded the Grade 3 concentra-

tion limit; for PM2.5 concentrations, no station exceeded the Grade 3 concentration limit. In summer and autumn, neither the PM10 nor the PM2.5 exceeded the Grade 3 concentration limit. Whereas in winter, both PM10 and PM2.5 exceeded the Grade 3 concentration limit, with no monitoring station reaching the Grade 1 concentration limit for PM10 (<50 μg/m3). However, Xingqing Community (XQ, 34.5 μg/m3) and Chang'an District (CA, 35.0 μg/m3) meet the Grade 1 PM2.5 concentration limit (<35 μg/m3) in summer. The maximum PM10 concentration difference between seasons was identified in Xiaozhai (XZ, 142.1 μg/m3), the minimum in Textile City (TC, 106.0 μg/m3). The maximum PM2.5 concentration difference was found in People's Stadium (PS, 100.4 μg/m3), the minimum in Yanliang District (YL, 84.5 μg/m3).


**Table 2.** The concentrations of PM10 and PM2.5 among monitoring stations in Xi'an annually and seasonally.

Notes: Max-Min was the maximum difference between the seasons. Units: μg/m3.

Overall, the concentrations of PM10 and PM2.5 were highest in People's Stadium (PS) and Caotan (CT), and lowest in the Xingqing Community (XQ), the Chang'an District (CA), the Lintong District (LT), and Guangyun Lake (GY).
