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. 2019 Nov 27;24:66. doi: 10.1186/s12199-019-0817-5

Fig. 3.

Fig. 3

Comparison of the outdoor NO2 concentrations in each district in each season. District A: box with stripes, district B: mesh box, district C: black box, district D: gray box; Asterisk indicates statistically significant (p < 0.05); air quality standard for the mean annual. NO2 concentration: 30 μg/m3 (16 ppb)