Comparison of Ozone and PM_2.5 Concentrations over Urban, Suburban, and Background Sites in China

Surface ozone (O₃) and fine particulate matter (PM_2.5) are dominant air pollutants in China. Concentrations of these pollutants can show significant differences between urban and nonurban areas. However, such contrast has never been explored on the country level. This study investigates the spatiotemporal characteristics of urban-to-suburban and urban-to-background difference for O₃ (ΔO₃) and PM_2.5 (ΔPM_2.5) concentrations in China using monitoring data from 1171 urban, 110 suburban, and 15 background sites built by the China National Environmental Monitoring Center (CNEMC). On the annual mean basis, the urban-to-suburban ΔO₃ is −3.7 ppbv in Beijing–Tianjin–Hebei, 1.0 ppbv in the Yangtze River Delta, −3.5 ppbv in the Pearl River Delta, and −3.8 ppbv in the Sichuan Basin. On the contrary, the urban-to-suburban ΔPM_2.5 is 15.8, −0.3, 3.5 and 2.4 μg m⁻³ in those areas, respectively. The urban-to-suburban contrast is more significant in winter for both ΔO₃ and ΔPM_2.5. In eastern China, urban-to-background differences are also moderate during summer, with −5.1 to 6.8 ppbv for ΔO₃ and −0.1 to 22.5 μg m⁻³ for ΔPM_2.5. However, such contrasts are much larger in winter, with −22.2 to 5.5 ppbv for ΔO₃ and 3.1 to 82.3 μg m⁻³ for ΔPM_2.5. Since the urban region accounts for only 2% of the whole country’s area, the urban-dominant air quality data from the CNEMC network may overestimate winter PM_2.5 but underestimate winter O₃ over the vast domain of China. The study suggests that the CNEMC monitoring data should be used with caution for evaluating chemical models and assessing ecosystem health, which require more data outside urban areas.