From traffic flow to ozone surges: Exploring threshold effects and interactive drivers of urban ozone pollution in arid regions

Ozone (O 3 ) pollution is an increasing barrier to air-quality improvement in arid cities and poses growing public-health risks because of rapid buildup and frequent exceedances. Unlike humid eastern China, arid cities typically experience high temperatures, low humidity, and intense solar radiation, together with rapid motorization, which can lead to complex nonlinear O 3 responses. We conducted hourly, synchronized monitoring of traffic flow, meteorological variables, and 11 air pollutants at a roadside site on a major arterial in Yinchuan (Northwest China). We used a random forest regression model with SHAP (Shapley Additive Explanations) to identify drivers of O 3 variability. Road traffic showed a clear weekday double-peak commuting pattern. Diurnal patterns differed across pollutants: primary pollutants (e.g., NO and CO) peaked during commuting hours, whereas O 3 increased with daytime radiation and peaked at 110.11 μg·m −3 in the afternoon. Model interpretation identified temperature and NO as dominant drivers, with methane and relative humidity (RH) also contributing. Threshold analysis showed that O 3 increased rapidly when air temperature exceeded 24.9 °C and traffic flow exceeded 386 veh·h −1 , whereas RH above 52% inhibited O 3 . SHAP interaction analysis indicated a climate-amplification pattern, with the strongest O 3 response under high-temperature, low-humidity conditions. The temperature–NO interaction dominated O 3 variability at low-to-moderate NO levels, whereas NO titration suppressed O 3 under high-NO conditions. Traffic emissions showed a chemistry-dependent dual role: they promoted O 3 accumulation under moderate loads but suppressed O 3 via NO titration under severe congestion and high NO x . Overall, we present an interpretable, data-driven framework that links road traffic and meteorology to nonlinear O 3 responses and actionable thresholds, supporting time-resolved traffic management and coordinated precursor control in arid cities.