Temporal Characteristics of Ozone (O3) in the Representative City of the Yangtze River Delta: Explanatory Factors and Sensitivity Analysis

Ozone (O-3) has attracted considerable attention due to its harmful effects on the ecosystem and human health. The Yangtze River Delta (YRD), China in particular has experienced severe O-3 pollution in recent years. Here, we conducted a long-term observation of O-3 in YRD to reveal its characteristics. The O-3 concentration in autumn was the highest at 72.76 ppb due to photochemical contribution and local convection patterns, with its lowest value of 2.40 ppb in winter. O-3 exhibited strong diurnal variations, showing the highest values in the early afternoon (15:00-16:00) and the minimum in 07:00-08:00, specifically, peroxyacetyl nitrate (PAN) showed similar variations to O-3 but PAN peak usually occurred 1 h earlier than that of O-3 due to PAN photolysis. A generalized additive model indicated that the key factors to O-3 formation were NO2, PAN, and temperature. It was found that a certain temperature rise promoted O-3 formation, whereas temperatures above 27 degrees C inhibited O-3 formation. An observation-based model showed O-3 formation was VOCs-limited in spring and winter, was NOx-limited in summer, and even controlled by both VOCs and NOx in autumn. Thus, prevention and control strategies for O-3 in the YRD are strongly recommended to be variable for each season based on various formation mechanisms.