Unraveling ozone formation sensitivity to ambient VOCs and NOx at urban and mountain sites in a typical city of eastern China.

Ozone (O) is a primary pollutant affecting air quality in China, whose formation rate was non-linearly based on volatile organic compounds (VOCs) and nitrogen oxides (NOx). A comprehensive understanding of the key drivers governing O formation and its sensitivity to precursor variations presents a persistent research challenge, stemming from the complex interplay of underlying photochemistry, meteorology, and topography. To address this knowledge gap, we conducted synchronous measurements of O and its precursors at both an urban (JPU) and a mountain (LM) site in a typical city in eastern China, enabling concurrent evaluation of O formation sensitivity and diagnostic analysis of its underlying mechanisms. Results indicated that O, and VOCs/NOx concentrations at LM site were ∼2 times higher and 3-6 times lower than those at JPU site, respectively. Unlike the pronounced diurnal cycles observed at most mountain and plain sites, the LM site showed attenuated diurnal variations. The distinct diurnal cycle of O was mainly induced by combined effects of 1) strong local emissions of VOCs and NOx at the JPU site, contrasting with limited emissions and consequently a suppressed NO titration effect at the LM site; 2) more favorable meteorological factors for in-situ O formation at JPU compared to LM; 3) influence of mountain-valley breeze and planetary boundary layer (PBL) dynamics. Further O budget showed notable differences existed in O depletion pathways. Moreover, the RIR and EKMA curve analysis identified that O formation at the JPU and LM sites occurred within VOCs-limited and VOCs-NOx transition regimes, respectively. This study yields a comprehensive understanding of complicated impacts of photochemistry, meteorology, and topography on O formation across contrasting basin and mountain sites in eastern China.