Diesel‐Related Combustion Drives Significant Missing OH Reactivity: Field Evidence From the Shanghai Megacity

Identifying ozone (O 3 ) production impacts and sources of unidentified reactive organic gases (ROGs) remains challenging. Here, missing OH reactivity (OHR), an indicator of unmeasured ROGs, defined as the gap between measured and calculated OHR, was quantified at a typical urban site in Shanghai megacity during April 2023. Measured and calculated OHR were 19.0 ± 8.8 s −1 and 13.8 ± 5.4 s −1 , respectively, yielding missing OHR of 5.2 s −1 (27.1% of measured OHR, averaging 37.0% in morning rush hours). Sensitivity analyses showed that omitting missing OHR in model simulation caused 2.2%–134.2% underestimations of the net ozone production rate ( P net [O 3 ]) and 11.1%–69.2% overestimations of ROG‐limited regime prevalence. This impact is sensitive to the chemical nature of unmeasured ROGs; the upper bound occurs if they possess radical‐regeneration efficiencies similar to measured photolabile organics. A robust correlation ( R = 0.90) indicates that the response of the NO X threshold (the NO X concentration at peak maximum daily 8‐hr average O 3 ) is quantitatively linked with P net (O 3 ). Primary anthropogenic emissions dominated (88.1%–92.2%) in missing OHR was identified consistently across different methods, while biogenic sources contributed 7.8%–11.9%. Further source apportionment resolved six anthropogenic sources, including diesel‐related combustion (29.3%), solvent usage (19.1%), gasoline vehicle exhaust (16.8%), fuel evaporation (13.0%), biomass combustion (9.4%) and industrial process (4.5%). Combustion sources (diesel, gasoline, and biomass) contributed substantially more to missing OHR (55.5%) than fugitive sources (solvent usage, fuel evaporation and industrial process) (36.7%). These findings underscore the need for further source speciation and integrated control targeting missing OHR particularly from diesel‐related combustion sources to mitigate urban ozone pollution.