Diagnosis of photochemical O3 production of urban plumes in summer via developing the real-field IRs of VOCs: A case study in Beijing of China.

This study mainly developed an estimate method for photochemical ozone (O3) production from urban plumes in hot season, through simulating O3 evolution from precursors locally emitted and determining the real-field O3 increment reactivity (IR) of volatile organic compounds (VOCs) based on the box chemical model. Our simulation on June-2019 indicated that Beijing local emissions produced O3 at the rate of 0.7-9.2 ppb/h and led to an O3 increase of 48.9 ppb during 05:00-18:00, accounting for 68.3% of the observed O3 increase. The maximum level and production rate of simulated O3 showed a linear response to VOCs, therefore we can use VOCs levels in urban plumes to quantify O3 formation in summer. The IR (g O3 formed per g VOCs) was calculated on the actual precursor and meteorology condition of this megacity, 0.12-4.90 g/g for individual VOCs and 1.49 g/g for comprehensive TVOCs. The weighted average of individual IRs agreed well with that of TVOCs, but these IRs were 34.5% of MIR values that were widely used in references. It's noteworthy that these IRs had greater sensitivity to precursor levels, and broadly remained stable under the fixed VOCs:NOx. Considering the synchronous reductions of precursors in Beijing, we applied these IRs to quantify chemical O3 evolution from Beijing local emissions in summer of recent years, declining from 63.5 ppb in 2016 to 44.0 ppb in 2020 for June. The contributions of the diagnosed chemical O3 to Beijing O3 better matched with the atmospheric transport paths on daily basis, higher than 100% when the transport paths starting from the clean neighbor cities, but lower to 45%-66% when the transport paths originating from the highly-polluted neighbor cities. This consistence indicated the reliability of our IR calculation method for quickly estimating chemical O3 production of urban plumes in summer.