Insight into decreased ozone formation across the Chinese National Day Holidays at a regional background site in the Pearl River Delta

Investigating the formation mechanism of ozone (O₃) during the Chinese National Day Holidays (CNDH, Oct. 1st to 7th) in the background area of megacity clusters provided a valuable opportunity for surveying the relative influence of anthropogenic and biogenic sources on O₃ pollution. Here we conducted an intensive observation campaign during Sep. 24 to Oct. 14, 2021 at a regional background site in the Pearl River Delta region, southern China. Results showed that O₃ concentrations exhibited a downtrend across the CNDH, with a decreasing rate of 14.8% from prior-CNDH to CNDH and 28.9% from CNDH to post-CNDH. Simulation of O₃ photochemical process by using a photochemical box model with the master chemical mechanism (PBM-MCM) indicated that the decreased atmospheric oxidation capacity and decelerated free radical cycling led to a decrease in the net O₃ production rate (PO₃) across the CNDH. O₃ isopleth diagrams and relative incremental reactivity (RIR) analysis revealed that O₃ formation (FO₃) in prior-CNDH and CNDH was jointly limited by VOCs and NO_x, while was VOC-limited in post-CNDH. VOCs was still shown to be the largest contributor to FO₃, suggesting VOCs reduction was the most effective way to reduce O₃ pollution. Further simulation by combining the PBM-MCM and positive matrix factorization (PMF) model showed that anthropogenic sources have a smaller contribution to the PO₃ in CNDH compared to the other periods, showing a significant holiday effect of “decreasing during holidays and rebounding after holidays”. However, due to the more biogenic emission during CNDH, the net PO₃ was higher than that post-CNDH. This study improved our understanding of the enhanced biogenic contributions to FO₃ in future scenarios of anthropogenic emissions reduction.