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

Abstract

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 NOx, 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.