Significant but Overlooked: The Role of Anthropogenic Monoterpenes in Ozone Formation in a Chinese Megacity

Abstract

Recent observations have revealed unexpectedly high concentrations of monoterpenes (MT) in urban areas, frequently surpassing those in forested regions. These findings suggest significant anthropogenic contributions (MT_ANT), challenging the traditional view that MT emissions are predominantly natural (MT_NAT) in current inventories. This oversight likely results in a substantial underestimation of MT’s role in urban ozone (O₃) production. Therefore, we developed a novel approach to generate a gridded emission inventory (EI) of MT_ANT, integrating flux measurements of MT and carbon monoxide (CO). Results show that MT_ANT emission rate in Beijing core areas exceeds MT_NAT by a factor of 1.83, with household volatile chemical products (VCPs) contributing 56% of total MT_ANT emissions. Incorporating MT_ANT emissions into the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) model significantly improved the simulation of diurnal MT variations (correlation coefficient, r = 0.985) and reduced the normalized mean bias (NMB) in surface MT concentration predictions by 53%. Notably, the combined effects of anthropogenic and biogenic MT emissions increased summertime maximum daily 8-h average (MDA8) O₃ levels by 12.8 ppb in Beijing core areas, with MT from household VCPs (MT_VCP) accounting for 62% of the MT_ANT-driven O₃ increase. This study provides a robust quantitative foundation for assessing the impact of anthropogenic MT emissions on urban air quality and highlights the urgent need for targeted regulatory measures to mitigate their growing contribution to O₃ pollution.