Investigating the role of anthropogenic terpenoids in urban secondary pollution under summer conditions by a box modeling approach†

Abstract

Terpenoids, including isoprene and monoterpenes, are highly reactive volatile organic compounds (VOCs) that play an essential role in atmospheric chemistry, contributing to the formation of ozone and secondary organic aerosols (SOAs). While known for decades for their biogenic origin, their anthropogenic origin is now well established in urban areas worldwide. Nevertheless, there is still a lack of clarity regarding the relative significance of these emissions and their impact on secondary pollution at the urban scale where biogenic and anthropogenic emissions coexist. The objective of this study is to evaluate the role of anthropogenic terpenoids in secondary pollution over the megacity of Paris, a typical northern mid-latitude urban area, using a box model. The model employs the Master Chemical Mechanism (MCM v3.3.1) to describe the gaseous reactivity. A physico-chemical scenario was developed to reproduce a typical summertime environment built upon in situ observations collected during the EU-MEGAPOLI campaign in Paris. Emission ratios of anthropogenic VOCs over carbon monoxide were used to parametrize the primary emissions of more than 60 species (including anthropogenic terpenoids). The comparison between in situ observations and modelled trace gas concentrations demonstrated the model's capacity to reproduce the levels and their temporal variability. Two sensitivity tests were conducted to quantify the impact of terpenoid emissions on ozone formation and their potential to form SOA mass concentration according to two simulations modulating anthropogenic and biogenic emissions of terpenoids based on the uncertainties associated with their estimation. Ozone concentration slightly increases by 1 (±0.5)% when increasing anthropogenic terpenoid emissions and by 3 (±2)% when increasing biogenic terpenoid emissions; the increase of O₃ with increasing VOCs is consistent with the high-NO_x chemical regime. Looking at the potential terpenoid derived SOA production, isoprene and limonene dominate. The estimated total mass concentration of SOAs produced over a 24 h period is 0.53 μg m⁻³, with a maximum hourly produced mass concentration of 0.045 μg m⁻³ observed in the morning. This modelling study suggests that the production of SOAs through the oxidation of terpenoids emitted from anthropogenic sources is competitive with that derived from their biogenic sources and remains significant at night.