Characterization of gaseous and particulate phase organic compounds during a winter-time air pollution event

 Abstract

Atmospheric particles are known to cause adverse health effects and premature deaths in European cities. To improve air quality, a detailed understanding of particle sources is thus essential in order to reduce their emissions. Secondary organic aerosols (SOA) produced from the oxidation of volatile organic compounds emitted by anthropogenic sources such as road vehicles and solid fuel combustion is an important air pollution source in urban areas. It is demonstrated that SOA contribute significantly to the atmospheric particle loading, and could even be the major contributor at specific locations. Yet, state of the art models are still not able to reproduce SOA formation despite recent advances. Clearly, further work is needed to improve our understanding of the processes related to SOA formation.

In this context, a field campaign has been conducted at a monitoring station in Cork City, Ireland during winter 2019 (26^th January to 8^th February). The chemical composition of organic compounds in both gas and particle phases was investigated online using a Time-of-Flight Chemical Ionisation Mass Spectrometer (ToF-CIMS) coupled with a Filter Inlet for Gases and Aerosols (FIGAERO). PM_2.5 concentration, ozone and nitrogen oxides (NO_x) were also monitored during the campaign, as well as meteorological parameters. Finally, air mass backward trajectories were computed using the HYSPLIT model.

A strong night-time air pollution event was observed during the field campaign, characterized by PM_2.5 concentrations up to 180 µg m^-3. Using iodide as reagent, the FIGAERO-ToF-CIMS detected hundreds of ions simultaneously in gas and particulate phases. Among the identified compounds were a range of well-known atmospheric tracers of solid fuel burning, including phenolic compounds such as guaiacol and catechol, and numerous oxygenated polycyclic aromatic hydrocarbons (OPAHs). A number of nitrated aromatic compounds were also detected. In this work, the gas/particle partitioning of some of these key compounds has been investigated to provide information on phase transfer of solid fuel emissions over time. The thermograms produced by the FIGAERO analysis are also used to determine the volatility of the species detected. Finally, the FIGAERO-ToF-CIMS data is used to explore the extent to which oxidation of the gaseous emissions by the nitrate radical (NO₃) leads to the formation of nitrated compounds in the particulate phase. This work thus provides unique insights into the night-time oxidation processes that can lead to SOA formation from anthropogenic sources.

 

Acknowledgments

This work was supported by the Irish Research Council (GOIPG/2017/1364) and by the European Union’s Horizon 2020 research and innovation programme (EUROCHAMP-2020, grant no. 730997; Marie Skłodowska-Curie grant agreement No. 751527).