Changing impacts of plant emissions on urban air quality in a changing climate

Particulate matter acts as a significant pollutant affecting climate and health, particularly in urban environments such as Beijing which experiences consistent poor air quality. The most dangerous pollutant to health in China is fine particulate matter (PM2.5), and substantial reductions are required to reduce pollution-related mortality. A large fraction of the sub-micron particulate matter (PM1) mass consists of secondary organic aerosols (SOA), which form from the oxidation of volatile organic compounds (VOCs) emitted from biogenic or anthropogenic sources.

In this study, non-target analysis was used to develop SOA tracer libraries containing the 100 most abundant compounds for 6 monoterpenes, 2 monoterpenols and 2 sesquiterpenes via photo-oxidation chamber experiments. PM2.5 filter samples were extracted and analysed using Ultra-High-Performance Liquid Chromatography with tandem Mass Spectrometry (UHPLC-MS2), and imported into the open source MZmine 4.3.0 software for library generation. To investigate ambient PM2.5, filter samples were collected from 11th-25th June 2017 at the Institute of Atmospheric Physics (IAP) in Beijing, China, and consequently imported into MZmine. A data processing workflow was set up using the ‘Spectral Library Search’ function on MZmine, allowing matching of the libraries to Beijing samples, leading to the identification of 311 biogenic SOA tracers.

For monoterpenoids, fragmentation processes generating species with less than 10 carbons dominated oxidation products for all libraries except linalool, which produced more functionalised C11-C20 products. Sesquiterpene libraries were also dominated by
fragmentation products, yielding mostly C1-C15 compounds. Tracer concentrations were
generally stable apart from a pronounced peak on 20th June, driven by p-cymene SOA, linked to southeasterly transport of biogenic emissions. Diurnal patterns showed morning maximums and night-time minimums, while correlation analysis indicated that atmospheric processes, including oxidant levels and NOₓ regime, were significant in controlling SOA formation. These findings demonstrate the influence of regional transport and atmospheric complexity in urban biogenic SOA diversity.