Abstract

A series of experiments were designed and conducted in the Manchester Aerosol Chamber (MAC) to study the photooxidation of single and mixed biogenic (isoprene and α-pinene) and anthropogenic (o-cresol) precursors in the presence of NOx and ammonium sulphate seed particles. Several online techniques (HR-TOF-AMS, Semi-Continuous GC-MS, NOx and O3 analyser) were coupled to the MAC to monitor the gas and particle mass concentrations. Secondary Organic Aerosol (SOA) particles were collected onto a quartz fibre filter at the end of each experiment and analysed using liquid chromatography ultra-high resolution mass spectrometry (LC-Orbitrap MS). The SOA particle chemical composition in single and mixed precursor systems was investigated using non-targeted accurate mass analysis of measurements in both negative and positive ionization modes, significantly reducing data complexity and analysis time, providing an more complete assessment of the chemical composition. This non-targeted analysis is not widely used in environmental science and never previously in atmospheric simulation chamber studies. Products from α-pinene were found to dominate the binary mixed α-pinene / isoprene system in terms of signal contributed and the number of particle components detected. Isoprene photooxidation was found to generate negligible SOA particle mass under the investigated experimental conditions and isoprene-derived products made a negligible contribution to particle composition in the α-pinene / isoprene system. No compounds uniquely found in this system contributed sufficiently to be reliably considered as a tracer compound for the mixture. Methyl-nitrocatechol isomers (C7H7NO4) and methyl-nitrophenol (C7H7NO3) from o-cresol oxidation made dominant contributions to the SOA particle composition in both the o-cresol / isoprene and o-cresol / α-pinene binary systems in negative ionization mode. In contrast, interactions in the oxidation mechanisms led to the formation of compounds uniquely found in the mixed o-cresol containing binary systems in positive ionization mode. C9H11NO and C8H8O10 made large signal contributions in the o-cresol / isoprene binary system. The SOA molecular composition in the o-cresol / α-pinene system in positive ionization mode is mainly driven by the large molecular weight compounds (e.g. C20H31NO4, and C20H30O3) uniquely found in the mixture. The SOA particle chemical composition formed in the ternary system is more complex. The molecular composition and signal abundance are both markedly similar to those in the single α-pinene system in positive ionization mode, with major contributions from o-cresol products in negative ionization mode.

Highlights

  • Secondary Organic Aerosol (SOA) particles were collected onto a quartz fibre filter at the end of each experiment and analysed using liquid chromatography ultrahigh resolution mass spectrometry (LC-Orbitrap MS)

  • The reduction of NOx will result from i) reaction between oxidants; the hydroxyl radical (OH) radicals and NO2 leading to HNO3 formation with subsequent loss to the chember walls or particles as inorganic nitrates. ii) termination reactions between NO and RO2 radicals or NO2 and RO2 radicals leading to formation of nitrogen-containing organic (NOROO2 and NO2ROO2) compounds(Atkinson, 2000)

  • The SOA chemical composition formed from the photooxidation of α-pinene, isoprene, o-cresol and their binary and ternary mixtures in the presence of NOx and ammonium sulphate seed particles was determined by non-targeted LC945 Orbitrap MS

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Summary

Introduction

Atmospheric aerosols affect climate directly through scattering or absorbing solar-radiation (Novakov and Penner, 1993; Andreae and Crutzen, 1997) and indirectly by acting as cloud condensation nuclei (CCN) (Mcfiggans et al, 2006). Exposure to particulate matter has been directly linked to adverse impacts on human health (WHO, 2016). Organic aerosol 60 significantly contributes to fine particulate matter (PM) in the atmosphere (Fiore et al, 2012; Jimenez et al, 2009),and can affect human health through the deep penetration of small aerosol particles into the lungs through inhalation, and the deposition of larger particles in the upper respiratory tract (Burnett et al, 2014). Fine PM has a wide variety of primary (e.g. agricultural operations, industrial processes, and combustion processes) and secondary sources. In addition to secondary inorganic contributions from nitrate and sulphate, secondary organic aerosol (SOA) formed from the oxidation of atmospheric volatile 65 organic vapours (VOCs) can make a major contribution (Hallquist et al, 2009)

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