Abstract

A comprehensive investigation of the photochemical secondary organic aerosol (SOA) formation and transformation in mixtures of anthropogenic (o-cresol) and biogenic (α-pinene and isoprene) volatile organic compound (VOC) precursors in the presence of NOx and inorganic seed particles was conducted. Initial iso-reactivity was used to enable direct comparison across systems, adjusting the initial reactivity of the systems towards the assumed dominant oxidant (OH). Comparing experiments conducted in single precursor systems at various initial reactivity levels (referenced to a nominal base case VOC reactivity) and their binary and ternary mixtures, we show that the molecular interactions from the mixing of the precursors can be investigated and discuss limitations in their interpretation. The observed average SOA yields in descending order were found for the α-pinene (32 ± 7 %), α-pinene/o-cresol (28 ± 9 %), α-pinene at ½ initial reactivity (21 ± 5 %), α-pinene/isoprene (16 ± 1 %), α-pinene at ⠓ initial reactivity (15 ± 4 %), o-cresol (13 ± 3 %), α-pinene/o-cresol/isoprene (11 ± 4%), o-cresol at ½ initial reactivity (11 ± 3 %), o-cresol/isoprene (6 ± 2 %) and isoprene systems (0 ± 0 %). We find a clear suppression of the SOA yield from α-pinene when it is mixed with isoprene, whilst the addition of isoprene to o-cresol may enhance the mixture’s SOA formation potential, however, the difference was too small to be unequivocal. The α-pinene/o-cresol system yield appeared to be increased compared to that calculated based on the additivity, whilst in the α-pinene/o-cresol/isoprene system the measured and predicted yield were comparable. However, in mixtures where more than one precursor contributes to the SOA mass it is unclear whether changes in the SOA formation potential are attributable to physical or chemical interactions, since the reference basis for the comparison is complex. Online and offline chemical composition and SOA particle volatility, water uptake and “phase” behaviour measurements that were used to interpret the SOA formation and behaviour are introduced and detailed elsewhere.

Highlights

  • The fine fraction of particulate matter (PM) plays the dominant role in the impact of air pollution on human health and of 40 aerosol on climate through direct radiative effects and cloud adjustments

  • Comparing experiments conducted in single precursor systems at various initial reactivity levels and their binary and ternary mixtures, we show that the molecular interactions from the mixing of the precursors 25 can be investigated and discuss limitations in their interpretation

  • This is an unavoidable feature of volatile organic compound (VOC) mixtures that will occur in the real atmosphere and such differences need to be carefully considered in the interpretation of the results

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Summary

Introduction

The fine fraction of particulate matter (PM) plays the dominant role in the impact of air pollution on human health and of 40 aerosol on climate through direct radiative effects and cloud adjustments. Our ability to predict the atmospheric burden and impacts of fine secondary aerosol particles (Kanakidou et al, 2005; Tsigaridis and Kanakidou, 2018) has been limited by basic understanding of the formation of this organic component (Hallquist et al, 2009). There are tens of thousands of isolated organic compounds in the atmosphere, ranging across more than 12 orders of magnitude in volatility (Goldstein and Galbally, 2007) with possible oxidation products running to many millions (Aumont et al, 2005). They have extensive biogenic and anthropogenic sources and are spatially heterogeneous. An unknown but substantial proportion of the organic compounds have the potential to act as SOA precursors and the degree to which this is influenced by the complex atmospheric mixture is unclear

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