Chemical transformations in monoterpene-derived organic aerosol enhanced by inorganic composition

M Riva,O Peräkylä,D Imre,M P Rissanen,M Ehn,T Petäjä,D M Bell,S Schallhart,L Heikkinen,A Zelenyuk,J A Thornton,Q Zha

doi:10.1038/s41612-018-0058-0

Abstract

Secondary organic aerosol (SOA) is known to impact both climate and air quality, yet molecular-level composition measurements remain challenging, hampering our understanding of SOA formation and evolution. Here, we reveal the importance of underestimated reaction pathways for the (trans)formation of SOA from monoterpenes, one of the largest SOA precursors globally. Utilizing mass spectrometric techniques to achieve a comprehensive characterization of molecular-level changes in the SOA, we were able to link the appearance of high-molecular weight (HMW) organic molecules to the concentration and level of neutralization of particulate sulfate. Interestingly, this oligomerization coincided with a decrease of highly oxygenated molecules (HOMs). Our findings highlight the role of particle-phase processing, and the underestimated importance of sulfate aerosol for monoterpene-SOA formation. The observations of these processes directly in the atmosphere reveal the need to account for the formation of HMW oligomers to fully understand the physicochemical properties of organic aerosol.

Highlights

The largest mass fraction of atmospheric fine particulate matter (PM2.5) is generally organic, dominated by secondary organic aerosol (SOA) formed from the gas-phase oxidation of volatile organic compounds (VOCs).[1,2,3] Biogenic VOCs (BVOCs), such as isoprene and monoterpenes, are typically the most abundant SOA precursors, especially in regions of dense terrestrial vegetation.Understanding the physical and chemical processes associated with SOA formation and properties is crucial to properly evaluate their impacts on climate and human health.[1,2,4] Despite decades of research, many substantial questions remain concerning how SOA forms and evolves in the atmosphere under different conditions
In order to investigate the impact of multiphase chemistry on the chemical composition and evolution of monoterpene-derived SOA, we targeted the boreal forest where monoterpenes are the primary source of SOA
Large signals were observed for some lower mass ranges, which might result from the thermal decomposition of high-molecular weight (HMW) oligomeric products in the instrument,[39,40] but this cannot be verified solely using our ambient data

Summary

Introduction

The largest mass fraction of atmospheric fine particulate matter (PM2.5) is generally organic, dominated by secondary organic aerosol (SOA) formed from the gas-phase oxidation of volatile organic compounds (VOCs).[1,2,3] Biogenic VOCs (BVOCs), such as isoprene and monoterpenes, are typically the most abundant SOA precursors, especially in regions of dense terrestrial vegetation.Understanding the physical and chemical processes associated with SOA formation and properties is crucial to properly evaluate their impacts on climate and human health.[1,2,4] Despite decades of research, many substantial questions remain concerning how SOA forms and evolves in the atmosphere under different conditions. Formation of HMW compounds (m/z 500–800 Th) in the presence of high concentration of sulfate was RESULTS AND DISCUSSION Evolution of monoterpene-SOA products in the boreal forest observed (Fig. 1d).

Results

Conclusion