Abstract

Chemical ionization mass spectrometry with nitrate reagent ion (NO3− CIMS) was used to investigate the products of nitrate radical (NO3) initiated oxidation of four monoterpenes in laboratory chamber experiments. α-Pinene, ÎČ-pinene, Δ-3-carene, and α-thujene were studied. The major gas-phase species produced in each system were distinctly different, showing the effect of monoterpene structure on the oxidation mechanism and further elucidated the contributions of these species to particle formation and growth. By comparing groupings of products based on ratios of elements in the general formula CwHxNyOz, the relative importance of specific mechanistic pathways (fragmentation, termination, radical rearrangement) can be assessed for each system. Additionally, the measured time series of the highly oxidized reaction products provide insights into the ratio of relative production and loss rates of the high molecular weight products of the Δ-3-carene system. Measured effective O : C ratio of reaction products were anti-correlated to new particle formation intensity and number concentration for each system; however, monomer : dimer ratio of products was positively correlated. Gas phase yields of oxidation products measured by NO3− CIMS correlated with particle number concentrations for each monoterpene system, with the exception of α-thujene, which produced a considerable amount of low volatility products but no particles. Species-resolved wall loss was measured with NO3− CIMS and found to be highly variable among oxidized reaction products in our stainless steel chamber.

Highlights

  • IntroductionThe largest uncertainty in modern climate models is attributed to the radiative effect of aerosols (Stocker et al, 2013)

  • Measured effective O:C ratio of reaction products were anti-correlated to new particle formation intensity and number concentration for each system; 10 monomer:dimer ratio of products was positively correlated

  • Gas phase yields of oxidation products measured by NO3– CIMS correlated with particle number concentrations for each monoterpene system, with the exception of α-thujene, which produced a considerable amount of low volatility products but no particles

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Summary

Introduction

The largest uncertainty in modern climate models is attributed to the radiative effect of aerosols (Stocker et al, 2013) Their chemical complexity makes it challenging to predict their formation as well as properties that determine their direct and indirect impacts on climate. Many SOA formation pathways have been widely studied, such as the ozone (O3) and hydroxyl radical (OH) initiated oxidation of biogenic volatile organic compounds (BVOCs) (Berndt et al, 2016; Lee et al, 2006; Atkinson and Arey, 2003). One such system that has been shown to contribute significantly to SOA formation, but has not been as comprehensively studied, is nitrate radical-initiated (NO3) oxidation of BVOCs (Ng et al, 2017).

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