Observations of speciated isoprene nitrates in Beijing:implications for isoprene chemistry

Reeves, Claire E. and Mills, Graham P. and Whalley, Lisa K. and Acton, W. Joe F. and Bloss, William J. and Crilley, Leigh R. and Grimmond, Sue and Heard, Dwayne E. and Hewitt, C. N. and Hopkins, James R. and Kotthaus, Simone and Kramer, Louisa J. and Jones, Roderic L. and Lee, James D. and Liu, Yanhui and Ouyang, Bin and Slater, Eloise and Squires, Freya and Wang, Xinming and Woodward-Massey, Robert and Ye, Chunxiang (2021) Observations of speciated isoprene nitrates in Beijing:implications for isoprene chemistry. Atmospheric Chemistry and Physics, 21 (8). pp. 6315-6330. ISSN 1680-7316

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Abstract

Isoprene is the most important biogenic volatile organic compound in the atmosphere. Its calculated impact on ozone (O3) is critically dependent on the model isoprene oxidation chemical scheme, in particular the way the isoprene-derived organic nitrates (IN) are treated. By combining gas chromatography with mass spectrometry, we have developed a system capable of separating and unambiguously measuring individual IN isomers. In this paper we use measurements from its first field deployment, which took place in Beijing as part of the Atmospheric Pollution and Human Health in a Chinese Megacity programme, to test understanding of the isoprene chemistry as simulated in the Master Chemical Mechanism (MCM) (v.3.3.1). Seven individual isoprene nitrates were identified and quantified during the campaign: two β-hydroxy nitrates (IHN), four δ-carbonyl nitrates (ICN), and propanone nitrate. Our measurements show that in the summertime conditions experienced in Beijing the ratio of (1-OH, 2-ONO2)-IHN to (4-OH, 3-ONO2)-IHN (the numbers indicate the carbon atom in the isoprene chain to which the radical is added) increases at NO mixing ratios below 2 ppb. This provides observational field evidence of the redistribution of the peroxy radicals derived from OH oxidation of isoprene away from the kinetic ratio towards a new thermodynamic equilibrium consistent with box model calculations. The observed amounts of δ-ICN demonstrate the importance of daytime addition of NO3 to isoprene in Beijing but suggest that the predominant source of the δ-ICN in the model (reaction of NO with δ-nitrooxy peroxy radicals) may be too large. Our speciated measurements of the four δ-ICN exhibit a mean C1 : C4 isomer ratio of 1.4 and a mean trans : cis isomer ratio of 7 and provide insight into the isomeric distribution of the δ-nitrooxy peroxy radicals. Together our measurements and model results indicate that propanone nitrate was formed from the OH oxidation of δ-ICN both during the day and night, as well as from NO3 addition to propene at night. This study demonstrates the value of speciated IN measurements in testing understanding of the isoprene degradation chemistry and shows how more extensive measurements would provide greater constraints. It highlights areas of the isoprene chemistry that warrant further study, in particular the impact of NO on the formation of the IHN and the NO3-initiated isoprene degradation chemistry, as well as the need for further laboratory studies on the formation and the losses of IN, in particular via photolysis of δ-ICN and hydrolysis.

Item Type:
Journal Article
Journal or Publication Title:
Atmospheric Chemistry and Physics
Uncontrolled Keywords:
/dk/atira/pure/subjectarea/asjc/1900/1902
Subjects:
ID Code:
145946
Deposited By:
Deposited On:
27 Apr 2021 08:55
Refereed?:
Yes
Published?:
Published
Last Modified:
16 Jun 2021 09:02