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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  27 Jan 2020

27 Jan 2020

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A revised version of this preprint is currently under review for the journal ACP.

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

Claire E. Reeves1, Graham P. Mills1, Lisa K. Whalley2, W. Joe F. Acton3, William J. Bloss4, Leigh R. Crilley4,a, Sue Grimmond5, Dwayne E. Heard6, C. Nicholas Hewitt3, James R. Hopkins7, Simone Kotthaus5,8, Louisa J. Kramer4, Roderic L. Jones9, James D. Lee7, Yanhui Liu1, Bin Ouyang9, Eloise Slater6, Freya Squires10, Xinming Wang11, Robert Woodward-Massey12, and Chunxiang Ye12 Claire E. Reeves et al.
  • 1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, UK
  • 2National Centre for Atmospheric Science, School of Chemistry, University of Leeds, UK
  • 3Lancaster Environment Centre, Lancaster University, Lancaster, UK
  • 4School of Geography, Earth and Environmental Sciences, the University of Birmingham, Birmingham, B15 2TT, UK
  • 5Department of Meteorology, University of Reading, Reading, UK
  • 6School of Chemistry, University of Leeds, UK
  • 7National Centre for Atmospheric Science, Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, UK
  • 8Institut Pierre Simon Laplace, Ecole Polytechnique, France
  • 9Department of Chemistry, University of Cambridge, UK
  • 10Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, UK
  • 11Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
  • 12Beijing Innovation Center for Engineering Science and Advanced Technology, State Key Joint Laboratory for Environmental Simulation and Pollution Control, Center for Environment and Health, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China
  • anow at: Department of Chemistry, York University, Toronto, Canada

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 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 report measurements from its first field deployment, which took place in Beijing as part of the Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-Beijing) programme, along with box model simulations using the Master Chemical Mechanism (MCM) (v.3.3.1) to assess the key processes affecting the production and loss of the IN.

Seven individual isoprene nitrates were identified and quantified during the summer campaign: two β-isoprene hydroxy nitrates (IHN); four δ isoprene carbonyl nitrates (ICN); and propanone nitrate. Whilst we had previously demonstrated that the system can measure the four δ-IHN, we found no evidence of them in Beijing.

The two β-IHN mixing ratios are well correlated with an R2 value of 0.85. The mean for their ratio ((1-OH, 2-ONO2)-IHN : (4-OH, 3-ONO2)-IHN) is 3.4 and exhibits no clear diel cycle (the numbers in the names indicate the carbon (C) atom in the isoprene chain to which the radical is added). Examining this in a box model demonstrates its sensitivity to nitric oxide (NO), with lower NO mixing ratios favouring (1-OH, 2-ONO2)-IHN over (4-OH, 3-ONO2)-IHN. This is largely a reflection of the modelled ratios of their respective precursor peroxy radicals which, at NO mixing ratios of less than 1 part per billion (ppb), increase substantially with decreasing NO. Interestingly, this ratio in the peroxy radicals still exceeds the kinetic ratio (i.e. their initial ratio based on the yields of the adducts from OH addition to isoprene and the rates of reaction of the adducts with oxygen (O2)) even at NO mixing ratios as high as 100 ppb. The relationship of the observed β-IHN ratio with NO is much weaker than modelled, partly due to far fewer data points, but it agrees with the model simulation in so far as there tend to be larger ratios at sub 1 ppb amounts of NO.

Of the δ-ICN, the two trans (E) isomers are observed to have the highest mixing ratios and the mean isomer ratio (E-(4-ONO2, 1-CO)-ICN to E-(1-ONO2, 4-CO)-ICN)) is 1.4, which is considerably lower than the expected ratio of 6 for addition of NO3 in the C1 and C4 carbon positions in the isoprene chain. The MCM produces far more δ-ICN than observed, particularly at night and it also simulates an increase in the daytime δ-ICN that greatly exceeds that seen in the observations. Interestingly, the modelled source of δ-ICN is predominantly during the daytime, due to the presence in Beijing of appreciable daytime amounts of NO3 along with isoprene. The modelled ratios of δ-ICN to propanone nitrate are very different to the observed.

This study demonstrates the value of speciated IN measurements to test our understanding of the isoprene degradation chemistry. Our interpretation is limited by the uncertainties in our measurements and relatively small data set, but highlights areas of the isoprene chemistry that warrant further study, in particular the NO3 initiated isoprene degradation chemistry.

Claire E. Reeves et al.

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Claire E. Reeves et al.

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Short summary
The impact of isoprene on atmospheric chemistry is dependent on how its oxidation products interact with other pollutants, specifically nitrogen oxides. Such interactions can lead to isoprene nitrates. We made measurements of the concentrations of individual isoprene nitrate isomers in Beijing and used a model to test current understanding of their chemistry. We highlight areas of uncertainty in understanding, in particular the chemistry following oxidation of isoprene by the nitrate radical.
The impact of isoprene on atmospheric chemistry is dependent on how its oxidation products...