Preprints
https://doi.org/10.5194/acp-2021-901
https://doi.org/10.5194/acp-2021-901

  11 Nov 2021

11 Nov 2021

Review status: this preprint is currently under review for the journal ACP.

Global simulations of monoterpene-derived peroxy radical fates and the distributions of highly oxygenated organic molecules (HOM) and accretion products

Ruochong Xu1,2,a, Joel A. Thornton1, Ben H. Lee1, Yanxu Zhang2, Lyatt Jaeglé1, Felipe D. Lopez-Hilfiker1,b, Pekka Rantala3, and Tuukka Petäjä3 Ruochong Xu et al.
  • 1Department of Atmospheric Sciences, University of Washington, Seattle, WA USA 91895
  • 2School of Atmospheric Sciences, Nanjing University, Nanjing 210023, China
  • 3Institute for Atmospheric and Earth System Research (INAR), University of Helsinki, Helsinki 00014, Finland
  • anow at: Department of Earth System Science, Tsinghua University, Beijing 100084, China
  • bnow at: Tofwerk AG, Thun Switzerland

Abstract. We evaluate monoterpene-derived peroxy radical (MT-RO2) unimolecular autoxidation and self and cross reactions with other RO2 in the GEOS-Chem global chemical transport model. Formation of associated highly oxygenated organic molecule (HOM) and accretion products are tracked in competition with other bimolecular reactions. Autoxidation is the dominant fate up to 6–8 km for first-generation MT-RO2 which can undergo unimolecular H-shifts. Reaction with NO can be a more common fate for H-shift rate constants < 0.1 s−1 or at altitudes higher than 8 km due to the imposed Arrhenius temperature dependence of unimolecular H-shifts. For MT-derived HOM-RO2, generated by multi-step autoxidation of first-generation MT-RO2, reaction with other RO2 is predicted to be the major fate throughout most of the boreal and tropical forested regions, while reaction with NO dominates in temperate and subtropical forests of the Northern Hemisphere. The newly added reactions result in ~4 % global average decrease of HO2 and RO2 mainly due to faster self-/cross-reactions of MT-RO2, but the impact upon HO2/OH/NOx abundances is only important in the planetary boundary layer (PBL) over portions of tropical forests. Within the bounds of formation kinetics and HOM photochemical lifetime constraints from laboratory studies, predicted HOM concentrations in MT-rich regions and seasons reach 10 % or even exceed total organic aerosol as predicted by the standard GEOS-Chem model. Comparisons to observations reveal large uncertainties remain for key reaction parameters and processes, especially the photochemical lifetime of HOM and associated accretion products. Using the highest reported yields and H-shift rate constants of MT-RO2 that undergo autoxidation, HOM concentrations tend to exceed the limited set of observations. Similarly, we infer that RO2 cross reactions rate constants near the gas-kinetic limit with accretion product branching greater than ~0.25 are inconsistent with total organic aerosol unless there is rapid decomposition of accretion products, the accretion products have saturation vapor concentrations > > 1 μg m−3, or modeled MT emission rates are overestimated. This work suggests further observations and laboratory studies related to MT-RO2 derived HOM and gas-phase accretion product formation kinetics, and especially their atmospheric fate, such as gas-particle partitioning, multi-phase chemistry, and net SOA formation, are needed.

Ruochong Xu et al.

Status: open (until 23 Dec 2021)

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Ruochong Xu et al.

Ruochong Xu et al.

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Short summary
Monoterpenes are emitted to the atmosphere by vegetation and by the use of certain consumer products. Reactions of monoterpenes in the atmosphere lead to low volatility products which condense to grow particulate matter or participate in new particle formation and thus affect air quality and climate. We use a model of atmospheric chemistry and transport to evaluate the global scale importance of recent updates to our understanding of monoterpene chemistry in particle formation and growth.
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