Showing papers by "Michael E. Jenkin published in 2020"

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume VII – Criegee intermediates

Abstract: . This article, the seventh in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers an extension of the gas phase and photochemical reactions related to Criegee intermediates previously published in ACP in 2006, and implemented on the IUPAC website up to 2020. The article consists of an introduction, description of laboratory measurements, discussion of rate coefficients for reactions of O3 with alkenes producing Criegee intermediates, rate coefficients of unimolecular and bimolecular reactions and photochemical data for reactions of Criegee intermediates, and overview of the atmospheric chemistry of Criegee intermediates. Summary tables of the recommended kinetic and mechanistic parameters for the evaluated reactions are provided. Data sheets summarizing information upon which the recommendations are based are given in two files, provided as a Supplement to this article.

CRI-HOM: A novel chemical mechanism for simulating highly oxygenated organic molecules (HOMs) in global chemistry-aerosol-climate models

Abstract: . We present here results from a new mechanism, CRI-HOM, which we have developed to simulate the formation of highly oxygenated organic molecules (HOMs) from the gas-phase oxidation of α -pinene, one of the most widely emitted biogenic volatile organic compounds (BVOCs) by mass. This concise scheme adds 12 species and 66 reactions to the Common Representative Intermediates (CRI) mechanism v2.2 Reduction 5 and enables the representation of semi-explicit HOM treatment suitable for long-term global chemistry–aerosol–climate modelling, within a comprehensive tropospheric chemical mechanism. The key features of the new mechanism are (i) representation of the autoxidation of peroxy radicals from the hydroxyl radical and ozone initiated reactions of α -pinene, (ii) formation of multiple generations of peroxy radicals, (iii) formation of accretion products (dimers), and (iv) isoprene-driven suppression of accretion product formation, as observed in experiments. The mechanism has been constructed through optimisation against a series of flow tube laboratory experiments. The mechanism predicts a HOM yield of 2 %–4.5 % under conditions of low to moderate NOx , in line with experimental observations, and reproduces qualitatively the decline in HOM yield and concentration at higher NOx levels. The mechanism gives a HOM yield that also increases with temperature, in line with observations, and our mechanism compares favourably to some of the limited observations of [HOM] observed in the boreal forest in Finland and in the southeast USA. The reproduction of isoprene-driven suppression of HOMs is a key step forward as it enables global climate models to capture the interaction between the major BVOC species, along with the potential climatic feedbacks. This suppression is demonstrated when the mechanism is used to simulate atmospheric profiles over the boreal forest and rainforest; different isoprene concentrations result in different [HOM] distributions, illustrating the importance of BVOC interactions in atmospheric composition and climate. Finally particle nucleation rates calculated from [HOM] in present-day and pre-industrial atmospheres suggest that “sulfuric-acid-free” nucleation can compete effectively with other nucleation pathways in the boreal forest, particularly in the pre-industrial period, with important implications for the aerosol budget and radiative forcing.

Estimation of rate coefficients for the reactions of O 3 with unsaturated organic compounds for use in automated mechanism construction

Abstract: . Reaction with ozone (O3) is an important removal process for unsaturated volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of O3 with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models. Updated and extended structure–activity relationship (SAR) methods are presented for the reactions of O3 with mono- and poly-unsaturated organic compounds. The methods are optimized using a preferred set of data including reactions of O3 with 222 unsaturated compounds. For conjugated dialkene structures, site specific rates are defined, and for isolated poly-alkenes rates are defined for each double bond to determine the branching ratios for primary ozonide formation. The information can therefore guide the representation of the O3 reactions in the next generation of explicit detailed chemical mechanisms.

Investigating the Impacts of Nonacyl Peroxy Nitrates on the Global Composition of the Troposphere Using a 3-D Chemical Transport Model, STOCHEM-CRI

Abstract: Nonacyl peroxy nitrates, RO2NO2, act as a reservoir species for NOx in the upper troposphere The low thermal stability of these compounds means that they only become a significant sink of NOx at the low temperatures observed in the upper troposphere The chemical processes involved with the formation and degradation of methyl peroxy nitrate (CH3O2NO2) and an additional 44 RO2NO2 have been incorporated into the global three-dimensional chemical transport model, STOCHEM-CRI The study investigates the effect of the addition of RO2NO2 chemistry on the budget of NOx, which in turn impacts the ozone, hydroxyl radical (OH), and nitrate radical (NO3) formation This investigation found that the addition of CH3O2NO2 led to an increase in the tropospheric burdens of NOx (+30%), ozone (+20%), OH (+40%), and NO3 (+88%) However, the other 44 RO2NO2 contribute a significant increment of tropospheric global burdens of NOx (+44%), ozone (+34%), OH (+55%), and NO3 (+111%) with the largest mixing ratios of NOx of up to 25%, ozone up to 14%, OH up to 20%, and NO3 up to 50% The increase in the global burden of oxidizing species such as OH due to the addition of 44 other RO2NO2 led to a significant decrease in the lifetimes of greenhouse gases such as methane (∼6%) The modeled mixing ratios of CH3O2NO2 were in reasonable agreement with measurements, the only extensive data set available