CRI-HOM: A novel chemical mechanism for simulating Highly Oxygenated Organic Molecules (HOMs) in global chemistry-aerosol-climate models

Abstract

<jats:p>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 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 4–6 % 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. 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 south east 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 sulphuric acid free nucleation can compete effectively with other nucleation pathways in the boreal forest, particularly in the pre-industrial, with important implications for the aerosol budget and radiative forcing. </jats:p>

The Common Representative Intermediates Mechanism Version 2 in the United Kingdom Chemistry and Aerosols Model

“…This evaluation has also neglected analysis of aerosols, whose formation rates will differ with CRI‐Strat due to changes in oxidant fields, and will be properly evaluated in the future. There is also scope to improve the coupling between CRI‐Strat and the GLOMAP‐mode scheme, for example updating reaction rates and extending the isoprene chemistry with inclusion of CRIv2.2 (Jenkin, Khan, et al., 2019), improving representation of BVOC environments, OH recycling and further expansion with HOM chemistry (Weber et al., 2020).…”

“…Overall, this method enables efficient representation of peroxy radical chemistry without adding an excessive number of reactions or transported tracers. This framework can be built on further, for example to parameterize the formation of RO 2 accretion products which are important for the formation of highly oxidized organic material (HOM) (Weber et al., 2020).…”

“…However, it is designed to produce a similar amount of SEC_ORG as the StratTrop mechanism in order to enable fair comparison of the gas‐phase chemistry between the mechanisms and it serves as a placeholder until a more complete coupling between gas‐phase chemistry and aerosol can be developed. Working on more explicit couplings between the organic gas‐phase chemistry and aerosol routines is ongoing and will build on CRI version 2.2 (Jenkin, Khan, et al., 2019) and CRIv2.2 with Highly Oxygenated Organic Material (CRI_HOM) (Weber et al., 2020) versions of the mechanism. In the long term, this potential for more realistic chemical coupling between gas‐phase organic chemistry and SOA formation is one of the key advantages of using a semi‐explicit mechanism such as CRI‐Strat over a simpler mechanism such as StratTrop.…”

Improvements to the representation of BVOC chemistry-climate interactions in UKCA (vn11.5) with the CRI-Strat 2 mechanism: Incorporation and Evaluation

“…The addition of CS2 also lays the groundwork for the incorporation of a novel chemistry scheme which describes the formation of the highly oxidised organic molecules (HOMs) derived from biogenic species such as α-pinene (e.g. CRI-HOM, Weber et al, 2020b). HOMs are crucial for new particle formation without sulphuric acid (Kirkby et al, 2016, Simon et al, 2020, a process which is an important source of new particles in the Amazonian free troposphere (Zhao et al, 2020) and has been simulated to have consequences for our understanding of pre-industrial aerosol burden (Gordon et al, 2016).…”

The Common Representative Intermediates Mechanism version 2 in the United Kingdom Chemistry and Aerosols Model

“…Overall, this method enables efficient representation of peroxy radical chemistry without adding an excessive number of reactions or transported tracers. This framework can be built on further, for example to parameterize the formation of RO 2 accretion products which are important for the formation of highly oxidized organic material (HOM) (Weber et al, 2020).…”