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What does $(+M)$ means in the following reaction from Chua et al. [1]?

$$\ce{NO2 + OH($+M$) -> HNO3($+M$)} \tag{R11}$$

It looks like $M$ could be a catalyst (maybe metal?), but usually catalysts are written above the arrow. Also, why is $(+M)$ allegedly associated with $\ce{OH}$ on the left and $\ce{HNO3}$ on the right?

Further, there is also a reaction

$$\ce{CO + OH(+O2) -> HO2 + CO2}\tag{R6}$$

Here the $\ce{(+O2)}$ participates in the reaction balance, so it is not a catalyst. Why isn't it just a reactant on the left?

Reference

  1. Chua, G.; Naik, V.; Horowitz, L. W. Exploring the Drivers of Tropospheric Hydroxyl Radical Trends in the Geophysical Fluid Dynamics Laboratory AM4.1 Atmospheric Chemistry–Climate Model. Atmos. Chem. Phys. 2023, 23 (8), 4955–4975. DOI: 10.5194/acp-23-4955-2023.
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2 Answers 2

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I think the OP is exploring this paper from 1975, behind paywall. Since the paper is about half a century old, there are some non-standard notations. However the context is clear from Karsten's answer plus their abstract.

paper

Most of the experiments have been carried out with $\ce{M}$ = $\ce{N2}$, but results have also been obtained with $\ce{M}$= $\ce{He}$, $\ce{Ar}$, $\ce{O2}$ and $\ce{SF6}$.

All it means that the main reaction was carried out in the presence of the above mentioned gases, and they did not form final products. It does not mean that gases did not form intermediates.

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    $\begingroup$ May be we can take it that $M$ is a variable , taking some values ? Here $M$ is some Molecule. $\endgroup$
    – Prem
    Commented Jul 15 at 10:54
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    $\begingroup$ To be clear, it's not that paper, it's this one: acp.copernicus.org/preprints/acp-2023-9/… But point taken. Thank you! $\endgroup$
    – jshrager
    Commented Jul 15 at 14:22
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    $\begingroup$ Please note I added the source to the question, so you might want to adjust the answer a bit. In the future, please try to ask OP for the reference in the comments first to spare the guesswork. N.B. Text should preferably be presented as text, not as image. Also, articles should be properly cited and DOI-linked, not via the publisher. $\endgroup$
    – andselisk ♦
    Commented Jul 16 at 18:36
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In gas phase reactions, it is not uncommon to have a non-reacting molecule provide or take up kinetic energy as part of the reaction. For example,

$$\ce{O2(g) + O(g) + M(g) -> O3(g) + M(g)}$$

Source e.g. https://pubs.acs.org/doi/10.1021/j100663a002

M is an unspecified molecule. You could also write it above the arrow like other catalysts.

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    $\begingroup$ Gas phase free radical combination reactions forming bonds require collisions to remove energy and stabilize the bond. M is a nonspecific inert gas presence in the reaction mix. This should have been mentioned and specified in the article. The 90 $\endgroup$
    – jimchmst
    Commented Jul 15 at 7:34
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    $\begingroup$ The inert molecule does not remove KE that is reduced to potential in the bond forming transition state The inert molecule probably collisionally deactivates a high energy vibrational state. [my idea, I have no reference for this] $\endgroup$
    – jimchmst
    Commented Jul 15 at 7:51
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    $\begingroup$ Thank you all for these helpful comments and guidance. BTW, the (+O2) doesn't appear to be a typo (it participates in the balance!), so I think that it's probably as @ACR implies, a participant someplace in a chain of reactions, although not in the first one....or something like that. $\endgroup$
    – jshrager
    Commented Jul 15 at 14:21

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